AQA Biology GCSE - Control in Plants

Plants produce hormones and respond to external stimuli, growing towards sources of water and light, which they need to survive.

Plant responses - called tropisms - help make sure that any growth is towards sources of light and water.

• positive tropism – the plant grows towards the stimulus.
• negative tropism – the plant grows away from the stimulus.

Light tropism (phototropism) - where the plant is sensitive to light.

Gravitational tropism (geotropism) - where the plant is sensitive to gravity.

Shoot

• Positive phototropism; negative geotropism.

Roots

• Negative phototropism; positive geotropism. 

Auxins are mostly made in the tips of the shoots and roots, and can diffuse to other parts of the shoots or roots. They change the rate of elongation in plant cells, controlling how long they become. Shoots and roots respond differently to high concentrations of auxins:

• cells in shoots grow more.
• cells in roots grow less.

Auxins are also involved in gravitropisms. In a root placed horizontally, the bottom side contains more auxin than the top side. This makes the bottom side grow less than the top side, causing the root to bend in the direction of the force of gravity.

In a shoot placed horizontally, the bottom side contains more auxin than the top side. This makes the bottom side grow more than the top side, causing the shoot to bend and grow against the force of gravity.