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Plant hormones
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Plants respond to light and gravity using hormones. People use plant hormones in farming. Separate Biology only.
Tropisms
Phototropism: growing towards or away from light. Shoots grow towards light.
Gravitropism (geotropism): roots grow down, with gravity; shoots grow up.
Gravitropism (geotropism): roots grow down, with gravity; shoots grow up.
Auxins
Auxins control growth. In shoots, light makes auxin move to the shaded side, so that side grows faster and the shoot bends towards the light.
In roots, auxin slows growth, so roots bend downwards.
In roots, auxin slows growth, so roots bend downwards.
Commercial uses
Auxins: weedkillers, rooting powder for cuttings.
Gibberellins: germination, fruit and flower production.
Ethene: ripening fruit.
Gibberellins: germination, fruit and flower production.
Ethene: ripening fruit.
Explain why a shoot bends towards a window.
- Light from one side makes auxin move to the shaded side.
- Cells on the shaded side elongate more.
- So the shoot bends towards the light.
Answer: Uneven auxin makes the shaded side grow faster.
Shoots are positively phototropic; roots are positively gravitropic. Auxin moves to the shaded side of shoots, making them bend to the light. Gibberellins and ethene have farming uses.
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