Photoregulation of hypocotyl growth: geotropic evidence for the operation of two photosystems

Abstract. The rate of curvature of etiolated cress ( Lepi-dium sativum L. ) hypocotyls in response to gravity (negative geotropism) was retarded by red or blue light; far-red irradiation was without effect. The timing of the irradiation period in relation to the presentation for geostimulus markedly affected the response. When seedlings were irradiated during the 1–2 h period of geostimulus, blue light was more effective than red at retarding curvature; when seedlings were irradiated prior to geostimulus, only red light affected geocurvature. These results are interpreted as a further example of the kinetically distinct effects of red and blue light on hypocotyl development. Blue light elicited a rapid, immediate response effective only during the period of irradiation; red light induced a response characterized by a lag period and persistence in subsequent darkness. Etiolated mustard seedlings showed similar responses to light and gravity. The results are discussed in relation to the possibility that two photosystems operate in hypocotyl growth.