Plant Growth and Development Class 11 Notes

Three Phases of Growth

• Meristematic phase — cells are small, thin-walled, dense cytoplasm, large nuclei, rich in protoplasm. They divide actively. Found at root and shoot apices (apical meristems) and cambium.
• Elongation phase — cells behind the meristematic zone enlarge rapidly, vacuoles form and enlarge, cell wall loosens. This is the zone of maximum water uptake and is responsible for increase in length.
• Maturation phase — cells differentiate and attain their final form and function. Vacuoles occupy most of the cell volume; specialised cell walls form. Also called the zone of permanent cell.

Growth can be measured by increase in cell number, cell size or fresh/dry weight. The rate of growth varies and can follow arithmetic or geometric patterns.

Arithmetic Growth

If only one daughter cell continues to divide while the other differentiates, the number of cells increases by a constant amount per unit time. A straight line is obtained when length is plotted against time.

Geometric Growth

When both daughter cells from each division continue to divide, the cell number doubles at each division, giving a characteristic S-shaped (sigmoid) curve when plotted on a linear scale. On a log scale the initial phase appears linear.

The sigmoid growth curve has three phases: lag (slow), log or exponential (rapid cell division), and stationary or senescence (rate slows as nutrients limit growth).

• Water — essential for cell elongation (turgidity), enzyme activity, and as a solvent for nutrients. Water deficit reduces growth markedly.
• Oxygen — required for aerobic respiration to generate ATP needed for growth. Anaerobic conditions severely retard root growth.
• Temperature — enzymatic reactions of growth have a Q₁₀ of about 2; most plants grow best at 25–35 °C. Extreme heat denatures enzymes.
• Light — not required by all cells for growth per se, but photomorphogenesis, photosynthesis, and photoperiodism depend on light quality, intensity and duration.
• Nutrients — mineral elements such as N, P, K, Ca, Mg and trace elements are needed for building cell walls, membranes, nucleic acids and enzymes.

Plant growth regulators (PGRs) are small organic molecules that act as signalling compounds to regulate growth and development. They are grouped into five major families based on their physiological effects.

Auxins (e.g. IAA — Indole-3-Acetic Acid)

• Synthesised in shoot apices, young leaves and developing seeds.
• Promote cell elongation (acid-growth hypothesis: proton pumps lower wall pH, expansins loosen cross-links).
• Apical dominance — high auxin from the tip suppresses lateral bud growth.
• Root initiation on stems, vascular differentiation, phototropism (lateral redistribution under unilateral light).
• Applications: rooting of cuttings, weed control (2,4-D as selective herbicide), parthenocarpy.

Gibberellins (GA)

• Discovered from the fungus Gibberella fujikuroi causing 'foolish seedling' disease in rice.
• Promote stem elongation by stimulating both cell division and elongation — can restore dwarf varieties to normal height.
• Break dormancy of seeds and buds; induce germination by stimulating α-amylase synthesis in the aleurone layer.
• Promote bolting (internode elongation) before flowering in rosette plants.
• Applications: malt production in brewing, thinning of fruit, increasing grape berry size.

Cytokinins

• First isolated from coconut milk; the natural form is zeatin (from maize endosperm).
• Promote cell division (cytokinesis) — often used together with auxin in tissue culture.
• Delay senescence (anti-ageing effect) — keep leaves green by mobilising nutrients toward them (Richmond–Lang effect).
• Stimulate lateral bud growth — antagonistic to auxin-mediated apical dominance.
• Applications: tissue culture media, preservation of cut flowers and vegetables.

Ethylene (C₂H₄)

• The only gaseous PGR; produced in all higher plant tissues, especially during ripening and senescence.
• Promotes fruit ripening — stimulates autocatalytic ethylene production and respiration climacteric.
• Triple response in seedlings: inhibition of stem elongation, radial swelling, horizontal growth (useful for navigating around obstacles in soil).
• Promotes abscission of leaves, flowers and fruits; induces flowering in pineapple (with auxin interaction).
• Applications: commercial ripening of bananas, mangoes and tomatoes in ripening chambers.

Abscisic Acid (ABA)

• A sesquiterpenoid; functions mainly as a growth inhibitor and stress hormone.
• Induces and maintains bud and seed dormancy — prevents precocious germination.
• Promotes stomatal closure during water stress by increasing cytosolic Ca²⁺ in guard cells.
• Inhibits growth, accelerates senescence and leaf abscission — sometimes called the 'stress hormone'.
• Promotes desiccation tolerance in developing seeds (seed maturation).

