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Plant Responses Notes

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Plant responses Hormones coordinate plant responses to environmental changes, some of which are commercially important. Candidates should be able to: •

Explain why plants need to respond to their environment in terms of the need to avoid predation and abiotic stress. While most animals are able to move around to avoiddanger or unpleasant environmental conditions, plants have to stay put and take whatever their environment throws at them. Abiotic stress factors, such as high temperatures or drought, simply have to be survived - they cannot be avoided. However, this does not mean that plants cannot respond to these threats or environmental changes. Plants have evolved a very wide range of responses to a large variety of stimuli, which help them to survive.

Define the term tropism. A response to a stimulus, brought about by growth, in which the direction of growth is related to the direction of the stimulus. POSITIVE tropism grows toward the direction of the stimulus, and NEGATIVE tropism grows away from the stimulus. Phototropism: Shoots grow towards light, to increase rate of photosynthesis. Geotropism: Shoots grow upwards and roots grow downwards towards the pull of gravity. Chemotropism: Pollen tubes grow towards chemicals given off by the ovary in a flower. Thigmotropism: Shoots grow towards other plants by responding to touch.

Explain how plant responses to environmental changes are coordinated by hormones, with reference to responding to changes in light direction. Sensor (receptor) -> Plant Hormone -> Effector Hormones coordinate plant growth. Hormones are produced in one part of a plant and are transported to target cells by active transport, diffusion and in the xylem/phloem.

Cytokinins - promote mitosis in the meristem.

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