COMBINED EFFECTS OF IRRADIANCE AND FIRST AUTUMN FROST ON CO2 ASSIMILATION AND SELECTED PARAMETERS OF CHLOROPHYLL-A FLUORESCENCE IN NORWAY SPRUCE SHOOTS

The state of some parameters of photosynthetic activity in Norway spru ce (Picea abies [L.] Karst.) seedlings during the first autumn tempera ture transition to frost was monitored in October 1991. The trees were grown under field conditions of the Beskydy Mts. (North Moravia, The Czech Republic), Simultaneous measurements of Chl a fluorescence and C O2 gas exchange revealed two phases in the functional transition of th e assimilatory apparatus. Immediately upon the temperature transition to frost a distinct decrease in the radiant energy saturated rate of C O2 uptake was observed and radiationless dissipation was indicated by higher values of the nonphotochemical quenching coefficient. The secon d period of the transition, a period with prolonged influence of frost together with a higher level of irradiance, was connected with a decr ease of photosynthetic efficiency. The overwhelming capacity for prote ctive non-photochemical energy dissipation anti the complete reduction of acceptor Q(A) occurred especially at medium and high incident irra diance documenting photoinhibitory damage to the photosynthetic appara tus.