OCR Unit 2 Module 4: Responding to the Environment - Plant Responses

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Unit 2 Module 4: Responding to the Environment – Plant responses

Plants need to respond adaptively to changes in their internal and external environment to avoid predators and abiotic (non-living) stress. This is done by:

· The seed needs to germinate in suitable conditions

· The plant must grow adaptively with respect to light intensity and variations in water content of the soil

· Must respond to seasons and flower at the right time

Tropism – A directional growth response in which the direction of the response is determined by the direction of the external stimulus. A response towards the stimulus is said to be positive and a response away from the stimulus is said to be negative. (E.g. a growth towards light it a positive phototropism)

If a plant is to respond to an internal or external stimulus it must have a receptor that is sensitive to that particular change. For example a positive phototropism must have a photoreceptor molecule that is affected by light.

An example of this is the phototropic response of cereal coleoptiles: its receptor region is in the tip of the coleoptile, it responds to wavelengths in the blue region of the spectrum. It is likely that riboflavin is the photoreceptor for phototropism however both riboflavin and carotene are molecules in the coleoptile tip that have absorption spectra in that region.

The plant's effector is in the region of cell elongation, just behind a shoot, coleoptiles or root tip. The cells on the shaded side elongate more than those on the illuminated side which makes the coleoptiles bend towards the source of light. Auxin produced by the dividing cells in the tip is transported to

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