Abstract. Photoinhibition of PSII and turnover of the D1 reaction-centre protein in vivo were studied in pumpkin leaves (Cucurbita pepo L.) acclimated to different growth irradiances and in low-light-grown moss, Ceratodon purpureus (Hedw.) Brid. The low-light-acclimated pumpkins were most susceptible to photoinhibition. The production rate of photoinhibited PSII centres (kw), determined in the presence of a chloroplast-encoded protein-synthesis inhibitor, showed no marked difference between the high-and low-light-grown pumpkin leaves. On the other hand, the rate constant for the repair cycle (kREc) of PSII was nearly three times higher in the high-light-grown pumpkin when compared to low-light-grown pumpkin. The slower degradation rate of the damaged D1 protein in the low-light-acclimated leaves, determined by pulsechase experiments with [35S]methionine suggested that the degradation of the D 1 protein retards the repair cycle of PSII under photoinhibitory light. Slow degradation of the D 1 protein in low-light-grown pumpkin was accompanied by accumulation of a phosphorylated form of the D1 protein, which we postulate as being involved in the regulation of Dl-protein degradation and therefore the whole PSII repair cycle. In spite of low growth irradiance the repair cycle of PSII in the moss Ceratodon was rapid under high irradiance. When compared to the high-or low-light-acclimated pumpkin leaves, Ceratodon had the highest rate of Dl-protein degradation at 1000 Izmol photons m 2 s 1. In contrast to the higher plants, the D1 protein of Ceratodon was not phosphorylated either under high irradiance in vivo or under in-vitro conditions, which readily phosphorylate the D1 protein of higher plants. This is consistent with the rapid degradation of the D1 protein in Ceratodon. Screening experiments indicated that D1 protein can be phosphorylated in the thyAbbreviations: Chl = chlorophyll; DI* = phosphorylated form of D1 protein; Fma x and Fv=maximal and variable fluorescence, respectively; k m and kREc=rate constants of photoinhibition and concurrent recovery, respectively; LHCII = light-harvesting chlorophyll a/b-protein of PSII; PFD =photon flux density Correspondence to: E. Rintamfiki; FAX: 358(21)6335549 lakoid membranes of angiosperms and conifers but not in lower plants. The postulated regulation mechanism of Dl-protein degradation involving phosphorylation and the role of thylakoid organization in the function of PSII repair cycle are discussed.

Abstract. Susceptibility of a moss, Ceratodon purpureus (Hedw.) Brid., to photoinhibition and subsequent recovery of the photochemical efficiency of PSII was studied in the presence and absence of the chloroplast-encoded protein-synthesis inhibitor lincomycin. Ceratodon had a good capacity for repairing the damage to PSII centers induced by strong light. Tolerance against photoinhibition was associated with rapid turnover of the D1 protein, since blocking of D 1 protein synthesis more than doubled the photoinhibition rate measured as the decline in the ratio of variable fluorescence to maximal fluorescence (Fv/Fmax). Under exposure to strong light in the absence of lincomycin a net loss of D1 protein occurred, indicating that the degradation of damaged D 1 protein in Ceratodon was rapid and independent of the resynthesis of the polypeptide. The result suggests that synthesis is the limiting factor in the turnover of D1 protein during photoinhibition of the moss Ceratodon. The level of initial fluorescence (Fo) correlated with the production of inactive PSII centers depleted of D1 protein. The higher the F o level, the more severe was the loss of D1 protein seen in the samples during photoinhibition. Restoration of Fv/Fma x at recovery light consisted of a fast and slow phase. The recovery of fluorescence yield in the presence of lincomycin, which was added at different times in the recovery, indicated that the chloroplast-encoded proteinsynthesis-deper~dent repair of damaged PSII centers took place during the fast phase of recovery. Pulse-labelling experiments with [35S]methionine supported the conclusion drawn from fluorescence measurements, since the rate of D1 protein synthesis after photoinhibition exceeded that of the control plants during the first hours under recovery conditions. Abbreviations: DTT = dithiothreitol; F o, Fmax, and Fv : initial, maximal and variable fluorescence, respectively; LHCII = lightharvesting chlorophyll a/b-protein of PSII; PFD = photon flux density Correspondence to: E. Rintam~iki; FAX: 358 (21) 6335549

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