The effect of mono- and divalent cations on the quantum yields for electron transport in chloroplasts

We have previously shown (Gross, E.L. and Hess, S.C. (1973) Arch. Biochem. Biophys 159, 832-836) that low concentrations of monovalent cations (3-10 mM) caused changes in chlorophyll a fluorescence indicative of a transfer of excitation energy from Photosystem II to Photosystem I. These effects were reversed by divalent cations or higher concentrations of monovalent cations. In this study, we have examined the effects of cations on the relative quantum yields for Photosystem II (dichlorophenolindophenol reduction) and Photosystem I (diphenylcarbazone disproportionation) with the following results. (1) Low concentrations of monovalent cations decreased the quantum yield for dichlorophenolindophenol reduction and increased that for diphenylcarbazone disproportionation. These results confirm that cations promote excitation energy transfer to Photosystem I. (2) Higher concentrations of monovalent cations (30-100 mM) had no effect on electron transport. Therefore, the increases in chlorophyll a fluorescence observed at these concentrations may be due to a decrease in the rate constant for radiationless decay. (3) In the absence of Tricine, divalent cations also promote energy transfer from the light harvesting pigments to Photosystem I. However, a direct inhibition of Photosystem I and divalent cations can reverse the Tricine effects.m II photochemistry cannot be rules out. (4) Tricine biases the system in favor of Photosystem I and divalent cations can reverse the Tricine effects.