Plant Growth and Development

Start with the NCERT definition, because a 2-mark question can be nothing more than this. Growth is an irreversible permanent increase in size of an organ or its parts, or even of an individual cell. It is generally accompanied by metabolic processes (both anabolic and catabolic) that occur at the expense of energy. Two words in the definition matter to the examiner: irreversible (a leaf swelling with water and shrinking again is not growth) and permanent. The expansion of a leaf is growth; the swelling of a wooden door in the monsoon is not, because it reverses when the wood dries.

Plant growth is generally indeterminate. Plants retain the capacity for unlimited growth throughout their life because they have meristems at certain locations in their body. Cells of these meristems have the capacity to divide and self-perpetuate: with every division one product remains meristematic while the other becomes a body cell. Because new cells are always being added to the body by the meristem, this form of growth, in which new cells keep being added, is called the open form of growth.

Three meristems are named. The root apical meristem and the shoot apical meristem are responsible for primary growth, which mainly contributes to elongation of the plant along its axis. In dicotyledonous plants and gymnosperms, the lateral meristems, the vascular cambium and the cork cambium, appear later in life and are responsible for secondary growth, the increase in girth of the organs. Intercalary meristems (at the base of internodes and leaf sheaths in grasses) explain why a lawn regrows after mowing; NCERT mentions them in the anatomy chapter, and they are a favourite explanation in questions about grass regenerating after grazing.

Contrast this with animals, whose growth is determinate: it stops once a fixed size is reached. In a plant, even a very old tree keeps adding new leaves, roots and wood every season. That contrast, indeterminate versus determinate, is the point of NCERT exercise question 1 (define determinate growth) and of the exemplar question on why plants have an open form of growth.

At the cellular level growth is principally a consequence of an increase in the amount of protoplasm. Because protoplasm cannot be measured directly, growth is measured through a variety of parameters, and this is the answer to NCERT exercise question 2 (why no single parameter is good enough). The parameters NCERT lists are increase in fresh weight, dry weight, length, area, volume and cell number.

The three numerical examples are asked as MCQs, so learn them exactly. A maize root apical meristem can produce more than 17,500 new cells per hour: here growth is best counted as cell number. Cells in a watermelon may increase in size by up to 3,50,000 times: here growth is an increase in cell size, measured as volume. In a pollen tube growth is measured in terms of length, and in a dorsiventral leaf it is measured as increase in surface area.

The reason no single parameter serves throughout the life of a flowering plant is that different organs and different stages grow in different ways. A root elongates (length), a leaf expands (area), a fruit swells (volume), a tuber adds stored food (dry weight), a meristem adds cells (cell number). Fresh weight is easy to take but rises and falls with water content, which is why dry weight is the more reliable measure of real growth for a whole plant. So the parameter chosen must suit the organ and the stage; one measure alone would miss growth happening in another form.

Write the answer to the 'why not one parameter' question with two parts: (i) growth is basically an increase in protoplasm, which cannot be measured directly, so we use proxies; (ii) each proxy suits a particular organ or stage, with the maize, watermelon, pollen tube and leaf examples.

The period of growth is generally divided into three phases: meristematic, elongation and maturation. NCERT illustrates them on a root tip (Fig. 13.2), and a 3-mark question usually asks you to describe the three phases with the characteristics of the cells in each. The same three phases exist in the shoot apex too.

Meristematic phase. The cells constantly dividing, both at the root apex and at the shoot apex, represent this phase. The cells in this region are rich in protoplasm and possess large, conspicuous nuclei. Their cell walls are primary in nature, thin and cellulosic, with abundant plasmodesmatal connections. These four features (dense protoplasm, large nucleus, thin primary cellulosic wall, many plasmodesmata) are the marking points; give all four.

Phase of elongation. The cells just proximal (just behind) to the meristematic zone are in this phase. It is marked by increased vacuolation, cell enlargement and new cell wall deposition. The rapid lengthening of a root or stem is mostly this phase, not the meristem: a dividing cell does not get much longer, but a cell taking in water into a growing vacuole can lengthen many times.

Phase of maturation. The portion of the axis further away from the apex, proximal to the phase of elongation, represents this phase. The cells attain their maximal size in terms of wall thickening and protoplasmic modifications. Most of the tissues and cell types we studied in the anatomy chapter (tracheids, vessels, fibres, sieve tubes) are cells in this phase. In a root the maturation zone is where root hairs appear, which is a good landmark to add to the diagram.

