Plant responses and hormones

Plants cannot move about, but they grow in response to their surroundings. Shoots grow towards light and roots grow down into the soil. These responses are controlled by plant hormones such as auxin, which change how fast cells grow. People also use plant hormones in farming and gardening.

Key facts

• A tropism is a growth response to a stimulus. Phototropism is a response to light; gravitropism (geotropism) is a response to gravity. Growing towards the stimulus is positive, away from it is negative.
• Shoots are positively phototropic and negatively gravitropic. Roots are positively gravitropic, which helps them reach water and minerals.
• Auxin is made in the tips of shoots and roots and moves to the area just behind the tip, where cells elongate.
• Light from one side makes auxin collect on the shaded side of a shoot. In shoots auxin speeds up cell elongation, so the shaded side grows faster and the shoot bends towards the light.
• In a root lying on its side, auxin collects on the lower side. In roots auxin slows cell elongation, so the upper side grows faster and the root bends downwards.
• Uses of auxins: selective weedkillers (they kill broad-leaved weeds but not narrow-leaved crops such as wheat), rooting powder for cuttings, and plant tissue culture.
• Gibberellins end seed dormancy so seeds germinate, make plants flower earlier and make fruit bigger. Ethene controls cell division and makes fruit ripen, so fruit can be picked unripe and ripened when it is sold.

Worked examples