Regulation of Photosynthetic Electron Transport in Intact Spinach Chloroplasts

Abstract

Relatively high concentrations of monovalent salts (150 millimolar) stimulated light-saturated uncoupled rates of O(2) evolution linked to oxaloacetic acid (OAA) reduction by intact chloroplasts 2-to 3-fold. In contrast, monovalent salts partially inhibited light-saturated rates of O(2) evolution coupled to CO(2) fixation and uncoupled rates of nitrite reduction. In the presence of high salt concentration, light-saturated rates of electron transport were about equivalent for all three terminal electron acceptors. It is inferred that exogenous monovalent salts have at least two effects on photosynthetic electron transport, independent of photophosphorylation and CO(2) metabolism: a partial inhibitory effect common to OAA, NO(2) (-) and CO(2) reduction and a marked stimulatory effect unique to the photoreduction of OAA.The stimulation of electron transport to OAA was effected by certain exogenous monovalent salts (KCl or NaCl, but not LiCl). Divalent salts (MgCl(2) or CaCl(2)) and high osmotic strength were ineffective. The salt-induced stimulation was eliminated by low concentrations of phosphate or sulfate (>/= millimolar) and by higher concentrations of magnesium (>/=30 millimolar). These results suggest that the ion content of the medium (or cytosol) is potentially important in modulating photosynthetic electron transport events in intact chloroplasts.