NEET Biology

Plant Growth And Development

Growth

Growth is an irreversible increase in size, volume, and weight of a part or whole of an organism. An irreversible increase in size, volume, or weight is called apparent growth as it is the external manifestation of growth. Formation at cellular materials or protoplasm is the real growth. Growth is a measurable or quantitative phenomenon that can be measured in relation to time. The growth of living beings is internal or intrinsic.

Plant growth is diffused like that of animals only during the early embryonic stages. Later on, plants develop specific areas, called meristem, for growth. On account of meristems, plant growth is localized.

Characteristics Of Plant Growth

1. Growth is localized.
2. Growth continues throughout life.
3. There is an increase in the number of parts.
4. It is open-ended.
5. The younger one or seedling can be quite different from an adult.
6. The juvenile stage may have different traits.

Plant Growth Differentiation: Growth is invariably associated with differentiation. Differentiation is a permanent localized qualitative change in size, biochemistry, structure, and function of cells, tissues, or organs, for example, fiber, vessel, tracheid, sieve tube, mesophyll, leaf, etc. Some examples of differentiation are as follows

1. Enlargement, lignocellulosic wall thickening, and emptying in case of tracheids
2. Loss of end wall in case of vessel elements
3. Loss of nucleus and perforation of end wall in sieve tube members
4. Deposition of suberin and tannins in cells
5. Differential wall thickening (in guard cells)
6. Secretion of mucilage in root cap

Development is the sequence of changes that occur in the structure and functioning of an organism, organ, tissue, or cell involving its formation, growth differentiation, maturation, reproduction, senescence, and death.

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Characteristics Of Growth

• Primary Growth: It is the formation of primary permanent tissues and organs. It is caused by the activity of apical and intercalary meristems.
• Secondary Growth: It is an increase in girth. It occurs by two types of lateral meristems, vascular cambium and cork cambium.
• Efficiency Index: It is the rate of growth. It is measured by calculating the increase in size, diameter, or area per unit time.

Growth Rate

An increase in growth per unit of time is called growth rate. Growth rate may result in arithmetic or geometric growth.

Arithmetic Growth: Arithmetic growth is a type of growth in which the rate of growth is constant and an increase in growth occurs in arithmetic progression 2, 4, 6, 8, 10, and 12. Arithmetic growth is found in root or shoot elongating at a constant rate.

The meristematic cells at the growing point divide in such a fashion that one daughter cell remains meristematic while the other cells grow and differentiate. The process continues. Mathematically, arithmetic growth is expressed as:

L_t = L₀ + rt

where L_t is the length after time t, L₀ is the length at the beginning, and r is the growth rate or elongation per unit time.

Geometric Growth: Geometric growth is quite common in unicellular organisms when grown in a nutrient-rich medium. Here, every cell divides. The daughter cells grow and divide. The granddaughters repeat the process and so on. The number of cells is initially small so the initial growth is slow. Later on, there is rapid growth at an exponential rate.

Geometric growth cannot be sustained for long. Some cells die. Limited nutrient availability causes a slowing down of growth. It leads to the stationary phase. (There may be actually a decline.) Plotting the growth against time will give a typical sigmoid or S-curve.

The S-curve of growth is typical of most living organisms in their natural environment. It also occurs in cells, tissues, and organs of plants.

Exponential Growth (Law Of Compound Interest): Growth is dependent on three factors: initial size (W₀), rate of growth (r), and time interval (t) for which the rate of growth can be retained.

W₁ = W₀e^rt

Here W₁ is the final size, W₀ is the initial size, r is the growth rate, 1 is the time of growth, while e is the base on natural logarithms. The magnitude of r or the rate of growth has been called the efficiency index by Blackman (1919).

Absolute And Relative Growth Rate: Quantitative comparisons between the growths of various systems can be made by measuring their absolute and relative growth rates.

Absolute Growth Rate: The absolute growth curve is the actual growth curve obtained by plotting growth against time. It is commonly S-shaped. The absolute growth rate is the total growth per unit time. A graph plotted for absolute growth rates for various times of the grand period of growth appears bell-shaped.

The peak is formed when the growth rate is the fastest. The period of increasing growth is depicted by the first part of the curve while the period of decreasing growth rate is shown by the second part of the curve.

Relative Growth Rate: It is the growth per unit time per unit initial growth.

Relative growth rate = \(\frac{\text{Growth in given time period}}{\text{Measurement at start of time period}}\)

Suppose two leaves have grown by 5 cm² in one day. The initial size of leaf A was 10 cm² while that of leaf B was 15 cm². Though their absolute growth is the same, the relative rate of growth is faster in leaf A.

Measurement Of Growth

To measure growth, various methods/instruments are used.

1. Direct method
2. Horizontal microscope
3. Auxanometer
   • Arc auxanometer
   • Pfeffer’s auxanometer
4. Crescograph (Developed By J.C. Bose): A highly sensitive growth-measuring instrument that can magnify growth by 10000 times.

Growth Hormones

In all plants, minute quantities of certain substances are found (plant growth regulators or phytohormones), which regulate growth and differentiation.

Five major types of growth substances are recognized: aux-ins, gibberellins, cytokinins, abscisic acid, and ethylene.

Auxins: Auxins are weak organic acids having unsaturated ring structures.

• Charles Darwin conducted his experiments concerning growth on canary grass (Phalaris canariensis) and found that the bending movement of coleoptiles in uni-lateral light was due to a chemical.
• Went is credited with the discovery of auxin.
• Auxins are synthesized mainly in apices and exhibit polar transport through parenchyma.
• KogI and Haagen Smith (1931) found that human urine contained a growth substance, which was isolated and given the name auxin-a (auxentriolic acid). In 1934, Kogl and coworkers isolated another compound, auxin-b (auxenolinic acid) from com germ oil and heteroauxin (now known as IAA or indole-3-acetic acid, C₁₀H₉O₂N), from human urine. It is the only natural auxin.
• The precursor of auxin is the tryptophan amino acid.
• The optimum concentration in the stem apex is 10 ppm while in the root apex, it is 0.0001 ppm.
• Auxin is active in a free state and can be easily extracted. Bound auxin is inactive and meant for storage. For example, IAA-aspartic acid, IAA-inositol, and IBA-alanine.

Functions Of Auxins

• Auxins promote cell elongation by loosening cell wall microfibrils, solubilization of carbohydrate reserve, and increased respiration.
• Responsible for phototropism and geotropism.
• Promote apical dominance (in the presence of an apical bud, the growth of lateral buds are inhibited due to auxin secreted by the apical bud).
• Promote root initiation in cuttings (by NAA, IBA).
• Delay of abscission of leaves by preventing the formation of an abscission layer.
• Prevention of lodging in cereals.
• Induce parthenocarpy (production of seedless fruits).
• Selective weedicide.
• Have feminizing effect (increase number of female flowers in plants, for example, Cannabis).
• Seasonal activity of cambium is promoted by auxin.
• Healing of injury is effected through auxin-induced division in cells around the injured area.
• Auxin induces negative potential in cell membranes.
• In legumes, IAA stimulates nodule formation.

Antiauxins: They inhibit auxin activity. For example, triiodobenzoic acid (TIBA), PCIB (p-chlorophenoxy isobutyric acid)

Bioassay Of Auxins

1. Avena curvature test
2. Split pea test
3. Root growth inhibition test

Applications Of Synthetic Auxins

• Rooting: IBA, IBA-alanine, and NAA are used.
• Parthenocarpy: IAA, IBA.
• Weedicide: 2,4-D, 2,4-5-T are used for killing broad-leaved weeds (generally dicot).
• Flowering: NAA and 2,4-D for litchi and pineapple.
• Storage: Methyl ester of NAA for the storage of potato.
• Pre-harvest Fruit Drop: 2,4-D for citrus fruits; NAA for tomato.
• Prevention Of Lodging: Naphthalene acetamide (NAAM).
• Vegetable Crops: Chlorophenoxypropionic acid is used to improve the quality of vegetable crops by inhibiting flower formation. For example, lettuce.

Dwarf Shoots: NAA is used for increasing dwarf shoots and a number of fruits in apples.

Gibberellins: In the early twentieth century, Japanese farmers noticed balance or foolish seedling disease of lice. As a result of the disease, certain rice seedlings grew excessively tall, the disease was caused by the fungus Gibherella fuikuroi (perfect state of Fusarium moniliform).

• Yabuta and Suxniki (1930) isolated the growth-inducing hormone and called it gibberellin.
• Chemically, all gibberellins are terpenes, a complex group of plant chemicals related to lipids. All are weak acids and have gibbane ring skeletons. GA₃ is the commonest. GA₂₄ and GA₂₅ are found only in fungi. The precursor is acetyl CoA.
• Gibberellins are synthesized in the apices of young leaves, embryos, buds, and roots and are transported through the xylem.

Applications Of Gibberellins

• Internodal Elongation: Like auxins, the main effect of gibberellins is on stem elongation. Gibberellins stimulate stem elongation and leaf expansion but do not affect roots. Thus, gibberellins restore normal-size arid growth to genetically dwarf varieties of pear and maize.
• Bolting: In many plants, leaf development is profuse, while internodal growth is retarded. This term of growth is called rosette, for example, cabbage, radish, and henbane. Just before the reproductive phase, internodes elongate enormously, causing a marked increase in height.

The stem sometimes elongates five to six times the original height of the plant. This is called bolting. Bolting requires either long days or cold nights and gibberellins treatment.

• Germination Of Seeds: Gibberellins promote seed germination (especially in cereals).
• Control Of Flowering: Gibberellins promote flowering in long-day plants and inhibit it in short-day plants. These also control sex expression in certain species. In general, the application of gibberellins promotes the production of male flowers in female plants of Cannabis.
• Control Of Fruit Growth: Along with gibberellins, auxins control fruit growth and development. Gibberellins cause parthenocarpy in pome fruits (apple, pear, etc.).
• Vernalization: Gibberellins can substitute vernalization.
• Dormancy: Gibberellins overcome the natural dormancy of buds, tubers, seeds, etc.

Commercial Application Of Gibberellins

• Fruit Growth: Increase the number and size of grapes, tomatoes, etc. Pomalin (a mixture of GA₃ and BAP) is used for such purpose.
• Malt: Increase the yield of malt from barley.
• Overcoming Dormancy: In photoelastic seeds of tobacco and lettuce
• Delay Ripening: in citrus.
• Induce Flowering: In long-day plants, in non-inductive periods.

Antigibberellins: Certain chemicals are antagonistic to gibberellins. For example, Phosphan-D, Amo-1618, CCC, and maleic hydrazide.

Bioassay Of Gibberellins

1. Induction of α-amylase in barley endosperm test
2. Dwarf maize test
3. Dwarf pea test

Cytokinins: They are basic hormones and are purine (adenine) derivatives. Cytokinins are substances that act primarily on cell division and have little or no effect on extension growth. In 1955, Miller et al. separated it from herring sperm DNA and yeast DNA and called it kinetin (because of its involvement in cell division, i.e., cytokinesis). Later on, the substance was identified as 6-furfuryl aminopurine. Subsequently, the term cytokinin was adopted by Letham.

• The first naturally occurring cytokinin to be chemically identified was from young maize (Zea mays) grains in 1963 and was called zeatin, which is benzyl amino purine (BAP). Cytokinins are a part of t-RNA.
• Cytokinins are mostly synthesized in roots, seeds, and developing fruits. Coconut milk and apple fruit extracts are rich in cytokinins. Some other cytokinins are dihydrozeatin, IPA (isopentanyl adenine).

Applications Of Cytokinins

• Cell-division: Cytokinins are quite abundant wherever rapid cell division occurs, especially in growing tissues.
• Morphogenesis: Cytokinins promote cell division. In the presence of auxins, cytokinins promote cell division even in non-meristematic tissues. In tissue cultures, mitotic divisions are accelerated when both auxin and cytokinin are present. The ratio of cytokinins to auxins also controls cell differentiation and morphogenesis.
• Apical Dominance: Cytokinins and auxins act antagonistically in the control of apical dominance.
• Delay In Senescence: Cytokinins delay the senescence of plant organs by controlling protein synthesis and mobilization of resources. This is called the Richmond-Lang effect. Cytokinins are also called anti-aging hormones.
• Flowering: Cytokinins also induce flowering in certain species of plants such as Lemna and Wolffia and are also responsible for breaking the dormancy of seeds of some plants.
• Favors Transport: Phloem transport is promoted.
• Favors Salt Accumulation: Accumulation of salts in the cells is promoted.
• Favors Sex-Expression: Promote femaleness.
• Temperature/disease Resistance: Increase resistance to low and high temperatures and diseases.

Commercial Applications of Cytokinins

1. Tissue culture
2. The shelf life of vegetables and cut flowers is increased
3. Overcoming senescence

Bioassay Of Cytokinins

1. Chlorophyll preservation test.
2. Cell division test

Ethylene: Ethylene is the only natural plant growth regulator in gaseous form and is effective in the concentration of 0.01-10 ppm.

Ethylene is produced by most or all plant organs but maximum production occurs in ripening fruits and during senescence. High concentrations of auxin induce the formation of ethylene. Though it is a gas, it does not generally move through air spaces in plants. Rather, it escapes from the plant surface. The precursor of ethylene is methionine.

Applications Of Ethylene

• Growth: Ethylene inhibits stem elongation and stimulates its transverse expansion. As a result, the stem looks swollen.
• Abscission: It accelerates the abscission of leaves, flowers, and fruits.
• Fruit Ripening: Its chief effects are on the ripening of fruits accompanied by a rise in the rate of respiration (climacteric). It causes the dehiscence of dry fruits.
• Flowering: The application of ethylene induces flowering in pineapple.
• A commercial compound “ethephon” breaks down to release ethylene in plants. It is particularly applied to rubber plants for the flow of latex.
• It decreases the sensitivity to gravity. Roots become apogeotropic; seedling develops a tight epicotyl hook.
• It has a feminizing effect.
• Bioassay: Triple response test.

Growth Inhibitors

• For a long time, it has been suspected that dormancy is caused by inhibitors. A group of scientists led by Wareing initiated studies to find them. In 1964, pure crystals of a substance were isolated called dormin. It was found to be similar to another compound isolated from young cotton fruits in 1963 by Addicott.
• This substance accelerated abscission and was called abscission 2. In 1967, it was decided to call it abscissic acid (ABA). Since then, it has been found in all groups of plants (from mosses to higher plants). In liverworts and algae, a compound lunular acid has been found to have activities similar to ABA.
• Chemically, ABA is a dextrorotatory cA-sesquiterpene. ABA is synthesized in leaves and transported through the xylem and phloem. It is a major inhibitor of growth in plants and is antagonistic to all three growth promoters. Its precursor is violaxanthin (in chloroplast).

Applications Of ABA

• Stoppage Of Cambial Activity: It inhibits mitosis in vascular cambium.
• Bud Dormancy: It induces axillary buds to become dormant as the winter approaches.
• It plays a role in seed development, maturation, and dormancy.
• Transpiration: It is a “stress hormone” and helps the plant to cope with adverse environmental conditions by closing stomata (antitranspirant).
• It may be sprayed on tree crops to regulate fruit drop at the end of the season.
• Application of ABA to green oranges turns them yellow by inducing the synthesis of carotenoids.
• It induces flowering in some short-day plants such as strawberries and blackcurrant.