Notice the order from the tip backwards: division, then enlargement, then maturation. A cell passes through the three in sequence, and the three zones exist together at the same time at any tip.

The increased growth per unit time is termed the growth rate. An organism, or a part of it, can produce more cells in a variety of ways, and NCERT describes two mathematical patterns: arithmetic and geometric.

Arithmetic growth. Following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures. The simplest expression is a root elongating at a constant rate. Plotting length against time gives a straight line (a linear curve, NCERT Fig. 13.3). Mathematically,

L_t = L₀ + rt

where L_t is the length at time t, L₀ is the length at time zero and r is the growth rate (elongation per unit time). Every unit of time adds the same fixed amount, so the increase is by addition.

Geometric growth. In most systems the initial growth is slow (lag phase), and it increases rapidly thereafter at an exponential rate (log or exponential phase). Here both daughter cells produced by mitosis retain the ability to divide and keep dividing. But with limited nutrient supply the growth slows down, leading to a stationary phase. If we plot the parameter of growth against time we get a typical sigmoid (S-shaped) curve (NCERT Fig. 13.4). A sigmoid curve is characteristic of a living organism growing in a natural environment, and it is typical for all cells, tissues and organs of a plant. The exponential growth can be expressed as

W₁ = W₀ e^rt

where W₁ is the final size (weight, height, number etc.), W₀ the initial size at the beginning of the period, r the growth rate, t the time of growth and e the base of natural logarithms. Here r is the relative growth rate, and it is also a measure of the ability of the plant to produce new plant material, referred to as the efficiency index. Hence the final size of W₁ depends on the initial size, W₀.

Absolute and relative growth rates. Quantitative comparisons between the growth of living systems can be made in two ways. Absolute growth rate is the measurement and comparison of total growth per unit time. Relative growth rate is the growth of the given system per unit time expressed on a common basis, for example per unit initial parameter. NCERT Fig. 13.5 shows two leaves, A and B, of different initial sizes that both add the same absolute area, 5 cm², in a given time; the smaller leaf, which grew from 5 cm² to 10 cm², shows the higher relative growth rate (100 per cent) than the larger leaf, which grew from 50 cm² to 55 cm² (10 per cent). Same absolute growth, different relative growth: that is the whole point of the figure.

A quick worked example of the kind a case-based question may set: a root 2 cm long grows to 8 cm in 3 days. Absolute growth rate = (8 − 2)/3 = 2 cm per day. Relative growth rate = (8 − 2)/2 = 3, i.e. 300 per cent over the 3 days, or 100 per cent per day. If the length is 2, 4, 6, 8 cm on successive days the growth is arithmetic; if it is 2, 4, 8, 16 it is geometric.

This is a short section but it is a ready-made 2-mark or 3-mark question: 'What are the necessary conditions for growth?' NCERT names them in one paragraph and you should reproduce every item.

Water, oxygen and nutrients are the very essential elements for growth. The plant cells grow in size by cell enlargement, which in turn requires water. Turgidity of cells helps in extension growth: a cell that has taken in water pushes on its wall and stretches it, which is why wilted plants stop elongating. Water also provides the medium for enzymatic activities needed for growth. Oxygen helps in releasing metabolic energy essential for growth activities through respiration. Nutrients (macro and micro essential elements) are required by plants for the synthesis of protoplasm and act as a source of energy.

Every plant organism can grow only within a suitable range of temperature; an optimum temperature is needed, and a deviation from it could be detrimental to survival. Finally, environmental signals such as light and gravity affect certain phases or stages of growth. Light does not just supply energy for photosynthesis; it also acts as a signal (phototropism, the greening of a seedling), and gravity orients roots and shoots (geotropism).

A neat way to write the answer is a list of five: water (turgidity plus medium for enzymes), oxygen (metabolic energy), nutrients (protoplasm synthesis and energy source), optimum temperature, and light and gravity as environmental signals for particular phases. Link each to why it is needed; a bare list earns half the marks.

These three definitions appear in NCERT exercise question 1 and in nearly every school paper, so learn them in NCERT's own words and keep the examples attached.

