Plant Growth and Development

Growth is defined as a permanent and irreversible increase in the size of an organ, of a part of an organ, or even of a single cell. The word irreversible is the important one. A wilted leaf that swells again after you water the plant has not grown; it has only taken back the water it lost. Growth is also not free. It is accompanied by a whole set of metabolic processes, both building up and breaking down, and all of them run at the expense of energy. So growth is best thought of as size increase that the plant has paid for and cannot undo.

Animals usually stop growing once they reach a certain adult size. Plants do not. A plant keeps small pockets of cells that never lose the power to divide, and these are the meristems. Because of them a plant has what is called an open form of growth, adding new organs to the body it already has for as long as it lives. The apical meristems sit at the tip of the root and at the tip of the shoot, and they are responsible for primary growth, which makes the plant taller and the root longer. Lateral meristems such as the vascular cambium and the cork cambium appear later in dicots and gymnosperms and are responsible for secondary growth, which adds girth. In grasses there is a third kind, the intercalary meristem, left behind at the bases of the internodes. It is the reason a lawn or a grazed field grows back after the tops have been cut off.

Growth is one of the few things in the plant body you can put a number on, and there is more than one number to choose from. You can follow an increase in length, as with a pollen tube growing down a style. You can follow an increase in surface area, which is the sensible unit for a flat dorsiventral leaf. You can count cells, and the numbers are startling: a single apical meristem in a maize root can give rise to more than 17,500 new cells in one hour. You can follow an increase in the size of one cell, and cells in a developing watermelon may enlarge by as much as 3,50,000 times. You can also weigh the material. Fresh weight is the weight of the living tissue as it is, so it moves up and down with the water in the tissue and can mislead you. Dry weight is what is left after the water has been driven off, so it measures the organic material the plant has actually accumulated. A germinating seed is the classic caution here: the seedling gets visibly longer and heavier in fresh weight while its dry weight is still falling, because it is spending stored food and has not yet begun to photosynthesise.

Growth in a root or shoot happens in three phases, and the neat thing is that all three can be seen at the same moment if you look along the organ. Closest to the tip is the meristematic phase. These cells are constantly dividing, they are rich in protoplasm, they have large conspicuous nuclei, their walls are thin, primary and cellulosic, and they are connected to each other by abundant plasmodesmata. Just behind them is the phase of elongation, where the cells stop dividing and instead swell. Vacuoles enlarge and run together, water is taken in, and new cell wall material is laid down so the enlarged cell keeps its shape. Further back still is the phase of maturation, where cells have reached their maximum size and now finish themselves off: walls thicken, the protoplasm is modified, and each cell takes on the structure that suits its job. What looks like three regions in space is really one story in time. Any single cell begins at the tip, is pushed backwards as newer cells appear behind it, and passes through elongation and then maturation as it goes.

Growth can be expressed as a rate, and the shape of that rate depends on what the dividing cells do with themselves. In arithmetic growth, after mitosis only one of the two daughter cells goes on dividing while the other one leaves the cycle, elongates and matures. The number of dividing cells therefore stays the same, a constant amount is added in each unit of time, and a plot of size against time is a straight line. An elongating root measured at fixed intervals behaves this way. The relation is written as Lt = L0 + rt, where L0 is the length at the start, r is the growth rate per unit time, t is time, and Lt is the length at time t.

In geometric growth, both daughter cells keep the power to divide, so the number of dividing cells doubles each round and the increase is proportional to how much is already there. This is what happens in the early life of a plant or in a cell culture, and the plot curves steeply upwards. Geometric growth is described by W1 = W0 e^rt, where W0 is the initial size, W1 is the final size after time t, e is the base of natural logarithms, and r is the relative growth rate. Here r is often called the efficiency index, because it tells you how efficiently the material already present is being used to make more material. The bigger the r, the faster the system grows, and comparing r values is a fair way to compare two plants or two varieties.

No real plant grows geometrically forever, because nutrients, water, space and light all run short. So growth in an organism placed in a natural environment goes through three stages one after the other. There is a slow beginning, the lag phase, where the plant is preparing and the numbers are small. Then comes the log or exponential phase, where growth is rapid and multiplies on itself. Finally there is the stationary phase, where limited supply slows growth until it levels off. Drawn as a graph of size against time this gives an S-shaped or sigmoid curve, and it is typical of cells, tissues, organs and whole plants alike.

