Increased leaf area compensated photosynthetic downregulation in response to elevated CO2 and warming in white birch

Predicting photosynthetic acclimation to elevated CO2 and warming is difficult because they have opposite effects. We investigated physiological and morphological responses in white birch (Betula papyrifera Marshall) to a combination of CO2 and temperature (ACT -- 400 mu mol.mol(-1) CO2, current temperature; ECT -- 750 mu mol.mol(-1) CO2, current + 4 C temperature). ECT reduced photosynthesis, maximum Rubisco carboxylation (Vcmax), maximum electron transport rate (Jmax), photorespiration, daytime respiration, leaf N, and stomatal and mesophyll conductance, but increased biomass, height, total leaf area, electron partitioning to carboxylation and oxygenation ratio, and CO2 compensation point. The photosynthetic acclimation is consistent with the optimal carbon gain theory (carbon gain drives the coordination of carboxylation, electron transport, and respiration). While the photosynthetic acclimation was similar to acclimation to elevated CO2, ECT reduced Jmax/Vcmax, which is consistent with the response to warming but opposite to the response to elevated CO2, suggesting that thermal acclimation may be the primary mechanism of photosynthetic acclimation to ECT and ECT probably altered N allocation between machinery for carboxylation and that for ribulose-1,5-bisphosphate regeneration. The increase in total leaf area by ECT more than offset the negative effect of photosynthetic downregulation on carbon sequestration, resulting in faster growth and greater biomass under ECT.