Plant hormones

In a nutshell

Plant hormones control plant growth. Auxins are plant hormones that control the growth at the tips of the shoots and roots. Another important plant hormone is gibberellin, this stimulates seed germination, stem growth and flowering. The ability of plant hormones to control growth can be used commercially, such as selective weedkillers using auxins.

Auxin

Auxin is a plant hormone that controls the growth of the tips of shoots and roots in response to light and gravity. Growth in response to light is called phototropism and growth in response to gravity is called gravitropism (or geotropism).

Auxin is produced in the tips of the plant and stimulates cell elongation below the tip. As auxin is produced in the tip, if the tip of the plant is removed, the shoot may stop growing. Too much auxin promotes shoot growth but inhibits root growth.

Growth in response to light

When the tip of the shoot is in the light, the auxin will accumulate on the shaded side. This means that the cells grow faster on the shaded side making the shoot bend towards the light.

Biology; Plant structures and their functions; KS4 Year 10; Plant hormones

Growth in response to gravity

Gravity causes an uneven distribution of auxins when the shoot and roots are horizontal.

Biology; Plant structures and their functions; KS4 Year 10; Plant hormones

A	Gravity (1.) causes an uneven distribution of auxins when the shoot is horizontal. This means that the lower side of the shoot will have more growth stimulation so it will grow faster and bend the shoot upwards.
B	The opposite happens when the auxin is in the root. This is because auxin inhibits root growth. The auxin gathers on the lower side of the root so the growth of these cells will be inhibited. The cells at the top of the root will grow faster, causing the root to bend downwards towards gravity (1.).

Uses of auxins- Higher tier only

The ability of auxins to control plant growth can be exploited commercially.

Use	Description
Killing weeds	Generally, weeds have broad leaves in comparison to useful plants like grasses and cereals which have narrow leaves. Auxins can be used to make selective weedkillers that only kill plants with broad leaves. Their growth will be disrupted but the useful narrow-leaved plants will be unaffected.
Rooting powder	Auxins can be used to make rooting powder. Rooting powder can make plant cuttings (parts of a plant that have been cut) grow in soil when they usually wouldn't. They will quickly produce roots and grow into clones of their parent plant.
Tissue culture	Auxins are added to growth medium to encourage the growth of cells. These will also produce clones of the parent plant.

Gibberellin- Higher tier only

Gibberellin is another plant hormone. Instead of controlling growth in roots and shoots, it stimulates seed germination, stem growth and flowering. Like auxin, it can be exploited commercially.

Use	Description
Controlling dormancy	Seeds can be dormant and not germinate until conditions are just right. Gibberellins can control the dormancy period. Farmers can use gibberellins to make seeds germinate at any time of year.
Flowering	Some plants need to experience a specific set of conditions in order to flower. They may need to experience longer days to know it is summer and a good time to flower. Gibberellins can be used to grow bigger flowers and induce flowering even if these specific conditions are not met.
Larger fruits	Gibberellin can be used to create larger fruits. Example Seedless grapes are usually smaller than seeded grapes but farmers can use gibberellins to encourage larger fruits.

Ethene- Higher tier only

Older plants can produce ethene gas which controls cell division and stimulates an enzyme that causes fruit to ripen. This can be exploited by humans to speed up fruit ripening (both on the plant and when they are on their way to the shops).

The use of ethene to ripen plants means that fruits can be picked whilst they are unripe. This is beneficial to farmers as the fruits are less likely to be damaged given they are usually harder when unripe.

The ability of ethene to ripen fruits can also be inhibited so ripening will be delayed.