Photosynthetic acclimation to elevated atmospheric carbon dioxide and UV irradiation in Pinus banksiana

Pinus banksiana seedlings were grown for 9 months in enclosures in greenhouses at CO₂ concentrations of 350 or 750 μmol mol⁻¹ with either low (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 treatment but was unaffected by CO₂ enrichment. High UV treatment also shifted 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 μmol mol⁻¹ CO₂ due to a UV induced increase in ribulose-1, 5-bisphosphate carboxylase/oxygenase efficiency and ribulose-1, 5-bisphosphate regeneration. Stomatal density was increased by high UV irradiance but was unchanged by CO₂ enrichment.