Effects of Magnesium on Intact Chloroplasts : II. CATION SPECIFICITY AND INVOLVEMENT OF THE ENVELOPE ATPase IN (SODIUM) POTASSIUM/PROTON EXCHANGE ACROSS THE ENVELOPE

Addition of exogenous Mg²⁺ (2 millimolar) to illuminated intact spinach (Spinacia oleracea L.) chloroplasts caused acidification of the stroma and a 20% decrease in stromal K⁺. Addition of K⁺ (10-50 millimolar) reversed both stromal acidification and K⁺ efflux from the chloroplast caused by Mg²⁺. These data suggested that Mg²⁺ induced reversible H⁺/K⁺ fluxes across the chloroplast envelope. Ca²⁺ and Mn²⁺ (2 millimolar) were as effective as 4 millimolar Mg²⁺ in causing K⁺ efflux from chloroplasts and inhibition of O₂ evolution. In contrast, 10 millimolar Ba²⁺ induced only a small amount of inhibition. The lack of strong inhibition by Ba²⁺ indicated that the effects of divalent cations such as Mg²⁺ cannot be attributed to generalized electrostatic interactions of the cation with the chloroplast envelope. With the chloroplasts used in this study, stromal acidification caused by 2 millimolar Mg²⁺ was small (0.07 to 0.15 pH units), but sufficient to account for the inhibition of O₂ evolution (43%) induced by Mg²⁺.