Differentiation. The cells derived from the root apical and shoot apical meristems and the cambium differentiate and mature to perform specific functions. This act leading to maturation is termed differentiation. During differentiation cells undergo few to major structural changes both in their cell walls and in the protoplasm. For example, to form a tracheary element the cells lose their protoplasm and develop very strong, elastic, lignocellulosic secondary cell walls, so that they can carry water to long distances even under negative pressure. Differentiation makes a cell specialised for a job and it usually means the cell stops dividing.

Dedifferentiation. Plants show another interesting phenomenon: living differentiated cells that have lost the capacity to divide can regain it under certain conditions. This is dedifferentiation. NCERT's examples are the formation of the interfascicular cambium and the cork cambium (phellogen) from fully differentiated parenchyma cells. In secondary growth, parenchyma cells of the medullary rays between the vascular bundles start dividing again to form the interfascicular cambium; cortical parenchyma dedifferentiates to form the cork cambium. Wound healing and callus formation in tissue culture are the same idea.

Redifferentiation. While doing so (after dedifferentiation), such cells or tissues can divide and produce cells that once again lose the capacity to divide but mature to perform specific functions, that is, they redifferentiate. The secondary xylem and secondary phloem cut off by the interfascicular cambium, and the cork cells cut off by the cork cambium, are redifferentiated cells.

Put the three in a chain to remember them: meristematic cell → (differentiation) → mature parenchyma → (dedifferentiation) → cambium that divides again → (redifferentiation) → secondary xylem, secondary phloem, cork.

Growth and differentiation are open. Recall that the growth in plants is open, meaning that new cells are always being added by the meristem. Differentiation in plants is also open, because cells or tissues arising out of the same meristem have different structures at maturity. The final structure at maturity of a cell or tissue is also determined by the location of the cell within the organ. For example, cells positioned away from the root apical meristem differentiate as root cap cells, while those pushed to the periphery mature as epidermis. The same meristem, different fates depending on position: that is what 'open' differentiation means, and it is the answer to NCERT exercise question 6.

Development is a term that includes all the changes an organism goes through during its life cycle, from germination of the seed to senescence. NCERT gives the sequence in Fig. 13.6 as a flow: a seed germinates into a seedling, the seedling grows into a mature plant, which flowers, produces fruits and seeds, and finally ages and dies (senescence). Development is the sum of growth and differentiation. The diagrammatic representation of the sequence of processes constituting the development of a cell of a higher plant is also shown: cell division → cell enlargement (growth) → differentiation → maturation → senescence and death.

Plasticity. Plants follow different pathways in response to environment or phases of life to form different kinds of structures. This ability is called plasticity. The NCERT examples must be quoted exactly. Heterophylly in cotton, coriander and larkspur: in these plants the leaves of the juvenile plant are different in shape from those of the mature plant. This is plasticity driven by the phase of life. In buttercup (Ranunculus), the leaves produced in air are different in shape from those produced in water. This is plasticity driven by the environment; the submerged leaves are finely dissected while the aerial leaves are broad and lobed. NCERT Fig. 13.7 shows both cases and it is worth a small sketch in a 3-mark answer.

Plasticity is why the same genotype can look different in different surroundings, and it is one reason plant development is described as open.

Factors controlling growth and development. Development in plants (i.e. both growth and differentiation) is under the control of intrinsic and extrinsic factors. The intrinsic factors are of two kinds: intracellular (genetic, the genome of the cell) and intercellular (chemical, the plant growth regulators). The extrinsic factors include light, temperature, water, oxygen, nutrition etc. The rest of the chapter deals with the intercellular intrinsic factors, the plant growth regulators.

The plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition. Because they are so varied chemically, they are grouped by their chemical class, and this grouping is a regular MCQ. They could be: indole compounds (indole-3-acetic acid, IAA); adenine derivatives (N⁶-furfurylamino purine, kinetin); derivatives of carotenoids (abscisic acid, ABA); terpenes (gibberellic acid, GA₃); or gases (ethylene, C₂H₄). PGRs are variously described as plant growth substances, plant hormones or phytohormones.

Do not call them hormones in the animal sense without a caveat. Animal hormones are made in a gland and travel in blood to a distant target; PGRs are made in many tissues, act at very low concentrations, and often act near where they are made. The word 'regulator' is deliberate.

