Photocontrol of anthocyanin formation in turnip seedlings : V. Differential response patterns of hypocotyls and cotyledons.

Kinetic studies of anthocyanin formation in far-red and red light given in various combinations have shown that the responses to the two wave-lengths are different and depend on the sequence of irradiation. Red light reduced the effectiveness of subsequent prolonged far-red regardless of whether red began the irradiation or was interposed after several hours of far-red. However, far-red eventually regained some activity to a degree which depended on seedling age and the duration of the initial exposure to red light. It is suggested that red light may act by destroying the labile form of phytochrome (presumed to be required for far-red action) and that the resumption of far-red activity may indicate new synthesis of such phytochrome.When given after several hours of far-red, the yield in prolonged red light was markedly increased and, in intact hypocotyls, anthocyanin synthesis then proceeded as rapidly in red as in far-red. Far-red also increased the yield in subsequent red light when given after an initial irradiation with read and, under these conditions, anthocyanin yield in hypocotyls was even greater if seedlings were transferred to red light than if the far-red was continued. Red light appeared to favour formation of anthocyanin in the hypocotyl and it is possible that the action of red is to block synthesis in the cotyledons thus leading to an increased translocation of substrate to the hypocotyl. A number of treatments increased the yield of cotyledons in red light but, unlike the hypocotyls, they never reached that obtained with far-red. Several observations indicated that the increased effectiveness of red, when given after far-red, might be due to the accumulation of substrate in far-red, which is then converted to anthocyanin by the action of the stable form of phytochrome.