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.