Effects of Magnesium on Intact Chloroplasts

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Abstract
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+.

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