Photosynthesis at low water potentials in sunflower: lack of photoinhibitory effects.

The losses in chloroplast capacity to fix CO(2) when photosynthesis is reduced at low leaf water potential (psi(1)) have been proposed to result from photoinhibition. We investigated this possibility in soil-grown sunflower (Helianthus annuus L. cv IS894) using gas exchange techniques to measure directly the influence of light during dehydration on the in situ chloroplast capacity to fix CO(2). The quantum yield for CO(2) fixation as well as the rate of light- and CO(2)-saturated photosynthesis were strongly inhibited at low psi(1). The extent of inhibition was the same whether the leaves were exposed to high or to low light during dehydration. When intercellular partial pressures of CO(2) were decreased to the compensation point, which was lower than the partial pressures resulting from stomatal closure, the inhibition of the quantum yield was also unaffected. Photoinhibition could be observed only after high light exposures were imposed under nonphysiological low CO(2) and O(2) where both photosynthesis and photorespiration were suppressed. The experiments are the first to test whether gas exchange at low psi(1) is affected by potentially photoinhibitory conditions and show that the loss in chloroplast capacity to fix CO(2) was entirely the result of a direct effect of water availability on chloroplast function and not photoinhibition.