Elevated CO2 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 CO2 environment, we examined photosynthetic induction in two poplar genotypes from two species, Populus koreana x 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 CO2. The plants were grown under three CO2 regimes (380, 700, and 1,020 mu mol CO2 mol(-1) in air) for approximately 2 months. CO2 gas exchange was measured in situ in the three CO2 regimes under a sudden PFD increase from 20 to 800 mu mol m(-2) s(-1). In both genotypes, plants grown under higher CO2 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 CO2 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 CO2 regimes than under the ambient CO2 regime, while the high CO2 effect was smaller in genotype cv. Peace. The results suggest that a high CO2 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 CO2 effect.