Phytochrome Action in Oryza Sativa L. I. Growth Responses of Etiolated Coleoptiles to Red, Far-Red and Blue Light

Under continuous irradiation, the growth of intact rice coleoptile was strongly inhibited by red light, and somewhat prevented by blue and far-red light. The inhibitory effect of red light on coleoptile elongation was caused by a low-energy brief irradiation, and a single exposure of 1.5 kiloergs × cm−2 incident energy of red light brought about the 50% inhibition. This photoinhibition of growth was observed only after the coleoptile had elongated to about 10 mm or longer. The red light-induced effect was reversed by an immediately following brief exposure to far-red light, and the photoresponses to red and far-red light were repeatedly reversible. The escape reaction of red lightinduced effect took place at a rate so that 50% of the initial reversibility was lost within 9 hr in darkness at 27°. The inhibition by blue light and reversal by far-red irradiation was also achieved repeatedly with successive treatments of the coleoptiles. The evidence for a low intensity red far-red reversible control of coleoptile growth, indicative of control by phytochrome, seems clearly established in etiolated intact seedlings. In contrast, the elongation of apically excised rice coleoptile segments was promoted by a brief exposure to red light in 0.02 M phosphate buffer, pH 7, and the effect was almost completely nullified by an immediately subsequent exposure to far-red light. It becomes evident that the growth of intact coleoptiles was inhibited by a exposure to red light, while that of excised segments in a buffer was rather promoted by red irradiation. The direction of red light induced responses, either promotive or inhibitory, depends upon the method of bioassay using intact coleoptiles or their excised segments.