Dormancy

In favorable conditions, if a viable seed fails to germinate, this condition is called dormancy and if the viable seed fails to germinate due to unfavorable conditions, it is called “quiescence.” Dormancy may be due to:

1. Seed coat impermeable to gases, for example, apple, or water, for example, Trigonella; or seed coat mechanically resistant, for example, Capsella, Amaranthus.
2. Immaturity of the embryo, for example, G. biloba.
3. Specific light requirement: Some seeds require light for germination and are called positive photoblastic seeds (for example, Lactuca sativa, Nicotiana tobaccum, Lythrum, etc.). Lettuce (Lactuca) is induced by red light and inhibited by far red light. Some seeds show inhibition in germination due to light exposure and are called negative photoblastic, for example, onions, lily, phlox, etc.
4. Dormancy due to chilling temperature requirement, for example, Polygonum.
5. After-ripening: Some seeds have a mature embryo but do not germinate immediately due to the absence of growth hormone. They require a period of after-ripening during which they attain the power to germinate. For example, oats, barley, wheat, etc.
6. Due To Germination Inhibitors: Some chemicals such as organic acids, phenolics, tannins, alkaloids, lactones, mustard oil, etc., inhibit germination (for example, ferulic acid in tomato pulp).

Methods To Break Dormancy

1. Scarification: It is a method of softening and weakening of seed coat by acids, alcohol, or knife.
2. Stratification: After ripening treatment, low temperature (0-10°C) with O₂.
3. Light exposure.
4. Low temperature + Gibbcrellin + O₂ treatment, etc.

Seed Germination

Seed germination is of two types:

1. Epigeal: Hypocotyl grows first, cotyledons come out of the soil as in cucurbits, mustard, castor, onion, tamarind, etc.
2. Hypodeal: Epicotyl grows first, cotyledons remain underground as in rice, maize, mango, Fabaceae, etc. Whenever a seed germinates inside the fruit, it is vivipary as in Rhizophora, Sonneralia, and Heritiera (mangrove plants).

Cytochrome

Borthwick and Hendrick, in the 1950s, plotted the action spectrum of wavelengths showing their relative effectiveness in stimu¬lating seed germination. The wavelength most effective for promoting germination was 660 nm (red) and for inhibition of germination about 730 nm (far red). They also demonstrated that only brief exposures of light were necessary and that the effects of red light were reversed by far red fight and vice versa.

• The pigment responsible for this was isolated in 1960 and was called phytochrome by Butler. It is a blue-green pigment existing in two interconvertible forms: P_FR or P₇₃₀ (absorbs far-red lights) and P_R or P₆₆₀ (absorbs red light).
• By absorbing red light, P_R is converted to P_FR rapidly. P_FR absorbing far-red light is converted to P_R rapidly. P_FR is the physiologically active form; P_R is inactive. The table describes the effects of red light and far red light on plant growth.

Effects Of Red Light And Far Red Light On Plant Growth

Phytochrome is responsible for various photomorphogenic processes in plants such as the growth and development of plant organs; germination of seed, pollen, and spores; flowering; differentiation of stomata; epinasty and abscission; etc.

Photoperiiqdiibm

The response of plants to changes in the relative lengths of day and night is called photoperiodism.

Photoperiod: The relative lengths of dark and light periods in a day vary from place to place and from season to season. The length of the light period is called the photoperiod. At equators, the day length is of 12 hours duration throughout the year.

Types Of Plants According To Photoperiodic Requirements For Flowering

For SDP, P_R/P_FR > 1 while for LDP, P_FR/P_R > 1 is critical for flowering.

Photoperiodic stimulus is perceived by the leaf. When proper photoperiod is perceived, a flowering hormone called florigen is synthesized in the leaf and is transported to the bud where flowering occurs. Florigen, a hypothetical hormone, is chemically similar to gibberellins.

Difference Between Long-Day And Short-Day Plants

Vernalization (Yarovization)

The term vernalization was coined by the Russian agronomist Lysenko to refer to the method of accelerating the flowering ability of biennials or winter annuals, by exposing their soaked seeds to low temperatures for a few weeks.

• However, presently the term is used in a wider sense to include the promotion of flowering in plants by exposing them to low temperatures at any stage in their life cycle. It has been found that some plants especially biennials and perennials are stimulated to flower by exposure to low temperatures.
• This promotive effect of temperature on flowering is called vernalization. The vernalization was first studied in Europe on the winter varieties of cereals such as wheat, barley, oats, and rye by Klippart.

Site Of Vernalization: The sites of vernalization are the shoot tip, embryo tip, and root apex. As a result of vernalization, a hormone called vemalin (by Melcher) is synthesized.

Requirements Of Vernalization

1. Low temperature: 0°C-5°C
2. Period of low temperature: A few hours to a few days
3. Actively dividing cells (meristematic cells)
4. Water
5. Aerobic condition
6. Proper nourishment

As a result of vernalization, the vegetative phase of the plant is shortened and flowering is initiated. Therefore, the duration of crops is reduced.

Senescence

Senescence is the study of aging in plants. It is of the following types:

Sequential Senescence: In many perennial plants, the apical meristems continue to produce new buds and leaves, while the older leaves and lateral organs undergo senescence and die. For example, Eucalyptus, mango.

Shoot Senescence: In some perennials such as banana and Gladiolus, the aboveground part of the shoot dies every year after producing flowers and fruits. However, the underground parts survive and give rise to new shoots again in the following year.

Simultaneous Or Synchronous Senescence: In some trees such as elm, Dalbergia, and maples, all the leaves are shed in late autumn (October).

Whole Plant Senescence: In monocarpic plants that flower and produce fruits only once in life, for example, wheat, rice, mustard, etc.

Abscission

It is a natural separation or shedding of leaves, foliage branches, fruits, floral parts, etc., from plants. It is generally seasonal. A special narrow zone develops in the area of abscission called as abscission zone. Two distinct layers develop in the abscission zone:

1. Separation layer (upper layer) and
2. Protective layer (lower suberized and lignitized layer).

Plant Growth And Development Points To Remember

Strasburger studied growth in roots by marking it at equal intervals with Indian ink.

1. Ethylene causes respiratory climacteric.
2. The most widely occurring cytokinin in plants is isopentenyl adenine (IPA).
3. Sleep disease (enrolling of petals in opened flowers) is caused by ethylene. Even 1 ppm of ethylene prevents the opening of flower buds.
4. Zinc is important for auxin synthesis.
5. Clinostat is an instrument used for eliminating the effect of gravity.
6. Phytochrome is a regulatory pigment. It regulates several light-dependent developmental processes in it called photomorphogenetic processes.
7. Ethylene is the most widely used plant growth regulator in agriculture.

Plant growth And Development Assertion Reasoning Questions And Answers

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1. Assertion: If a plant is kept horizontally, auxin accumulates on the lower surface.

Reason: The displacement of statoliths and other cell organelles to lower surfaces modifies the translocation pattern of auxins.

Answer: 1. If both Assertion and Reason are true and the Rea¬son is the correct explanation of the Assertion.

Question 2. Assertion: Only bud and embryo can be vernalized.

Reason: Vernalization requires dividing cells.

Answer: 3. If Assertion is true, but Reason is false.

Question 3. Assertion: Phytochrome, a protein, has regulatory functions.

Reason: Various morphogenetic processes are regulated by it.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 4. Assertion: Auxin treatment causes acidification of the cell wall and helps in cell elongation.

Reason: Loosening of cell wall microfibrils occurs.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 5. Assertion: Cytokinins are anti-aging hormones.

Reason: They cause changes in osmotic potential by increasing the volume of mature cells.

Answer: 4. If both Assertion and Reason are false.

Photosynthesis In Higher Plants

Photosynthesis: Photosynthesis (photo = light, synthesis = putting together) is the process of the formation of simple sugars by green plants, some bacteria, and some protistans from water, soil, or from carbon dioxide in the air in the presence of sunlight and chlorophyll.

• By the process of photosynthesis, solar energy is trapped by autotrophic organisms and stored in the form of chemical energy. Annually, about 75 x 10¹² kg of carbon (in the form of CO₂) is fixed through photosynthesis, producing about 1700 million tonnes of dry matter.
• About 90% of it is carried out in the oceans (largely by phytoplanktons and algae). Only 0.2% of the light energy falling on earth is utilized by photosynthetic organisms. Photosynthesis first appeared in cyanobacteria.

Contributions By Some Scientists

Joseph Priestley: Vegetation purifies (dephlogiston) air.

Jan Ingen-Housz: Discovered photosynthesis.

Lavoisier: Phlogiston is CO₂ and dephlogiston is O₂.

de Saussure: H₂O is involved in photosynthesis, CO₂ is used and O₂ is evolved.

Dutrochet: The established connection between chlorophyll and photosynthesis.

Ruben, Kamen: Using O¹⁸ (H₂O), he found that O₂ involved in the photosynthesis comes from H₂O (in Chlorella).

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Photosynthetic Pigments

The two types of photosynthetic pigments present in higher plants are chlorophyll (discovered by Pelletier and Caventau) and carotenoids.

Chlorophyll: Chlorophylls are specialized lipid molecules embedded in the thylakoid membrane. They are the main pigment concerned with the harvesting or trapping of solar energy. Amoff and Allen (1966) recognized nine types of chlorophyll:

1. Chlorophyll-a,
2. Chlorophyll-b,
3. Chlorophyll-c,
4. Chlorophyll-d,
5. Chlorophyll-e,
6. Bacteriochiorophyll-a,
7. Bacteriochlorophyll-b,
8. Chlorobium chlorophyll-650, and
9. Chlorobium chlorophyll-666.

Out of these, the two important types of chlorophyll found in green plants are chlorophyll-a and chlorophyll-b.

Chlorophyll-a: universal pigment found in all green plants (C₅₅H₇₂O₅N₄Mg).

Chlorophyll-b: found in green algae and higher plants (C₅₅H₇₀O₆N₄Mg).

Chlorophyll-c: found in diatoms, dinoflagellates, and brown algae (C₅₅H₃₂O₅N₄Mg).

Chlorophyll-d: found in red algae.

The chlorophyll molecule is asymmetrical and consists of two parts:

1. Porphyrin head (15 x 15 Å; hydrophilic)
2. Phylol fail (20 Å; hydrophobic)

The porphyrin ring is a Mat. square structure containing four smaller pyrrole rings (tetrapyrrole) with a magnesium atom at the center. The porphyrin ring has several side groups which alter the properties of the pigment. In chlorophyll-a, there is a -CH₃ (methyl) group, while in chlorophyll-b, it is replaced by – CHO (aldehyde) group.

An isocyclic cyclopentanone ring is also attached to the third pyrrole group. The head is joined to a long hydrocarbon tail (phylol). Phylol esterification is absent in chlorophyll-c.

Chloiophyll-a is the most abundant photosynthetic pigment It is the only pigment found in all photosynthetic plants Chlorophylls absorb light near both ends of the visible spectrum, the blue and red light, and transmit or reflect green light, Therefore, these appear to be green in color.

Chlorophylls are synthesized from the precursor protochlorophyll. Its synthesis starts from glycine and succinyl CoA.

Fluorescence: Isolated chlorophyll-a when dissolved in organic solvents and exposed to light emits red color immediately. It is called fluorescence. It is, in fact, a manifestation of loss of light energy absorbed in excess. Actually, when the absorbed energy is not utilized for a purpose (phosphorylation), it is emitted as radiations of longer wavelength in green plants. Of course, this is utilized to synthesize chemical energy (ATP).

Carotenoids: Carotenoids arc yellow to orange pigments and are specialized lipids that absorb light strongly in the blue-violet range. These are called shield pigments as they protect chlorophyll from photo-oxidation (photobleaching) by the light of high intensity and also from oxygen produced during photosynthesis. These absorb light and transfer it to chlorophyll for use in pho¬tosynthesis, and therefore, function as a light-harvesting complex. Carotenoids are of two types:

1. Carotenes (C₄₀H₅₆) are orange in color and xanthophylls are yellow in color. Carotenes are hydrocarbons, most of them being tetraterpenes, for example, lycopene
2. Xanthophylls (C₄₀H₅₆O₂) are very similar to carotenes but contain oxygen, for example, violaxanthin, and fucoxanthin.

Phycobilins: Phycobilins are proteinaceous pigments found in phycobilisomes. They are soluble in hot water and do not contain magnesium and tail. They are generally of two types: phycocyanin (for example, blue-green algae) and phycoerythrin (for example, red algae). These pigments are accessory pigments and also help in chromatic adaptation (Gaidukov phenomenon).

Quantasome: Park and Biggins (1964) loaned the term quantasome for a group of pigment molecules required for carrying out a photochemical reaction. These are situated as small units on the membranes of thylakoids. Each quantasome consists of about 230 chlorophyll molecules, carotenoids, quinone compounds, sulpholipids, phospholipids, proteins, etc.

Absorption And Action Spectra

Different plant pigments absorb only certain wavelengths of light and these wavelengths are not absorbed at the same rate. A curve obtained by plotting the amount of absorption to different wavelengths of light by a particular pigment is called an absorption spectrum.

An action spectrum is a curve showing the effectiveness of different wavelengths of light in stimulating the process being investigated. The effectiveness of different wavelengths of light on photosynthesis is measured and plotted by quantum yield or amount of action (expressed through CO₂ reduction, O₂ evolution, etc.)

Hill Reaction: Hill working on Stellana media demonstrated that the evolution of O₂ occurs from illuminated chloroplast in the presence of hydrogen or electron acceptors but in the absence of CO₂, it produces assimilatory power for use in CO₂ assimilation. Hill reaction is now considered to be equivalent to light reaction. Hill oxidants are hydrogen acceptors. The common ones are ferricyanide, benzoquinone, dichlorophenol indophenol, NADP⁺ (natural H⁺ acceptor in photosynthesis).

Quantum Yield: It is the number of O₂ produced per quantum of light absorbed. It represents the rate of photosynthesis.

Quantum Requirement: It is the number of light quanta needed for the production of one O₂ molecule or reduction of one O₂ molecule.

Red Drop: Emerson measured quantum yield in Chlorella by exposing it to different monochromatic light. The yield was almost constant in all wavelengths but dropped suddenly in red light. It is called red drop.

Enhancement Effect: Emerson worked on chlorella and gave the concept of two photosystems. When Emerson and other scientists provided monochromatic light, then quantum yield suddenly dropped down (called red drop). When Emerson gave combined light, then photosynthesis increased. This is called the Emerson enhancement effect.

1. 680 nm ↑: Red drop effect
2. 680 nm ↓ + 680 nm ↑: Emerson effect

Thus, both lights had a synergistic effect, i.e., one helped the other. It leads to the conclusion that two groups of pigments, one absorbing in the lower range and the other absorbing in the higher range of wavelengths, are involved in photosynthesis and are called pigment systems.

