Phototropia
In Biology, Phototropia is movement (tropism) in organisms in response to light. Growth toward a light source (e.g., the Sun) is positive phototropia, while the reverse is negative phototropia. Leaves and other parts of the plant that require light for photosynthesis exhibit positive phototropism, while roots, shoots, and other parts needing shelter use negative phototropism to some small extent, but gravitropism usually plays a larger role in their development.Phototropism in plants such as Arabidopsis thaliana is regulated by several hormones called phototropins, photosensitive receptors such as phytochrome A, phytochrome B, and phytochrome E.
Different organs (parts of the plant) may exhibit different phototropic reactions to different wavelengths (colors) of light. Stem tips exhibit positive phototropic growth to blue light, while root tips exhibit negative phototropic growth to blue light. Both root tips and stem tips exhibit positive phototropism to red light. Not all phototropic effects are visible. Chloroplasts change their positions within cells depending on red-light intensity.