Two ways of reporting a rate are used, and mixing them up is a common mistake. Absolute growth rate is the total growth per unit time, measured and compared just as it is. Relative growth rate is the growth per unit time expressed on a common basis, usually per unit of the starting size. Take two leaves, one of 5 square centimetres and one of 50 square centimetres, and suppose that in the same period each one adds 5 square centimetres. Their absolute growth rates are identical. But the small leaf has doubled itself while the large one has added a tenth, so the relative growth rate of the small leaf is far higher. Absolute rate answers how much was added; relative rate answers how much was added for what was already there.

Growth also needs the right conditions, and every one of them can become the limiting factor. Water is needed for cell enlargement, because it is turgor pressure that stretches the wall of an elongating cell, and water also supplies the medium in which enzymes work. Oxygen is needed because respiration releases the metabolic energy that growth runs on. Nutrients, both macronutrients and micronutrients, are needed as raw material for protoplasm and as a source of energy. Every plant has an optimum temperature range for growth, for most plants somewhere between about 25 and 35 degrees Celsius, and growth is reduced both above and below that range. On top of these, environmental signals such as light and gravity affect particular phases and stages of growth, deciding not so much how fast the plant grows as which way it grows and what it becomes.

Cells produced by the root apical meristem, the shoot apical meristem and the cambium do not stay alike. They mature and take on the structure that fits the job they will do, and this process is called differentiation. Differentiation involves major and often drastic changes in both the cell wall and the protoplasm. The clearest example in a plant body is the making of a tracheary element. To become a vessel element or a tracheid, a living cell loses its protoplasm altogether and lays down a strong, elastic, lignocellulosic secondary wall. Only after it has emptied and reinforced itself in this way can it carry water over long distances under the extreme tension that pulling water up a tall plant involves.

Plants also do something animals rarely manage. A living, fully differentiated cell that had stopped dividing can regain the capacity to divide, and this is called dedifferentiation. When the interfascicular cambium forms in a dicot stem, it forms out of ordinary parenchyma cells sitting between the vascular bundles, and those parenchyma cells had already finished differentiating. The cork cambium arises in the same way, from fully differentiated cells of the outer cortex. Once such a meristem has formed, the cells it produces go on to lose the power to divide once more and mature into something specific, and that second round of specialisation is called redifferentiation. Secondary xylem and secondary phloem produced by the vascular cambium, and the cork produced by the cork cambium, are all redifferentiated tissues.

Growth and differentiation in plants are described as open, and part of what that means is that where a cell ends up decides what it becomes. Cells from the very same meristem can finish as completely different things. A cell pushed forward, ahead of the root apical meristem towards the very tip, matures as a root cap cell, while a cell pushed sideways to the outer surface of the growing axis matures as epidermis. The two cells came from the same meristem; only their positions differed. Position, not ancestry alone, settles the outcome.

Development is the wider word. It covers all the changes an organism passes through during its life cycle, from the germination of the seed right through to senescence. A convenient way to remember it is that development = growth + differentiation. Both intrinsic and extrinsic factors control it. The intrinsic ones are intracellular, meaning the genes, and intercellular, meaning the chemical messengers that pass between cells, which is where plant growth regulators come in. The extrinsic ones are the surroundings: light, temperature, water, oxygen and nutrition.

Because a plant is fixed in one place and cannot walk away from bad conditions, it has to answer them by building itself differently. This ability is called plasticity: plants follow different pathways in response to their environment or to their phase of life, and so form different kinds of structures. The standard illustration is heterophylly, which simply means bearing more than one kind of leaf on the same plant, and it comes in two flavours. In cotton, coriander and larkspur the leaves made by a juvenile plant are a different shape from the leaves the same plant makes when it is mature, so here the trigger is the age or phase of the plant. In buttercup the trigger is the surroundings instead: leaves formed under water are finely dissected into narrow thread-like segments, while leaves the same plant forms in air are broad and lobed. The genes have not changed in either case. What has changed is which of the possible programmes the plant is running.

