A cryptochrome is a member of a light-sensitive protein family involved in biological timing and development. In plants, blue-light perception by cryptochromes contributes to responses including growth regulation and entrainment of the circadian clock.
A light input that resets the circadian clock can change the timing of a later response even after illumination ends. This distinguishes a clock-mediated effect from a response that simply follows the current light level; photoperiodism can depend on both.
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