Immediate Action of Phytochrome in Light-stimulated Lettuce Seeds

Abstract

SEEDS of Lactuca sativa (var. ‘Grand Rapids’), which we have used in these studies, require a short exposure to red light in order to germinate at 25° C. The promotive effect of red light can be completely nullified if the latter is followed closely by an exposure to far-red light. This involves the reversion of the active form of phytochrome (P_FR), produced by red light, to its inactive form (P_R). If a dark interval of more than a certain critical duration is intercalated between the red and far-red irradiations, some action of P_FR is permitted which is expressed subsequently as the germination of a number of seeds. After a certain period in darkness (the escape time) after red light, short far-red fails to have any inhibitory effect¹ and all the seeds germinate. The escape time therefore gives a physiological measure of the course of P_FR action in the seed population, as shown in Fig. 1 for our sample.