Effects of the Relative Extrachloroplastic Concentrations of Inorganic Phosphate, 3-Phosphoglycerate, and Dihydroxyacetone Phosphate on the Rate of Starch Synthesis in Isolated Spinach Chloroplasts

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Abstract
The effect of external inorganic phosphate (Pi) on starch synthesis in isolated spinach (Spinacia oleracea American Hybrid No. 424) chloroplasts in the presence of millimolar concentrations of 3-phosphoglycerate (PGA) and/or dihydroxyacetone phosphate (DAP) was examined. Whereas CO2 fixation was relatively constant as the ratio of the external phosphate to the PGA + DAP varied from 1:3 to 3:1, starch synthesis varied from 17% to 2% of the CO2 fixation rate. With DAP alone, maximal starch synthesis was about 10% of the CO2 fixation rate. The data demonstrate that the Pi/(PGA + DAP) ratio in the cytoplasm of plant cells could serve to regulate the flow of newly fixed carbon into starch without alterations in the rate of CO2 fixation.