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phytochromea type of plant pigment that occurs in two forms, P660 (Pr) which absorbs red light (600–700 nm) and P725 (Pfr) which absorbs far-red light (700–760 nm). P72 5 is biologically active in that it stimulates enzymic reactions, whereas P660 is biologically inert. The conversion of one form into the other occurs simultaneously. See Fig. 252 .
Daylight contains a mixture of red and far-red light, with a predominance of red light. During the day the proportion of P725 builds up relative to P660, whereas during the night there is a gradual conversion of P725 to P660. Thus phytochrome provides the plant with a method of detecting day and night, with P72 5 predominant when it is day, and P660 predominant when it is night. This mechanism is the basis of PHOTOPERIODISM.