Effects of External Mg2+ and Ph on the Photosynthetic Activities and Stromal pH of Intact Chloroplasts Isolated From C4 Plants

Exogenous Mg2+ inhibited PGA- and OAA-dependent photosynthetic O2 evolution by isolated mesophyll chloroplasts from Zea mays and also HCO3--dependent O2 evolution by chloroplasts from Panicum miliaceum bundle sheath cells. Inhibition of 50-75% was observed with 3-5 mM Mg2+; this varied to some extent with pH but was not reversed by K+ . Inhibition of HCO3-- and PGA-dependent O2 evolution by the divalent cation ionophore A23187 was reversed by adding Mg2+ . Notably, HCO3-- dependent O2 evolution by isolated bundle sheath cells from P. miliaceum was not inhibited by [Mg2+] up to 20 mM. Addition of Mg2+ in the light decreased the stromal pH of mesophyll chloroplasts by less than 0.2 units but reduced apparent stromal volume by as much as 25%. At least for bundle sheath chloroplasts, stromal pH varied with external pH over the range from 7 to 8 but remained about 0.3 units higher throughout this range. Oxygen evolution by isolated mesophyll chloroplasts, and bundle sheath cells and chloroplasts, was relatively insensitive to external pH in the range from 7 to 8. The results are considered in terms of likely mechanisms for the effects of exogenous Mg2+ and pH on photosynthesis by isolated chloroplasts and the physiological significance of Mg2+ effects.