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Further Studies on the Bicarbonate Stimulation of Photophosphorylation in Isolated Chloroplasts ¹

^a Division of Biological Sciences, Cornell University, Ithaca, New York 14850

The bicarbonate effect in stimulating the rate of photophosphorylation by isolated spinach (Spinacia oleracea var. Virginia blight-resistant savoy) chloroplasts at a pH below the optimum has been re-examined. Its seasonal nature may be related to the hormonal status of the plants. Bicarbonate anions stimulate adenosine 5'-triphosphate synthesis if added in the final, adenosine 5'-triphosphate-forming stage of either a postillumination or an acid-base experiment. They also stimulate the membrane-bound, Mg²⁺-dependent adenosine 5'-triphosphatase of chloroplasts, and the Ca²⁺-dependent adenosine 5'-triphosphatase of detached coupling factor. These and other data point to the interaction between energized thylakoid membranes and the coupling factor as the probable site of action of bicarbonate anions when they stimulate photophosphorylation.

¹ This research was supported by National Institutes of Health Grant GM-14479 and a National Science Foundation Postdoctoral Fellowship (49039) to W.S.C.