The essence of photoperiodism is the measurement of time, the durations of day and night. Early experiments showed that the night was especially important for many species. Interrupting the night with even a brief period of light (seconds to an hour or two, depending on species and light intensity) stops the short-day response or promotes the long-day response. If the total of light plus dark adds up to more or less than twenty-four hours, it is the dark period that seems to be important. More recent experiments, however, show that photoperiod-sensitive plants measure the durations of both day and night. Time measurement in photoperiodism is clearly related to circadian leaf movements and other manifestations of the biological clock.
How do the plants know when it is light or dark? The pigment phy-tochrome, so important in many plant responses, couples the light environment to the mysterious biological clock. Phytochrome exists in two forms, both of which absorb certain wavelengths (colors) of light. One form, called Pr, absorbs red light, which converts it to the other form, Pfr. Pfr absorbs longer wavelengths of light, called far red, which convert it back to Pr. During the day, red light predominates so most of the pigment is in the Pfr form, signaling to the clock that it is light; the clock measures how long it is light. As it begins to get dark, the Pfr begins to break down, and some of it is spontaneously converted to Pr. This drop in Pfr level signals the clock that it is getting dark, and the clock begins to measure the length of the dark period. When the lengths of both day and night are right for the particular species, the next steps in the response to photoperiod are initiated; for example, florigen may begin to be synthesized.
Much study has gone into understanding these phenomena, and recent work has emphasized the role of specific genes in the flowering process.
florigen a substance that promotes flowering circadian "about a day"; related to a day florigen a substance that promotes flowering
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