• Short-day plants (SDP) flower when the night length exceeds a critical duration (long nights, short days). E.g. rice, soybean, chrysanthemum, cocklebur, potato.
• Long-day plants (LDP) flower when the night length is shorter than a critical duration (short nights, long days). E.g. wheat, barley, spinach, lettuce, clover.
• Day-neutral plants (DNP) flower irrespective of night length. E.g. tomato, cucumber, maize, some varieties of rice.

Example: A root tip has cells that are small, dense, thin-walled and actively dividing. In which phase of growth are these cells? Name the zone in which they are located.

Solution: These cells are in the meristematic phase of growth. They are found in the zone of cell division (apical meristem) at the root tip. These cells have dense cytoplasm, large nuclei and no large central vacuole, which are hallmarks of actively dividing meristematic cells.

Example: Explain why a short-day plant like soybean fails to flower when grown under long-day conditions in summer, but flowers when days become shorter in autumn.

Solution: Soybean is a short-day plant that requires a continuous dark period exceeding a critical length to flower. In summer, nights are short (less than the critical duration), so the plant remains vegetative. As autumn approaches, nights lengthen beyond the critical threshold, Pfr levels decline sufficiently during the long dark period, and the floral stimulus is generated at the shoot apex, inducing flowering.

Exercises (NCERT Q1–Q10)

• 1) Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate.
• 2) Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?
• 3) Describe briefly: (a) Arithmetic growth (b) Geometric growth (c) Sigmoid growth curve (d) Absolute and relative growth rates
• 4) List five main groups of natural plant growth regulators. Write a note on discovery, physiological functions and agricultural/horticultural applications of any one of them.
• 5) Why is abscisic acid also known as stress hormone?
• 6) ‘Both growth and differentiation in higher plants are open’. Comment.
• 7) ‘Both a short day plant and a long day plant can produce can flower simultaneously in a given place’. Explain.
• 8) Which one of the plant growth regulators would you use if you are asked to: (a) induce rooting in a twig (b) quickly ripen a fruit (c) delay leaf senescence (d) induce growth in axillary buds (e) ‘bolt’ a rosette plant (f) induce immediate stomatal closure in leaves.
• 9) Would a defoliated plant respond to photoperiodic cycle? Why?
• 10) What would be expected to happen if: (a) GA3 is applied to rice seedlings (b) dividing cells stop differentiating (c) a rotten fruit gets mixed with unripe fruits (d) you forget to add cytokinin to the culture medium.

