The effect of magnesium and phosphorylation of light-harvesting chlorophyll [formula omitted] on the yield of P-700-photooxidation in pea chloroplasts

Abstract

The yield of P-700 photooxidation has been studied in isolated chloroplast membranes by measuring the extent of the flash-induced absorption increase at 820 nm ( ΔA 820) in the microsecond time range. The extent of ΔA 820 induced by non-saturating laser flashes was increased by the following treatments. (1) Suspension of chloroplast membranes in Mg 2+ free medium (plus 15 mM K +) which leads to unstacking of grana (as detected by a decrease in chlorophyll fluorescence). (2) Reduction of Q, the primary acceptor of Photosystem II, in the presence of 20 μM 3-(3,4 dichlorophenyl)-1,1-dimethylurea by a saturating xenon flash, fired 300 ms before the laser flash. (3) Phosphorylation of light harvesting chlorophyll a b - protein complex, which occurs in the presence of ATP after activation of protein kinase in the dark with NADPH and ferredoxin. We conclude that the Mg 2+ concentration, the redox state of Q and the protein-phosphorylation all can control the photochemical efficiency of P-700 photooxidation in isolated chloroplasts, and we discuss these results in relation to control of excitation energy distribution between the two photosystems. We also discuss the significance of these results in relation to the regulation of photosynthetic electron transport in vivo.