PHOTOSYNTHETIC ACCLIMATION TO ELEVATED ATMOSPHERIC CARBON-DIOXIDE ANDUV IRRADIATION IN PINUS-BANKSIANA

Pinus banksiana seedlings were grown for 9 months in enclosures in gre enhouses at CO2 concentrations of 350 or 750 mumol mol-1 with either l ow (0.005 to 0.3 W M-2) or high (0.25 to 0.90 W M-2) ultraviolet-B (UV -B) irradiances. Total seedling dry weight decreased with high UV trea tment but was unaffected by CO2 enrichment. High UV treatment also shi fted biomass partitioning in favor of leaf production. Both CO2 and UV treatments decreased the dark respiration rate and light compensation point. High UV light inhibited photosynthesis at 350 but not at 750 m umol mol-1 CO2 due to a UV induced increase in ribulose-1,5-bisphospha te carboxylase/oxygenase efficiency and ribulose-1,5-bisphosphate rege neration. Stomatal density was increased by high UV irradiance but was unchanged by CO2 enrichment.