Pigment Systems

It is a group of 250-100 pigment molecules having chlorophyll- a, chlorophyll-b, carotenoids, etc. One molecule of chlorophyll-a functions as a reaction center or trapping center and the other acts as a light-harvesting complex (LHC) or antenna molecule. These antenna molecules collect light energy and transfer this energy (by inductive resonance process) to the reaction center where the primary photochemical act occurs (quantum conversion), i.e., absorption of a photon and release of an electron.

Differences Between Pigment Systems 1 And 2

Photolysis Of H₂O

The photolysis of water occurs in the lumen of the thylakoid and requires minerals Mn⁺, CI^–, Ca^+2, and an oxygen-evolving complex (OEC).

2H₂O → 4H⁺ + 4e^– + O₂

Released electrons are attracted toward reduced reaction center P₆₈₀ of PS-2, while H⁺ accumulates in the lumen and finally reduces NADP.

Mechanism Of Photosynthesis

Photosynthesis occurs In Two Phases:

1. Light Reaction: Solar energy is trapped by chlorophyll and is stored in the form of chemical energy (ATP) and as reducing power (NADPH).
2. Bark Reaction: Reducing capacity or NADPH and the energy of ATP are utilized in the conversion of CO₂ to carbohydrates.

Warburg and others, using reaction inhibitors, proved that light and dark reactions occur independently.

1. Light Reaction: Emerson and Arnold (1932) subjected unicellular algae, Chlorella, to brief flashes of light or continued light at high intensities and observed O₂ evolution The extent of photosynthesis (per unit of light energy provided to plant) was higher when light and dark periods were alternated (photosynthetic efficiency increases in intermittent light).

• It showed that the process of photosynthesis consisted of distinct light and dark reactions (the products of the light phase are not utilized immediately in the dark phase, so photosynthetic efficiency decreases in continuous saturating light).
• During Photosynthesis, 264 g of CO₂ and 216 g of water are utilized to produce 108 g of water and 192 g of O₂.

Redox Potential: It is the tendency of an atom/molecule having low redox potential to lose electrons (electron donors) while those with high redox potential to accept electrons (gain electrons). Hence, electrons move from substances having low redox potentials to those having high redox potentials.

Assimilatory Power: NADPH + H⁺ and ATP produced in light reaction constitute assimilatory power.

Photophosphorylation: The generation of ATP in light reaction is called photophosphorylation. It was first explained by P. Mitchell who gave chemiosmotic theory. As a result of photolysis and quinone pump, H⁺ concentration in the thylakoid lumen increases by 1000-2000 times, and as a result, a proton motive force develops. This force is responsible for ATP synthesis on the head of CF₀-CF₁ a particle plugged into the thylakoid membrane.

Difference Between Photophosphorylation

2. Dark Reaction (C₃ Cycle, Calvin Cycle): The biosynthetic phase (dark reaction) is so-called because it is independent of light. The ATP and NADPH produced by the light reactions are utilized in the dark reaction to reduce carbon dioxide to carbohydrates by a process called carbon fixation. It occurs in the stroma. The process comprises a series of reactions controlled by enzymes. The sequence of these reactions was determined in Chlorella and Scenedesmus by Calvin, Benson, and

Bassham uses radioactive carbon ¹⁴C and techniques such as chromatography and autoradiography. Therefore, it is also known as the Calvin cycle or Calvin Benson cycle. The enzyme for CO₂ fixation is RuBisCo. It is a large enzyme found in stroma. It is the most abundant protein on earth and constitutes 16% of the chloroplastic protein. It shows a bifunctional nature having both carboxylase and oxygenase activity. The dark reaction is also known as the black reaction. The whole reaction can be studied in three parts

1. Carboxylation (acceptance of CO₂ by RuBP—CO₂ acceptor)
2. Glycolytic reversal
3. Regeneration of RuBP

The formation of a molecule of glucose requires 18 ATPs and 12 NADPH + H⁺ (or to reduce 60O₂).

Photorespiration

Since 1920, it has been known that higher concentrations of oxygen inhibit photosynthesis (Warburg effect). However, the reason was discovered in 1971. It was shown that RuBP carboxylase (RuBisCo) accepts not only CO₂ but also oxygen as a substrate. If oxygen is accepted, the following reaction occurs:

Oxygen is a competitive inhibitor of CO₂ fixation. Any increase in O₂ concentration would favor the uptake of oxygen rather than CO₂, and thus inhibit photosynthesis.

• The phosphoglycolate is immediately converted to glycolate. The peroxisomes present in the cell metabolize the glycolate into glycine and glycine into seine and carbon dioxide without the production of ATP or NADPH. This process is called photorespiration. Photorespiration is defined as a light-dependent uptake of oxygen and the output of carbon dioxide.
• Photorespiration has no relation with normal respiration called dark respiration. Both of these resemble only in one point, i.e., O₂ is used and CO₂ is released. Photorespiration depends upon light for the supply of RuBP. RuBP is available only when photosynthesis is operating, as RuBP is regenerated in the Calvin cycle.
• The function of photorespiration is to recover some of the carbon from the excess glycolate. However, there is a wasteful loss of carbon as CO₂ (when glycine is oxidized to serine) and energy. Although ATP is produced when glycine is oxidized to serine, the overall process is energy-consuming. Up to one-fourth of the photosynthetically fixed CO₂ may be lost by photorespiration.

With the increase in temperature, light intensity, and oxygen concentration, the affinity of RuBisCo for CO₂ decreases, and its affinity for oxygen increases. Thus, a rise in the temperature means more loss of photosynthetically fixed carbon by photorespiration. Photorespiration reduces the potential yield of plants growing in the tropics by 30-40%.

Differences Between Normal Respiration And Photorespiration

Dicarboxylic Acid Cycle (Cooperative photosynthesis)

H.P. Kortschak and C.E. Hartt (1965) found that in sugarcane (a tropical plant), leaves removed CO₂ more efficiently from the atmosphere, and the first products of photosynthesis were acids containing four-carbon acids (for example, malic, oxaloacetate, and aspartic), rather than the 3C acid PGA.

Since then, the same has been found true for many important tropical plants including monocots (such as maize, Sorghum, and Eleusine) as well as dicots (such as Amaranthus and Euphorbia sp.). These plants are called C₄ plants. On the other hand, the plants in which the first product for photosynthesis is O₃ acid PGA are called C₃ plants. Calvin cycle, in fact, is a C₃ pathway.

In 1966, two Australian scientists, Hatch and Slack, showed that C4 plants were much more efficient in CO₂ utilization than C₃ plants. Such plants do not show photorespiration. In 1967, Hatch and Slack explained the manner of CO₂ fixation and reduction in such plants. The new carbon pathway in C₄ plants is called the Hatch-Slack pathway.

• The C₄ plants possess a characteristic leaf anatomy. Their vascular bundles are surrounded by two rings of cells. The inner ring, called bundle sheath cells, contains starch-rich chloroplasts lacking grana which differ from those in the mesophyll cells that make the outer ring.
• The chloroplasts in these plants are, therefore, called dimorphic. This peculiar anatomy is called kranz anatomy because kranz means crown or halo, which refers to two distinct rings of cells. Correlation between Kranz anatomy and C₄ photosynthesis was established by Dowton and Treguna.

Since every CO₂ molecule has to be fixed twice, hence C₄ pathway is more energy-consuming than the C₃ pathway. The C pathway requires 18 ATP and 12 NADPH₂, for the synthesis of one molecule of glucose.

On the other hand, the C₄ pathway requires 30 ATP and 12 NADPH₂. However since otherwise tropical plants lose more than one-fourth of the photosynthetic carbon in photorespiration, the C₄ pathway is an adaptive mechanism for minimizing the loss.

Cam Cycle (In Succulent Plants)

The leaves of plants belonging to the Crassulaceae family. for example. certain cacti, orchids, pineapple, Kahuwhoe, and Scdum undergo crassulacean acid metabolism and such plants are called Crassulacean acid metabolism (CAM) plants.

1. All CAM plants are succulent inhabit. As such, stomata remain open in night and closed during the day (scotoactive stomata).
2. CO₂ is fixed during the night (dark) to malic acid via PEP carboxylase. This CO₂ comes from respiration (breakdown of starch) as well as from the atmosphere. Malic acid gets stored in vacuoles.
3. The CAM plants also contain the enzyme of the Calvin cycle. Thus, during the daytime, malic acid breaks into pyruvate and CO₂. While CO₂ enters the Calvin cycle, pyruvate is used up to regenerate PEP.
4. The succulents, therefore, synthesize plenty of organic acids from CO₂ during the night (when stomata are open) and plenty of carbohydrates during the day (when stomata are closed).
5. Like the Calvin cycle, the CAM cycle also operates in the mesophyll cell. None of these have shown chloroplast dimorphism as is found in C₄ plants.
6. It should be remembered that the slow-growing desert succulents exhibiting the CAM cycle have the slowest photosynthetic rate, while the species possessing C₄ pathway possess the highest rates. Thus, CAM plants are although not as efficient as C₄ plants, they are definitely better suited to adverse conditions (i.e., conditions of extreme dissipation).

Blackman (1905) proposed the law of limiting factor which states that “when a biological process is conditioned as to its rapidity by a number of separate factors, the role of the process is limited by the pace of the slowest factor.”

Factors Affecting Photosynthesis

Photosynthesis is regulated by many factors.

Light: Light is the most important factor for photosynthesis because it is used as a source of energy. Normally, plants utilize sunlight, but marine algae also use moonlight. Photosynthesis even occurs in electric light.

1. Quality Of Light: Photosynthesis occurs in the visible part of the spectrum. Photosynthesis is maximum in polychromatic light or white light.
2. Intensity Of Light: Only 1-4% of light is utilized in photosynthesis. In general, the rate of photosynthesis is higher in intense light than in diffused light. (Upto 10% light is utilized in sugar cane, i.e., the most efficient converter). For a complete plant, the rate of photosynthesis increases with an increase in light intensity, except for very high light intensity where the “solarization” phenomenon occurs, i.e., photo-oxidation of different cellular components including chlorophyll occurs.

CO₂: The normal concentration of CO₂ in the atmosphere is 360 ppm. By increasing CO₂ concentration 15-20 times, the rate of photosynthesis increases, but after that it decreases. C₄ plants show saturation at about 360 ppm.

Temperature: Generally photosynthesis is inhibited at 0°C, but some conifers even photosynthesize up to 35°C. Opuntia photosynthesizes up to 55°C. The Q₁₀ value for photosynthesis is 2 up to about 40°C, and above it, the denaturation of enzymes begins, and hence, the rate of photosynthesis decreases.

H₂O: As less than 1% of the total water absorbed by the plant is utilized in photosynthesis, so water rarely acts as a limiting factor. In water-deficient conditions, photosynthesis is found to be decreased.

Chlorophyll Content: I fall other factors are favorable, an increase in chlorophyll leads to an increase in photosynthesis.

Accumulation Of End Products: It results in a decrease in the rate of photosynthesis.

Photosynthesis In Higher Plants Points To Remember

Photosynthesis is an anabolic endergonic, oxidation-reduction process.

• PAR (photosynthetically active radiation) is the wavelength included in the range 400-700 nm.
• Destruction of chlorophyll due to high light intensity is called solarization.
• The most common limiting factor for photosynthesis is CO₂.

Light Compensation Point: The light intensity at which the rate of photosynthesis is equal to the rate of respiration (in the morning and evening).

• DCMU (dichlorophenyl dimethyl urea)—a herbicide—inhibits PS-2 and oxygen release in the light phase.
• In C₃ plants, more CO₂ is released in light than in dark due to photorespiration.
• The yield of C₃ plants is increased by increasing CO₂ in the atmosphere but not in C₄ plants.
• Chlorophyll-b absorbs more of a red wavelength than chlorophyll-a.
• Chloride ions stimulate the quick release of electrons from water.
• Phaeophytin is chlorophyll-a without Mg⁺⁺.
• The size of quant some is about 180 x 155 x 100 Å
• In C₄ plants, the bundle sheath cells possess chloroplasts without grana.
• Atriplex hastata show’s Calvin cycle whereas A triplex roseus shows the Hatch-Slack cycle.
• The reaction center in bacteria is B-890 and the reducing agent is NADH₂.
• In bacterial photosynthesis, the type of photosynthesis is anoxygenic and cyclic.

CO₂ Compensation Point: The CO₂ concentration at which the rate of photosynthesis is equal to the rate of respiration.

For C₃ plants → 50-100 ppm

For C₄ plants → 0-10 ppm

Photosynthesis In Higher Plants Assertion Reasoning Type Question And Answers

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1. Assertion: In C₄ plants, the chloroplasts of bundle sheath cells are granular.

Reason: PS-2 is mostly found in the appressed part of granum.

Answer: 4. If both Assertion and Reason are false.

Question 2. Assertion: Dark reactions of photosynthesis are temperature-controlled processes.

Reason: Most of the reactions are enzymatic in nature.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 3. Assertion: Dark acidification of cytoplasm occurs in CAM plants.

Reason: Organic acids are decarboxylated during the night.

Answer: 3. If Assertion is true, but Reason is false.

Question 4. Assertion: Assimilatory power in photosynthesis is generated in ETS occurring in 23 thylakoid membranes.

Reason: They are needed for CO reduction.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 5. Assertion: Light-harvesting complexes (LHC) on thylakoid membranes broaden the range of light absorption.

Reason: They transfer electrons to the reaction center.

Answer: 3. If Assertion is true, but Reason is false.

Transport In Plant

Importance Of Water

Water is one of the most important constituents of the protoplasm. It makes up 80-90% of the fresh weight of most herbaceous plants and over 50% of the fresh weight of woody plants. It acts as a solvent in which the oases, minerals, and other solutes enter the plant cells and move from cell to cell and from organ to organ.

• Water is a reactant or reagent in many important processes such as the hydrolysis of starch to sugar. Water maintains turgidity which is essential for cell enlargement, growth, and maintaining the form of herbaceous plants.
• Turgor is also important in the opening and closing of stomata, movement of leaves, flower petals, and various specialized plant structures. Water is also needed for the hydration of protoplasm, in the mobility of gametes, dehiscence, etc.

Diffusion: Diffusion is the movement of particles (or molecules or ions) of a substance from a region of its higher concentration to the region of lower concentration down the concentration gradient until the molecules are evenly distributed throughout the available space.

The rate and direction of a diffusing substance depend upon the concentration of that substance at different spots and is independent of the presence of other diffusing substances. The rate of diffusion is affected by many factors, for example,

Rate ∝ Temperature

Diffusion Pressure (DP): It is the pressure exerted by a substance due to the tendency of its particles to diffuse. It is also defined as the potential ability of a molecule or ion to diffuse. It is directly proportional to the diffusing particles.

DP ∝ Concentration

DP ∝ Temperature

The DP of pure water is maximum.

Facilitated Diffusion: Particles that are lipid soluble can easily pass directly through the cell membrane as it is mainly made of it. The hydrophilic solutes find it difficult to pass through the membrane. Their movement has to be facilitated. For this, the membranes possess aquaporins and ion channels.

Some carrier proteins allow transport only if two types of molecules move together. This is called cotransport. It is of two types. In the symport method of cotransport, both molecules cross the membrane in the same direction at the same time. In the antiport method of cotransport, both molecules move in opposite directions. When a molecule moves across a membrane independent of another molecule, the process is called uniport.

