Influence of glycerate on photosynthesis by wheat chloroplasts

Glycerate was found to effect photosynthetic O2 evolution in wheat chloroplasts by its conversion to triose phosphate and by influencing the rate of photosynthesis through the reductive pentose phosphate pathway. In the absence of bicarbonate, the photosynthetic O2 evolution with glycerate was low (10 to 25 mumol mg chlorophyll-1 h-1), and only about 15% of the rate of bicarbonate-dependent O2 evolution under optimum conditions. This corresponds to a rate of glycerate conversion to triose phosphate of 20 to 50 mumol mg chlorophyll-1 h-1, which appears sufficient to accommodate flux through the glycolate pathway in vivo. Pi was required for this glycerate-dependent O2 evolution; rates remained relatively constant between 0.1 and 40 mM Pi, and proceeded with little lag upon illumination (less than 0.5 min). Evidence for O2 evolution due to glycerate conversion to triose phosphate could be conclusively demonstrated by addition of glycolaldehyde, an inhibitor of the regenerative phase of photosynthesis, which prevents CO2 fixation. The effect of glycerate on photosynthesis in the presence of bicarbonate was determined by measuring both photosynthetic O2 evolution and 14CO2 fixation at varying Pi concentrations. Low concentrations of glycerate (micro- to millimolar levels) prevented inhibition of photosynthesis by Pi. With 1 mM bicarbonate and pH 8.2, which is favorable for glycolate synthesis, maximum rates of photosynthesis were obtained at low Pi (25 microM), whereas strong inhibition of photosynthesis occurred at only 0.2 mM Pi. Addition of glycerate relieved the inhibition of photosynthesis by Pi, indicating the possible importance of glycerate metabolism in the chloroplast under photorespiratory conditions. The initiation of photosynthesis by glycerate at inhibitory Pi levels occurred with little reduction in the ratio of CO2 fixed/O2 evolved, and the main effect of glycerate was on carbon assimilation. While the basis for the beneficial effect of glycerate on CO2 assimilation under moderate to high Pi levels is uncertain, it may increase the concentration of 3-phosphoglycerate (PGA) in the chloroplast, and thus make conditions more favorable for induction of photosynthesis and reduction of PGA to triose phosphate.