Surface Charge-Mediated Effects of Mg on K Flux across the Chloroplast Envelope Are Associated with Regulation of Stromal pH and Photosynthesis.

Studies of Spinacia oleracea L. were undertaken to characterize further how Mg(2+) external to the isolated intact chloroplast interacts with stromal K(+), pH, and photosynthetic capacity. Data presented in this report were consistent with the previously developed hypothesis that millimolar levels of external, unchelated Mg(2+) result in lower stromal K(+), which somehow is linked to stromal acidification. Stromal acidification directly results in photosynthetic inhibition. These effects were attributed to Mg(2+) interaction (binding) to negative surface charges on the chloroplast envelope. Chloroplast envelope-bound Mg(2+) was found to decrease the envelope membrane potential (inside negative) of the illuminated chloroplast by 10 millivolts. It was concluded that Mg(2+) effects on photosynthesis were likely not mediated by this effect on membrane potential. Further experiments indicated that envelope-bound Mg(2+) caused lower stromal K(+) by restricting the rate of K(+) influx; Mg(2+) did not affect K(+) efflux from the stroma. Mg(2+) restriction of K(+) influx appeared consistent with the typical effects imposed on monovalent cation channels by polyvalent cations that bind to negatively charged sites on a membrane surface near the outer pore of the channel. It was hypothesized that this interaction of Mg(2+) with the chloroplast envelope likely mediated external Mg(2+) effects on chloroplast metabolism.