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Biological Orientation Responses in Plants. Tropisms. A growth response towards or away from an environmental stimulus coming from one direction A shoot growing towards the light is positively phototropic A root growing towards gravity is positively geotropic.
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Tropisms • A growth response towards or away from an environmental stimulus coming from one direction • A shoot growing towards the light is positively phototropic • A root growing towards gravity is positively geotropic. • Roots in soil grow away from copper pipes, we say these are negatively chemotropic.
Nastic Responses • The response of plants to diffuse stimuli that do not come from any particular direction, e.g. temperature, humidity and light that surround a plant. • E.g. the opening and closing of flowers to different light intensities. • Photonasty – a response to alterations in the light intensity. • Thermonasty – a response to changes in the temperature.
The Control of Plant Growth • Plants have a “guidance system” or mechanism designed to make sure they grow in a suitable manner. • This presupposes 3 things: • A sensing system • A response system • A mechanism connecting the 2.
The Light-Sensing System • Plants have at least 3 distinct photoreceptors. • Phytochrome– (blue green) that controls the development of plants and flowering. • Chlorophyll – (green) and other yellow substances called Carotenoids, that run photosynthesis. • Riboflavin (yellow) and Beta Carotene (orange)
The Light-Sensing System • Coleoptiles contain riboflavin and beta carotene. • When pieces of membrane from coleoptile cells were examined they had the same absorption spectrum, and the only chemical found in the membrane was flavin. • So it seems the light detector in phototropism is a membrane-bound flavin pigment system.
