Magnesium, potassium flux and photosynthesis.

Photosynthetic capacity of chloroplasts is regulated by stromal pH. The pH of the stroma must be maintained higher than external pH during illumination in order to optimize photosynthetic carbon reduction cycle activity. Stromal pH is affected by K+/H+ counterexchange across the chloroplast envelope. Magnesium ion external to the chloroplast affects K+/H+ counterfluxes and hence stromal pH and photosynthesis. K+/H+ counterfluxes across the envelope are facilitated by K+ and H+ conducting ion channels, and an H+ pumping envelope ATPase. External magnesium affects K+/H+ exchange by binding to negative surface charges on the envelope membrane. Magnesium binding restricts currents through K+ channels. Magnesium block of K+ uptake through this channel impairs H+ efflux, inhibiting photosynthesis. Under conditions which lead to net K+ efflux through the channel, envelope-bound magnesium restricts K+ efflux, reducing the driving force for electroneutral H+ uptake. External magnesium has a profound effect on chloroplast photosynthetic capacity by an indirect control of H+ movement across the envelope and hence of stromal pH.