Read and Learn More NEET Biology Notes

Osmosis (Diffusion Through Membrane)

Osmosis may be defined as “the passage of solvent molecules from a region of their higher concentration to a region of their lower concentration through a semipermeable membrane.” In all biological systems, the solvent is water.

Types Of Membranes

• Permeable Membranes: Such membranes allow diffusion of both solvent and solute molecules or ions through them, for example, the cellulose wall of cells. Lignified cell walls are also quite freely permeable to water.
• Impermeable Membranes: Such membranes prohibit the diffusion of both solvent and solute particles through them, for example, heavily cutinized or suberized cell walls in plants.
• Semipermeable Membranes: Such membranes allow the diffusion of solvent molecules but do not allow the passage of solute molecules. Such membranes form a perfect partition between two osmometers, for example, membranes of collodion, parchment paper, and copper ferrocyanide membranes.

Differentially Permeable Membranes: However, all membranes found in plants allow some solutes to pass through them along with the solvent molecules. Such membranes are called differentially permeable membranes, for example, all biological membranes.

Osmotic Pressure

Osmotic pressure (OP) is the “maximum pressure which develops in a solution when it is separated from pure water by a semipermeable membrane.” It is also defined as “the pressure needed to prevent the passage of pure water into an aqueous solution through a semipermeable membrane thereby preventing an increase in the volume of the solution”.

The osmotic pressure depends upon

1. The concentration of solute particles,
2. Ionization of solute particles,
3. Hydration of solute particles, and
4. Temperature.

An increase in the concentration of solutes in the solution increases the osmotic pressure. If the solute ionizes in the solution, the number of particles increases, thus raising the osmotic pressure. If the solute molecules are hydrated, the water molecules bound with the solute will be unable to diffuse and hence increase the osmotic pressure.

• An increase in the temperature raises the osmotic pressure of the solution.
• Plant cells exhibit a considerable range of variations in osmotic pressure. In land plants, it varies from 5 to 30 atm.
• In aquatic plants, it varies from 1 to 3 atm. Plants of arid regions possess high osmotic pressure. The highest osmotic pressure is recorded in the halophytic plant, Atriplex confertifolia, i.e., 202.5 atm.
• The osmotic pressure of an electrolyte is two to three times greater than a non-electrolyte.
• Osmotic pressure can be calculated by OP = miRT where m is the molar concentration, i is the ionization constant, R is the gas constant, and T is the temperature (273 °C) (π). Osmotic pressure is numerically equal to osmotic/solute potential (ψ_s) but has a positive value ψ_s = π.

Measurement Of Osmotic Pressure: The methods for measuring osmotic pressure are

1. Plasmolytic method by de Vries
2. Pleffer osmometer
3. Berkeley/Hartley osmometer
4. Cryoscopic osmometer

Turgidity And Turgor Pressure

If a plant cell is placed in a hypotonic solution or pure water (i.e., a solution of higher water potential), the water starts moving into the cell by osmosis. As the volume of the protoplast increases, it begins to exert pressure against the cell wall and thus stretches it.

• The pressure exerted by the protoplast against the cell wall is called turgor pressure (TP). The cell wall, being rigid, exerts an equal and opposite pressure on the protoplast, which is called wall pressure (WP).
• The two pressures are equal and opposite in direction. As the turgor pressure of the cell increases, the cell becomes turgid. However, a stage is reached when osmotic pressure is exactly balanced by turgor pressure.
• At this point, the amount of water leaving the cell equals that entering the cell. Hence, there is no further net movement of water and the cell is said to be in equilibrium with the exterior solution.

Diffusion Pressure Deficit (Suction Pressure)

The diffusion pressure of a pure solvent is maximum. The addition of solutes results in a decrease in the diffusion pressure. This deficit is termed diffusion pressure deficit (DPD). The greater the concentration of a solution, the greater is its DPD.

• Such a solution will tend to take up water and become normal; therefore, water moves from low DPD to high DPD. The value of DPD for a cell is always positive. DPD for a cell is always positive and is calculated as (QP – TP).
• DPD is an index of the water-absorbing capacity of cell. In a flaccid cell, TP = 0. Therefore, DPD = OP.
• In a turgid cell, the entry of water into the cell causes the development of TP. At a point, TP = OP. Therefore, DPD = 0. Therefore, in a fully turgid cell, there is no net movement of water into the cell.

Chemical Potential

Chemical potential is the free energy of one mole of a particular chemical in a multi-component system. The lamer the chemical potential of a substance, the greater its tendency to undergo a chemical reaction. The chemical potential of water is called water potential and is given the value of zero at prevailing temperature and pressure.

Water Potential

Currently, the term water potential is used by biologists to describe the tendency of water molecules to move from one place to another. It is denoted by the symbol “ψ” (the Greek letter, psi).

Components Of Water Potential: Water potential is the difference in the free energy per unit molal volume of water in the pure state and in a system. For a solution, the value of water potential is always less than zero or negative. Water potential is determined by the following relation

Water Potential (ψ) = Solute potential (ψ_s) + Pressure potential (ψ_p)

Solute Potential (ψ_s): Also called osmotic potential, it is defined as the amount by which T_yr is reduced as a result of the addition of solute. It is always negative.

Pressure Potential (ψ_p): It is equal to TP and is positive except in plasmolyzed cell and in xylem vessels where it is negative. In terms of water potential, osmosis can be redefined as “the movement of water molecules from a region of higher water potential to the region of lower water potential through a differentially permeable membrane.”

Water potential may be regarded as the tendency of water to leave a system. The more the water potential, the greater is the tendency to leave. If two systems are in contact, water moves from the system with higher water potential to that with lower water potential or less negative ψ_w to more negative ψ_w.

Importance Of Osmosis: Osmosis is involved in

1. Water absorption by roots
2. Maintenance of turgidity
3. Turgor movements in Mimosa and Desmodium.
4. Stomatal movement
5. Self-dispersal of fruits (autochory)
6. Drought and frost resistance

Plasmolysis

When a cell is placed in a hypertonic solution, exosmosis occurs. Due to the loss of water to the external solution, TP of the cell decreases, hence the cell shape slightly changes. This stage is called limiting plasmolysis.

• If the cell continues to be in a hypertonic solution, more and more water is lost, therefore protoplast starts shrinking. It first leaves the comers of the cell and this stage is called as incipient plasmolysis.
• If more water is lost, the protoplast shrinks but remains in contact with the wall in one or two places. The space between the cell wall and the protoplast is occupied by the outer solution. This stage is called evident plasmolysis. The turgor pressure of a cell at limiting plasmolysis is equal to zero, whereas at incipient plasmolysis and evident plasmolysis, TP of the cell is negative.

Imbibition

The adsorption as well as absorption of a liquid by a solid without forming a solution is called as imbibition. The solid sub¬stance is called imbibing, whereas liquid is called as imbibe. The liquid particles are held in between the particles of solid by adsorption and capillarity.

The phenomenon depends upon the affinity of imbibant to imbibe. As a result of imbibition, the following events occur

Volume Change: The volume of the system increases, i.e., swelling. The total volume of water imbibed, plus the imbibing material is less after imbibition than before.

Heat Release: This is due to the compact arrangement of water molecules which liberate some of their kinetic energy.

Development Of Pressure: If the imbibing is confined, pressure may be developed. For example, in dry pea seeds, it is 1000 bars.

Imbibants present in plants are generally hydrophilic in nature. Among plant imbibing, hydrocolloids (for example, agar-agar, etc.) are the best imbibing followed by proteins, starch, and cellulose. Imbibition is affected by a number of factors such as temperature, pressure, etc., against the imbibing surface, the texture of imbibing, electrolytes, and pH.

Hypotonic Hypertonic And Isotonic Solutions

A solution with a higher concentration of solutes is said to be the hypertonic solution. A lower concentration of solutes in a solution is called a hypotonic solution, and two solutions having the same concentration of solutes are called isotonic solutions.

If a cell is placed in pure water or hypotonic solution, there is a net movement of water into the cell (endosmosis); if it is placed in a hypertonic solution, there is a net movement of water from the cell outwards (exosmosis); if the cell is placed in an isotonic solution, the amount of water leaving the cell equals that entering the cell and therefore, there is no net movement of water.

Classification Of Soil Moisture

The amount of water that can be held by soil depends upon the total pore space in the SCA. Water is present in the spaces between the soil. Various types of groundwater are

Run-off Water: Water that flows along the surface of the soil and reaches to the nearest water body constitutes run-off water. This water is not available to the plants.

Gravitational Water: Water that percolates down through the soil macropores (50 piM diameter) under the influence of gravity and reaches to the water table is called gravitational water. This type of water is also not available to the plants.

Capillary Water: Water retained by soil micropores (20 pm diameter) in the form of fine capillaries constitutes capillary water. This type of water is available to the plants.

Hygroscopic Water: Water held in the form of a very thin film around the soil particles due to adsorption constitutes hygroscopic water. This type of water is not available to the plant.

Chemically Combined Water: Water that combines with inorganic salts of the soil in the form of water of hydration constitutes chemically combined water. This type of water is also not available to the plants.

Water Vapors: They are present in soil air spaces. Normally, they are not available to the plants. Under certain conditions, they are useful in the phenomenon of night recovery.

Absorption Of Water And Pathway Of Water Across The Root

Water is mainly absorbed by the root hair zone. Root hairs are tubular elongations of the external wall of epiblema cells. They range in length from less than a millimeter to about a centimeter and are usually 10 pm in diameter. They have a vacuole filled with salt, sugars, and organic acid.

Hence, the osmotic pressure of the root hair cell is high (approx. 3-8 atm) as compared to that of soil sap (approx, less than 1 atm). The movement of water from the root hair cell to the xylem may occur by two methods.

1. Apoplast Pathway: In this method, water passes from the root hair cell to the xylem through the walls of intervening cells without crossing any membrane or cytoplasm. The apoplastic movement of water beyond the cortex is blocked due to the presence of Casparian strips in the endodermal cells. The major movement of water through cortical cells occurs by this method, as cortical cells offer the least resistance.
2. Symplast Pathway: In this method, water passes from cell to cell by crossing the plasma membrane. Therefore, it is also known as a transmembrane pathway. This may occur by two methods
   • Non-Vacuolar Symplast Pathway: In this method, water passes between adjacent cells through plasmodesmata. It does not enter into the vacuoles.
   • Vacuolar Symplast Pathway: In this method, water crosses the tonoplast surrounding the vacuole. This pathway offers a lot of resistance. Beyond the cortex through the endodermis and pericycle water is forced to move through the symplast pathway.

Mechanism Of Water Absorption: Water is mainly absorbed by two different mechanisms

1. Passive (water is absorbed through roots)
2. Active (water is absorbed by roots)

Water absorption by rapidly transpiring plants is called passive absorption because forces responsible for water uptake develop in shoots and is transmitted to roots through which water enters. Active absorption depends on forces developing in roots and is found in low-transpiring plants. The table describes the differences between passive and active absorption

Difference Between Passive Absorption And Active Absorption

Factors Affecting Water Absorption: The rate of water absorption decreases with

1. Increased salt concentration in soil
2. Decreased soil temperature
3. Decreased soil aeration (O₂)
4. Decreased soil moisture

Ascent Of Sap: The water absorbed by roots has to be conducted upwards so as to meet the needs of tissues there. This vertical conduction of water from the root to aerial parts of the plant is called the ascent of water or ascent of sap. Various theories explaining the ascent of sap have been put under three categories

1. Vital Force Theories: These theories explain the ascent of sap by the force developing in living cells.
   • Relay Pump Theory Or Clambering Theory (By Godlewski): According to it, water rises in the stem by the rhythmic changes in osmotic pressure of the xylem parenchyma and medullary rays. The water is relayed in a staircase-like manner to the next higher cell. Now it has been established that water moves through tracheids and vessels. Hence, this theory has been rejected.
   • Pulsation Theory (By J.C. Bose): According to this theory, water rises in the stem due to pulsatory activity in the innermost layer of the cortex. This theory is not accepted nowadays, as neither these pulsations are universal in occurrence nor they are able to explain the magnitude of the ascent of sap.
   • Root Pressure Theory (By Priestley): Due to the movement of water from the soil into the root hairs and from there to xylem cells, hydrostatic pressure develops. This is called root pressure (the term given by Stephan Hales).
     • This pressure pushes the water up in the xylem vessels. The root pressure can be easily observed and measured when a freshly cut stump continues to exude water (bleeding) from its xylem vessels. The development of root pressure is an active process.
     • Stocking has defined root pressure as a pressure developing in tracheary elements of the xylem as a result of the metabolic activity of roots. Root pressure depends upon the active secretion of salts or other solutes into the xylem sap, thereby lowering its osmotic potential.
     • This involves the utilization of metabolically produced energy and is inhibited by respiratory inhibitors such as cyanide, lack of oxygen, and low temperatures. The positive hydrostatic pressure generated by root pressure (maximum 2 atm) is not sufficient to push up water to more than a few meters.
     • It cannot account for water movement up the xylem in tall trees. Also, actively transpiring plants and tall trees, especially conifers, do not generate root pressure. But it is a contributing factor in many plants. It may be sufficient for the transport of water in slowly transpiring herbaceous plants.
2. Physical Force Theories: According to these theories, some physical force or dead cells are responsible for the ascent of sap.
   • Capillarity Theory (By Boehm): This theory explains the rise of the water column due to the joint force of capillarity and atmospheric pressure, as the xylem vessels have a narrow diameter and act as capillaries. This theory has been rejected as it does not explain the ascent of sap in tall trees because capillary force developed by a narrow vessel of 0.03 mm diameter would support a water column of about 1 m only.
   • Imbibition Theory (By Such): This theory explains the rise of the water column due to the force of imbibition. If it were true, water must rise through the walls of xylem vessels and not through their lumen as it always occurs. Hence, the concept was rejected.
3. Cohesion-tension Or Transpiration Pull Theory: This is the most widely accepted theory proposed by Dixon and Joley (1894). The main features of the theory are as follows
   • Right from the root hairs to the leaves on top, water forms a continuous column in the plants.
   • Water molecules have high cohesive forces between them, i.e., these tend to stick to each other, because, being polar, they are electrically attracted to each other by hydrogen bonds. The high cohesive force means that a relatively large tension is required to break the water column.
   • In other words, the water column has a great tensile strength. The magnitude is generally 10-30 MPa.
   • The lignin cellulosic cell walls of xylem vessels have a strong affinity (adhesion) for water molecules. Therefore, a strong adhesive force exists between the walls of the xylem vessels and water, i.e., water tends to “stick” to the vessel wall.
   • As the water is lost from the leaf surface by transpiration, the DPD of the leaf cells increases.
   • As a result, the cells develop low water potential and water from the leaf veins (xylem) moves into leaf cells. The xylem vessels, in turn, draw water from the xylem of the main stem. A negative (pulling) pressure is thus exerted by all the leaves on the stem. The combined pressure, called transpiration pun, is strong enough to pull up the column of water to great heights.
   • The whole column of water moves. Water potential as low as -3 MPa or -30 bars has been measured in the leaves borne on tree tops.
   • Such a low water potential is sufficient to create pulling pres-sure which can overcome the gravitational pull and resistance offered by the capillaries of xylem vessels.
4. Criticism Of Transpiration Pull Theory: The theory assumes tracheids to be more efficient than vessels. Dixon believed that partition walls of the tracheids confer stability on tensile stressed transpiration stream.

