Plant Growth and Development: Class 11 Biology · Chapter 13

Throw a mango stone onto damp ground as the monsoon begins and, a few weeks later, a reddish shoot with soft, drooping leaves stands where it fell. Given decades, that shoot can become a tree that flowers each spring, fruits in summer, and finally ages and dies. None of this happens at random: roots come before leaves, leaves before flowers, flowers before fruit, and each organ appears in its proper place (Fig. 13.1).

A root-hair cell, a xylem vessel and a petal cell look and work nothing alike, yet each traces back to the same single cell, the zygote. What turned one cell into so many kinds, and what tells a plant when to stop making leaves and start making flowers? Biologists answer with two ideas that together make up development (परिवर्धन): growth, which makes the plant bigger, and differentiation, which makes its cells different.

Everything starts with germination, yet a seed is choosy about when. Grass seeds on a dusty roadside in May lie unchanged for weeks, alive but nearly idle, in a state of suspended growth. The first good shower changes that: the seeds drink, respiration and enzyme activity restart, and the root pushes out. Which signals does a plant read before it grows, flowers or rests? Some come from inside it (intrinsic factors, such as genes and chemical messengers) and some from outside (extrinsic factors, such as light, temperature and water). This chapter looks at both.

13.1 Growth

Ask anyone how to tell that a plant is alive and 'it grows' will be among the first answers. Biologists use the word in a narrow sense: growth (वृद्धि) is an irreversible, permanent increase in size of a cell, an organ or a whole organism. It is paid for by metabolism, with anabolic reactions building new protoplasm and wall material and catabolic reactions such as respiration supplying the energy. A jackfruit swelling on the trunk over several months is growing, because it will never shrink back to its first size.

13.1.1 Plant Growth Generally is Indeterminate

Most animals stop growing once they reach adult size. Many plants never do: a banyan can keep adding roots, branches and wood for centuries. They manage this by keeping pockets of permanently young tissue, the meristems (विभज्योतक). When a meristem cell divides, one daughter usually stays in the meristem while the other is pushed aside, soon stops dividing and matures into part of the body. So the meristem renews itself even as it builds the plant.

Since the meristems go on supplying cells for as long as the plant lives, botanists call plant growth open, or indeterminate. Not every organ is open-ended, though: a leaf, a flower or a fruit grows to a set size and stops, which is determinate growth.

Where the meristems sit decides which way the plant grows (Fig. 13.2). The meristems at the two tips, the shoot apical meristem and the root apical meristem, lengthen the plant along its axis: this is primary growth. Dicots and gymnosperms add a second set once the plant is established: the vascular cambium and the cork cambium, called lateral meristems because they run along the sides of stems and roots. Their divisions add layers sideways, so stems and roots get thicker; this is secondary growth. Grasses and bamboo also have intercalary meristems at the bases of their internodes and leaves, which is why a mown lawn grows back from below.

13.1.2 Growth is Measurable

What actually piles up when a cell grows is protoplasm, but nobody can put the protoplasm of a root tip on a balance. Instead we track a proxy that rises along with it, such as fresh weight, dry weight, length, area, volume or the number of cells, and the right proxy depends on what is growing (Table 13.1).

No single yardstick works for a plant's whole life. A soaked chickpea gains fresh weight quickly because it takes up water, yet its dry weight falls for the first few days: it burns stored food before its leaves can photosynthesise. Late in life, leaf fall and seed shedding make weight records drop while the plant is still alive. So biologists match the measure to the organ and the stage, and often use more than one.

13.1.3 Phases of Growth

Slice a growing root tip lengthwise and you see growth laid out in space (Fig. 13.3). Just behind the root cap, cells divide again and again: the meristematic phase. A little further back, the new cells stop dividing and stretch: the elongation phase. Further back still, cells take on their final form: the maturation phase. A shoot tip shows the same sequence. Every cell passes through all three phases in turn, so the zones are a timeline frozen along the root.

13.1.4 Growth Rates

How fast something grows is its growth rate: the increase in size per unit time. Behind it can lie two very different patterns of cell division (Fig. 13.4).

In arithmetic growth, each division leaves one daughter that goes on dividing and one that stops and matures. The dividing cell works like a press that turns out one finished cell per round, so the organ gains the same amount in every interval. Think of a root that adds the same length every hour: plotted against time, its length gives a straight line (Fig. 13.5):

In geometric growth, both daughters keep dividing, so the cell number doubles each round. Real systems cannot keep this up for long. Growth starts slowly in a lag phase, speeds up in the log or exponential phase, then levels off in a stationary phase as nutrients run short. A graph of size against time is then S-shaped, a sigmoid curve. Cells in culture, organs and whole plants growing in natural surroundings all trace it. An embryo uses both patterns: its first divisions are geometric, then only some regions keep dividing while the rest differentiate.

