Phytochrome Control of Nitrate Reductase Activity and Anthocyanin Synthesis in Light‐grown Sinapis Alba (L.). Differential Responses of Cotyledons and Hypocotyls

S ummary In 40 h old cotyledons of etiolated Sinapis alba (L.) red/far‐red photoreversibility and the effect of continuous far‐red light were used to demonstrate the involvement of phytochrome in the control of in vivo nitrate reductase activity and anthocyanin synthesis. Over a period of several days during the development of S. alba seedlings, in the light, in vivo nitrate reductase activity and anthocyanin synthesis were determined in the cotyledons and hypocotyls. Seedlings were grown under two light environments; fluorescent white light, establishing an estimated phytochrome photo‐equilibrium, φE, of 0.69, simulating direct sunlight and fluorescent white light with added far‐red light, establishing an estimated phytochrome photo‐equilibrium of 0.26, simulating ‘shade’ or ‘canopy’ light. The time courses of both responses were greatly influenced by these different light qualities. Furthermore, in the case of nitrate reductase activity the cotyledons and hypocotyls showed marked differences in their responses to light quality. In the cotyledons a light source establishing a low photo‐equilibrium was initially more effective in eliciting the response, but with time, a light source establishing a high photo‐equilibrium became the more effective. In the hypocotyls a light source establishing a high photo‐equilibrium was always the more effective. The anthocyanin response showed the crossing over of effectiveness of high and low photo‐equilibrium in both tissues. The results are discussed in terms of the transition from the ‘high irradiance reaction’ of young seedlings to a possible steady state of phytochrome control in the mature plant.