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.

The Ilomantsi greenstone belt is a Neoarchaean, c. 2.75–2.70 Ga volcanic–sedimentary complex in which metamorphic grade increases from staurolite grade in the SW of the belt to sillimanite grade in the NE. In the staurolite zone, prograde garnet zoning indicates pressure and temperature increases from 480–500°C at 2–4 kbar to 560–570°C at 6–7 kbar. Within the sillimanite zone temperatures peaked at 660–670°C at pressures of around 6 kbar. The U–Pb age determinations on monazite from the sillimanite zone yielded both Archaean and Proterozoic ages. One sample contains an exclusively Archaean monazite population of 2620±24 Ma, while another sample has two generations of monazite, with ages of 2664±33 Ma and 1837±13 Ma. The monazite data confirm that the Ilomantsi greenstone belt was metamorphosed simultaneously with the surrounding Neoarchaean migmatite complexes. The apparent clockwise PT path and medium P/T-type metamorphism are consistent with collisional tectonic settings, but the two distinct metamorphic events recorded by monazite indicate that a second, Palaeoproterozoic thermal event caused recrystallization and new mineral growth, in line with previous evidence from other isotopic systems. Accordingly, great care is necessary in defining metamorphic evolutionary P–T–t paths in rocks with complex mineral assemblages, to ensure correct identification of truly coeval mineral assemblages.

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