The exponential phase follows a simple law:

The constant $r$ in this law is the relative growth rate: new material made per unit of existing material per unit time. It measures how efficiently the plant turns what it already has into more of itself, so it is also called the efficiency index. Notice that $W_1$ is proportional to $W_0$: with the same $r$ and $t$, a plant that starts twice as big ends twice as big. Taking natural logarithms gives $r=\frac{1}{t}\ln (W_1\mathrm{/}{W}_0)$, and the size doubles every $\ln 2\mathrm{/}r$ units of time.

Two growing systems can be compared in two ways. The absolute growth rate is the total gain per unit time. The relative growth rate divides that gain by the starting size, so that a seedling and a full-grown plant can be compared fairly:11For a short interval the two forms agree. Over a long stretch of exponential growth, use $r=\frac{1}{t}\ln (W_1\mathrm{/}{W}_0)$ instead, because the base keeps growing during the interval.

13.1.5 Conditions for Growth

What would a seedling on a windowsill die without? Each of its requirements has a definite job:

• Water. Cells enlarge by taking up water, and the turgor it creates pushes the wall outward during extension growth. The enzyme reactions that build new material also need an aqueous medium.
• Oxygen. Aerobic respiration needs it to release the energy that building new cells consumes.
• Nutrients. Macronutrients and micronutrients supply the raw material for new protoplasm, and some also serve as a source of energy.
• Temperature. Every plant grows best within an optimum range; move too far either side and growth slows, stops or the plant dies.
• Light and gravity. These act as signals that steer particular stages, such as which way a shoot bends or when a seedling turns green.

13.2 Differentiation, Dedifferentiation and Redifferentiation

Compare a cell in a root apical meristem with a vessel element in the wood of a teak trunk. Both came from meristems, yet one is a small living box packed with protoplasm, and the other is a dead, hollow pipe. Its thick lignocellulosic secondary wall, strong yet elastic, keeps it from collapsing while water is pulled up through it under strong tension. The journey from the first kind of cell to the second, or to any other specialised cell, is differentiation (विभेदन). Some cells change only slightly on the way, others drastically, and the changes can involve the wall, the protoplasm, or both.

Plant cells can also run the film backwards. When a woody dicot stem begins to thicken, some parenchyma cells that had long since stopped dividing switch division back on: those lying between the vascular bundles become the interfascicular cambium, and some in the cortex become the cork cambium. A mature, living cell that resumes division like this has undergone dedifferentiation (निर्विभेदन). The cells these new meristems cut off soon settle down, stop dividing and take up specific roles; that second maturing is redifferentiation (पुनर्विभेदन). Secondary xylem, secondary phloem, cork and secondary cortex are all its products (Fig. 13.8).

Openness applies to differentiation as well as to growth (Section 13.1.1). A meristem does not turn out one standard product: its derivatives mature into quite different cells, and the place where a cell ends up has a big say in its fate. At a root apex, cells displaced towards the tip, beyond the meristem, become root-cap cells, whereas cells displaced sideways to the surface become epidermis. The vascular cambium follows the same rule sideways: its derivatives on the inner side become xylem and those on the outer side become phloem.

13.3 Development

Development covers every change in an organism's life, from the moment its seed germinates until it grows old and senesces. For a single cell the sequence runs: cell division, growth of protoplasm (plasmatic growth), expansion or elongation, differentiation, maturation, and finally senescence and death (Fig. 13.9). Tissues and organs pass through the same stages.

A plant's body plan is not fixed in advance: the same plant can build differently shaped organs depending on its age or its surroundings, a flexibility called plasticity (सुघट्यता). The classic case is heterophylly, leaves of more than one shape on one plant (Fig. 13.10). Coriander's first leaves near the ground are broad and lobed, but those made later, higher up, are cut into thin threads; cotton and larkspur also switch between juvenile and adult leaf shapes, so here the trigger is the life phase. In buttercup the trigger is the surroundings: a shoot growing up through a pond forms finely divided leaves under water and broad ones once it reaches the air.

