Long-term growth of soybean at elevated [CO₂] does not cause acclimation of stomatal conductance under fully open-air conditions

Accurately predicting plant function and global biogeochemical cycles later in this century will be complicated if stomatal conductance (g_s) acclimates to growth at elevated [CO₂], in the sense of a long-term alteration of the response of g_s to [CO₂], humidity (h) and/or photosynthetic rate (A). If so, photosynthetic and stomatal models will require parameterization at each growth [CO₂] of interest. Photosynthetic acclimation to long-term growth at elevated [CO₂] occurs frequently. Acclimation of g_s has rarely been examined, even though stomatal density commonly changes with growth [CO₂]. Soybean was grown under field conditions at ambient [CO₂] (378 μmol mol^-1) and elevated [CO₂] (552 μmol mol^-1) using free-air [CO₂] enrichment (FACE). This study tested for stomatal acclimation by parameterizing and validating the widely used Ball et al. model (1987, Progress in Photosynthesis Research, vol IV, 221-224) with measurements of leaf gas exchange. The dependence of g_s on A, h and [CO₂] at the leaf surface was unaltered by long-term growth at elevated [CO₂]. This suggests that the commonly observed decrease in g_s under elevated [CO₂] is due entirely to the direct instantaneous effect of [CO₂] on g_s and that there is no longer-term acclimation of g_s independent of photosynthetic acclimation. The model accurately predicted g_s for soybean growing under ambient and elevated [CO₂] in the field. Model parameters under ambient and elevated [CO₂] were indistinguishable, demonstrating that stomatal function under ambient and elevated [CO₂] could be modelled without the need for parameterization at each growth [CO₂].