Plant growth regulators, also called plant hormones or phytohormones, are small, simple molecules that a plant makes in one place and that act, often in another place, in extremely small amounts. They have almost nothing in common chemically. Auxin is an indole compound, indole-3-acetic acid. Kinetin is an adenine derivative, that is a modified purine. Abscisic acid is derived from carotenoids. Gibberellic acid is a terpene. Ethylene is a gas, and the simplest molecule of the five. What unites them is not their structure but their job.

By the kind of job they do, they fall into two broad groups. The growth promoters are auxins, gibberellins and cytokinins, and they are involved in cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation. The growth inhibitors are concerned with dormancy, abscission and responses to stress, and abscisic acid is the classic member. Ethylene is awkward to place, since it can go into either group, but it acts mostly as an inhibitor of growth activities. In practice a plant almost never uses one regulator alone. What a tissue does is decided by the balance between several regulators present at once, and by how sensitive that particular tissue is to them.

Every one of the five was found by chasing an oddity in plant behaviour. Auxin begins with Charles Darwin and his son Francis, who noticed that the coleoptile of canary grass bent towards light coming from one side. When they cut off the tip, or covered it with an opaque cap, the bending stopped, even though the bend itself takes place lower down. They concluded that some influence travelled from the tip to the region below. Much later F. W. Went isolated the substance from the tips of oat coleoptiles by standing cut tips on blocks of agar and then placing the agar, now carrying the diffused chemical, on decapitated coleoptiles, which curved as though the tip were still there.

Gibberellin came out of a disease of rice called bakanae, or the foolish seedling disease, in which affected seedlings shot up abnormally tall, thin and pale and then flopped over. E. Kurosawa showed that the cause was a fungus, Gibberella fujikuroi, and that filtrates of the fungus, with no fungus in them at all, produced the same symptoms in healthy rice. The active substance was gibberellic acid or GA₃, and more than a hundred gibberellins are now known from very different organisms, named GA₁, GA₂, GA₃ and so on.

Cytokinin was found by F. Skoog and his co-workers, who were growing tobacco stem pith in culture. The tissue would form a callus, that is a mass of undifferentiated dividing cells, only if auxin was supplied along with something extra: extracts of vascular tissue, or yeast extract, or coconut milk, or DNA. Miller and co-workers later isolated the active compound from autoclaved herring sperm DNA and named it kinetin. Kinetin is a modified adenine, and it does not occur naturally in plants. The natural cytokinin, zeatin, was later obtained from corn kernels and from coconut milk.

Abscisic acid was discovered three times over. Three groups working on different problems each isolated an inhibitor and gave it a different name, inhibitor-B, abscission II and dormin. When the three substances turned out to be chemically identical they were renamed abscisic acid. Ethylene was traced by Cousins, who confirmed that ripened oranges released a volatile substance which hastened the ripening of unripe bananas stored with them. That volatile substance was ethylene, and it remains the only gaseous plant growth regulator.

Auxin was the first of these substances to be isolated, and the term now covers a family of compounds that behave like indole-3-acetic acid. Indole-3-acetic acid, or IAA, and indole butyric acid, IBA, are natural auxins found in plants. Naphthalene acetic acid, NAA, and 2,4-dichlorophenoxyacetic acid, 2,4-D, are synthetic ones. Auxins are produced by the growing apices of stems and roots, and they move from there to the region where they act.

Inside a plant, auxin makes cells in the elongating zone below the shoot tip take in water and stretch. This is why unequal distribution of auxin bends a shoot. When light falls on one side of a coleoptile, more auxin ends up on the shaded side, the cells there elongate more than those on the lit side, and the shoot curves towards the light. Auxin is also strongly concentration dependent, and a concentration that promotes elongation in a shoot can inhibit elongation in a root. That is why the same substance can bend two organs in opposite directions: in a plant laid on its side, auxin collects along the lower flank of both organs, the lower side of the shoot elongates faster and the shoot curves upward, while in the root that same higher concentration slows the lower side and the root curves downward.

