Elevated CO2 Shifts Photosynthetic Constraint from Stomatal to Biochemical Limitations During Induction in Populus tomentosa and Eucalyptus robusta

The relative impacts of biochemical and stomatal limitations on photosynthesis during photosynthetic induction have been well studied for diverse plants under ambient CO₂ concentration (<i>C</i>_a). However, a knowledge gap remains regarding how the various photosynthetic components limit duction efficiency under elevated CO₂. In this study, we experimentally investigated the influence of elevated CO₂ (from 400 to 800 μmol mol^-1) on photosynthetic induction dynamics and its associated limitation components in two broadleaved tree species, <i>Populus tomentosa</i> and <i>Eucalyptus robusta</i>. The results show that elevated CO₂ increased the steady-state photosynthesis rate (<i>A</i>) and decreased stomatal conductance (<i>g</i>_s) and the maximum carboxylation rate (<i>V</i>_cmax) in both species. While <i>E. robusta</i> exhibited a decrease in the linear electron transport rate (<i>J</i>) and the fraction of open reaction centers in photosynthesis II (<i>q</i>_L), <i>P. tomentosa</i> showed a significant increase in non-photochemical quenching (NPQ). With respect to non-steady-state photosynthesis, elevated CO₂ significantly reduced the induction time of <i>A</i> following a shift from low to high light intensity in both species. Time-integrated limitation analysis during induction revealed that elevated CO₂ reduces the relative impacts of stomatal limitations in both species, consequently shifting the predominant limitation on induction efficiency from stomatal to biochemical components. Additionally, species-specific changes in <i>q</i>_L and NPQ suggest that elevated CO₂ may increase biochemical limitation by affecting energy allocation between carbon fixation and photoprotection. These findings suggest that, in a future CO₂-rich atmosphere, plants productivity under fluctuating light may be primarily constrained by photochemical and non-photochemical quenching.