HORMONAL COORDINATION
PLANT HORMONES
Plants require hormones to coordinate control and growth, as well as to respond to specific stimuli. One example of this is tropisms, such as phototropism and gravitropism, which are the responses of a plant's shoots or roots to light and gravity, respectively.
AUXINS
Auxins are plant hormones that are produced in both shoots and roots:
- In shoots, auxin is concentrated on the shaded side and stimulates cell growth.
- In roots, auxin is concentrated on the lower side and inhibits cell growth.
PHOTOTROPISM
Light causes auxin to accumulate on the shaded side of the shoot tip, which then spreads down the shoot. As a result, cells on the shaded side grow faster than cells on the lighter side, causing the shoot to grow towards the light and form a curved shape. Shoots are positively phototropic as they grow towards the light, while roots are negatively phototropic as they grow away from the light.
GEOTROPISM/GRAVITROPISM
Auxin is produced in the root, but gravity causes it to concentrate on the lower side. In roots, auxin inhibits cell growth, causing the lower side to grow more slowly than the upper side. This results in the roots growing towards the force of gravity. Roots are positively geotropic as they grow towards the ground, while shoots are negatively geotropic as they grow away from the ground.
EXAMPLE QUESTION
GIBBERELLINS & ETHENE:
- Gibberellins are hormones that are important for starting the germination of seeds.
- Ethene is a chemical that controls cell division and the ripening of fruits.
If asked to state which direction the plant shoot in the image above will grow, and to explain why, the response would be that the shoot will grow towards the left. This is because the light is on the left, causing auxins to accumulate on the right side (in the shade). As auxins stimulate growth in shoots, the right side of the shoot will grow faster, causing the shoot to lean over to the left.