Answers at a glance

1. 1
   Q1 → Growth is the irreversible permanent increase in the size of a plant organ, and it may be measured as an increase in length, area, volume, mass or in the number of the units, that is the leaves, the flowers and the branches. Its essential properties are that it is an irreversible and permanent increase, that it is measured as a quantitative increase in some parameter, and that it results from the division of the meristematic cells followed by their enlargement; and two qualifications are that growth is a quantitative phenomenon, an increase in size, whereas differentiation is qualitative, and that only the meristematic cells divide, so the growth of a plant is localised at the meristems. Differentiation is the maturation of the cells of a tissue to perform a specific function, the cells of the permanent tissue coming to specialise, so that a meristematic, undifferentiated and actively dividing cell gives rise to the specialised permanent tissues; it is a qualitative process, a change in the nature and not merely in the size of the cell, which is the exact contrast with growth, and the examples are the meristematic cell becoming a permanent tissue cell, the guard cell of the epidermis being differentiated from the epidermal cell, the cambium forming the secondary xylem and the secondary phloem, and the tracheary element, in which the cell loses its protoplasm and develops a strong, elastic, lignocellulosic secondary wall. Development is the sum of all the changes that the plant undergoes during its whole life from the germinating seed to the senescent plant, so it is a wider term than either growth or differentiation and includes both, and it also includes the interaction of the two, and it is the study of the whole of the cellular and molecular behaviour of the plant, its signalling, its gene expression and its hormone action. Dedifferentiation is the reacquisition by a mature, permanent and fully differentiated tissue cell of the capacity to divide again, so that it reverts to a less specialised and more meristematic-like state, and redifferentiation is the reverse change, the conversion of such a dedifferentiated cell once more into a specialised tissue cell, that is the restoration of the differentiated state. Determinate growth is the growth in which the extent of the growth is limited to a fixed or predetermined limit, the organ ceasing to grow at that limit and then undergoing senescence, so that the growth of the whole plant is determinate, though the shoot and the root apices may show indeterminate growth. A meristem is a tissue of actively dividing cells which produces the new cells for the growth and for the replacement of the lost or the worn-out tissues, and it is of two types, the apical meristem at the tips of the stem and the root and the lateral meristem, the vascular cambium and the cork-cambium, in the girth. Finally, the growth rate is the increased growth per unit time, and NCERT expresses that rate in two ways: the absolute growth rate, which is the measurement and the comparison of the total growth per unit time, that is the increase in the length or the mass in a given time, and the relative growth rate, which is the growth of the given system per unit time expressed on a common basis, for example per unit initial parameter. The distinction matters, because the relative rate is the only one of the two that allows a large plant and a small plant to be compared on the same basis, and it is the relative rate that is used in the analysis of the growth curves.
2. 2
   Q2 → No single parameter is good enough to demonstrate growth throughout the life of a flowering plant, because the rate of growth of a plant is not constant through its life. The parameters that could be used are the length, measured as the height of the plant or the length of one organ, the area of the leaf or the diameter of the stem, the fresh or the dry mass, the volume, and the number of the leaves, the flowers and the branches. The difficulty is not that these cannot be measured but that none of them shows the growth continuously. To demonstrate this, a plant is grown from a seed and one of the parameters, the height, is measured at regular intervals and the values are plotted against time. The curve obtained is not a straight line. In the early period the seedling grows only very slowly, because the seed is small, the plant has few leaves and so little photosynthetic surface, and it is living on the reserve of the seed. It then enters a phase of very rapid growth, growing exponentially, and this is the fastest phase, the curve of the length against the time being of the logarithmic or the exponential type. Then the growth slows and the plant enters a phase of declining growth, the rate falling progressively, until at last the growth ceases, the plant reaches maturity, the leaves begin to fall, the flowers open and the plant enters senescence and dies. The plant therefore passes through an initial slow phase, a phase of most vigorous growth, a phase of declining growth, a period of maturity and a period of senescence, and the graph of any single parameter against time has the same general shape, a curve that rises slowly, rises fastest, rises slowly again, becomes level and finally falls. So any parameter that measures the increase will show first a slow increase, then a rapid one, then a slowing one, then no increase, and finally a decrease, and no single figure of it can represent what is happening across the whole life. A true demonstration of growth throughout the life therefore requires the measurement of the plant at successive intervals, the plotting of the parameter against time, and the interpretation of the phases of the resulting curve, and it is precisely because of this succession of phases that the growth of a plant has to be studied as a growth curve rather than as a single measurement.
3. 3
