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The perception of light direction and orientation responses in chloroplasts.

We may summarise what is known, in stationary cells, about perception of light direction. Cells have evolved three different mechanisms to transform light direction into an absorption gradient: light attenuation, light refraction, and dichroic orientation of the photoreceptors. Furthermore, these mechanisms can be combined and can cooperate in the same cell. With the photochromic pigment phytochrome, a regular change in dichroism upon photoconversion is an important improvement for effectively establishing an absorption gradient. However, perception of the direction of light, while a necessary condition, is not solely sufficient for a directional response. Equally important is a strongly localised transduction chain effecting localised control of the intracellular motor apparatus. Slowly-diffusing ions seem to fulfil this requirement, but further work needs to be done to elucidate the mechanisms completely.