GABA, V., Division of Biosphere Sciences, Kings College, London, United Kingdom, Department of Virology, Institute of Plant Protection, Volcani Institute, ARO, P. O. B. 6, Bet Dagan, 50-250, Israel, University of London, Kensington Campus, Campden Hill Rd, London, W8 7AH, United Kingdom
BLACK, M., Division of Biosphere Sciences, Kings College, London, United Kingdom, University of London, Kensington Campus, Campden Hill Rd, London, W8 7AH, United Kingdom
CANAANI, O., Department of Biochemistry, Weizmann Institute of Science, Rehovot, 76100, Israel
ATTRIDGE, T.H., Department of Biosciences, Polytechnic of East London, Romford Rd., Stratford, London, E15 4LZ, United Kingdom

We investigated the role of photosynthesis in the photocontrol of extension growth of the hypocotyl of light‐grown Cucumis sativus L. Previous work [Gaba and Black (1985b) Plant Physiol.79, 1011] demonstrated that the inhibition of cucumber hypocotyl elongation is a fluence rate dependent response in red light. However, the relative contributions of phytochrome and photosynthesis to the photon flux dependent inhibition response were not clear. Here we have shown that photoperception by the foliar cotyledons as well as the hypocotyl itself are responsible for fluence rate dependence in red light. The inhibitor of photosynthesis diuron [3‐(3,4‐dichlorophenyl)‐1, 1‐dimethylurea] reduced both the magnitude of inhibition and the fluence rate dependency in red light, indicating an involvement of photosynthesis. Furthermore, the growth of non‐pigmented seedlings (treated with the herbicide norflurazon) was less inhibited by red light, with no fluence rate dependency. In particular, inhibition due to cotyledon photoperception was completely lost in non‐pigmented (norflurazon‐treated) plants, and much reduced by diuron treatment. Hypocotyl‐perceived red light inhibition was only slightly reduced by treatment with norflurazon and diuron. Photosynthesis was compared directly to photo‐inhibition of growth: the photon flux response curve of oxygen evolution of green Cucumis cotyledons was distinctly different from that of hypocotyl inhibition. In conclusion, photosynthesis is an essential requirement for the cotyledon‐perceived inhibition, but the response itself is not due to photosynthesis. Copyright © 1991, Wiley Blackwell. All rights reserved