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Articles

Effects of Magnesium on Intact Chloroplasts

I. EVIDENCE FOR ACTIVATION OF (SODIUM) POTASSIUM/PROTON EXCHANGE ACROSS THE CHLOROPLAST ENVELOPE ¹

² United States Department of Agriculture, Science and Education Administration, Agricultural Research, Department of Crop Science, North Carolina State University, Raleigh, North Carolina 27650, ³ Department of Botany, North Carolina State University, Raleigh, North Carolina 27650

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⁺.

External K⁺ or Na⁺ also prevented the inhibition of CO₂-dependent O₂ evolution by Mg²⁺, whereas choline chloride was less effective. The combination of Mg²⁺ and K⁺ stimulated O₂ evolution at suboptimal pH, inhibited O₂ evolution at optimal and superoptimal pH, and prevented the inhibition of photosynthesis caused by acetate. In the absence of added K⁺, Mg²⁺ was most inhibitory to O₂ evolution at suboptimal pH.

The results suggested that Mg²⁺ activated a reversible (Na⁺)K⁺/H⁺ exchange across the chloroplast envelope. It is postulated that changes in the stromal pH may explain the inhibition of photosynthesis caused by the presence of exogenous Mg²⁺.

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				C.-P. Song, Y. Guo, Q. Qiu, G. Lambert, D. W. Galbraith, A. Jagendorf, and J.-K. Zhu A probable Na+(K+)/H+ exchanger on the chloroplast envelope functions in pH homeostasis and chloroplast development in Arabidopsis thaliana PNAS, July 6, 2004; 101(27): 10211 - 10216. [Abstract] [Full Text] [PDF]