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GIBBERELLIC acid is known to mimic the morphogenetic effects of red light1 on certain plants. Brian2 has suggested that gibberellins are synthesized in plants after the absorption of red light and Kohler3 has shown that there is a significant increase in the content of gibberellic acid in etiolated peas after 24 h of irradiation with red light. This and other work4 suggests that phytochrome in the Pfr form (after irradiation with red light) initiates synthesis, or at least brings about increases in the content of gibberellin in plants and seeds. Many responses of phytochrome, however, can be observed a few hours (or in some cases in less than 1 h (ref. 5)) after illumination with red light. Thus to explain effects of phytochrome in terms of the action of gibberellin, it is necessary to show that gibberellins are synthesized during or very soon after the end of a period of red irradiation. This is what we have tried to do.

IT has been shown that the synthesis of mRNA and protein is a prerequisite for phytochrome -mediated synthesis of anthocyanin1,2, and that irradiation of plant tissue with red light stimulates the production of several enzymes3,4. We recently demonstrated that red light irradiation of segments from etiolated barley leaves would quickly induce the synthesis of gibberellin-like substances5. Subsequently we have investigated the possibility that this red light-induced gibberellin synthesis requires the formation of enzymes and RNA before the production of the gibberellins themselves.