Effects of uncouplers on Mg(2+)-dependent fluorescence quenching in isolated chloroplasts

Uncoupling concentrations (about 1 μmol l(-1)) of desaspidin or carbonyl cyanide-4-trifluoromethoxyphenyl hydrazone reverse the slow light-induced, Mg(2+)-dependent quenching of fluorescence of chlorophyll a in isolated (intact and broken) spinach chloroplasts. Likewise, uncoupling inhibits the light-induced increase of the Mg(2+) concentration in the stroma of intact chloroplasts, as determined with Eriochrome Blue SE. Addition of higher amounts of the uncouplers to the chloroplasts leads to a slow, light-dependent fluorescence lowering which appears to be promoted by high light intensities and is not reversed in the dark. The reversal of the fluorescence quenching by uncoupling is interpreted to reflect exchange of protons for Mg(2+) ions at negative sites of the inner thylakoid face, caused by the collapse of the proton gradient across the membrane. The secondary fluorescence lowering caused by high levels of the uncouplers and high light intensities is suggested to be related to an inhibition of non-cyclic photosynthetic electron transport.