CONTROL OF EXCITATION TRANSFER IN PHOTOSYNTHESIS .2. MAGNESIUM ION-DEPENDENT DISTRIBUTION OF EXCITATION ENERGY BETWEEN 2 PIGMENT SYSTEMS IN SPINACH CHLOROPLASTS

1. 1. Effects of metal ions on the yields of chlorophyll a fluorescence were investigated in spinach chloroplasts. At room and liquid-nitrogen temperatures, addition of Mg2+ increased the yields of two emissions of chlorophyll a, having peaks at 684 and 695 nm which were emitted from pigment system II and decreased the yield of another emission of chlorophyll a at about 735 nm which was emitted from pigment system I. Mg2+ also changed the pattern of the fluorescence induction at room temperature to increase the variable component of fluorescence. 2. 2. Effects of Mg2+ on the quantum yields of the two photoreactions were investigated in a region of weak light intensities. Mg2+ accelerated the rate of photoreaction II measured by the Hill reaction with 2,6-dichlorophenolindophenol and depressed the rate of photoreaction I measured by the NADP+ reduction in the presence of reduced 2,6-dichlorophenolindophenol and 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea. 3. 3. These facts indicate that Mg2+ controls the distribution of excitation energy between the two pigment systems and that the control consists mainly in change in rate of excitation transfer from the bulk chlorophyll a molecules in pigment system II to those in pigment system I; namely, Mg2+ suppresses the spillover of excitation energy from pigment system II to pigment system I. © 1969.