Put the three ideas together and you get a compact formula: development = growth + differentiation. What steers it? Some controls come from within the plant and some from outside:

13.4 Plant Growth Regulators

13.4.1 Characteristics

Chemically, plant growth regulators (पादप वृद्धि नियामक), or PGRs, are a mixed bag, but all are small, simple molecules, and between them they steer growth and development. You will also meet them as plant hormones, phytohormones or plant growth substances. The five major classes and their chemical nature are listed in Table 13.3.

By function, PGRs fall into two broad groups. Growth promoters (the auxins, gibberellins and cytokinins) drive activities such as dividing and enlarging cells, laying down patterns, tropic bending, and forming flowers, fruits and seeds. The second group, the growth inhibitors, helps the plant cope with wounding and with stress, whether from pests and pathogens or from drought and cold, and it brakes growth through dormancy and abscission; abscisic acid is its chief member. Ethylene refuses to sit neatly in either camp, though most of its actions hold growth back.

13.4.2 The Discovery of Plant Growth Regulators

None of the five classes was found by design: each turned up while scientists were chasing another question. The story begins with Charles Darwin and his son Francis. They saw that the coleoptiles (the sheaths around the first leaf) of canary grass seedlings bend towards light coming from one side, a response called phototropism. By covering or removing different parts, they concluded that the tip senses the light and sends a signal down to the region that bends (Fig. 13.11). Later, F. W. Went collected that influence from the tips of oat coleoptiles in small blocks of agar, showing it was a chemical: auxin.

Rice farmers in Japan knew a disease called bakanae, 'foolish seedling', in which infected seedlings shoot up tall and thin. It is caused by the fungus Gibberella fujikuroi. In 1926 E. Kurosawa showed that a sterile filtrate of the fungus, with no living fungus in it, produced the same symptoms in healthy seedlings. Chemists later showed that the active compounds were gibberellic acid.

F. Skoog's team was trying to keep pieces of tobacco stem growing in culture. Auxin alone was not enough: the pieces formed a callus, a mass of undifferentiated cells, that kept proliferating only if something extra went into the medium, whether coconut milk, yeast extract, an extract of vascular tissue, or DNA. Hunting for the active ingredient, C. O. Miller and co-workers crystallised it in 1955; because it made cells undergo cytokinesis, they named it kinetin.

Abscisic acid has three birth certificates. By the mid-1960s, separate groups had purified growth inhibitors from different sources and given them different names: dormin (linked with bud dormancy), abscisin II (linked with the shedding of young cotton fruits) and inhibitor-B. Chemical analysis showed that all three were one molecule, renamed abscisic acid (ABA). A clue to the fifth class had come earlier, from stored fruit: in 1910 H. H. Cousins reported that gases given off by ripe oranges made unripe bananas kept with them ripen early. Years later chemists showed that the gas responsible was ethylene.

13.4.3 Physiological Effects of Plant Growth Regulators

13.4.3.1 Auxins

Auxin takes its name from the Greek auxein, 'to grow'. Oddly, the first auxin anyone isolated came not from a plant but from human urine. Today the word covers a whole family: indole-3-acetic acid (IAA) and indole butyric acid (IBA), which plants make, and laboratory-made compounds such as naphthalene acetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) that act in the same way. Inside a plant, auxin is made mostly at the tips of growing shoots and roots and travels from there to the cells it acts on. Farmers and gardeners put the whole family to work:

• Weed control. 2,4-D kills dicot weeds but does not affect mature monocots, so gardeners use it to keep lawns weed-free.
• Rooting cuttings. Treating the cut end of a stem cutting with auxin makes roots start, a standard way to propagate plants.
• Seedless fruit. Auxins induce parthenocarpy, fruit formation without fertilisation, for example in tomato.
• Flowering. They promote flowering, for example in pineapple.
• Holding on, then letting go. They stop young fruits and leaves from dropping too soon, yet make old, mature leaves and fruits shed (abscise).
• Inside the plant. Auxin also governs how xylem differentiates and assists cell division.

Look at a sunflower stem and you will see buds sitting idle in the angle of each leaf. They are held back by the growing bud at the top, an effect called apical dominance. Snip off that apical bud (decapitation) and the buds below the cut usually start growing into side branches (Fig. 13.12). Tea pluckers exploit this: by removing the shoot tips again and again, they keep the bushes waist-high and crowded with tender new shoots. Clipping a hedge works the same way, making it thick and dense.

