The effects of irradiation on photosystem (PS)-I submembrane particles using intense white light (2000 μE·m−2·s−1) at chilling temperature (4°C) were studied. PSI-dependent oxygen uptake activity was stable during the first 3 h of photoinhibitory illumination in the presence of added superoxide dismutase (SOD). Without added SOD, the oxygen uptake almost doubled during this period, presumably due to the denaturation of native membrane-bound SOD or its release from the PSI membranes. The total chlorophyll (Chl) content and the magnitude of light-induced absorbance changes at 830 nm (ΔA830) were also barely affected during the first 3–3.5 h of photoinhibitory treatment. However, further exposure to strong light markedly accelerated Chl breakdown followed by a decline in oxygen uptake rate and ΔA830. This corresponded with the disappearance of the bands attributed to PsaA/B polypeptides on electrophoretic gels. Despite the invariant maximum magnitude of ΔA830 during the first 3–3.5 h of photoinhibitory treatment, the light–response curves of P700 oxidation gradually altered, demonstrating a several-fold increase in the ability of weak actinic light to oxidize P700. The major Chl a–protein 1 (CP1) band gradually disappeared during the first 4 h of light exposure with a corresponding increase in the Chl content of a band with lower electrophoretic mobility ascribed to the formation of oligomers containing CP1, light-harvesting complex I (LHCI)-680 and LHCI-730. This aggregation of Chl–protein complexes, likely caused by photoinhibitory-induced cross-linking favoring light harvesting, is proposed to explain the enhanced capacity of weak light to oxidize P700 in photoinhibited PSI submembrane fractions compared with untreated ones.
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Vol. 77 • No. 3