Inhibition of photosynthetic carbon dioxide fixation in isolated spinach chloroplasts exposed to reduced osmotic potentials.

Reduced osmotic potentials inhibited the rate of CO(2) 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 CO(2) 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 CO(2), as CO(2) 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.CO(2) 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.