Transpiration

Less than 5% of absorbed water is used by plants and the rest is lost in transpiration.

Types Of Transpiration

• Cuticular: Loss of water vapor from the general surface through the cuticle. Commonly, it is 3-10% of the total but in herbs and ferns it may be up to 50%.
• Lenticular: Loss of water vapor from lenticels or aerating pores in the bark of trees or fruits, etc. It is hardly 0.1% of the total transpiration.
• Bark: It is approximately 1% of the total transpiration.
• Stomatal: Major form of transpiration constituting 50-97% of the total transpiration.

Number And Distribution Of Stomata: In most plants, there are 1000-60000 stomata per square centimeter of leaf surface. The total pore area is approximately 1-2% of the total leaf area. On the basis of their distribution, stomata are divided into the following categories

• Apple Type (Mulberry Type): Stomata are present only on the lower surface of the leaf, for example, apple.
• Potato Type: Stomata are present on both surfaces of the leaf but more on the lower surface, for example, potato, pea, tomato, and many other dicot plants.
• Oat Type: The number of stomata is equal on both surfaces of the leaf, for example, oat and many other monocots.
• Water Lily Type: The stomata are present only on the upper surface of the leaf, for example, water lily and most floating plants.
• Potamogeton Type: Stomata arc cither absent or vestigial, for example, Potamogeton.

Structure Of Stomata: Each stoma is surrounded by two specialized cells called guard cells. They have chloroplasts and arc green and are surrounded by two or more specialized epidermal cells called accessory or subsidiary cells. Guard cells are of two types: kidney-shaped and dumbbell-shaped. Dumbbell-shaped guard cells are found in the family Gramineae and Cyperaceae. Kidney-shaped guard cells have an inner thick wall and an outer thin and elastic wall.

Dumbbell-shaped guard cells have thin-walled ends and thick-walled middle regions. The cell wall bordering the stomatal pore is thicker than that of next to the surrounding cells. Lotfield classified stomata as follows on the basis of their daily opening and closing movement

• Alfalfa Type: Open the whole day, closed in night. For example, pea, radish, mustard, turnip, apple, grapes.
• Potato Type: Open the whole day and night except for a few hours in the evening. For example, onion, potato, banana.
• Barley Type: Open only for a few hours in day. For example, maize, wheat, and other cereals.
• Equisetum Type: Always open.

Mechanism Of Opening And Closing Of Stomata: Stomata function as turgoroperated valves. When the osmotic concentration of guard cells increases, water comes in guard cells becomes turgid and stomata are open. Whenever the osmotic concentration of the guard cell decreases, water moves out, and guard cells become flaccid, and hence become closed. This increase and decrease in osmotic concentration is explained by a number of theories mentioned below.

Photosynthetic Theory (von Mohl And Schwendener): This theory proposes that in the morning, as soon as light is available, the chloroplasts of guard cells start photosynthesis. As a result, sugars are produced, which increase the osmotic concentration of guard cells. Water comes in, guard cells become turgid, and stomata open. This theory was not accepted because the photosynthetic activity of guard cell chloroplasts seems to be negligible and sugar does not occur in detectable quantity in guard cells.

Starch ⇔ Sugar Interconversion Hypothesis (Classical Theory): The theory was proposed by Sayre and Scarth and was modified by Steward on the basis of the activity of the phosphorylase enzyme.

According to this theory, guard cells contain starch and phosphorylase enzyme, which causes the conversion of starch into glucose at higher pH which is developed by the utilization of CO₂ in photosynthesis.

Glucose increases the osmotic concentration of guard cells, raising the turgor pressure and causing the opening of stomata. At night, CO₂ accumulates in cell and intercellular spaces, thus lowering the pH at which phosphorylase causes the conversion of glucose to starch. The osmotic concentration of guard cells decreases. Hence, water moves out and, therefore, stomata are closed.

Objections: In some families, starch is absent, for example, onion. Glu¬cose does not appear in detectable quantity in the guard cells of open stomata, rather malic acid accumulates.

Active K⁺ Ion Uptake Theory (Levitt 1974): According to this theory, the pH of guard cells rises in the day due to the assimilation of CO₂ in the photosynthesis and the uptake of H⁺ ions by chloroplast and mitochondria from cytoplasm. At this higher pH, starch is converted into phosphoenolpyruvate (PEP). PEP combines with CO₂ with the help of PEPCase (phosphoenol pyruvate carboxylase) and forms oxaloacetic acid which gets changed into malic acid. Malic acid dissociates into malate and H⁺ ions.

• There occurs an efflux of H⁺ ions and the influx of K⁺ ions, which forms potassium malate after reacting with malate. Potassium malate is a highly osmotically active substance which is stored in vacuoles. This raises the osmotic concentration of guard cells, water moves in, cells become turgid, and stomata are open. In the night, the process is reversed.
• The potassium, chloride, and malate ions have a prominent role in stomatal opening. The ions accumulate in the vacuole of guard cells, lowering the water potential and thereby increasing water uptake and subsequently opening the stomata.

Factors Affecting Transpiration: Blue light induces maximum opening of stomata. Red and blue lights constitute the action spectrum of transpiration.

A rise in the temperature of the leaf increases the vaporization of water within the leaves resulting in a greater DP gradient of vapors, which diffuses out into the air rapidly. The rate of transpiration is generally doubled with about every 10° = C rise in temperature. High CO₂ closes stomata and cytokinins open stomata.

• Wind Speed: If the wind velocity is high, the rate of transpiration is also high.
• Relative Humidity: If relative humidity is low, there would be a high rate of transpiration and vice versa.
• Root/shoot Ratio: An increase in the root/shoot ratio causes an increase in the rate of transpiration. Suolt plants have a high root/shoot ratio.
• Plant Factors: Number and distribution of stomata, number of open stomata, water status of the plants, canopy structure, etc.

Significance Of Transpiration: The following roles have been given in relation to transpiration by different biologists

• Since plants absorb far more amounts of water than is actually used by plants, transpiration helps in the removal of excess water.
• Removal of water in the form of vapors has a cooling effect on the leaves. Transpiration, thus, does not allow the leaf temperature to rise to detrimental levels.
• The transpiration pull created by transpiration is responsible for the ascent of water in tall trees.
• Passive absorption of water through the roots takes place due to the negative pressure developing in the shoots as a result of transpiration.
• Development of mechanical tissues, growth of root system, increasing ash and sugar content of fruits, and development of resistance are other beneficial effects of transpiration.
• Many chemicals (antitranspirants) have been found to reduce the rate of transpiration without affecting CO₂ uptake, for example, phenylmercuric acetate (PMA) abscisic acid (ABA), and CO₂. Silicon emulsion and low viscosity wax cover stomata as a film, allowing CO₂ and O₂ but resisting diffusion of water.

Guttation

Plants growing under humid conditions in moist warm soil often exhibit droplets of water along the margins of their leaves. This phenomenon is commonly seen in oats, tomatoes, cucumbers, garden nasturtium, saxifrage, etc. The loss of water in the form of liquid is called guttation.

In moist and humid conditions, the rate of absorption of water greatly exceeds transpiration. The root pressure is built up which pushes the water up in the xylem ducts, from where it comes out on the leaf surface through special structures called hydathodes.

Hydathodes are present at the tips of veins in the leaves. A hydathode consists of a pore in the epidermis followed by large intercellular spaces and loosely arranged parenchyma called epithem and blindly ending xylem elements. Guttated water contains inorganic and organic salts and is not pure.

Translocation Of Organic Solutes

Organic solutes such as glucose, and sucrose produced during photosynthesis are translocated through phloem tissue. The transport of photosynthates from the production centers (leaves) to the consumption centers (apices, roots, fruits, tubers) through the phloem is called translocation of organic solutes or long-distance transport. Translocation through phloem occurs in upward, downward, and radial directions from the source (leaves) to the sink (apices, roots, fruits, tubers, etc.)

Chemical analysis of the phloem sap revealed the presence of sugars up to 90%. Sucrose constitutes 5-15% of the total sugars. Other sugars such as raffinose (triose). staehyose (tetrose), and verbascose (pentose) are also present in small quantities.

• This analysis strongly suggests that the phloem is the tissue concerned with the translocation of organic solutes. Phloem sap may be collected by using sap-sucking aphids.
• The tracer technique (Rabideau and Bun, 1945) supplied ¹⁴CO₂ to a leaf during photosynthesis. Sugars synthesized in this leaf were labeled with ¹⁴C (tracer). The presence of labeled sugars (radioactivity) in the phloem showed that solutes are translocated through the phloem.

Theories Of Translocation Of Organic Solutes: Protoplasmic Streaming Hypothesis: This theory was proposed by a Dutch botanist, Hugo de Vries, in 18S5 and was supported by C.F. Curtis (1935). According to this theory, the protoplasm of the sieve tubes shows continuous streaming from one end to the other.

• Organic solutes (sugars) that enter the sieve tube are passively carried by the streaming protoplasm from one end of the sieve tube to the other. Solutes move from one sieve tube to the next sieve tube by diffusion through the pores of the sieve plate.
• Thus, the streaming protoplasm acts as a conveyor belt or two-way escalator. Different substances move in different directions at the same time in the same sieve tube.

Pressure Flow Or Mass Flow Hypothesis: According to this theory proposed by Munch (1929) and elaborated by Crafts (1938), organic solutes are translocated “en masse’’ through the sieve tubes from the supplying end or source leaves to the consumption end or sink (roots, fruits, tubers).

• Mesophyll cells synthesize sugars during photosynthesis. As these get dissolved in the cell sap, the osmotic concentration and DPD of mesophyll cells increase (ψ_w decreases). Water enters the mesophyll cells from the xylem. The turgor pressure or pressure potential (ψ_p) of the mesophyll cells increases.
• Sugars dissolved in water move from mesophyll cells into the symplast system of sieve tubes. Solutes are carried “en masse” through the symplast to finally reach the consumption centers.
• At the consumption end, food materials (solutes) are either used up (as in roots) or are stored in an insoluble form (as in fruits, or tubers). Hence, the osmotic concentration and. consequently, the turgor pressure in these cells will be low. Thus, a continuous turgor pressure gradient gets established across the symplast between the cells of the source (leaf) and the cells of the sink (root).
• Water returns to the source (leaf) through the apoplast system.

Object To Mass Flow Hypothesis: The bidirectional transport of solution in the same sieve tube needs explanation. The slime content and other fibrils of the sieve tube reduce the speed of the flow of solutes even under high pressure.

Mass flow is not a purely physical process as described by Munch Phloem cells utilize 0.1 0.5% of sucrose translocated through them. This is evidence to show that phloem translocation is an active process and requires metabolic energy.

Factors Affecting Translocation Of Solutes

• Temperature: The optimum temperature for translocation is between 20° and 30°C. The file rate of translocation increases with an increase in temperature. The temperature influences the root more than the shoot since it acts as a sink for the sugars.
• Light: The root/shoot dry weight ratio increases with increased light intensities. This indicates that translocation to root increases as compared to shoot when light intensity is increased.
• Metabolic Inhibitors: The metabolic inhibitors inhibit carbohydrate translocation. These include dinitrophenol (DNP), arsenite, azide, fluoride, and hydrogen cyanide.
• Mineral Deficiencies: The absorption and translocation of sucrose by a leaf is facilitated by boron. It helps sucrose to move easily through the cell membranes in the form of the boron-sucrose complex.
• Hormones: Sucrose is much more efficiently translocated when growth regulators are applied, such as kinetin, IAA, and gibberellic acid.

Transport In Plant Points To Remember

The unit for water potential is bar or Pascal (1 MPa = 10 bar) The osmotic pressure of 1 molar solution of a non-electrolytic would be 22.4 atm at 0°C. The equimolar concentrations of two solutions of non-ionizing substances will have the same osmotic pressure.

• The value of the osmotic potential of an electrolyte will be greater by the degree of its dissociation into ions at a given temperature.
• Bacteria do not survive in salted pickles because these get plasmolyzed.
• Common salt kills weeds by plasmolysis.
• Plasmolysis can be demonstrated in the epidermal peel of the Rhoeo discolor leaf.
• A plasmolyzed cell regains normal condition if placed in a hypotonic solution. It is called deplasmolysis.
• The government of H₂O occurs from the high value of ψ_w to low value of ψ_w, i.e., from less negative value to more negative value of ψ_w.
• The auxin-treated cells show an increase in their metabolism. Respiration in these cells increases and more energy is provided for the absorption of water.
• At low temperatures, water in the intercellular spaces freezes into ice, thus having higher OP. It causes exosmosis of water from cells causing desiccation.
• If a freshwater plant is transferred to marine water, it dies due to exosmosis.
• The approximate water potential of root hairs is -3 to -43 bar.
• Ringing experiments to prove the ascent of water through the xylem were first conducted by Malpighi (1672), Stephan Hales (1727), and Hartig.
• Root pressure is measured by a manometer.
• Stocking (1956) considered root pressure as an active process responsible for guttation and bleeding in plants.
• Maximum root pressure recorded in plants is 2 bar which is sufficient to raise the water column to a height of 20 m.
• Root pressure is absent in gymnosperms (some of the tallest trees are gymnosperms).
• Photometers are used for measuring/comparing the rates of transpiration.

CoCl₂ paper method is also used to compare the rates of transpiration. Moisture coming out of stomata turns blue CoCl₂ paper to pink. Porometers are used for assessing the total pore area (stoma).

• Generally, stomata are photoactive (open during the day and close at night). But in succulents such as Bryophyllum, Opimtia, and cacti, stomata close during the day and open at night (scotoactive).
• Transpiration in old stems and fruits occurs through lenticels.
• The fresh weight of a plant or leaf would be maximum in the morning and minimum in the afternoon.
• If half of the total number of stomata on a leaf closes down, the rate of transpiration is not reduced by half.
• Stomata are absent or non-functional in submerged hydrophytes.
• Cytokinins help in the opening of stomata while ABA (abscisic acid) and low O₂ close stomata.
• Curtis (1926) considered transpiration “as a necessary evil” in plants.
• Plants growing at high altitudes show xeromorphy (adaptation to minimize transpiration).

Transpiration Ratio: the amount of water lost per unit of dry matter produced during the growing season of a plant.

• In Saxifraga, the rate of guttation is high during flowering.
• In Colocasia antiquarian, the rate of guttation is very high.
• Mechanical shock causes stomatal closure.

Stomatal index = \(\frac{E}{E + S}\) x 100 where E is the number of epidermal cells and S is the number of stomata.

Transpiration index = Leaf test tuneAVater test time.

A psychrometer is used to measure relative humidity and rate of transpiration.

The diameter of tree decreases during the day. It is due to the narrowing of tracheary elements due to the development of negative pressure. It is measured by dendrograph.

Matric Potential, ψ_m: It is used for surfaces that bind water. It is also negative. For example, soil particles, cell wall, cytoplasm, etc.

