Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO2 and/or O3

Leaf gas exchange parameters and the content of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the leaves of two 2-year-old aspen (Populus tremuloides Michx.) clones (no. 216, ozone tolerant and no. 259, ozone sensitive) were determined to estimate the relative stomatal and mesophyll limitations to photosynthesis and to determine how these limitations were altered by exposure to elevated CO2 and/or O-3. The plants were exposed either to ambient air (control), elevated CO2 (560 p.p.m.) elevated O-3 (55 p.p.b.) or a mixture of elevated CO2 and O-3 in a free air CO2 enrichment (FACE) facility located near Rhinelander, Wisconsin, USA. Light-saturated photosynthesis and stomatal conductance were measured in all leaves of the current terminal and of two lateral branches (one from the upper and one from the lower canopy) to detect possible age-related variation in relative stomatal limitation (leaf age is described as a function of leaf plastochron index). Photosynthesis was increased by elevated CO2 and decreased by O-3 at both control and elevated CO2. The relative stomatal limitation to photosynthesis (l(s)) was in both clones about 10% under control and elevated O-3. Exposure to elevated CO2 + O-3 in both clones and to elevated CO2 in clone 259, decreased l(s) even further - to about 5%. The corresponding changes in Rubisco content and the stability of C-i/C-a ratio suggest that the changes in photosynthesis in response to elevated CO2 and O-3 were primarily triggered by altered mesophyll processes in the two aspen clones of contrasting O-3 tolerance. The changes in stomatal conductance seem to be a secondary response, maintaining stable C-i under the given treatment, that indicates close coupling between stomatal and mesophyll processes.