Influence of Inorganic Phosphate on Photosynthesis of Wheat Chloroplasts

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Abstract
Isolated wheat chloroplasts were pre-incubated in the dark in the presence of various concentrations of inorganic phosphate with or without carbon dioxide, oxaloacetate, glycerate, and 3-phosphoglycerate. The effect of subsequent illumination on photosynthetic oxygen evolution, ribulose bisphosphate carboxylase activity, ATP content, and ribulose bisphosphate content was investigated. Inorganic phosphate had little effect on ribulose bisphosphate carboxylase activity in darkness or during the initial phase of illumination, but it prevented the decline in activity that occurred during later stages of illumination, when photoreduction of CO2 was decreasing in rate. Addition of inorganic phosphate to chloroplasts illuminated without phosphate restored the ribulose bisphosphate carboxylase activity, increased the ATP, and decreased the ribulose bisphosphate in the organelles. The responses to CO2, oxaloacetate, glycerate, and 3-phosphoglycerate suggest that the decreased activity of ribulose bisphosphate carboxylase during photosynthesis results from ATP consumption. Purified ribulose bisphosphate carboxylase was activated by inorganic phosphate, but this activation did not occur in the presence of ATP. ATP inhibited ribulose bisphosphate carboxylase when it was present in combination with various photosynthetic metabolites. Inactivation of ribulose bisphosphate carboxylase in chloroplasts, illuminated in the absence of inorganic phosphate, is not due to lack of activation by inorganic phosphate or ATP. It may result from decreased stromal pH.

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