The best known auxin effect is apical dominance. In most higher plants the growing apical bud stops the axillary or lateral buds below it from growing out, so the plant runs up as a single main axis. Remove the shoot tip, which is called decapitation, and the source of auxin goes with it, so the lateral buds are released and start to grow. Gardeners use this deliberately. Tea bushes are repeatedly pruned and hedges are trimmed so that they branch out and become dense instead of tall. Auxin also initiates rooting in stem cuttings, which is the basis of a great deal of vegetative propagation, and it promotes flowering in pineapple. It prevents fruit and leaf drop at early stages, yet it promotes the abscission of older, mature leaves and fruits, so its effect on abscission depends on the age of the organ. Auxin induces parthenocarpy, the setting of fruit without fertilisation, as in tomato. It also controls xylem differentiation and takes part in cell division. Synthetic auxins are used as weedkillers; 2,4-D is used to kill dicotyledonous weeds without harming mature monocotyledonous plants, which is how a weed-free lawn of grass is maintained.

Gibberellins are a large group, all of them acidic, of which GA₃ is the most studied. Their signature effect in a plant is elongation of the axis. Sprayed on grapes, gibberellin lengthens the stalks so the bunch is looser and the berries larger. Applied to apples it makes the fruit elongate and improves its shape. Gibberellins delay senescence, so treated fruits can be left hanging on the tree longer and the marketing period is extended. Sprayed on sugarcane, which stores its sugar in the stem, gibberellin lengthens the stem and can raise the yield by as much as 20 tonnes per acre. Sprayed on juvenile conifers it shortens the wait for maturity and brings on early seed production.

Two more gibberellin effects are worth fixing in memory. The first is bolting, which is the sudden elongation of the internodes just before flowering. Plants with a rosette habit, such as beet and cabbage, normally sit as a flat cluster of leaves close to the ground, and gibberellin makes the axis between those leaves shoot upward. The second is in the germinating cereal grain. Gibberellin released by the embryo acts on the aleurone layer and makes it produce hydrolysing enzymes such as alpha-amylase, which digest the stored starch of the endosperm into sugars the growing embryo can use. This is exactly what the brewing industry exploits when GA₃ is used to speed up the malting of barley. Notice the pattern: gibberellin gets stored reserves unlocked and axes stretched, while auxin decides which bud grows and in which direction.

Cytokinins are named for their effect on cytokinesis, the division of the cell itself. The natural cytokinins, such as zeatin, are made wherever rapid cell division is going on, so the richest sources in a plant are root apices, developing shoot buds and young fruits. Their effects follow from that. Cytokinin helps produce new leaves, promotes the formation of chloroplasts in leaves, encourages lateral shoot growth and brings about adventitious shoot formation. It helps overcome apical dominance, so cytokinin applied to a suppressed axillary bud can start it growing even while the shoot tip is still in place. Set that beside auxin and the pair make sense as opposites acting on the same bud.

Cytokinin also delays leaf senescence, and it does so by promoting nutrient mobilisation. Nutrients are drawn towards the tissue that has the cytokinin, so a treated leaf keeps its supply of amino acids and stays green while untreated leaves around it yellow. Cut flowers and leafy vegetables are kept fresh on the same principle. In tissue culture the ratio between cytokinin and auxin decides what the callus becomes: a high proportion of cytokinin favours shoot formation, while a high proportion of auxin favours root formation. This is one of the clearest demonstrations that a plant response is set by the balance of regulators, not by any single one.

Ethylene is a gas, and it is produced in large quantities by tissues that are ageing and by fruits that are ripening. In a germinating dicot seedling pushing up through soil, ethylene causes horizontal growth of the axis, swelling of the axis and the formation of the apical hook that protects the tender growing tip. Ethylene promotes senescence and abscission of plant organs, and it is especially effective on leaves and flowers. It is the main regulator of fruit ripening, and it raises the respiration rate of the fruit during ripening, an increase known as the respiratory climactic, also written respiratory climacteric. Ethylene is also autocatalytic, meaning that ripening tissue makes ethylene which makes more tissue ripen and produce still more ethylene, which is why one ripe fruit closed in a bag will hurry along the unripe ones with it.

Ethylene breaks seed and bud dormancy, starting germination in peanut seeds and sprouting in potato tubers. In deep water rice it promotes rapid elongation of the internodes and petioles as the flood rises, so that the leaves and the upper part of the shoot stay above water. It promotes root growth and root hair formation, which increases the absorbing surface of the root system. Growers use it to initiate flowering and to synchronise fruit set in pineapple, and to induce flowering in mango. Auxin is also listed as promoting flowering in pineapple, and the two statements do not clash: both regulators can trigger the response, but it is ethylene, released from an applied compound, that growers actually spray to bring a whole pineapple field into flower at once. The commonest source compound is ethephon, which in aqueous solution is readily absorbed, is transported within the plant, and releases ethylene slowly. Ethephon hastens the ripening of tomatoes and apples, accelerates abscission in flowers and fruits, which is used to thin cotton, cherry and walnut, and promotes female flowers in cucumber, raising the yield.

