Carbon Dioxide and the Reduction of Indophenol and Ferricyanide by Chloroplasts

Pea chloroplasts isolated in salt media show decreased rates of 2:6 dichlorophenolindophenol (DCPIP) and ferricyanide reduction when depleted of CO(2) at pH values below 7.5. The greatest effect of CO(2) was on uncoupled systems. The incorporation of 10(-2), 2 x 10(-2) and 4 x 10(-2)m sodium acetate into the reaction mixtures progressively increased the bicarbonate concentration required for half maximal rates of reduction of DCPIP. The reaction was saturated by bicarbonate concentrations of 1 to 4 x 10(-2)m. With both DCPIP and ferricyanide, the addition of bicarbonate to illuminated chloroplast systems depleted of CO(2) gave very rapid increases in the rates of reduction. Bicarbonate also stimulated oxygen uptake by the illuminated chloroplasts when added hydrogen acceptors had been reduced. There was no effect of bicarbonate on ferricyanide reduction at low light intensities, but with DCPIP reduction, the apparent magnitude of the effect was independent of light intensity. This suggests that DCPIP reacts with the chloroplast electron transport chain at a site nearer to a photochemical stage than does ferricyanide. It also suggests that CO(2) has at least 2 sites of action.