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Articles

Photosynthesis at Low Water Potentials in Sunflower: Lack of Photoinhibitory Effects ¹

Department of Plant Biology, University of Illinois, Urbana, Illinois 61801, Department of Agronomy, University of Illinois, Urbana, Illinois 61801, United States Department of Agriculture, Agricultural Research Service, University of Illinois, Urbana, Illinois 61801

The losses in chloroplast capacity to fix CO₂ when photosynthesis is reduced at low leaf water potential (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 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 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.

³ Present address: Department of Soil and Crop Sciences, Texas A & M University, College Station, TX 77843-2474.

¹ Supported by National Science Foundation grant PCM 79-0979 to J. S. B.

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