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 CO 2 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 CO 2 enrichment. High UV treatment also shifted biomass partitioning in favor of leaf production. Both CO 2 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 CO 2 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 CO 2 enrichment.