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Not an open-or-shut case
It is something that all plant biologists know: drought limits photosynthesis in C3 plants. However, there is no universal agreement about the mechanism for this limitation. Intuitively, it might be supposed that stomatal closure would be the main reason and we might knowingly contrast this situation with that in C4 plants. But it is not as simple as that: there is also evidence that low water potential may directly inhibit components of the biochemical and photochemical machinery and several authors have suggested this to be the prime cause of drought-induced photosynthetic inhibition. The situation is further complicated by the differing severities of lowered leaf water potential at which different C3 plants exhibit photosynthetic limitation. In an attempt to resolve these ambiguities, Flexas and Medrano (University of the Balearics, pp. 183-189) have reanalysed an extensive set of previously published data, including their own. They have looked at a very wide range of studies of photosynthetic rates in relation to drought and have plotted changes in photosynthetic biochemistry and photochemistry against stomatal aperture at light saturation (g). (Note that in most previous studies, these changes were plotted against leaf water potential or leaf water content.) The results of this synthesis are fascinating: across a wide range of plants with differing sensitivities to drought and across a range of biochemical and photochemical responses, photosynthetic limitation is strongly correlated with g, especially in the early stages of the response. Nevertheless, there is also evidence for more direct effects on photosynthetic biochemistry, especially photophosphorylation. These latter effects, which may be caused by either inhibition of or damage to the biochemical/photochemical machinery, become more important as the severity of the water deficit increases. Thus, this analysis provides important insights into the effects of drought, the most important nonbiotic factor limiting world crop productivity.
Professor J.A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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