Photoperiodism is dependence of a biological response on the daily timing of light and darkness. In plants, flowering can depend on whether illumination coincides with an internally timed sensitive phase, as described by the external coincidence model.
Equal total hours of light need not produce equal flowering responses if the light occurs at different phases of the circadian clock. Light-dependent regulation of CONSTANS provides a molecular example connecting the timing of illumination to FLOWERING LOCUS T activity.[ref]
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