Photoperiodism

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]

CONSTANS is an Arabidopsis thaliana transcriptional regulator involved in photoperiodism. Its daily expression pattern and light-dependent protein regulation help activate FLOWERING LOCUS T when internal timing coincides with suitable illumination.

The useful comparison is not merely long days versus short days: changing the phase of light relative to the regulator's availability tests the coincidence mechanism. This explains how a plant can use a daily light pattern as seasonal information.^[ref]

A model in which a light-sensitive internal phase produces a response when it coincides with the external light interval. Equal total light need not produce equal responses if timing differs.