   Q3 → Arithmetic growth is that in which the increase in the measured parameter is constant in unit time, so that the rate of growth is constant and the total growth is linear with time, which NCERT expresses as Lt equals L0 plus rt, where L0 is the length at time zero, Lt is the length at time t and r is the growth rate, that is the elongation per unit time, and the graph of the length against time is a straight line. The biological case of this is a root elongating at a constant rate, which is the example NCERT gives, and the condition holds only over a limited range and not for the whole life of the plant. Geometric growth is that in which the increase in the parameter is itself proportional to the value already reached, so the rate of growth is not constant but keeps increasing and the growth is exponential, which NCERT writes as W equals W0 times e raised to the power rt, where W0 is the initial size, W the final size, r the growth rate, here the relative growth rate, and e the base of the natural logarithms, and the graph is a curve of the exponential or the logarithmic type that rises ever more steeply. Its biological case is the early phase of the growth of a cell, tissue or organ, in which the daughter cells following each mitotic division retain the ability to divide and continue to do so, and a constant relative rate of increase can continue only for a limited time, since the plant cannot grow indefinitely at an increasing rate and the tissue would become impossibly large, so the geometric growth is the growth of the young phase only. The sigmoid, that is the S-shaped, growth curve is the sum of the two, an arithmetic growth phase followed by a geometric growth phase followed by an arithmetic growth phase again, and it is the growth curve of an organ or of a whole plant through its life. Its three phases are: the initial, or lag, or slow phase, which is arithmetic, in which the growth is slow because the plant is living on the reserve of the seed and has little photosynthetic surface; the log, or exponential, or acceleration phase, which is geometric, in which the growth is at its maximum and proportional to the existing size and the curve rises steeply; and the deceleration, or declining, or stationary phase, which is arithmetic again, in which the rate falls progressively, the growth slows, the curve flattens and the organ reaches its full size. The curve is called sigmoid from its S shape, with an inflection at the transition from the slow to the rapid phase and another at the transition from the rapid to the slow phase, and it is important both for the growth of an organ and for the growth of the whole plant, and its practical significance is that the final size of the organ, the point at which the curve flattens, is set by the balance between the factors that promote the growth and those that limit it. The absolute growth rate is the measurement and the comparison of the total growth per unit time, obtained simply by subtraction, so the change in the value of the parameter with time, and a plant gaining five centimetres in a week has an absolute growth rate of five centimetres a week. The relative growth rate is the growth of the given system per unit time expressed on a common basis, for example per unit initial parameter, so that systems of very different size can be compared. The comparison of the two is the point to be noted, and NCERT illustrates it with two leaves, A and B, of different sizes which both increase their area by the same five square centimetres in the same given time, so that they have the same absolute growth rate; only one of them shows a much higher relative growth rate, and that is the smaller leaf, the leaf B, because the same five square centimetres is a much larger fraction of its smaller initial area than of the large initial area of the leaf A. So the leaf of smaller initial size shows the higher relative growth rate, and it is the relative rate, being expressed on a common basis, that allows a fair comparison between a large plant and a small one.
4. 4
   Q4 → The five main groups of the natural plant growth regulators are the auxins, the gibberellins, the cytokinins, the abscisic acid and the ethylene. The commonest natural auxin is the indole 3-acetic acid, the IAA, the term auxin being applied to the IAA and to the other natural and synthetic compounds with growth-regulating properties; the IAA and the indole butyric acid have been isolated from plants while the NAA and the 2,4-D are synthetic. Of the gibberellins, of which there are more than a hundred, GA1, GA2, GA3 and so on, the gibberellic acid, GA3, was the first to be discovered and is the most intensively studied. The cytokinins were discovered as the kinetin, a modified form of adenine, and kinetin does not occur naturally in plants, the natural substance with cytokinin-like activity being the zeatin, isolated from the corn-kernels and the coconut milk. The abscisic acid, a derivative of the carotenoids, acts as a general plant growth inhibitor and an inhibitor of the plant metabolism. The ethylene is a gas, and it could fit either of the two groups but is largely an inhibitor of the growth activities. So the auxins, the gibberellins and the cytokinins are the growth promoters, and the abscisic acid and the ethylene belong to the inhibiting side. The note on the auxin is as follows. The discovery of each of the five major groups of the PGRs has been accidental, and the auxin story began with the observation of Charles Darwin and his son Francis Darwin, that the coleoptiles of the canary grass responded to unilateral illumination by growing towards the light source, the phototropism; after a series of experiments it was concluded that the tip of the coleoptile was the site of a transmittable influence that caused the bending of the entire coleoptile, so the tip is the source of the auxin, and the auxin was then isolated by F. W. Went from the tips of the coleoptiles of the oat seedlings. The chemical note is that the auxin, from the Greek auxein meaning to grow, was first isolated from the human urine, that the auxins are generally produced by the growing apices of the stems and the roots from where they migrate to the regions of their action, and that all these auxins have been used extensively in the agricultural and the horticultural practices. The physiological functions of the auxin are that it initiates the rooting in the stem cuttings, that it controls the xylem differentiation and helps in the cell division, that it helps to prevent the fruit and the leaf drop at the early stages but promotes the abscission of the older mature leaves and fruits, that it promotes the flowering, for example in the pineapples, and that it induces the parthenocarpy, the development of the fruit in the absence of pollination, as in the tomatoes, and it is widely used as the herbicide, the 2,4-D killing the dicotyledonous weeds without affecting the mature monocotyledonous plants, so that the gardeners use it to prepare the weed-free lawns. The agricultural and horticultural applications are accordingly the rooting of the stem cuttings, and so the propagation by the layering, the grafting and the tissue-culture plantlets, the promotion of flowering, for example in the pineapples, the production of the seedless fruit in the tomatoes by the parthenocarpy, the prevention of the fruit and the leaf drop at the early stages, and the use of the 2,4-D as a weedicide, which kills the dicotyledonous weeds without affecting the mature monocotyledonous plants, so that a lawn of grass is sprayed with it and the dicotyledonous weeds are killed while the grass is unharmed. The two synthetic auxins to name are the naphthaleneacetic acid, the NAA, and the 2,4-dichlorophenoxyacetic acid, the 2,4-D. The auxin is thus the growth regulator most widely used in the agriculture and in the horticulture, and the reason is that it is the one with the widest range of effects, from the rooting to the flowering to the fruit set to the weed control.
