RESPONSES OF PHOTOSYSTEM 2 PHOTOCHEMISTRY AND PIGMENT COMPOSITION IN NEEDLES OF NORWAY SPRUCE SAPLINGS TO INCREASED RADIATION LEVEL

To reveal the dynamics of short-term photosynthetic acclimation to inc reased irradiance, the light response of photochemical (q(P)), non-pho tochemical (q(N)) and F0 (q0) quenchings of chlorophyll (Chl) fluoresc ence and Chl and carotenoids composition in Norway spruce needles were monitored within three days after transfer of saplings from low diffu se irradiance (maximum photosynthetic photon fluence density PPFD 50 m umol m-2 s-1) to direct sun radiation (maximum PPFD 2000 mumol m-2 s-1 ). Irradiance responses of fluorescence quenching coefficients reveale d the occurrence of substantial changes in partitioning of excitation energy between photochemical reactions and radiationless dissipation w ithin two days. The saturating irradiance for q(N) and the capacity of non-radiative dissipation processes was shifted from about 450 mumol m-2 S-1 to 1620 mumol M-2 s-1. Whereas immediately after exposure to f ull sunlight Q(A) was completely reduced at 1620 mumol m-2 s-1, two da ys later 40% Of Q(A) was still present in oxidized form at this irradi ance. A fast pigment photobleaching at noon prevented the over-reducti on Of Q(A) and thus it was one of the possible short-term acclimation processes. No severe photoinhibition of photosystem 2 (PS 2) photochem istry occurred within the period of investigation as can be judged fro m the high F(v)/F(m) value.