Gravity Potential, ψ_g: It denotes the effect of gravity on ψ_w. It depends on the height (h) of water above the reference state of water, the density of water, and acceleration due to gravity. The value of ψ_m negligible up to a height of 5 m from the reference level and also the value of ψ_m is ignored.

∴ ψ_w = ψ_s + ψ_p

Transport In Plant Multiple Choice Questions And Answers

Question 1. What determines the diffusion of water from one cell to another?

1. OP
2. WP
3. DPD
4. TP

Answer: 3. DPD

Question 2. The term “water potential” was used for the first time by

1. Slatyer and Taylor
2. Stocking
3. Sachs
4. Boehm

Answer: 1. Slatyer and Taylor

Question 3. In a hypertonic solution, the water potential of a cell

1. Increases
2. Decreases
3. First increases and then decreases
4. No chance occurs

Answer: 2. Decreases

Question 4. As a result of endosmosis, of a cell

1. Increases
2. Decreases
3. Remains same
4. Become zero

Answer: 1. Increases

Question 5. Which of the following equations is wrong?

1. ψ_s = -π
2. DPD = OP + TP
3. ψ_w = ψ_s + ψ_p
4. OP = CRT

Answer: 2. DPD = OP + TP

Question 6. When a cell is fully turgid, which of the following will be zero?

1. Osmotic pressure
2. Turgor pressure
3. Wall pressure
4. Suction pressure

Answer: 4. Suction pressure

Question 7. Osmotic potential is numerically equal to

1. TP
2. DPD
3. OP
4. WP

Answer: 3. OP

Question 8. The first sign of shrinkage of cell is detectable at

1. Limiting plasmolysis
2. Incipient plasmolysis
3. Evident plasmolysis
4. Permanent plasmolysis

Answer: 2. Incipient plasmolysis

Question 9. Osmotic pressure depends upon

1. Concentration of solutes
2. Temperature
3. Ionization
4. All of these

Answer: 4. All of these

Question 10. The water potential of a plasmolyzed cell will be

1. ψ_w = -ψ_s + ψ_p
2. ψ_s = ψ_p
3. ψ_w = 0
4. ψ_w  = -ψ_s – ψ_p

Answer: 4. ψ_w  = -ψ_s – ψ_p

Question 11. The water potential of soil at field capacity and wilting point are, respectively,

1. —1.5 MPa and-0.01 MPa
2. -0.1 MPa and -0.01 MPa
3. -0.01 MPa and-1.5 MPa
4. -0.01 MPa and -0.1 MPa

Answer: 3. -0.01 MPa and-1.5 MPa

Question 12. Which is not a characteristic of imbibition?

1. It is a reversible phenomenon.
2. Heat is generated.
3. Involve capillarity and adsorption.
4. It is a property of hydrophobic and lyophobic colloids.

Answer: 4. It is a property of hydrophobic and lyophobic colloids.

Question 13. The concept of apoplast and symplast imbibition was given by

1. Munch
2. Kramer
3. Renner
4. Dixon

Answer: 1. Munch

Question 14. The type of soil water that is available to roots is

1. Gravitational water
2. Hygroscopic water
3. Capillary water
4. Chemically combined water

Answer: 3. Capillary water

Question 15. Passive absorption is controlled by

1. Transpiration
2. Capillarity
3. Presence of solutes in soil
4. Temperature of atmosphere

Answer: 1. Transpiration

Question 16. If the texture of soil becomes fine, then the rate of movement of water through it will be

1. Faster
2. Slower
3. Nil
4. Same

Answer: 2. Slower

Question 17. At low temperatures, the rate of water absorption decreases due to

1. Decreased viscosity of water
2. Increased permeability
3. Reduced rate of diffusion
4. Increased root growth

Answer: 3. Reduced rate of diffusion

Question 18. Water absorption in plants is enhanced by

1. Increased transpiration
2. Decreased transpiration
3. Decreased salt absorption
4. Increased photosynthesis

Answer: 1. Increased transpiration

Question 19. The phenomenon of water uptake at the expense of energy by the cell and usually again osmotic phenomenon is known as

1. Osmosis
2. Active absorption
3. Passive absorption
4. Imbibition

Answer: 2. Active absorption

Question 20. Root pressure is measured by

1. Osmometer
2. Manometer
3. Barometer
4. Auxanometer

Answer: 2. Manometer

Question 21. When the temperature of soil is 0°C, then

1. The absorption of water increases
2. The absorption of water is not affected by temperature
3. The absorption of water decreases
4. The soil will lose capillary water

Answer: 3. The absorption of water decreases

Question 22. The osmotic theory for active water absorption was given by

1. Bennet Clarks
2. Thimann
3. Atkin and Priestley
4. Dixon

Answer: 3. Atkin and Priestley

Question 23. Root pressure is maximum when

1. Transpiration is high and absorption is very low
2. Transpiration is very low and absorption is high
3. Transpiration is very high and absorption is also high
4. Transpiration and absorption both are low

Answer: 2. Transpiration is very low and absorption is high

Question 24. The pulsation theory of the ascent of sap was given by

1. Godlewski
2. Dixon
3. Tansley
4. Sir J.C. Bose

Answer: 4. Sir J.C. Bose

Question 25. Which is not true for root pressure?

1. Positive hydrostatic pressure
2. Maximum during the day and minimum during night
3. Magnitude is 1-2 bars
4. Develops due to the metabolic activity of root.

Answer: 2. Maximum during the day and minimum during night

Question 26. A tension (transpiration pull) of 1 atm can pull water to a height of approx

1. 10 feet
2. 10 m
3. 1 m
4. 1 feet

Answer: 2. 10 m

Question 27. Amphistomatic leaves are generally found in

1. Dicots
2. Monocots
3. CAM plants
4. Aquatic plants

Answer: 2. Monocots

Question 28. The cutinized wall of epidermal cells is

1. Permeable
2. Semipermeable
3. Impermeable
4. Selective permeable

Answer: 3. Impermeable

Question 29. The stomata are widely open in

1. Red light
2. Blue light
3. Greenlight
4. Yellow light

Answer: 2. Blue light

Question 30. When transpiration is rapid

1. ψ_w of epidermal cells decreases
2. A negative pressure develops in the xylem vessel
3. Water is absorbed through the root passively
4. All of these

Answer: 4. All of these

Question 31. Which is not true regarding stomata?

1. They are turgor-operated valves.
2. They have differentially thickened walls in guard cells.
3. They open when OP of the guard cell decreases.
4. They show photoactive openings in CAM plants.

Answer: 3. They open when OP of the guard cell decreases.

Question 32. Cuticular transpiration is approx

1. 50% in herbs and ferns
2. 97% in most of the plants
3. 1% of the total transpiration
4. 50% in most of the plants

Answer: 1. 50% in herbs and ferns

Question 33. The rate of transpiration and stomatal movement are measured by, respectively,

1. Porometer and photometer
2. Potometer and pyrometer
3. Potometer and tensiometer
4. Hygrometer and porometer

Answer: 2. Potometer and pyrometer

Question 34. According to starch ⇔ sugar interconversion theory of stomatal opening, a stomatal opening is preceded by

1. Increase in H⁺ concentration
2. Decrease in the pH of the guard cell
3. Increase in the pH of guard cell
4. Inactivation of phosphorylase enzyme

Answer: 3. Increase in the pH of guard cell

Question 35. When stomata open in night only, they are called

1. Photoactive stomata
2. Scotoactive stomata
3. Hydathodes
4. All of these

Answer: 2. Scotoactive stomata

Question 36. Which of the following does not happen during stomatal opening?

1. Accumulation of K⁺ ions in guard cell
2. Lowering of osmotic pressure of guard cell
3. Creation of water potential gradient between guard cell and subsidiary cell
4. Increased thickening of the inner wall of the guard cell

Answer: 4. Increased thickening of the inner wall of the guard cell

Question 37. High amount of malate in guard cell during stomatal opening is due to

1. Import from subsidiary cell
2. Hydrolysis of starch
3. Photosynthesis in guard cell
4. Hydrolysis of proteins

Answer: 4. Hydrolysis of proteins

Question 38. Active K⁺ exchange mechanism for the opening and clos¬ing of stomata was given by

1. Darwin
2. Levitt
3. Scarth
4. Fujino

Answer: 2. Levitt

Question 39. Which of the following is a metabolic antitranspirant?

1. PMA, CO₂
2. Colorless plastics and waxes
3. Aspirin, ABA
4. Both (1) and (3)

Answer: 4. Both (1) and (3)

Question 40. Which is not an advantage of transpiration?

1. Development of mechanical tissues
2. Development of root system
3. Distribution of minerals
4. Fixation of nitrogen

Answer: 4. Fixation of nitrogen

Question 41. The plant factor which affects the rate of transpiration is

1. Leaf area
2. Temperature
3. Humidity
4. Wind speed

Answer: 1. Leaf area

Question 42. In guttation, water is lost in the form of

1. Water vapors
2. A dilute solution of sugars
3. Pure liquid water
4. Dilute solution of salts and organic substances

Answer: 4. Dilute solution of salts and organic substances

Question 43. Which of the following is an effective adaptation for better gaseous exchange in plants?

1. Presence of multiple epidermis
2. Presence of hair on the lower epidermis
3. The presence of a waxy cuticle covering the epidermis of the leaves
4. Location of stomata on the lower surface of leaves and side turned away from direct sun rays

Answer: 4. Location of stomata on the lower surface of leaves and side turned away from direct sun rays

Question 44. The conditions under which transpiration would be most rapid are

1. Excess of water in soil
2. Low humidity, high temperature, turgid guard cells, and moist soil
3. Low velocity of the wind
4. High humidity

Answer: 2. Low humidity, high temperature, turgid guard cells, and moist soil

Question 45. The transpiration index is equal to

1. \(\frac{\text{Leaf test time}}{\text{Water test time}}\)
2. \(\frac{\text{Water test time}}{\text{Leaf test time}}\)
3. The amount of water lost per unit of dry matter produced during the growing season
4. \(\frac{\text{PWP}}{\text{Field capacity}}\)

Answer: 1. \(\frac{\text{Leaf test time}}{\text{Water test time}}\)

Question 46. Translocation of photosynthates occurs in the form of

1. Starch
2. Glucose
3. Sucrose
4. 3PGA

Answer: 3. Sucrose

Question 47. Although a girdled (up to bast) tree may survive for some time, it will eventually die because

1. Water will not move upward
2. Water will not move downward
3. Sugars and other organic solutes will not move downward
4. Sugars and other organic solutes will not move upward

Answer: 3. Sugars and other organic solutes will not move downward

Question 48. When stomata open, the pH of guard cells.

1. Increases
2. Decreases
3. Remains same
4. Both (1) and (2)

Answer: 1. Increases

Question 49. Water lost in pulsation is

1. Pure water
2. Impure water
3. In vapor form
4. Either (1) and (2)

Answer: 2. Impure water

Question 50. What will happen if plant cells are placed in a hypertonic solution

1. Turgid
2. Plasmolyzed
3. Deplasmolyzed
4. Lysed

Answer: 2. Plasmolyzed

Question 51. Loss of water from the tips of leaves is called

1. Bleeding
2. Gustation
3. Respiration
4. Transpiration

Answer: 2. Gustation

Question 52. Root pressure is measured by

1. Manometer
2. Potomcter
3. Auxanomcter
4. Osmometer

Answer: 1. Manometer

Question 53. Which of the following apparatus is commonly used to measure the rate of transpiration?

1. Porometer
2. Altimeter
3. Potomcter
4. Luxmeter

Answer: 3. Porometer

Question 54. Leaves of the Nehnnbo plant are

1. Epistomatic
2. Hypostomatic
3. Amphistomatic
4. None of these

Answer: 1. Epistomatic

Question 55. 0.1 M solution has the water potential of

1. -2.3 bars
2. 0 bar
3. 22.4 bars
4. +2.3 bars

Answer: 1. -2.3 bars

Question 56. A small mesophytic twig with green leaves is dipped into water in a big beaker under sunlight. It demonstrates

1. Photosynthesis
2. Respiration
3. Transpiration
4. None of the above

Answer: 3. Transpiration

Question 57. Which one is not related to transpiration?

1. Regulation of plant body temperature
2. Absorption and distribution of mineral salt
3. Circulation of water
4. Bleeding

Answer: 4. Bleeding

Question 58. Stomata can open at night also in

1. Xerophyte
2. Gamelophyte
3. Hydrophyte
4. None of these

Answer: 1. Xerophyte

Question 59. Who had said that “transpiration is a necessary evil”?

1. Curtis
2. Steward
3. Andersen
4. J.C. Bose

Answer: 1. Curtis

Question 60. Stomata open during day because the guard cells have

1. Thin outer walls
2. Kidney shape structure
3. Chlorophyll
4. Large nuclei

Answer: 1. Thin outer walls

Question 61. Stomata open and close due to

1. Turgor pressure change
2. Hormone change
3. Temperature change
4. All of the above

Answer: 1. Turgor pressure change

Question 62. In plasmolyzed cell, the space between cell wall and protoplasm is occupied by

1. Hypotonic solution
2. Hypertonic solution
3. Isotonic solution
4. Distil water

Answer: 2. Hypertonic solution

Question 63. In CAM plants, stomata are

1. Closed at night and open during the day
2. Closed during the day and open at night
3. Never closes
4. Never opens

Answer: 2. Closed during the day and open at night

Question 64. The real force responsible for the movement of water from cell to cell is

1. OP
2. TP
3. DPD
4. WP

Answer: 3. DPD

Question 65. Which of the following have sunken stomata?

1. Nerium
2. Mangifera
3. Hydrilla
4. Zea mays

Answer: 1. Nerium

Question 66. When a plasmolyzed cell is placed in a hypotonic solution then water will move inside the cell. Which force causes this?

1. DPD
2. OP
3. WP
4. None of these

Answer: 1. DPD

Question 67. Rate of transpiration is measured by

1. Manometer
2. Auxanometer
3. Potometer
4. Barometer

Answer: 3. Potometer

Question 68. If a cell shrinks when placed in a solution, the solution becomes

1. Hypotonic
2. Hypertonic
3. Isotonic
4. Pure solvent

Answer: 2. Hypertonic

Question 69. If cell A with DPD 4 bar is connected to cell B, C, and D whose osmotic pressure and turgor pressure are, respectively, 4 and 4, 10 and 5, 7 and 3 bar, the flow of water will be

1. B to A, C, and D
2. A to D, B, and C
3. C to A, B, and D
4. A to B, C, and D

Answer: 1. B to A, C, and D

Question 70. Guard cell controls

1. The intensity of light entering
2. Photosynthesis
3. Closing and opening of stomata
4. Change in green color

Answer: 3. Closing and opening of stomata

Question 71. Active transport

1. Releases energy
2. Requires energy
3. Produces ATP
4. Produces a toxic substance

Answer: 2. Requires energy

Question 72. Velamen tissues are associated within

1. Haustorial function
2. Assimilation
3. Absorption of moisture
4. Nutrition

Answer: 3. Absorption of moisture

Question 73. Cohesion tension theory regarding the ascent of sap was given by

1. Dixon and Jolly
2. J.C. Bose
3. Cristian Wolf
4. Godlewski

Answer: 1. Dixon and Jolly

Question 74. Velamen tissue is found in

1. Mesophytes
2. Epiphytes
3. Hydrophytes
4. Xerophytes

Answer: 2. Epiphytes

Question 75. In a fully turgid plant cell, which one is zero?

