Picosecond fluorescence kinetics in spinach chloroplasts at room temperature. Effects of Mg 2+

Abstract

Single-photon timing with picosecond resolution is used to investigate the effect of Mg 2+ on the room-temperature fluorescence decay kinetics in broken spinach chloroplasts. In agreement with an earlier paper (Haehnel, W., Nairn, J.A., Reisberg, P. and Sauer, K. (1982) Biochim. Biophys. Acta 680, 161–173), we find three components in the fluorescence decay both in the presence and in the absence of Mg 2+. The behavior of these components is examined as a function of Mg 2+ concentration at both the F 0 and the F max fluorescence levels, and as a function of the excitation intensity for thylakoids from spinach chloroplasts isolated in the absence of added Mg 2+. Analysis of the results indicates that the subsequent addition of Mg 2+ has effects which occur at different levels of added cation. At low levels of Mg 2+ (less than 0.75 mM), there appears to be a decrease in communication between Photosystem (PS) II and PS I, which amounts to a decrease in the spillover rate between PS II and PS I. At higher levels of Mg 2+ (about 2 mM), there appears to be an increase in communication between PS II units and an increase in the effective absorption cross-section of PS II, probably both of these involving the chlorophyll a b light-harvesting antenna.