Coordinated responses of plant hydraulic architecture with the reduction of stomatal conductance under elevated CO2 concentration.

Stomatal conductance (gs) generally decreases under elevated CO2 concentration (eCO2) and its sensitivity varies widely among species, yet the underlying mechanisms for these observed patterns are not totally clear. Understanding these underlying mechanisms, however, is critical for addressing problems regarding plant-environment interactions in a changing climate. We examined gs, water transport efficiency of different components along the whole-plant hydraulic system and allometric scaling in seedlings of six tree species grown under ambient and eCO2 treatments (400 and 600 ppm, respectively). Growth under eCO2 caused gs to decrease in all species but to highly variable extents, ranging from 13% (Populus tremuloides Michx.) to 46% (Gymnocladus dioicus (L.)). Accompanying this significant decrease in gs, substantial changes in plant hydraulic architecture occurred, with root hydraulic conductance expressed both on leaf area and root mass bases overall exhibiting significant decreases, while stem and leaf hydraulic efficiency either increased or showed no consistent pattern of change. Moreover, significant changes in allometry in response to eCO2 affected the whole-plant water supply and demand relations. The interspecific variation in gs response among species was not correlated with relative changes in stem and leaf hydraulic conductance but was most strongly correlated with the relative change in the allometric scaling between roots and leaves, and to a lesser extent with the intrinsic root hydraulic conductance of the species. The results underscore that allometric adjustments between root and leaf play a key role in determining the interspecific sensitivity of gs responses to eCO2. Plant hydraulics and their associated allometric scaling are important changes accompanying gs responses to eCO2 and may play important roles in mediating the interspecific variations of leaf gas exchange responses, which suggests that mechanistic investigations regarding plant responses to eCO2 need to integrate characteristics of hydraulics and allometric scaling in the future.