1. Turgor pressure
2. Wall pressure
3. Suction pressure
4. None of these

Answer: 3. Suction pressure

Question 76. Who proposed the cohesion theory of the ascent of sap?

1. Strasburger
2. Godlewski
3. Western
4. Oixon and Jolly

Answer: 4. Oixon and Jolly

Question 77. The most accepted theory for ascent of sap is

1. Relay pump theory
2. Pulsation theory
3. Root pressure theory
4. Transpiration pull cohesion theory

Answer: 4. Transpiration pull cohesion theory

Question 78. The transport of water and salt is mediated by

1. Xylem
2. Sieve tubes
3. Sclerenchyma
4. Phloem

Answer: 1. Xylem

Question 79. The removal of a ring of tissue outside the vascular cambi¬um from the tree trunk kills it because

1. Water cannot move up
2. Food does not travel down and roots become starved
3. Shoots become starved
4. Annual rings are not produced

Answer: 2. Food does not travel down and roots become starved

Question 80. Wilting of the plant is present in

1. Moss
2. Fern
3. Algae
4. Angiosperm

Answer: 4. Angiosperm

Question 81. Root hair absorbs water from the soil on account of

1. Turgor pressure
2. Osmotic pressure
3. Suction pressure
4. Root pressure

Answer: 3. Suction pressure

Question 82. Increased humidity in the atmosphere decreases the rate of

1. Transpiration
2. Photosynthesis
3. Glycolysis
4. Growth

Answer: 1. Transpiration

Question 83. In osmosis, there is movement of

1. Solute only
2. Solvte only
3. Both (1) and (2)
4. Neither solute nor solvent

Answer: 2. Solvte only

Question 84. Guttation takes place through

1. Lenticles
2. Pneumatophores
3. Stomata
4. Hydathodes

Answer: 4. Hydathodes

Question 85. Which of the following statements is correct?

1. Cell membrane is involved only in exosmosis
2. Cell membrane is involved only in endosmosis
3. Cell membrane is involved both in exosmosis and endosmosis
4. None of the above

Answer: 3. Cell membrane is involved both in exosmosis and endosmosis

Question 86. The root hairs absorb which of the following types of water?

1. Capillary water
2. Hygroscopic water
3. Gravitational water
4. All of the water

Answer: 1. Capillary water

Question 87. If flowers are cut and dipped in dilute NaCl solution, then

1. Transpiration will become low
2. Endoosmosis occurs
3. No bacterial growth takes place
4. Absorption of solute inside flower cell takes place

Answer: 1. Transpiration will become low

Question 88. A plant cell is plasmolyzed in a solution that is

1. Hypotonic
2. Hypertonic
3. Isotonic
4. Concentration no means

Answer: 2. Hypotonic

Question 89. Turgidity in guard cells is controlled by

1. Chloride
2. Malic acid
3. Potassium
4. Potassium, chloride, and malic acid

Answer: 4. Potassium, chloride, and malic acid

Question 90. Stomata are not found in

1. Algae
2. Mosses
3. Ferns
4. Liverworts

Answer: 1. Algae

Question 91. In which of the following, the rate of transpiration is high?

1. CAM plant
2. C₃ plants
3. C₂ and C₄ plants
4. C₄ plants

Answer: 1. CAM plant

Question 92. Cell sap is found in which cell organelle?

1. Nucleolus
2. Chloroplast
3. Vacuole
4. Golgi apparatus

Answer: 3. Vacuole

Question 93. Which one of the following fixes nitrogen?

1. TMV
2. Yeast
3. Nostoc
4. Denitrifying bacteria

Answer: 3. Nostoc

Question 94. Active transport of ions by the cell requires

1. High temperature
2. ATP
3. Alkaline pH
4. Salts

Answer: 2. ATP

Question 95. To initiate cell plasmolysis, the salt concentration must be

1. Isotonic
2. Hypotonic
3. Hypertonic
4. Atonic

Answer: 3. Hypertonic

Question 96. The basis of stomatal opening is

1. Endosmosis
2. Plasmolysis of guard cells
3. Decrease in cell up concentration
4. Exosmosis

Answer: 1. Endosmosis

Question 97. Plants absorb carbon dioxide from

1. Vegetative
2. Heterocyst
3. Both vegetative and heterocyst
4. None of these

1. Millets
2. Cereals
3. Carbohydrates present in the soil
4. Atmosphere

Answer: 4. Atmosphere

Question 98. Transpiration will increase with the increase of

1. Humidity
2. Temperature
3. Carbon dioxide
4. Sulfur dioxide

Answer: 2. Temperature

Question 99. If it is possible to drop a small particle through the stomata of a leaf, what will you conclude?

1. It will fall on the earth surface.
2. It will stop on the lower epidermis.
3. It will stop on mesophyll cells.
4. It will stop on vascular tissue.

Answer: 3. It will stop on mesophyll cells.

Question 100. During transpiration, turgidity in guard cells is controlled by

1. Potassium
2. Bromine
3. Sodium
4. Oxalic acid

Answer: 1. Potassium

Question 101. Apparatus used for measuring the transpiration

1. Evapometer
2. Potometer
3. Osmometer
4. Tensiometer

Answer: 2. Potometer

Question 102. Which of the following theories gives the latest explanation for the closure of stomata?

1. ABA theory
2. Munch theory
3. Starch-glucose theory
4. Active KT transport theory

Answer: 1. ABA theory

Question 103. The sugarcane plant has

1. Dumbbell-shaped guard cells
2. Pentamerous flowers
3. Reticulate venation
4. Capsular fruits

Answer: 1. Dumbbell-shaped guard cells

Transport In Plant Assertion Reasoning Questions And Answers

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1. Assertion: Xerophytes have high water-retaining capacity.

Reason: They have high OR.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 2. Assertion: There is an indirect relationship between the rate of respiration and water absorption.

Reason: Increased metabolism increases mineral uptake.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 3. Assertion: Root pressure is a dynamic and always a positive hydrostatic pressure.

Reason: It is a universal phenomenon and develops under absorption lag.

Answer: 3. If Assertion is true, but Reason is false.

Question 4. Assertion: Stomata has delegated the task of providing food while preventing thirst.

Reason: They are made for gaseous exchange.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 5. Assertion: During stomata opening, there is a relative change in TP of the guard cell and subsidiary cell.

Reason: TP of the subsidiary cell decreases during the opening.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Mineral Nutrition Introduction

The sum total of various processes by which an organism with-draws and utilizes the substances required for the development and sustaining life-related processes is called nutrition, and these substances are called nutrients.

• The absorption, distribution, and metabolism of various mineral elements by plants is called mineral nutrition. About 60 elements have been reported from plant ash (obtained after heating dry plant matter at 600°C). Out of these, 30 are present in all the plants.
• Inorganic nutrients are classified as essential and non-essential elements. Seventeen elements have been placed in essential elements. These are the elements without which the reproduction and life cycle of a plant cannot be completed. The essential elements are C, H, O, N, P. K. S, Mg, Ca, Fe, Mo, Mn, Ni, Zn, B, Cl, and Cu.

Amon and Stout proposed the following criteria of the essentiality of an element:

• The element must be absolutely necessary for supporting normal growth and reproduction.
• The requirement of the element must be specific and not replaceable by another element.
• The element must be directly involved in the nutrition of the plant.
• A deficiency of minerals causes diseases in plants. Magnesium. for example, is a constituent of the chlorophyll molecule and is essential for photosynthesis. It cannot be replaced by any other element for the same function. It is also required as a cofactor by many enzymes involved in cellular respiration and metabolic pathways. Similarly, iron is a constituent of cytochromes.

Depending Upon The Quantity In Which These Elements Are Present In Cell, They Are Classified As:

Macronutrients: They are the elements that are present in a quantity of 1—10 mg/g dry wt. of the cell. They are C, H, O, N, P, S, K, Mg, and Ca.

Micronutrients: They are the elements that are present in a quantity equal to or less than 0.1 mg/g dry wt. of the cell (often less than 1 ppm). They are Fe, Mn, Mo, Ni, Zn, B, Cl, and Cu.

Mineral Nutrition Culture Medium

Soils normally contain sufficient quantities of essential minerals. However, three important elements need to be replenished in crop fields as they are depleted by repeated cultivation. These x fertilizer elements called critical elements are nitrogen, phosphorus, and potassium (NPK).

• The common sources of these elements used in India are nitrate of sodium, ammonium sulfate, ammonium nitrate, ammonium chloride, urea, etc. The NPK fertilizers comprising bags of fertilizers are labeled 17-18-19 or 15-15-15 or other combinations. These numbers refer to the percentage by weight of nitrogen, phosphorus, and water-soluble potassium
• To determine what elements are essential tor plant growth and deficiency symptoms of an essential element, a well-defined nutrient medium has to be used. Seeds are grown in highly washed pure sand in a glass, glazed porcelain, or plastic container and supplied with a carefully made-up nutrient solution.

Arnon And Hoagland’s Medium: Amon and Hoagland prescribed a medium containing micronutrients. Iron was earlier supplied as ferrous sulfate, but it often precipitated out. This problem has now been solved by dissolving ferrous sulfate along with a chelating agent Na-EDTA (disodium salt of ethylene diamine tetraacetic acid).

Read and Learn More NEET Biology Notes

Mineral Nutrition Solution Culture

Solution culture is performed in glass jars or polythene bottles. The container is covered with black paper after pouring the solution into them. Black paper has two functions

1. Prevention of the growth of algae.
2. Prevention of reaction of roots with light.

Seeds are allowed to germinate over the cork. Cotyledons are removed after seedling formation. The plant is properly sup¬ported with the help of split cork. The solution is aerated at regular intervals and is changed after two to three days.

Mineral Nutrition – Hydroponics

The commercial technique of soil-less culture is called hydroponics, which was first developed by Goerick (1940). The culture is performed in large tanks of metal or RCC tanks. The tanks are covered with wire mesh. Tanks are provided with aerating and circulating techniques. Seeds are suspended in the solution from the wire mesh with the help of threads. As the plant grows, additional support is provided. it depicts the diagrammatic views of the hydroponic technique.

Liebig’s Law Of The Minimum

As per this law, “the yield of a crop plant is determined by the amount of the necessary element which is present in minimum quantity in proportion to the demand of the plant.”

General Functions Of Mineral Elements

Frame Work Elements: They form carbohydrates that form cell walls, for example, C, H, and O.

Protoplasmic Elements: They form protoplasm, for example, C, H, O, N, S, P.

Catalytic Elements: Fe, Cu, Zn, Mo, Mg, Mn, K (activator of over 40 enzymes)

Balancing Elements: Ca, Mg, and K counteract the toxic effect of other minerals.

Storage Elements: C, N, S, P.

Critical Elements: N, P, K.

Elements Affecting Cell Permeability: Monovalent cations (Na⁺, K⁺) increase the permeability of membranes while divalent and trivalent cations decrease it.

Toxic Elements: Al, As, Hg, Pb, Ag.

Non-mineral Elements: C, H, O, N. Nitrogen is a mineral as well as a non-mineral element.

Functional Elements: They are non-essential in most plants but have a definite activity in some species, for example, silicon in grasses.

Nitrogen Fixation

Atmosphere is the ultimate source of nitrogen. Nitrogen is a highly inert gas. It cannot be used directly but has to be combined with C, H, N, and O to form compounds before being used. Higher plants utilize nitrogen in the oxidized forms such as nitrate (NO₃^–) and nitrite (NO₂^–) or in the reduced form (NH₄) made available to plants by the nitrogen fixers.

The best-known nitrogen-fixing symbiotic bacterium is Rhizobium. This bacterium lives in the soil to form root nodules in plants belonging to the family Leguminosae such as beans, gram, groundnut, and soya bean. Root nodules are little outgrowths on roots.

• When a section of the root nodule is examined, it appears pinkish due to the presence of a pigment called leghemoglobin. Like hemoglobin, leghemoglobin is an oxygen scavenger. The enzyme that catalyses the fixation of nitrogen is nitrogenase which functions under anaerobic conditions. Leghemoglobin combines with oxygen and protects nitrogenase.
• Free-living microorganisms such as the cyanobacteria can also fix nitrogen. Some cyanobacteria also have symbiotic as¬sociation with plants. They are found in lichens. Anthoccros. Azollcu and coralloid roots of Cycas.
• In the process of biological nitrogen fixation, the dinitrogen molecule is progressively reduced by the addition of pairs of hydrogen atoms so that the three bonds between the two nitrogen atoms are cleaved and ammonia is formed.

These reactions occur only in the presence of a single enzyme nitrogenase. The process of nitrogen fixation is energy-intensive.

Requirements Of N₂ Fixation

• Nitrogenase enzyme complex (synthesized by nif genes of bacteria) is the seat of nitrogen reduction and contains Mo, Fe, and S.
• It is a strong reducing agent, for example, NADPH₂, FMNH₂, ferredoxin, etc.
• It takes place under anaerobic conditions.
• The energy source is ATP.
• Cofactors included are TPP, CoA, Mg⁺².
• Electron and H⁺ donor (generally glucose-6-phosphate)

Process Of Nodule Formation

Nodules are little out growths on the roots. When a root hair of a legume comes in contact with Rhizobium, there occurs an ex-change of plant and bacterial signals. Bacteria secrete nod fac-tors which result in the curling of root hair tips.

The plant responds by forming an infection thread that grows inward and carries the bacteria to the cortical cells of the root. Some of bacteria enlarge to become membrane-bound structures called bacteroids which are the seat of N₂ fixation. Plant flavones act as the inducers of nod genes which specify early events of nodulation.

Cortical cells are stimulated to divide rapidly. It is due to auxin secreted by plants and cytokinin contributed by bacteria. Nodules are pink in color due to leghemoglobin which is an oxygen scavenger and protects nitrogenase. Its heme comes from bacteria and globin from legumes.

Process Of N₂ Fixation Can Be Summarized As: N₂ + 8e^– + 8H⁺ + 16 ATP → 2NH₃ + H₂ + 16ADP + 16P_i

Ammonium ions can be taken up by higher plants but plants are more adapted to absorb nitrate (NO₃^–) than ammonium ions (NH₄⁺) from the soil. Soil bacteria such as Nitrosomonas and Nitrococcus convert ammonia to nitrite (NO₂^–) ions. Nitrobacter oxidizes nitrite to nitrate. This process of converting ammonia into nitrate, a form of nitrogen more available to plants, is called nitrification.

This process is an oxidation process and releases energy which is used by nitrifying microbes.

Nitrate Assimilation

The process of nitrate reduction to ammonia is called nitrate assimilation and is accomplished in two steps mediated by two specific enzymes. First, the nitrate is reduced to nitrite by an enzyme called nitrate reductase. This enzyme is a flavoprotein and contains molybdenum.