Two groups by activity. NCERT divides PGRs into two groups based on their functions in a living plant body. One group is involved in growth-promoting activities, such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation; these are the plant growth promoters: auxins, gibberellins and cytokinins. The PGRs of the other group play an important role in plant responses to wounds and to stresses of biotic and abiotic origin; they are also involved in various growth-inhibiting activities such as dormancy and abscission. The PGR abscisic acid belongs to this group of growth inhibitors. The gaseous PGR, ethylene, could fit either of the groups, but it is largely an inhibitor of growth activities.

So the five groups are auxins, gibberellins, cytokinins, ethylene and abscisic acid (this is the first half of NCERT exercise question 4). Learn the pairing 'name, chemical class, example' as a table because MCQs test it in every direction: which PGR is a terpene (GA₃), which is a carotenoid derivative (ABA), which is an adenine derivative (kinetin, zeatin), which is an indole (IAA), which is a gas (ethylene).

Interestingly, all five major groups of PGRs were discovered accidentally, and each discovery has a plant, a scientist and an observation attached. Questions pair any two of these, so learn all three for each.

Auxin. The discovery of the first PGR began with the observations of Charles Darwin and his son Francis Darwin that the coleoptiles of canary grass responded to unilateral illumination by growing towards the light source (phototropism). After a series of experiments it was concluded that the tip of the coleoptile was the site of transmittable influence that caused the bending of the entire coleoptile (NCERT Fig. 13.8). F.W. Went (1928) isolated auxin from the tips of coleoptiles of oat seedlings: he placed cut tips on agar blocks, and the agar block alone, placed on one side of a decapitated coleoptile, made it bend, proving a diffusible chemical was responsible. The name auxin is from the Greek auxein, to grow.

Gibberellin. The 'bakanae' (foolish seedling) disease of rice seedlings was caused by a fungal pathogen, Gibberella fujikuroi. Infected seedlings grew abnormally tall and thin and fell over. E. Kurosawa reported the appearance of the disease symptoms in uninfected rice seedlings when they were treated with sterile filtrates of the fungus. The active substances were later identified as gibberellic acid.

Cytokinin. F. Skoog and his co-workers observed that from the internodal segments of tobacco stems the callus (a mass of undifferentiated cells) proliferated only if, in addition to auxins, the nutrient medium was supplemented with one of the following: extracts of vascular tissues, yeast extract, coconut milk or DNA. Skoog and Miller later identified and crystallised the cytokinesis-promoting active substance that they termed kinetin. The source was autoclaved herring sperm DNA, a detail NEET likes.

Abscisic acid. During the mid-1960s, three independent researches reported the purification and chemical characterisation of three different kinds of inhibitors: inhibitor-B, abscission II and dormin. Later all three were proved to be chemically identical, and the compound was named abscisic acid (ABA). Note the three roles hidden in the three names: growth inhibition, abscission and dormancy.

Ethylene. H.H. Cousins confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas. Later this volatile substance was identified as ethylene, a gaseous PGR.

In a 5-mark question on the discoveries, write five short paragraphs with the scientist, the plant or organism, the observation and the substance, exactly in this order. In a 1-mark question the trap is usually the plant: canary grass for Darwin, oat for Went, rice for Kurosawa, tobacco callus and herring sperm DNA for Skoog and Miller, oranges and bananas for Cousins.

Auxins (from Greek auxein, to grow) were first isolated from human urine, a fact NCERT states and MCQs use. The term auxin is applied to the indole-3-acetic acid (IAA) and to other natural and synthetic compounds having certain growth-regulating properties. They are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action. Auxins like IAA and indole butyric acid (IBA) have been isolated from plants; these are the natural auxins. NAA (naphthalene acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid) are synthetic auxins. All these auxins have been used extensively in agricultural and horticultural practices.

