A level Biology B · Edexcel 9BI0 · Topic 9E
Plant responses
Key points
- Plants respond to stimuli using tropisms (directional growth responses): phototropism (light) and gravitropism (gravity).
- Auxin (IAA) is made in shoot tips and causes cell elongation in shoots. In phototropism, auxin moves to the shaded side, so that side grows faster and the shoot bends towards light.
- In roots, high auxin inhibits elongation, so roots grow away from light and towards gravity.
- Gibberellins cause stem elongation and seed germination (stimulating amylase to break down starch); abscisic acid closes stomata and maintains dormancy.
- Phytochromes detect red (660 nm) and far-red (730 nm) light; the ratio tells the plant day length and controls flowering and germination.
- Plant growth regulators are used commercially, e.g. auxins as weedkillers and rooting powder.
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Plants cannot move away from danger or towards food, but they respond to light, gravity and the seasons by changing how they grow. These responses are coordinated by growth substances (plant hormones) and by the light-sensitive pigment…
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