13.4.3.2 Gibberellins

Gibberellins are the largest family of promoters: over 100 are known, from fungi as well as flowering plants. Instead of names they carry numbers ($\mathrm{G}{\mathrm{A}}_1$, $\mathrm{G}{\mathrm{A}}_2$, $\mathrm{G}{\mathrm{A}}_3$ and so on), and every one of them is an acid. The best known and most intensively studied is gibberellic acid, $\mathrm{G}{\mathrm{A}}_3$, one of the first to be found. What gibberellins do, and how growers use them:

• Bolting. They promote bolting, rapid internode elongation just before flowering, in beet, cabbage and other plants with a rosette habit.
• Sugarcane. The crop stores sugar in its stem; spraying it with gibberellins lengthens the stem and can add up to 20 tonnes of cane per acre to the harvest.
• Longer axes. They lengthen the axis, so growers spray grapes to get longer bunch stalks.
• Fruit shape. They make fruits such as apple elongate and take a better shape.
• Delayed senescence. Fruit can stay on the tree longer, stretching the market period.
• Early seed in conifers. Spraying juvenile conifers hastens maturity, so they produce seed sooner.
• Brewing. $\mathrm{G}{\mathrm{A}}_3$ speeds up malting in the brewing industry.

13.4.3.3 Cytokinins

As the name suggests, cytokinins act on cytokinesis, the division of the cytoplasm. Plants make them where cells divide fast: in root apices, developing shoot buds and young fruits. The first cytokinin to be found, though, was not a plant product at all. Kinetin, a modified adenine (a purine), was obtained from herring sperm DNA that had been heated in an autoclave, and it does not occur naturally in plants. The hunt for a natural compound with the same activity ended with zeatin, from maize (corn) kernels and coconut milk, and many more natural and synthetic cytokinins have been identified since. Their effects:

• They promote nutrient mobilisation into the leaf, which delays leaf senescence.
• They help overcome apical dominance, working against auxin from the tip.
• They promote lateral shoot growth and the formation of adventitious shoots.
• They help produce new leaves, and chloroplasts in leaves.

13.4.3.4 Ethylene

Ethylene ($\mathrm{C}{\phantom{X}}_2\mathrm{H}{\phantom{X}}_4$), the one gas among the five classes, is best known as the ripening hormone. Fruits that are ripening and tissues that are ageing pour it out in large amounts, and the gas in turn speeds ripening: it drives up the fruit's rate of respiration in a surge called the respiratory climacteric (Fig. 13.13). It hastens the ageing and shedding of plant organs, leaves and flowers above all. On dicot seedlings it has a striking threefold effect: the seedling grows sideways, its axis swells, and its tip curls into an apical hook.

Ethylene has several other effects:

• it makes the internodes and petioles of deep-water rice elongate rapidly, keeping the leaves and upper shoot above the rising water;
• it promotes root growth and root-hair formation, enlarging the absorbing surface;
• it breaks seed and bud dormancy, starting germination in peanut seeds and the sprouting of potato tubers;
• it is used to start flowering and synchronise fruit set in pineapple, and it induces flowering in mango.

Because it regulates so many processes, ethylene is among the PGRs farmers use most. They rarely handle the gas itself. Instead they spray ethephon, a compound that dissolves in water, is quickly taken up and moved through the plant, and then breaks down to give off ethylene gradually. It speeds the ripening of tomatoes and apples, makes flowers and fruits drop to thin out crops of cotton, cherry and walnut, and raises cucumber yields by increasing the share of female flowers.

13.4.3.5 Abscisic acid

When a leaf starts to run short of water, abscisic acid is its alarm signal. ABA makes the guard cells close the stomata (Fig. 13.14) and raises the plant's tolerance of drought, cold and other stresses, so it is nicknamed the stress hormone. In seeds it matters from start to finish: it guides seed development and maturation, then imposes dormancy, which lets the seed survive drying out and wait through bad seasons, and it keeps the seed from germinating. More broadly it slows growth and metabolism. It usually opposes the gibberellins, so it is described as a GA antagonist. Its name recalls the roles, in abscission and dormancy, for which it was first discovered.

No stage of a plant's life is left to a single regulator. Sometimes two PGRs push the same way (complementary or synergistic action), sometimes they pull against each other (antagonism), and sometimes one acts on its own. Dormancy, abscission, senescence and apical dominance are each settled by a balance among several PGRs (Table 13.4).

Keep PGRs in perspective, though. The genes in each cell set what a plant can do, and the environment decides much of what it actually does; PGRs are often the messengers in between. Light and temperature, for instance, act through PGRs to control seed germination, dormancy, plant movements, flowering and vernalisation.