Energy-dependent cation-induced control of chlorophyll a fluorescence in isolated intact chloroplasts

Abstract

Slow chlorophyll a fluorescence-yield changes occur in illuminated intact chloroplasts which are independent of the redox state of the photosystem 2 (PS2) traps. The changes seem to be associated with cation transport from the thylakoid compartment to the stromal space, powered by coupled electron flow. It is suggested that the active uptake of H + into the thylakoids, by either noncyclic or tetramethyl phenylene diamine-catalyzed cyclic flow in the presence of dichlorophenyl dimethyl urea, induces the cation displacement. Experiments involving the use of various ionophores, with whole chloroplasts suspended in a medium containing no inorganic cations and with broken chloroplasts treated with specific cations so as to show the slow fluorescence effects, indicate that Mg 2+ is the most likely cation involved in the in vivo quenching process. The results suggest that the Mg 2+-sensitive binding sites (p K around 5.0), which control the fluorescence changes, exist on the inner surface of the thylakoid membranes. The energy-dependent Mg 2+ sensitivity of chlorophyll a fluorescence is discussed in terms of the spillover model for control of electron transport through the two photosystems.