5. 5
   Q5 → The abscisic acid is also known as the stress hormone because it accumulates in the plant whenever the plant is subjected to any kind of stress, the response being general rather than specific to any one kind of stress. It was discovered for its role in regulating the abscission and the dormancy, and during the mid-1960s three independent researches reported the purification and the chemical characterisation of three different kinds of inhibitors, inhibitor-B, abscission II and dormin, which were later all proved to be chemically identical, and it was then named abscisic acid, the ABA. The reason it is also called the stress hormone is that, like the other PGRs, it has a wide range of effects on the plant growth and the development, so that it acts as a general plant growth inhibitor and an inhibitor of the plant metabolism, it inhibits the seed germination, it stimulates the closure of the stomata, and it increases the tolerance of the plants to the various kinds of stresses, and it is for that effect that it is called the stress hormone. It also plays an important role in the seed development, the maturation and the dormancy, and by inducing the dormancy the ABA helps the seeds to withstand the desiccation and the other factors unfavourable for the growth. The two stresses to consider are the water stress and the high salinity. In the water stress, or the drought, the plant being short of water, there is a fall in the water content of the leaf and the leaf cells lose their turgidity; this loss of turgidity is sensed and triggers a rise in the concentration of the ABA, which is made principally in the mature leaves and in the roots; the ABA then brings about the rapid and immediate closure of the stomata, within minutes, by causing the guard cells to lose their turgor, so that the loss of water by the transpiration is checked and the water content of the leaf is restored. The closure of the stomata also checks the entry of the carbon dioxide, so the photosynthesis is reduced, but this is a secondary sacrifice made so that the plant may be saved from desiccation, which is the point, that the plant sacrifices its carbon fixation in order to save its water. The ABA also induces the maturing seed to synthesise the proteins of the germination inhibitors, so that the seed is held in dormancy and does not germinate until the water is available, which is a second function of the water stress. In the high salinity, the decrease in the osmotic potential of the soil means that the movement of the water from the soil into the root is much reduced, or the water may even move out of the root and into the soil, so the root cells lose their turgidity and the same sequence follows, the ABA being synthesised and its concentration rising, the stomata closing and the transpiration being checked; and in the salt-tolerant species the ABA also induces the synthesis of the proteins of the salt tolerance, so that the cell accumulates the solutes, its osmotic potential is brought down again and its turgidity is restored. The ABA is therefore the hormone of the stress response in general, of the water deficit and of the salinity, and it is the general nature of its response that gives it the name of the stress hormone.
6. 6
   Q6 → To say that the growth and the differentiation in the higher plants are open is to say that neither is final, that a cell which has already grown and differentiated is not irrevocably committed but can be induced to divide again and to form a different tissue, which is unlike the animal, in which the fate of the cell is largely fixed and irreversibly determined. The NCERT reason for calling even the differentiation open is that the cells and the tissues arising out of the same meristem have different structures at maturity, and that the final structure at maturity of a cell or a tissue is also determined by the location of the cell within the organ: the cells positioned away from the root apical meristems differentiate as the root-cap cells, while those pushed to the periphery mature as the epidermis, so that the one meristem gives the root-cap on one side and the epidermis on the other. The meristem is the obvious case of the growth being open, since a meristematic cell retains the power of division for the whole life of the plant and continually produces new cells, whereas most of the cells of an animal lose the power of division once differentiated. The more striking evidence, and the proof that the differentiation is open, is that the fully differentiated cells can be induced to revert. The tracheary element, the xylem vessel, is a dead cell, the mature vessel having lost its nucleus and its protoplasm, and it is consequently the most differentiated and the least living tissue of the plant, and yet it can be induced to revert to a living, dividing, meristematic-like cell and to give rise to a whole plant; the mature sieve tube element, which has also lost its nucleus, has likewise been induced to divide, and the cells of the leaf, of the cortex and of the pith can be induced to divide and to form a callus, and in the plant body itself the living differentiated cells that have lost the capacity to divide can regain it, the examples being the interfascicular cambium and the cork cambium formed from the fully differentiated parenchyma. The technique that demonstrates all of this is the tissue culture. A small piece of the plant tissue, the explant, is taken and cultured on a sterile nutrient medium of sugars, mineral salts and growth regulators, and the cells of the explant, though fully differentiated, divide to form an undifferentiated mass of cells called the callus, and that is the dedifferentiation. The callus is then transferred to a fresh medium in which the proportions of the auxin and the cytokinin are adjusted, and the ratio determines what the callus gives rise to: a high