RESPONSE OF PHOTOSYNTHETIC CARBON ASSIMILATION OF NORWAY SPRUCE EXPOSED TO LONG-TERM ELEVATION OF CO2 CONCENTRATION

Young (12 years old) Norway spruce (Picea abies [L.] Karst.) trees wer e exposed to ambient CO2 or ambient + 350 mu mol(CO2) mol(-1) continuo usly over 2 growing seasons in open-top chambers, under field conditio ns of a mountain stand. Comprehesive analysis of CO2 assimilation was performed after 4 and 22 weeks of the second growing season to evaluat e the influence of elevated atmospheric CO2. A combination of gas exch ange and a mathematical mo del of ribulose-1,5-bisphosphate carboxylas e/oxygenase (RuBPCO) activity was used. After 4 weeks of exposure no s tatistically significant stimulation of the radiant energy and CO2 sat urated rate of CO2 uptake (P-Nsat) by the elevated CO2 concentration w as found. Yet after 24 weeks a statistically significant depression of P-Nsat (38 %) and carboxylation efficiency (32 %) was observed. Depre ssion of photosynthetic activity by elevated CO2 resulted from a decre ase in the RuBPCO carboxylation rate. The electron transport rate was also modified similarly to the rate of RuBP formation. An accompanying decrease in nitrogen content of the needles (by 12 %) together with a n increase in total saccharides (by 34 %) was observed after 24 weeks of exposure to enhanced CO2.