Abscisic acid acts as a general plant growth inhibitor and an inhibitor of plant metabolism. It inhibits seed germination. It stimulates the closure of stomata in the epidermis, which is its most important short-term effect: when a plant is short of water, abscisic acid accumulates and acts on the guard cells, they lose solutes and water, go flaccid, and the pore closes so that water loss by transpiration is cut. Because it increases the tolerance of plants to various kinds of stress, abscisic acid is also called the stress hormone. It plays an important part in seed development, maturation and dormancy, and by inducing dormancy it lets seeds withstand desiccation and other conditions unfavourable for growth. In most situations abscisic acid acts as an antagonist to the gibberellins, which is neatly seen in a seed: gibberellin pushes towards germination and mobilising reserves, abscisic acid holds the seed shut until conditions are right. One caution about the name. Abscisic acid was first linked with abscission, and that is where the name came from, but the shedding of leaves and fruits is now credited mainly to ethylene, while abscisic acid is remembered for stomatal closure, dormancy and stress tolerance.

A plant that flowers at the same time every year is not simply counting how big it has grown. It is reading the calendar off the sky. The response of plants to periods of day and night is called photoperiodism, and by it plants sort into three groups. Long day plants flower only when the light period they receive exceeds a certain well-defined critical duration. Short day plants flower only when the light period is shorter than that critical duration. Day neutral plants show no relation at all between the duration of light exposure and flowering, and will flower once they are mature whatever the season.

Two points here catch students out. First, the critical duration is not the same number for every species; each species has its own, and the same day length can be long for one plant and short for another. That is why a plant is not called a long day plant because the days are long in an absolute sense, but because the days must be longer than that plant's own critical value. Second, and more subtly, what the plant actually measures is the length of the continuous dark period. A short day plant is really a long night plant. If its long night is interrupted by even a brief flash of light in the middle, the plant will not flower, even though the total hours of daylight have hardly changed. Interrupting the light period with a spell of darkness does not have the same effect.

Where does the plant sense this? Not at the shoot apex, even though that is where the flower is finally produced. The site of perception of light and dark is the leaf. This has been shown by exposing only the leaves of a plant to the right photoperiod while the buds are kept under a different regime, and by removing the leaves so that no signal can be produced. The conclusion is that a hormonal substance is responsible for flowering, that it is made in the leaf only when the plant receives the inducing photoperiod, and that it then migrates from the leaf to the shoot apices to bring flowering about. So a plant with no leaves, or with leaves kept on the wrong photoperiod, will not flower however favourable everything else is.

Light is not the only signal from the seasons. Some plants will not flower, or will flower only poorly, unless they have first passed through a period of low temperature, and this promotion of flowering by a spell of cold is called vernalisation. Its value to the plant is that it prevents precocious reproductive development late in a growing season and makes sure the plant has enough time to reach maturity before it commits itself to flowering.

The standard examples are the cereals. Wheat, barley and rye all have two kinds of varieties. The spring varieties are planted in spring, and they come to flower and produce grain before the growing season ends. The winter varieties, if they were planted in spring, would normally fail to flower or fail to produce mature grain within that season. So they are sown in autumn instead. They germinate before the cold sets in, pass the winter as small seedlings, resume growth in spring and are harvested around the middle of summer. The cold they lived through is what made the difference. The other set of examples is the biennials. Biennials are monocarpic plants that normally flower and die in their second season, and sugarbeet, cabbage and carrot are common ones. Giving a growing biennial a cold treatment stimulates a subsequent photoperiodic flowering response, which shows how the two signals work in sequence rather than in competition.

Keep the two apart. Photoperiodism responds to the lengths of light and dark, is perceived by the leaf, and can act in the same season. Vernalisation responds to low temperature, is perceived by the actively dividing cells of the plant, that is the growing shoot tip or the embryo of a germinating seed, and does not itself make a flower appear. It only makes the plant competent, so that when the right photoperiod arrives afterwards, the plant is ready to answer it.