cytokinin to auxin ratio promotes the formation of the shoot and induces shoot buds, a high auxin to cytokinin ratio promotes the formation of the root, and a balanced ratio maintains the callus. So the same undifferentiated mass of cells can be made to form roots or shoots merely by changing the proportion of the two hormones, and this is the strongest possible demonstration that the differentiation of the higher plants is open, since the same cells can be directed to become several quite different tissues. A complete plant can then be regenerated from this tissue, so that a single explant of a single leaf can be made to give a complete new plant, and a large number of genetically identical plants can be produced from that one explant, which is the clonal propagation of the horticulture. The qualification is that the openness is not unlimited, for the genome is unchanged and the cell cannot be made to form a tissue of a wholly different type, so it is open within the limits set by the genetic programme of the plant.
7. 7
   Q7 → A short day plant is one in which the flowering is induced by a day shorter than a critical duration, so the photoperiod must be shorter than the critical photoperiod, and a long day plant is one in which the flowering is induced by a day longer than the critical duration; a third category, the day-neutral plant, is one in which the photoperiod has no influence, and it is with this that the two are contrasted. The key to the question is the critical photoperiod. Every photoperiodic plant has its own critical photoperiod, that is its own critical duration of light, and this is a characteristic of the species rather than of the individual, it is fixed and genetically determined, and the plant is induced to flower only if the day it receives is longer than its critical photoperiod, in the case of a long day plant, or shorter than it, in the case of a short day plant. So the critical photoperiod is the threshold, and the flowering is simply a comparison of the received day with that threshold, and the day is not short or long in itself at all, it is short or long only in relation to the critical value of the particular plant; the two terms therefore describe not the day but the relation of the plant to the day, and that is the whole answer to the question. Since short and long are defined only in relation to the critical photoperiod, and since that value differs from species to species, the same day of the same length may be longer than the critical photoperiod of one species and shorter than the critical photoperiod of another, so that the same day is a long day for the second and a short day for the first. A short day plant and a long day plant growing in the same place therefore receive the same photoperiod and yet the one experiences it as a short day and the other as a long day, and both are induced to flower, and they flower simultaneously. For example, a short day plant with a critical photoperiod of ten hours will flower in a day of nine hours, while a long day plant with a critical photoperiod of eight hours will flower in that same day of nine hours, because for the first the day is shorter than its critical value and for the second it is longer than its critical value. The practical importance is that the sowing time of a crop is decided by the critical photoperiod of that crop, so a knowledge of the critical photoperiod is what enables a farmer to choose the date of sowing such that the crop flowers at the right time; and the mechanism is the same in both, the photoperiod being perceived by the leaves through the phytochrome, which then induces the production of the florigen in the leaves, and the florigen being transported to the shoot apex where the flowering is brought about.
8. 8
   Q8 → (a) To induce rooting in a twig, the auxin is used, since the auxin at the appropriate concentration initiates the rooting in the stem cuttings, a low concentration of the synthetic auxin, the NAA, being applied to the basal end of the twig, which is the standard practice in the propagation of the plants by the cuttings and by the layering and in the grafting, the roots arising as the adventitious roots at the cut surface. (b) To ripen a fruit quickly, the ethylene is used, being the gaseous hormone and the ripener, applied commercially to the fruits that are picked unripe, as in the banana, the mango, the citrus and the tomato, and used also on the canned and the stored fruit, a single dose of the gas ripening a whole consignment at once; and the action of the auxin in the same circumstance is the opposite, since the auxin prevents the ripening and the senescence of the fruit and is used to hold the fruit on the tree, so the ethylene and the auxin form a useful contrast here. (c) To delay the senescence of the leaf, the cytokinin is used, since it is the anti-senescence hormone and markedly delays the ageing of the leaves, holding on the leaf that would otherwise fall, and the mechanism is the mobilisation of the nutrients, the amino acids and the mineral solutes, out of the ageing leaf and into the sink, that is the young growing parts; the rich sources of the cytokinins, the regions of rapid cell division and the fruits and the seeds, are consistent with this, for they are the parts that are drawing on the nutrients of the old leaves. (d) To induce the growth of the axillary buds, the cytokinin is used, since the cytokinins promote the growth of the lateral, that is the axillary, buds and so release the apical dominance, the auxin of the apical bud having suppressed them, the two being in antagonism, and this is the basis of the tissue-culture technique in which a large cytokinin to auxin ratio in the medium promotes the shoot formation, so that a single explant yields a large number of shoots; the horticultural consequence is that a plant from which the apical bud has been removed becomes bushy when the cytokinin is supplied to the axillary buds. (e) To bolt a rosette plant, the gibberellin is used, that is the gibberellic acid, GA3, since bolting, the rapid elongation of the internodes that occurs just before the flowering, is induced by the gibberellin, so that a rosette of leaves at ground level is carried up on a stem and the flowers are lifted into the light and the air for the pollination; the species quoted are the cabbage, the beet, the Beta vulgaris, and the Hyoscyamus, and the commercial use that follows is the bolting of the onion and the seed production of the rosette plants, since the flowering is a precondition of the seed formation. (f) To induce the immediate closure of the stomata in the leaves, the abscisic acid is used, since the ABA stimulates the closure of the stomata rapidly and immediately, within minutes of the onset of the water stress, by causing the guard cells to lose their turgor, which checks the transpiration and so saves the water; and it is to be noted that the ethylene, the gaseous PGR that is largely an inhibitor of the growth activities, is the one used in the horticulture to break the seed and the bud dormancy and to initiate the germination, while it is the ABA and not the ethylene that closes the stomata.
