Phytochrome and photosystem I interaction in a high-energy photoresponse.

AT LEAST TWO PHOTOREACTIONS CAN BE DEMONSTRATED IN PLANT DEVELOPMENTAL RESPONSES: the low-energy requiring phytochrome system and the high energy reaction. The action of these photoreactions on the formation of anthocyanin by turnip seedlings is discussed. The synthesis of small amounts of anthocyanin can be controlled solely by phytochrome, as evidenced by the red-far-red photoreversible effect of brief irradiations. Appreciable synthesis requires prolonged irradiations, the duration of irradiation being more important than intensity. The data presented suggest that the energy dependence of anthocyanin synthesis arises through photosynthesis. A mechanism for the interaction between photosynthesis and phytochrome is suggested. Under conditions of natural illumination of plants, the concentration of the species of phytochrome that absorbs far-red light may be lower than previously realized.