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

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Abstract
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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