Plant Hormones

This content is only required for triple sciences! It’s all about plant hormones, the chemicals that help shoots grow upwards and roots anchor into the soil.

Control and coordination in plants

Plants also use hormones to respond to their environment, allowing plants to grow in response to light (phototropism) and gravity (geotropism).

• Shoots grow towards light (positive phototropism) - this allows them to maximise the amount of light they absorb for photosynthesis

• Shoots grow against the force of gravity (negative geotropism) - this ensures they grow upwards, towards the light

• Roots grow away from light (negative phototropism) - ensuring that they grow deeper into the soil, anchoring the plant into the ground

• Roots grow in the direction of gravity (positive geotropism) - this also helps to anchor the plant into the ground.

These responses are controlled by the hormone auxin which is produced by actively dividing cells in the plant shoot.

• Auxin accumulates in the shaded side of the plant, away from light, and causes cells to grow longer (cell elongation).

• The longer cells on the shaded side make the shoot heavier on this side, causing it to bend towards the light.

 Other hormones are involved in plant growth, including:

• Gibberellins – important for initiating seed germination

• Ethene – controls cell division and fruit ripening

Uses of plant hormones

Humans use plant hormones to control the growth of plants in gardening and agriculture. For example, ethene is used in the food industry to control the ripening of fruit during storage and transport. It allows seasonal fruit and veg to be available all year round. Auxins are used as weed killers, rooting powders and for stimulating the growth of plant cells in tissue culture. Gibberellins can be used to promote flowering, end seed dormancy and increase fruit size.