The physiological effects, in NCERT order, are the answer to any 3-mark or 5-mark question on auxins:

1. They help to initiate rooting in stem cuttings, an application widely used for plant propagation (nurseries dip cuttings in IBA or NAA powder).
2. Auxins promote flowering, e.g. in pineapples.
3. They help to prevent fruit and leaf drop at early stages but promote the abscission of older, mature leaves and fruits. Both halves of this sentence are needed; the second half is what makes it a good Assertion-Reason item.
4. Apical dominance. In most higher plants the growing apical bud inhibits the growth of the lateral (axillary) buds. Removal of the shoot tips (decapitation) usually results in the growth of lateral buds. This is widely applied in tea plantations and hedge-making, where bushes are kept dense by repeated pruning.
5. Auxins also induce parthenocarpy, e.g. in tomatoes (fruit formed without fertilisation, hence seedless).
6. They are widely used as herbicides. 2,4-D is widely used to kill dicotyledonous weeds; it does not affect mature monocotyledonous plants. It is used to prepare weed-free lawns by gardeners.
7. Auxin also controls xylem differentiation and helps in cell division.

A note on concentration that helps with Assertion-Reason questions: auxin promotes shoot growth at a concentration that already inhibits root growth, which is why the same hormone can be read as a promoter in one organ and an inhibitor in another. For the exam, keep to the NCERT list above and quote pineapple, tomato, tea plantations and 2,4-D by name.

Gibberellins are another kind of promotory PGR. There are more than 100 gibberellins reported from widely different organisms such as fungi and higher plants. They are denoted as GA₁, GA₂, GA₃ and so on. Gibberellic acid (GA₃) was one of the first gibberellins to be discovered and remains the most intensively studied form. All GAs are acidic, and chemically they are terpenes.

They produce a wide range of physiological responses in plants, and the NCERT list should be reproduced in order:

1. Their ability to cause an increase in length of the axis is used to increase the length of grapes stalks, so that the bunch is looser and the berries are larger.
2. Gibberellins cause fruits like apple to elongate and improve their shape.
3. They also delay senescence, so that fruits can be left on the tree longer, which extends the market period.
4. GA₃ is used to speed up the malting process in the brewing industry (GA induces the aleurone layer of barley grain to produce amylase, which converts starch to sugar).
5. Sugarcane stores carbohydrate as sugar in its stems; spraying sugarcane crop with gibberellins increases the length of the stem, thus increasing the yield by as much as 20 tonnes per acre.
6. Spraying juvenile conifers with GAs hastens the maturity period, thus leading to early seed production.
7. Gibberellins also promote bolting (internode elongation just prior to flowering) in beet, cabbages and many plants with rosette habit.

Bolting is a definition asked on its own: rosette plants like cabbage and beet have very short internodes in their vegetative stage; just before flowering the internodes elongate suddenly and the flowering stalk shoots up. GA applied to a rosette plant induces this 'bolting' even without the usual cold or long-day requirement, which is why the answer to 'which PGR would you use to bolt a rosette plant' is gibberellin.

Remember also that GA and ABA are antagonistic in many situations (seed germination for instance: GA promotes it, ABA inhibits it), which NCERT states at the end of the ABA section. GA₃ applied to rice seedlings makes them grow abnormally tall, reproducing the bakanae symptom, which is part (a) of NCERT exercise question 8.

Cytokinins have a specific effect on cytokinesis, the division of the cytoplasm, which is where the name comes from. They were discovered as kinetin (a modified form of adenine, a purine) from the autoclaved herring sperm DNA. Kinetin does not occur naturally in plants. The search for natural substances with cytokinin-like activity led to the isolation of zeatin from corn kernels and coconut milk. Since the discovery of zeatin, several naturally occurring cytokinins and some synthetic compounds with cell-division-promoting activity have been identified.

Natural cytokinins are synthesised in regions where rapid cell division occurs, for example root apices, developing shoot buds and young fruits. Note that the root apex is a major site of synthesis; cytokinins made in the roots travel up in the xylem to the shoot.

The physiological effects listed in NCERT:

1. They help to produce new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation.
2. Cytokinins help overcome apical dominance. Auxin from the apex suppresses lateral buds; cytokinin applied to the lateral buds lets them grow. The ratio of auxin to cytokinin decides the outcome, and the same ratio decides whether a tissue-culture callus makes roots (high auxin) or shoots (high cytokinin).
3. They promote nutrient mobilisation, which helps in the delay of leaf senescence. A leaf treated with cytokinin stays green longer because nutrients are drawn towards it rather than exported out of it.

