Magnesium Ion Effects on Chloroplast Photosystem Ii Fluorescence and Photochemistry

Abstract. –Although there have been several reports that divalent cations, especially Mg2+, can significantly affect chloroplast photoprocesses, the molecular mechanism of cation interaction is not well understood. We have investigated the interaction of Mg2+with Photosystem II photoprocesses by studying cation effects on chloroplast fluorescence and the Hill reaction. Our results are summarized as follows. 1. Mg2+stimulation of background fluorescence (20–30%) saturates at about 0.5 mM Mg2+, while Mg2+stimulation of variable fluorescence (250%) saturates at about 2.5 mM Mg2+. 2. Addition of Mg2+to chloroplasts treated with 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea or dithionite causes a doubling in the amount of total (background + variable) fluorescence. 3. Studies on chloroplasts in the presence of 2,6‐dichlorophenolindophenol indicate that Mg2doubles the relative yield of variable fluorescence under light‐limiting conditions. 4. Mg2causes large (70–120%) increases in the light‐limited rate of the DCIP Hill reaction. We interpret these results in terms of a model involving two components of chloroplast emission. Our analysis indicates that Mg2+increases the effective absorption cross section (size) of the pigment array associated with Photosystem II photochemistry.