Chemical Coordination in Plants

Animals respond fast using a nervous system, but plants have no nerves and no muscles. Instead they coordinate using chemical messengers called plant hormones, or phytohormones. These are made in one part of the plant (often a growing tip), then diffuse slowly to other parts where they control growth. Because the message travels by diffusion and acts by changing how fast cells grow, the response is slow and is usually a permanent bend or growth, not a quick twitch. This is why a sunflower turning to face the sun takes hours, not seconds.

There are four hormones to know. Auxin causes cells to elongate and is behind tropic bending. Gibberellin promotes stem and shoot growth and breaks the dormancy of seeds and buds. Cytokinin promotes cell division, delays the ageing of leaves and helps stomata open. The fourth, abscisic acid, is the odd one: it inhibits growth, closes stomata during water stress and promotes dormancy and the falling (abscission) of leaves. The first three are growth promoters; abscisic acid is a growth inhibitor.

A tropism is directional growth controlled by an outside stimulus. Phototropism is growth in response to light: shoots grow towards light (positive), roots grow away (negative). Geotropism (or gravitropism) is growth in response to gravity: roots grow down (positive), shoots grow up (negative). Hydrotropism is growth towards water, shown strongly by roots. Thigmotropism is growth in response to touch, seen when a tendril coils around a support. In each case the part bends because cells on one side grow more than the other.

When light hits a shoot from one side, auxin made at the tip moves away from the bright side and collects on the shaded side. Auxin makes cells grow longer, so the cells on the shaded side elongate more than those on the lit side. One side growing faster than the other forces the whole shoot to curve towards the light. This single mechanism, uneven cell growth driven by uneven auxin, explains phototropism and is a favourite explanation question in the exam.