photoperiodism

photoperiodism

 [fo″to-pēr´e-o-dizm]

the physiologic and behavioral reactions brought about in organisms by changes in the duration of daylight and darkness.

pho·to·per·i·od·ism

(fō-tō-pēr'ē-ō-dizm),

The periodic (seasonal or diurnal) activities, behavior, or changes in plants or animals brought about by the action of light.

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.

photoperiodism

(fō′tō-pîr′ē-ə-dĭz′əm)

n.

An organism's response or ability to respond to changes in photoperiod.

photoperiodism

the 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.

Phytochromes are also present in the seed coat, the correct balance of P₆₆₀ and P₇₂₅ stimulating enzyme activation and hence GERMINATION. See also FLORIGEN.

photoperiodism

the physiological and behavioral reactions brought about in organisms by changes in the duration of daylight and darkness, e.g. in reproductive activity, shedding of hair. Birds respond to longer daylight hours by increased sexual activity. Use is made of the phenomenon by using artificial light to stimulate egg production.