Phototropism
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Phototropism or heliotropism are botanical terms for an organism response to light (in the case of heliotropism, specifically the light from the Sun). These are one of the many plant tropisms or movements in response to extermal stimuli. Growth toward a light source is a positive phototropism, while the reverse is called negative phototropism or skototropism. Leaves and other parts of the plant that require light for photosynthesis exhibit positive phototropism, while roots usually exhibit negative phototropism, although gravitropism may play a larger role in their behavior and growth.
Phototropism in plants such as Arabidopsis thaliana is regulated by blue light receptors called phototropins. Other photosensitive receptors in plants include phytochromes that sense red light and cryptochromes that sense blue light. Different organs of the plant may exhibit different phototropic reactions to different wavelengths of light. Stem tips exhibit positive phototropic reactions to blue light, while root tips exhibit negative phototropic reactions to blue light. Both root tips and stem tips exhibit positive phototropism to red light.
Not all phototropic effects are easily observed. Within a plant cell, chloroplasts change their positions depending upon light intensity. When the light intensity is high, chloroplasts move to the edge of the cell to reduce photobleaching (destruction of chlorophyll). In low light, chloroplasts tend to spread out within the protoplasm to maximize their capture of light energy.
See Also
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