The nitrite ions are then reduced to ammonia by an enzyme called nitrite reductase. Ferredoxin is the most detect source of electrons for nitrite reduction and hence it occurs specifically in leaves. Therefore, nitrite ions formed in other parts of the plant are transported to leaves arid and further reduced to ammonia. Nitrite reductase does not require molybdenum but contains copper and iron.

Synthesis Of Amino Acids

There are two main processes for amino acid synthesis.

1. Reductive Animation: From glutamic acid all other amino acids can be synthesized by the process of transamination.

2. Transamination: It involves the transfer of amino groups from one amino acid to the keto group of the keto acid. The reactions are catalyzed by the enzyme transaminase.

Mineral Absorption

Mineral Absorption Occurs By Two Ways:

1. Passive and
2. Active.

1. Passive Mineral Absorption: The main theories of passive mineral absorption. In most cases, the movement of mineral ions into the root occurs by diffusion. Molecules or ions diffuse from a region of their higher concentration to a region of their lower concentration. The movement of mineral ions into root cells as a result of diffusion is called passive absorption.
   • Donnan Equilibrium Theory: This theory was proposed by Donnan (1911). The entry of ions into the cell across the plasma membrane to maintain electrical equilibrium is called Donnan equilibrium. Some anions/cations get firmly attached to the inner surface of the plasma membrane (fixed and non-diffusible ions). To neutralize these, ions of opposite charges gain entrance in the cell passively against the concentration gradient without energy expenditure.
   • Ion Exchange Theory: It was proposed by Jenny and Overstreet (1938). The exchange of anions and cations absorbed on a colloidal fraction of the soil (clay and humus) with the ions adsorbed on the root surface is referred to as ion exchange. The small space in which the cations and anions attached to the surface of roots and particles oscillate is called oscillation volume. It is of two types
   • Contact Exchange: This includes the exchange of cation and anions from the root with similarly charged ions of soil solution.
   • Carbonic Acid Exchange: This includes the exchange of H⁺ and CHO₃^– ions from the root with similarly charged ions of the soil solution.
   • Bulk Flow/Mass Flow Theory: It was proposed by Hylmo (and supported by Kramer). According to it, the movement of ions occurs through roots along with the stream of water under the influence of transpiration.
2. Active Mineral Absorption: The uptake of mineral ions against a concentration gradient is called active absorption. Such movement of minerals requires the expenditure of energy by the absorbing cell. This energy is derived from respiration and is supplied through ATP. Hence, when the roots are deprived of oxygen, they show a sudden drop in the active absorption of minerals. Mostly minerals are absorbed by active mechanisms. Various theories are given for the active uptake of minerals.
   • Carrier Concept (By Van den Honert): Carriers are specific proteins in membranes that form complexes with ions. This complex is capable of moving across the membrane and releasing ions on the inner side. There are separate carriers for cations and anions.
   • Cytochrome Pump (By Lundegardh And Burstrom): Cytochromes are iron-containing proteins in the membrane. They transport electrons from the inner surface to the outer surface of the membrane and also transport anions from the outer surface to the inner surface. Anions are transported actively while cation transport is passive. Anion uptake increases the rate of respiration called salt respiration.
   • Protein Lecithin Carrier (by Bennet and Clark): Carrier is a protein associated with a phosphatide called lecithin. It is an amphoteric earner and transports both cations and anions.

Mobility

The elements that cannot move freely in the plants (for example, Ca, B, S, Fe, etc.) are called immobile elements. Deficiency symptoms of such elements first appear in young leaves. Those elements that can move from old leaves to young leaves and growing tips (N, P, K, Cl, Mg) are called mobile elements. Their deficiency affects old leaves.

Mineral Nutrition Points To Remember

Sodium (Na) is required by a desert shrub Atriplex and not by most of the other species.

1. Efforts are being made to develop varieties of plants that can mine metals from the soil so that that soil can be reclaimed for agriculture practice, called phytore¬mediation.
2. Foliar application of Fe, Mn, and Cu is more efficient than application through the soil.
3. Aeroponics (by Soifer Hillel and David Durger) is a system where roots are suspended into some plastic vessel and misted with oxygenated, nutrient-en-riched water.
4. Winogradsky (1891) discovered biological nitrogen fixation.
5. Cobalt is found mostly in hydathodes and is required in N₂ fixation.
6. Selenium is found in Astragalus.
7. Excess of manganese may, in fact, induce the deficiency symptom of iron, magnesium, and calcium (toxicity of micronutrients).
8. Gallium accumulates in Lcinnci and Aspergillus.
9. Phytotron: When a plant is grown in controlled conditions of temperature, and light. pH, etc.
10. Asparagine and glutamine are two amides formed from aspartic acid and glutamic acid, respectively, in a plant. Amides are the storage form of nitrogen.
11. Stem nodules are found in Sesbania.
12. Leaf nodules are found in Pavetta. Dioscorea.
13. Associative symbiosis (loose symbiosis) is found in tropical grass (with Azotobacter) and maize and sorghum (with Azospirillum).
14. True humus plant is Wullschleigelia aphylla.
15. Single ion channels (discovered by Neher and Sakmann) are transmembrane proteins meant for the entry of specific ions.
16. Aquaporins are water-filled pores in membranes.

Mineral Nutrition Multiple Choice Questions And Answers

Question 1. Which is not a criterion for the essentiality of a mineral?

1. Direct role in metabolism
2. Requirement is specific
3. Deficiency causes hunger signs
4. Dispensable for growth

Answer: 4. Dispensable for growth

Question 2. Essential elements are

1. Only macronutrients
2. Only micronutrients
3. Both macro and micronutrients
4. C, H, O, and N only

Answer: 3. Both macro and micronutrients

Question 3. Which is not a trace element?

1. Mn
2. Cu
3. Mo
4. K

Answer: 4. K

Question 4. Which is not a true statement regarding macronutrients?

1. Macronutrients form plant structure.
2. Macronutrients become toxic when present in excess.
3. Macronutrients have no role in electron transfer.
4. Macronutrients develop osmotic potential.

Answer: 2. Macronutrients become toxic when present in excess.

Question 5. Choose the false statement regarding micronutrients.

1. Micronutrients become toxic in excess.
2. Micronutrients do not cause osmotic potential.
3. Micronutrients have little role in protoplasmic structure.
4. Micronutrients play a secondary role in enzyme activation.

Answer: 4. Micronutrients play a secondary role in enzyme activation.

Question 6. Deficiency in plant growth and disorders caused by the reduced availability of a critical element is called

1. Critical deficiency
2. Secondary deficiency
3. Primary deficiency
4. Complete deficiency

Answer: 3. Primary deficiency

Question 7. Who prescribed a medium containing microelements for the first time?

1. Gericke
2. Arnon Ploagland
3. Knop
4. Stout

Answer: 2. Arnon Ploagland

Question 8. Excess of manganese may induce the deficiencies of

1. Iron
2. Calcium
3. Magnesium
4. All of these

Answer: 4. All of these

Question 9. A partial mineral element is

1. N
2. P
3. K
4. S

Answer: 1. N

Question 10.The deficiency of which element causes the deficiency of nitrogen?

1. Mo
2. K
3. Mn
4. S

Answer: 1. Mo

Question 11. Minerals associated with redox reactions are

1. N, Cu
2. Fe, Cu
3. Fe, K
4. Mn, Mo

Answer: 2. Fe, Cu

Question 12. Minerals that maintain cation-anion balance in cells are

1. Cl, K
2. Fe, Cu
3. K, P
4. Ca, Fe

Answer: 1. Cl, K

Question 13. Interveinal chlorosis is due to the deficiency of

1. Fe
2. Mn
3. N
4. B

Answer: 1. Fe

Question 14. Match columns A and B correctly.

1. (1) → (C), (2) → (A), (3) → (D), (4) → (B)
2. (1) → (C), (2) → (A), (3) → (B), (4) → (D)
3. (1) → (C), (2) → (B), (3)→(D), (4)→ (A)
4. (1) → (A), (2) → (B), (3) → (C), (4) → (D)

Answer: 2. (1) → (C), (2) → (A), (3) → (B), (4) → (D)

Question 15. Which of the following groups of elements are mobile?

1. Fe, Ca, B
2. N, P, K
3. B, K, Ca
4. Ca, Mg, K

Answer: 2. Fe, Ca, B

Question 16. Which of the following elements are required for chlorophyll synthesis?

1. Fe and Mg
2. Mo and Ca
3. Cu and Ca
4. Ca and K

Answer: 1. Fe and Mg

Question 17. If chloroplast is burnt, then which of the following is left?

1. Magnesium
2. Manganese
3. Iron
4. Sulfur

Answer: 1. Magnesium

Question 18. Which one of the following is a sulfur-containing amino acid?

1. Valine
2. Methionine
3. Tryptophan
4. Phenylalanine

Answer: 2. Methionine

Question 19. Copper deficiency leads to

1. Exanthema
2. Whiptail of cauliflower
3. Little leaf condition
4. Interveinal chlorosis

Answer: 1. Exanthema

Question 20. Phosphorus is found maximum in

1. Roots
2. Fruits
3. Flowers
4. None of these

Answer: 2. Fruits

Question 21. Which of the following is required for auxin synthesis?

1. Calcium
2. Zinc
3. Sugars
4. Proteins

Answer: 2. Zinc

Question 22. Reversible binding of cations, a property possessed by clay particles, is known as

1. Retentive capacity
2. Cation exchange
3. Adsorption
4. Chelation

Answer: 2. Cation exchange

Question 23. A characteristic of ion channels is/are

1. They are transmembrane proteins functioning a selective pores
2. They were discovered by Neher and Sakman
3. They are gated canners
4. All of these

Answer: 4. All of these

Question 24. The theory of Donnan equilibrium explains the process of some

1. Fixed diffusible cations on the inner side
2. Fixed non-diffusible anions on the inner side
3. Non-fixed diffusible anions on the inner side
4. Non-fixed diffusible cations on the inner side

Answer: 2. Fixed non-diffusible anions on the inner side

Question 25. Mineral salts which are absorbed by the roots from the soil, are in the from of

1. Very dilute solution
2. Dilute solution
3. Concentrated solution
4. Very concentrated solution

Answer: 1. Very dilute solution

Question 26. The movement of electrolytes through the roots is a general

1. Against the electrochemical gradient and requires energy
2. Along the electrochemical gradient and does not require energy
3. A passive process
4. Dependent on aquaporins

Answer: 1. Against the electrochemical gradient and requires energy

Question 27. Ionic uptake against electrochemical gradient without the expenditure of metabolic energy can be explained by

1. Ion exchange
2. Donnan equilibrium
3. Carrier proteins
4. Both (1) and (2)

Answer: 4. Both (1) and (2)

Question 28. Transpiration pull or water tension in leaf is responsible for which one of the following methods of absorption of minerals by the plants from soil?

1. Active absorption of minerals
2. Mass flow
3. Donnan equilibrium
4. Ionic exchange

Answer: 2. Mass flow

Question 29. If nitrogen is bubbled in the rooting medium, active absorption of minerals will

1. Increase
2. Decrease
3. Remain same
4. Stop immediately

Answer: 2. Decrease

Question 30. During ionic flux, the uptake of ions into inner space is

1. Active
2. Passive
3. Energy-dependent
4. Both (1) and (3)

Answer: 4. Both (1) and (3)

Question 31. Carrier proteins for active salt uptake

1. Have pores
2. Form complex with ions
3. Function under transpiration pull
4. All of these

Answer: 2. Form complex with ions

Question 32. The translocation of solute is

1. Equal to the rate of translocation of water
2. Dependent on transpiration pull
3. Through xylem vessel
4. None of these

Answer: 4. None of these

Question 33. Find the odd one (with respect to the critical element).

1. Nitrogen
2. Potassium
3. Nickel
4. Phosphorus

Answer: 3. Nickel

Question 34. The process of conversion of NH₄ → NO₂  → NO₃ is called

1. Ammonification
2. Nitrification
3. N₂ fixation
4. Denitrification

Answer: 2. Nitrification

Question 35. Which of the following is/are diazotrophs?

1. Rhizobitnn and Azotobacter
2. Frankia and Klebsiella
3. Anabaena and Nos toe
4. All of these

Answer: 4. All of these

Question 36. Which is not true for nitrogenase enzyme in root nodules in legumes?

1. Synthesized by nit genes of Rhizobium
2. Site of reduction of N₂ into NH₃
3. It is a Mo-Fe protein
4. Resistant to O₂ concentration

Answer: 4. Resistant to O₂ concentration

Question 37. Cell division in root nodules is promoted by se creted by plants and secreted by bacteria.

1. Auxin, Cytokinin
2. Cytokinin, Auxin
3. Auxin, Leghemoglobin
4. Nitrogenase, Leg hemoglobin

Answer: 1. Auxin, Cytokinin

Question 38. Conversion of NO₃ → NO₂ → NH₄ is called is catalysed by

1. Nitrate assimilation; nitrate and nitrite reductase
2. Nitrification; nitrate and nitrite reductase
3. Ammonification; glutamate dehydrogenase
4. Denitrification; transaminase

Answer: 1. Nitrate assimilation; nitrate and nitrite reductase

Question 39. Transported and storage forms of nitrogen in plants are

1. Amides
2. Polypeptides
3. Amino acids
4. α-ketoglutaric acids

Answer: 1. Amides

Question 40. The amino acid which plays a central role in nitrogen metabolism is/are

1. Glutamic acid
2. α-ketoglutaric acid
3. Aspartic acid
4. Double-aminated keto acids

Answer: 1. Glutamic acid

Question 41. The amino acid which plays a central role in nitrogen metabolism is/are

1. Anthoceros
2. Aulosira
3. Nostoc
4. Groundnut

Answer: 4. Groundnut

Question 42. Nitrite reductase enzyme is used to convert

1. Nitrate into nitrite ion
2. Nitrogen of the atmosphere into ammonia
3. Ammonia into nitrates
4. Nitrite to ammonium ion

Answer: 4. Ammonia into nitrates

Question 43. What do hemi parasites absorb from the host?

1. Water and minerals
2. Sugar
3. Both (1) and (3)
4. Nothing

Answer: 2. Sugar

Question 44. The small rootless aquatic herb in which a portion of the leaf forms a tiny sac or bladder that traps water insects is

1. Dionaea
2. Utricularia
3. Sarracenia
4. Drosera

Answer: 2. Utricularia

Question 45. The process of conversion of NO₂, NO₃, NH₃ to N₂ is called and is done by

1. Nitrification, Nitrosomouas
2. Denitrification, Pseudomonas
3. Nitrate assimilation, Nitrogenase
4. Ammonification, Bacillus

Answer: 4. Ammonification, Bacillus

Question 46. Which of the following elements are essential for the photolysis of water?

1. Ca and Cl
2. Mn and Cl
3. Zn and I
4. Cu and Fe

Answer: 2. Mn and Cl

Question 47. Which of the following is related with the transfer of food material?

1. Xylem
2. Collenchyma
3. Phloem
4. Parenchyma

Answer: 3. Phloem

Question 48. Which of the following elements is most mobile in plant metabolism?

1. Calcium
2. Phosphorus
3. Carbon
4. Magnesium