LONG-TERM EXPOSURE OF NORWAY SPRUCE TO ELEVATED CO2 CONCENTRATION INDUCES CHANGES IN PHOTOSYSTEM-II MIMICKING AN ADAPTATION TO INCREASED IRRADIANCE

Fifteen-year-old Norway spruces (Picea abies [L.] Karst.) were grown i n open top chambers (OTC) at ambient (A) and elevated (i.e. ambient 350 mu mol(CO2)mol(-1)) concentrations of CO2 (E) for four growing sea sons (1992-1995). During this time period several examples of the depr ession of photosynthetic activities were observed for E needles. In or der to better characterize the nature of this depression the gas excha nge and fluorescence parameters were analyzed on current year needles during the last season (July 1995). The photon flux density response c urves of CO2 uptake (P-N) revealed a significantly reduced stimulation of P-N for E needles as compared with short-term exposure to doubled CO2. Moreover, the sudden exposure of E shoots to 350 mu mol(CO2)mol(- 1) at saturating irradiance revealed a depression of both P-Nmax (by 2 0 %) and quantum yield of PS II (by 32 %) compared with A shoots measu red at 350 mu mol(CO2)mol(-1). The data supporting the diminished ligh t harvesting system of photosystem II (PS II) in E shoots compared wit h A shoots were obtained from pigment analysis, low temperature fluore scence spectra and Chl a fluorescence induction kinetics. The relative proportion of inactive reaction centres of PS II determined from F-pl of the fluorescence induction was 20 % higher for E needles. These ch anges found for E needles mimicked an adaptation of PS II to increased irradiance compared with A needles. As the irradiance exposure was th e same for the examined needles from both E and A spruces we suggest t hat these changes reported for E needles resulted from the feed-back l imitation of photochemical reactions due to suppressed electron transp ort through the plastoquinone pool.