Water stress inhibits plant photosynthesis by decreasing coupling factor and ATP

Water stress substantially alters plant metabolism, decreasing plant growth and photosynthesis(1-4) and profoundly affecting ecosystems and agricultur e, and thus human societies(5). There is controversy over the mechanisms by which stress decreases photosynthetic assimilation of CO2. Two principal e ffects are invoked(2,4): restricted diffusion of CO2 into the leaf, caused by stomatal closure(6-8), and inhibition of CO2 metabolism(9-11). Here we s how, in leaves of sunflower (Helianthus annuus L.), that stress decreases C O2 assimilation more than it slows O-2 evolution, and that the effects are not reversed by high concentrations of CO212,13. Stress decreases the amoun ts of ATP(9,11) and ribulose bisphosphate found in the leaves, correlating with reduced CO2 assimilation(11), but the amount and activity of ribulose bisphosphate carboxylase-oxygenase (Rubisco) do not correlate. We show that ATP-synthase (coupling factor) decreases with stress and conclude that pho tosynthetic assimilation of CO2 by stressed leaves is not limited by CO2 di ffusion but by inhibition of ribulose biphosphate synthesis, related to low er ATP content resulting from loss of ATP synthase.