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photoperiodism/pho·to·pe·ri·od·ism/ (fo″to-pēr´e-ah-dizm) the physiologic and behavioral reactions brought about in organisms by changes in the duration of daylight and darkness.
photoperiodismthe response observed in an organism to the relative length of light and dark periods. While all organisms can be affected by PHOTOPERIODS (See also BIOLOGICAL CLOCK the term is most commonly used in connection with higher plants, particularly their flowering. The regulation of flowering is rather complex and difficult to generalize about, but it is possible to divide plants into three categories: LONG-DAY PLANTS, SHORT-DAY PLANTS and DAY-NEUTRAL PLANTS (the latter being unaffected by photoperiod).
Plants appear to detect the photoperiod with the pigment PHYTOCHROME which can exist in two forms in the leaves, P660 in red light and P725 in far-red light. If there is more P660 than P725 in a short-day plant, or P725 than P660 in a long-day plant, then flowering is induced as a vegetative apical MERISTEM is changed to a flowering one.