Auxin and tropisms
- Plants produce hormones to coordinate and control growth, and their responses to light (phototropism) and gravity (gravitropism, or geotropism).
- Auxin is made at the tips of shoots and roots. An unequal distribution of auxin causes unequal growth.
- In shoots, light makes auxin collect on the shaded side. The cells there grow longer, so the shoot bends towards the light.
- In roots, auxin collects on the lower side and inhibits growth there, so the root bends downwards, towards gravity.
Other plant hormones
- Gibberellins are important in starting seed germination.
- Ethene controls cell division and the ripening of fruits.
Using plant hormones
- Auxins are used as weedkillers (selective ones kill broad-leaved weeds but not grasses), in rooting powders to help cuttings grow roots, and to promote growth in tissue culture.
- Ethene is used in the food industry to control the ripening of fruit during storage and transport.
- Gibberellins are used to end seed dormancy, promote flowering and increase fruit size.
Required practical: light and seedlings
- Plant cress seeds in several dishes and let them germinate.
- Put one dish in darkness, one in light from all around and one with light from one side only. Keep the temperature and water the same.
- After a few days, measure the height and observe the direction of growth. Seedlings lit from one side bend towards the light.
Key terms
- Auxin
- A plant hormone that controls growth in response to light and gravity.
- Phototropism
- Growth in response to light.
- Gravitropism
- Growth in response to gravity (also called geotropism).
- Gibberellins
- Plant hormones that start seed germination.
- Ethene
- A plant hormone that controls fruit ripening.
- Rooting powder
- A powder containing auxin that helps cuttings grow roots.
- Selective weedkiller
- A weedkiller containing auxins that kills broad-leaved weeds but not grasses.
- Dormancy
- A resting state in which a seed does not germinate.