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Evidence of a Low Stromal Mg²⁺ Concentration in Intact Chloroplasts in the Dark

I. STUDIES WITH THE IONOPHORE A23187

United States Department of Agriculture, Science and Education Administration, Agricultural Research, University of Illinois, Urbana, Illinois 61801, Department of Agronomy, University of Illinois, Urbana, Illinois 61801

The loss of Mg²⁺ upon the addition of the ionophore A23187 in the dark was prevented by less than 0.1 millimolar MgCl₂ with intact chloroplasts suspended in a sorbitol medium, but required 1 to 3 millimolar MgCl₂ if the chloroplasts were in a K⁺ -gluconate medium. Measurements of stromal pH in the dark indicated that, in the K⁺ -gluconate medium, the stromal pH is nearly the same as that of the medium, whereas in the sorbitol medium it is much more acidic as reported previously. These observations suggest that the free Mg²⁺ concentration in the stroma in the dark is between 1 and 3 millimolar. Other experiments on the inihibition by A23187 of CO₂ fixation in the light and in a system capable of catalyzing CO₂ fixation in the dark, and on the Mg²⁺ binding properties of thylakoid membranes, are consistent with this conclusion. The results provide further support for the hypothesis that light-induced Mg²⁺ concentration changes occur in the stroma that are important in the light-dark regulation of CO₂ fixation.

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