Light-stress-related changes in photosystem I (PS I) were analyzed in photoautotrophic cultured cells of Marchantia polymorpha L. High light treatment (30 h; 1300 mol photons · m )2 · s )1 ) reduced the PS I-mediated electron-transport rate by more than 50% and the photochemical efficiency of photosystem II (PS II) by about 35%. In photoinhibited cells, 76% of the PS II centers remained closed in low light, which is in agreement with a preferential impairment of PS I. Our data indicate that excessive linear electron transport is a cause of the loss in PS I activity. Two PS I forms could be isolated by sucrose-gradient ultracentrifugation of mildly solubilized thylakoid membranes. After high-light treatment one of these forms, which showed a larger light-harvesting complex (LHC) I antenna and a specific association of LHC IIb, was enriched. The effect could be suppressed by blockage of linear electron transport. It is suggested that PS I inactivation and state transitions caused the change in PS I organisation.Abbreviations: Chl = chlorophyll; DCMU = 3-(3,4-dichlorphenyl)-1,1-dimethylurea; Fo, Fs, F′m = basal, steady-state (light) and maximum chlorophyll fluorescence yield; LHC = light-harvesting complex; PFD = photon flux density; PS I u , PS I l = upper and lower PS I-containing bands after ultracentrifugation Correspondence

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