Two of the NCERT exercise items rest on cytokinin. Question 7(c), delay leaf senescence: cytokinin. Question 7(d), induce growth in axillary buds: cytokinin. Question 8(d), if you forget to add cytokinin to the culture medium: the callus will not divide and proliferate properly and no shoots will differentiate, because cytokinesis and shoot formation need cytokinin (Skoog's original observation).

Ethylene is a simple gaseous PGR. It is synthesised in large amounts by tissues undergoing senescence and by ripening fruits. Because it is a gas it diffuses through the air in a store or a basket, which is why one rotten fruit spoils the lot (NCERT exercise question 8(c)).

Effects of ethylene on plants include horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings. These three together are the classical 'triple response' of dark-grown seedlings, though NCERT does not use that name. Ethylene promotes senescence and abscission of plant organs, especially of leaves and flowers. Ethylene is highly effective in fruit ripening. It enhances the respiration rate during ripening of the fruits; this rise in the rate of respiration is called respiratory climactic. Ethylene breaks seed and bud dormancy, initiates germination in peanut seeds and sprouting of potato tubers. Ethylene promotes rapid internode and petiole elongation in deep-water rice plants, which helps the leaves and upper parts of the shoot remain above water. It also promotes root growth and root hair formation, thus helping the plants to increase their absorption surface.

Ethylene in agriculture. It is used to initiate flowering and for synchronising fruit-set in pineapples; it also induces flowering in mango. Since ethylene regulates so many physiological processes, it is one of the most widely used PGRs in agriculture. The most widely used compound as a source of ethylene is ethephon. Ethephon in an aqueous solution is readily absorbed and transported within the plant and releases ethylene slowly. Ethephon hastens fruit ripening in tomatoes and apples and accelerates abscission in flowers and fruits (thinning of cotton, cherry, walnut). It promotes female flowers in cucumbers, thereby increasing the yield.

For a 5-mark answer on ethylene, group the effects: seedling effects (three), senescence and abscission, fruit ripening and respiratory climactic, dormancy-breaking (peanut, potato), elongation in deep-water rice, root growth and root hairs, and then the agricultural uses with ethephon (pineapple, mango, tomato, apple, thinning, cucumber). Fruit sellers ripen bananas with ethylene or by keeping a ripe fruit in the basket, a scenario the case-based question often uses.

As mentioned earlier, abscisic acid (ABA) was discovered for its role in regulating abscission and dormancy. But like other PGRs it also has other wide-ranging effects on plant growth and development. It acts as a general plant growth inhibitor and an inhibitor of plant metabolism. ABA inhibits seed germination. ABA stimulates the closure of stomata in the epidermis and increases the tolerance of plants to various kinds of stresses. Therefore, it is also called the stress hormone. That single line is the full answer to NCERT exercise question 5 (why ABA is called the stress hormone): under water stress ABA is produced, it closes the stomata to cut water loss, and it raises the plant's tolerance to drought, salinity and cold.

ABA plays an important role in seed development, maturation and dormancy. By inducing dormancy, ABA helps seeds to withstand desiccation and other factors unfavourable for growth. In most situations, ABA acts as an antagonist to GAs: GA promotes germination, growth of the axis and bolting; ABA inhibits germination and growth and keeps buds and seeds dormant. A seed that stays dormant through a dry summer and germinates only when it rains is showing the ABA and GA balance in action.

PGRs act together, and with the environment. NCERT closes the chapter with a point that is easy to skip and easy to ask. Many events in the life of a plant are affected by more than one PGR: dormancy in seeds and buds, abscission, senescence, apical dominance, and also seed germination, plant movements and flowering. The PGRs act in synergy (auxin and cytokinin together in callus growth) or in antagonism (ABA against GA in germination, auxin against cytokinin in apical dominance). Also, the roles of PGRs are of many kinds in a plant's life, and the effect seen depends on the concentration, the tissue and the presence of other PGRs, which is why it is difficult in an experiment to assign an observed effect to any single hormone (an NCERT exemplar question). In addition to the intrinsic control by PGRs and the genome, extrinsic factors such as light and temperature control flowering and other developmental events, which links this chapter back to the extrinsic factors listed in Section 13.3.

For the exam, the two lines to memorise for ABA are 'general growth inhibitor, inhibits metabolism and seed germination' and 'closes stomata, increases stress tolerance, stress hormone; seed dormancy and desiccation tolerance; antagonist of GA'.