9. 9
   Q9 → A defoliated plant cannot respond to the photoperiodic cycle, and the reason is that the leaf is the site of the perception of the photoperiod. The mechanism of the response is that the light is perceived by a photoreceptor in the leaf, the phytochrome, which exists in two interconvertible forms, the Pr that absorbs the red light of about 660 nanometres and is converted to the Pfr, and the Pfr that absorbs the far-red light of about 730 nanometres and is converted back to the Pr, the Pfr being the physiologically active form that initiates the response. The activated phytochrome then acts on the leaf to induce the production of the floral stimulus, the florigen, in the leaf, and this hormone-like molecule is transported from the leaf through the phloem to the shoot apical meristem, where it brings about the flowering when the day is of the required length. Since both the phytochrome and the site of the production of the florigen are in the leaf, a plant that has had all its leaves removed has neither the receptor nor the source of the florigen, so it cannot perceive the length of the day and cannot form the florigen, and it will therefore not flower in response to the photoperiod; both the perception and the production are lost with the leaves, and the whole photoperiodic response is lost with them. The experiment that demonstrates the point is the one in which a single leaf of a short day plant is retained on an otherwise defoliated plant, and the plant then does respond, which shows that the leaf is the site of the perception and that the response of the shoot apex is mediated by a stimulus translocated from the leaf. Two qualifications complete the answer without weakening it. The shoot apical meristem is still present, and it is the organ that will carry the flowers, so a defoliated plant is not in every respect incapable of flowering; and a plant that had already been induced before it was defoliated will go on to develop its flowers and set its fruit, because the induction has already been completed and the leaves are no longer needed for that step, and it will continue to respire and to grow on the reserves of the stem for a while. And a partly defoliated plant may still respond, since one retained, photosynthetically active leaf is enough to perceive the photoperiod and to produce the florigen. But the induction of the flowering, the photoperiodic response itself, cannot take place in a fully defoliated plant, because the perception of the day length and the production of the florigen are both carried out in the leaf, and the defoliation removes both of them.
10. 10
    Q10 → (a) If the GA3 is applied to the rice seedlings, the internodes will show a dramatic elongation, the rice plant growing tall and slender, since the internodes of the rice are short and the gibberellin causes their rapid elongation, this being the bolting effect. But the more specific and more important consequence is that the elongation would occur without the plant flowering, for the elongation of the internodes is promoted independently of the flowering, so the GA3 would cause the elongation of the internodes in the absence of any flowering. The practical significance is a hazard in the rice, for the lodging, that is the falling over, of the crop is caused by the elongation of the internodes, so that if the GA3 is applied at the wrong time to the rice, the plant elongates, does not flower properly and falls over, and the crop is lost, and this is why the GA3 must be used with care in the rice. (b) If the dividing cells stop differentiating, the plant is deprived of the differentiated cells it needs, since the normal growth of the plant is a balance between the division and the differentiation, so the meristematic cells would continue to divide and accumulate and the result would be a mass of undifferentiated cells, a callus, with no organs and no normal structure. The plant would therefore be unable to form its tissues and its organs, since the meristematic cells would persist as such and the process of the maturation to a specific function, which is the differentiation, would be abolished, and the plant would show a proliferative tumour-like growth, which is in fact the undifferentiated state that the whole of the tissue-culture technique deliberately maintains. (c) If a rotten fruit is mixed with the unripe fruits, the rotten fruit gives off the ethylene, which is the gaseous ripening hormone, and the ethylene diffuses through the atmosphere of the container and reaches the unripe fruits, which are induced to ripen, so that the whole lot ripens together and quickly. This is exactly what happens in practice, for the fruits that are to be ripened are kept with a ripe or a rotten fruit in a closed box, or ethylene gas is injected into the store, and the reason is that the ethylene is a gas and so spreads through the packed fruit and reaches every one of them, and a rotten fruit produces it in quantity. The practical advice that follows is that a single rotten apple spoils the whole lot and should be removed from the store immediately, which is why the fruits are graded and the damaged ones culled. (d) If the cytokinin is omitted from the culture medium, the ratio of the auxin to the cytokinin is raised, and since a high auxin to cytokinin ratio promotes the root formation while the cytokinin is what promotes the formation of the shoot buds, the callus would give rise to roots only, or at best a poor root formation with no shoot buds at all, so the plantlets would fail to develop and no complete new plant would be produced. The culture would therefore yield a mass of root-like tissue and no aerial shoot, and the propagation would fail and the experiment would be wasted, the remedy being to add the cytokinin, for the shoots require it and the roots do not; and it should be added that a medium with only the auxin gives the root, so the cytokinin is the specific requirement of the shoot, which is exactly what the ratio experiment of Skoog and Miller demonstrated.

