Effects of Magnesium on Intact Chloroplasts: I. EVIDENCE FOR ACTIVATION OF (SODIUM) POTASSIUM/PROTON EXCHANGE ACROSS THE CHLOROPLAST ENVELOPE

Exogenous Mg²⁺ (2 millimolar) altered the stromal pH of intact spinach chloroplasts. Without added KCl in the medium, Mg²⁺ decreased the stromal pH in the light by approximately 0.3 pH unit. External KCl (25 millimolar) largely prevented the acidification caused by Mg²⁺. Effects on the stromal pH were not caused by changes in H⁺ pumping across the thylakoid membrane because Mg²⁺ had no effect on the light-induced quenching of atebrin fluorescence by intact chloroplasts. However, Mg²⁺ affected H⁺ fluxes across the envelope. Addition of Mg²⁺ to intact chloroplasts in the dark caused a significant acidification of the medium that was dependent on the presence of K⁺.