Influence of Varying CO2 and Orthophosphate Concentrations on Rates of Photosynthesis, and Synthesis of Glycolate and Dihydroxyacetone Phosphate by Wheat Chloroplasts

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Abstract
Intact chloroplasts of wheat (T. aestivum) were isolated from mesophyll protoplasts. With decreasing concentrations of bicarbonate from 10 to 0.3 mM (pH 8.0), the optimal concentration of Pi for photosynthetic 02 evolution decreased from a value of 0.1 to 0.2 mM to 0 to 0.025 mM. The extremely low Pi optimum for photosynthesis at the low bicarbonate levels of 0.3 mM was increased by lowering the O2 concentration from 253 (21% gas phase) to 72 .mu.M (6% gas phase). The relative amount of glycolate and dihydroxyacetone phosphate (DHAP) synthesized under high and low levels of bicarbonate and varying levels of Pi was determined. At low levels of bicarbonate, glycolate was the main product, whereas at high bicarbonate levels. DHAP was the main product. Most of the DHAP and glycolate was present in the extrachloroplastic fraction. The rate of photosynthesis at low levels of bicarbonate (0.3 mM) was as high as 75-95% of that at high levels of bicarbonate (10 mM) at the respective optimal levels of Pi. At low bicarbonate levels, and without Pi, there was little lag in photosynthetic O2 evolution upon illumination in comparison to that of high bicarbonate levels and optimal levels of Pi. Evidently, conditions which favor glycolate synthesis allow photosynthesis to continue without a depletion of internal Pi, whereas consumption of Pi may occur in the chloroplast during the net synthesis of organic phosphates under high levels of bicarbonate and without addition of Pi. At low bicarbonate levels, the extreme susceptibility of photosynthesis to inhibition by Pi may be due to excessive export of C from the chloroplast in the form of glycolate and triose phosphate.