Elevated CO ₂ differentially affects photosynthetic induction response in two Populus species with different stomatal behavior

To understand dynamic photosynthetic characteristics in response to fluctuating light under a high CO ₂ environment, we examined photosynthetic induction in two poplar genotypes from two species, Populus koreana × trichocarpa cv. Peace and Populus euramericana cv. I-55, respectively. Stomata of cv. Peace barely respond to changes in photosynthetic photon flux density (PFD), whereas those of cv. I-55 show a normal response to variations in PFD at ambient CO ₂. The plants were grown under three CO ₂ regimes (380, 700, and 1,020 μmol CO ₂ mol ^-1 in air) for approximately 2 months. CO ₂ gas exchange was measured in situ in the three CO ₂ regimes under a sudden PFD increase from 20 to 800 μmol m ^-2 s ^-1. In both genotypes, plants grown under higher CO ₂ conditions had a higher photosynthetic induction state, shorter induction time, and reduced induction limitation to photosynthetic carbon gain. Plants of cv. I-55 showed a much larger increase in induction state and decrease in induction time under high CO ₂ regimes than did plants of cv. Peace. These showed that, throughout the whole induction process, genotype cv. I-55 had a much smaller reduction of leaf carbon gain under the two high CO ₂ regimes than under the ambient CO ₂ regime, while the high CO ₂ effect was smaller in genotype cv. Peace. The results suggest that a high CO ₂ environment can reduce both biochemical and stomatal limitations of leaf carbon gain during the photosynthetic induction process, and that a rapid stomatal response can further enhance the high CO ₂ effect.