How Plants Control Growth Through Tropisms

Plants control and coordinate growth using plant hormones, especially auxin. Auxin can become unevenly distributed in a root or shoot, causing cells on different sides to grow at different rates. This makes the plant bend towards or away from a stimulus.

What is a tropism?

A tropism is a plant’s directional growth response to a stimulus:

• A positive tropism is growth towards the stimulus.
• A negative tropism is growth away from the stimulus.
• Phototropism is a response to light.
• Gravitropism (or geotropism) is a response to gravity.

Phototropism in shoots

Plant shoots are positively phototropic, so they grow towards light. When light comes from one direction, auxin accumulates on the shaded side of the shoot. Auxin causes the cells on this side to elongate more than those on the illuminated side. The unequal growth makes the shoot bend towards the light.

This helps the leaves receive more light for photosynthesis.

Gravitropism in roots and shoots

Gravity also causes auxin to become unevenly distributed.

• Shoots are negatively gravitropic, so they grow away from gravity and upwards.
• Roots are positively gravitropic, so they grow towards gravity and downwards.

In roots, a higher auxin concentration reduces cell elongation. Cells on the upper side therefore elongate more than cells on the lower side, making the root bend downwards.

Worked example

A seedling is placed horizontally in darkness. Gravity causes auxin to collect on the lower sides. The shoot bends upwards because auxin promotes shoot-cell elongation, while the root bends downwards because auxin reduces root-cell elongation.

Common exam mistake: Do not say that auxin always increases growth. It promotes cell elongation in shoots but reduces elongation in roots at higher concentrations.

This answer assumes AQA GCSE Biology separate science. Plant control and coordination is biology-only content, although the main tropism ideas apply at both Foundation and Higher tier.