MCQ Questions

1 Mark Questions

1. 1
   What is differentiation? (Ans: the process by which cells become structurally and functionally specialised)
2. 2
   Name the gaseous plant hormone. (Ans: ethylene)
3. 3
   Define growth. (Ans: an irreversible permanent increase in size, weight or volume or in number of cells)
4. 4
   Which hormone is used to break dormancy of potato tubers? (Ans: gibberellins)

2 Mark Questions

1. 1
   Differentiate between growth and development. (Ans: growth is irreversible increase in size; development includes growth, differentiation, maturation)
2. 2
   What are long-day and short-day plants? Give one example of each. (Ans: flower when day longer/shorter than critical length)
3. 3
   Write two functions of ethylene. (Ans: fruit ripening, senescence/abscission, response to stress)

3 Mark Questions

1. 1
   Describe the phases of growth and the sigmoid growth curve. (Ans: lag, log/exponential, stationary phases; S-shaped curve studied via arithmetic/geometric growth)
2. 2
   Explain the physiological effects of auxins in plants. (Ans: cell elongation, apical dominance, rooting, fruit development, abscission, weed control)
3. 3
   What is seed dormancy and how is it broken? (Ans: inactive state of seeds; broken by temperature, light, scarification, hormones like GA)

5 Mark Questions

1. 1
   Give a comparative account of the effects of auxins, gibberellins, cytokinins, ethylene and abscisic acid on plant growth and development, with one application of each.
2. 2
   Describe photoperiodism and vernalisation, explaining how day-length and low temperature influence flowering, with examples of SDP, LDP and dual-day-length plants. (Ans: Xanthium/tobacco SDP; wheat/hibiscus LDP; beet dual)