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Inhibition of Photosynthetic Carbon Dioxide Fixation in Isolated Spinach Chloroplasts Exposed to Reduced Osmotic Potentials ¹

^a The Volcani Institute of Agricultural Research, Bet Dagan, Israel

Reduced osmotic potentials inhibited the rate of CO₂ fixation by isolated intact spinach (Spinacia oleracea) chloroplasts. This inhibition was observed immediately after transfer of chloroplasts from a solution containing 0.33 M sorbitol to higher sorbitol concentrations, and the depressed rate remained constant. The inhibited CO₂ fixation could not be attributed to a decreased rate of photosynthetic electron transport, since NADP reduction was unaffected by subjecting the chloroplasts to low potentials. It could also not result from restricted permeability to CO₂, as CO₂ concentrations had no effect on the relative inhibition induced by the lowered potential.

A procedure was developed for the determination of several enzymes of the photosynthetic carbon reduction cycle in the intact chloroplast without their being extracted. The activities of the combined three enzymes: ribose-5-phosphate isomerase, ribulose-5-phosphate kinase, and ribulose-1,5-diphosphate carboxylase and of ribulose-1,5-diphosphate carboxylase alone were found to be inhibited at low osmotic potentials. Analysis of the photosynthetic products showed that the formation of glycerate-3-phosphate was inhibited to a greater extent than the formation of all other products.

CO₂ fixation was partly resumed when chloroplasts were returned from a 0.67 M sorbitol to a 0.33 M sorbitol solution, regardless whether the transfer occurred in the light or in the dark.