Studies on the Control of Excitation Energy Distribution between the Two Photosystems in Pea Thylakoids by Mg2+ and LHCP-phosphorylation

Abstract

Photosynthetic organisms, in which two photosystems cooperate in series during photosynthetic electron transport, are able to adapt to light of different wavelengths so as to maximize photochemical efficiency (see Myers 1971). In vitro both a decrease in cation level and phosphorylation of light harvesting chlorophyll a/b-protein (LHCP) have been suggested as mechanisms which promote excitation of photosystem I (PS1) at the expense of PSII (see Barber 1983). The changes in amount of energy going to PS1 have been determined both by measurement of chlorophyll fluorescence and steady state electron transfer rates. At room temperature the former method is indirect as a decrease in fluorescence from PSII indicates only a relative change in excitation and not an actual increase in PS1 activity. However fluorescence spectra measured under non-physiological conditions (77K) do indicate that both absence of Mg2+ and phosphorylation of LHCP increase excitation of PS1 (Krause, Behrend 1983). The effect of cations on steady-state electron transfer rates have yielded conflicting results because cations also affect interaction of electron transport chain components (see Barber 1976). Recently however 10–15% increases in electron transport through PS1, due to phosphorylation of LHCP, have been reported (Horton, Black 1982; Farchaus et al. 1982). In experiments designed to eliminate secondary electron transfer steps which might be affected by cation levels, Melis and Ow (1982) have recently demonstrated no effect of Mg2+ on the initial rate of photooxidation of P700. They attribute the decreased level of PS11 fluorescence that is seen in the absence of cations to increased radiationless decay rather than energy transfer to PS1. Because of these conflicting reports we have used a different approach. We have measured the relative quantum yield of flash-induced P700 oxidation by the change in absorbance at 820 nm due to P700+. We have shown that both absence of Mg2+ and LHCP-phosphorylation increase the photochemical efficiency of P700 oxidation induced by non-saturating flashes.