Photosynthesis at Low Water Potentials in Sunflower: Lack of Photoinhibitory Effects

Abstract

The losses in chloroplast capacity to fix CO2 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 CO2. The quantum yield for CO2 fixation as well as the rate of light- and CO2-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 CO2 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 CO2 and O2 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 CO2 was entirely the result of a direct effect of water availability on chloroplast function and not photoinhibition.