Mechanism of photosystem-I photoinhibition in leaves of Cucumis sativus L.

It was recently shown that the site of photoinhibition in leaves of Cucumis sativus L. at low temperatures is Photosystem I (PSI), not PSII (I. Terashima et al. 1994, Planta 193, 300-306). In the present study, the mechanisms of this PSI photoinhibition in vivo were examined. By lowering the photon flux density during the photoinhibitory treatment of leaves at 4°C for 5 h to less than 100 μmol·m^-2s^-1, we were able to separate the steps of the destruction of the electron-transfer components. Although P-700, the reaction-center chlorophyll, was almost intact in this low-light treatment, the quantum yield of the electron transfer through PSI and photochemically induced absorption change at 701 nm were markedly inhibited. This, along with the results from the measurements of the light-induced absorption changes in the presence of various concentrations of methyl viologen, an artificial electron acceptor, indicates that the component on the acceptor side of the PSI, A₁ or F_x, is the first site of inactivation. When the photon flux density during the treatment was increased to 220 μmol·m^-2s^-1, the destruction of P-700 itself was also observed. Furthermore, the partial degradation of the chlorophyll-binding large subunits was observed in photoinhibited leaves. This degradation of the subunits was not detected when the treatment was carried out under nitrogen atmosphere, the condition in which the electron transfer is not inhibited. Thus, the photoinhibitory processes in the reaction center of PSI go through three steps, the inactivation of the acceptor side, the destruction of the reaction-center chlorophyll and the degradation of the reaction center subunit(s). The similarities and the differences between the mechanisms of PSI photoinhibition and those of PSII photoinhibition are discussed.