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Water stress substantially alters plant metabolism, decreasing plant growth and photosynthesis 1, 2, 3, 4 and profoundly affecting ecosystems and agriculture, and thus human societies 5. There is controversy over the mechanisms by which stress decreases photosynthetic assimilation of CO 2. Two principal effects are invoked 2, 4: restricted diffusion of CO 2 into the leaf, caused by stomatal closure 6, 7, 8, and inhibition of CO 2 metabolism 9, 10, 11. Here we show, in leaves of sunflower (Helianthus annuus L.), that stress decreases CO 2 assimilation more than it slows O 2 evolution, and that the effects are not reversed by high concentrations of CO 2 12, 13. Stress decreases the amounts of ATP 9, 11 and ribulose bisphosphate found in the leaves, correlating with reduced CO 2 assimilation 11, but the amount and activity of ribulose bisphosphate carboxylase-oxygenase (Rubisco) do not correlate. We show that ATP …