Plant Growth and Development

Growth is an irreversible, permanent increase in the size of a part or the whole organism, usually accompanied by an increase in dry weight. In plants, growth is special because it continues throughout life — this is called indeterminate growth — thanks to special regions of dividing cells called meristems at the root and shoot tips and in the thickening tissues.

Growth at the cell level happens in three phases: the phase of cell division (in the meristem), the phase of cell enlargement (cells grow larger and take up water in a vacuole), and the phase of cell maturation (cells take their final shape and function). If you plot size against time, growth typically follows an S-shaped (sigmoid) curve — slow at first, then rapid, then slowing as it levels off.

Cells then become specialised for particular jobs — this is differentiation (for example, a simple cell becoming a water-carrying xylem vessel). Two related terms are dedifferentiation (a mature cell regaining the ability to divide, e.g. forming cork cambium) and redifferentiation (those cells maturing again into a new tissue). The whole sequence of growth, differentiation and maturation that leads to the adult plant is called development. Plants also show plasticity — they can change their form in response to the environment (for example, growing different-shaped leaves in water and in air).

Plants control their growth and development using chemical messengers called plant growth regulators (PGRs) or plant hormones — substances made in one part of the plant and active in tiny amounts. There are five main groups, broadly divided into growth promoters and growth inhibitors.

• Auxins — promote cell elongation; control phototropism and geotropism (bending toward light and gravity), promote rooting in cuttings, and cause apical dominance (the main shoot tip suppresses side branches). Used to prevent fruit drop and as weedkillers.
• Gibberellins (GA) — promote stem elongation (can make dwarf plants tall), help seeds germinate by breaking dormancy, and promote bolting and flowering.
• Cytokinins — promote cell division, help form new shoots/buds, and delay ageing of leaves (delay senescence).
• Ethylene — a gaseous hormone; ripens fruits, promotes ageing and falling of leaves and fruits (abscission), and breaks dormancy.
• Abscisic acid (ABA) — the main growth inhibitor; called the "stress hormone" because it closes stomata during water shortage, promotes dormancy of seeds and buds, and counters the growth promoters.

In real plants, growth is decided not by one hormone alone but by the balance between promoters and inhibitors. Farmers and gardeners use PGRs to root cuttings, ripen fruit, increase yield, and control weeds.

Plants do not flower at just any time — many time their flowering to the length of day and night, or to a spell of cold. These responses help a plant flower in the right season.

Photoperiodism is the response of plants to the relative lengths of day and night (the photoperiod). Based on their flowering needs, plants are grouped as:

• Long-day plants — flower when the day is longer than a critical length (e.g. wheat, spinach).
• Short-day plants — flower when the day is shorter than a critical length (e.g. rice, chrysanthemum).
• Day-neutral plants — flowering does not depend on day length (e.g. tomato, cucumber).

The day length is sensed by the leaves (through a pigment), which then send a signal (a hormone, sometimes called florigen) to the shoot tips to make them flower.

Vernalisation is the promotion of flowering by a period of low temperature (cold treatment). Some plants, especially certain winter varieties of wheat and other crops, will not flower unless they first experience a cold spell. This makes sure they flower at the proper time after winter, and not too early. Both photoperiodism and vernalisation are examples of how plants "read" their environment to develop at the right time.