Rhythmic oscillations of phytochrome and its pelletability in Cucurbita pepo L.

Abstract

MANY reactions of plants (such as flower initiation, growth and chlorophyll synthesis) show a rhythmically changing sensitivity to light¹. According to Bünning's hypothesis, the photoperiodic reactions are determined by the interaction between an endogenous rhythm and phytochrome: both components are essential for a photoperiodic response. The endogenous rhythm determines the changing sensitivity of the plant to P_fr, the physiologically active form of phytochrome. These characteristic changes in the sensitivity of the system towards light are probably not attributable to different P_fr molecules. On the basis of the theory that the binding of P_fr to a membrane is the primary reaction of phytochrome, it has been suggested² that such sensitivity changes could be achieved by varying the interaction of phytochrome molecules with their receptor sites. We have investigated the association of phytochrome with pelletable structures^3,4 and found that after a 12-h white light induction the total amount of phytochrome and its pelletability oscillate in the dark with frequency periods of 24,12 and 6 h.