Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO2 and/or O-3

Leaf gas exchange parameters and the content of ribulose-1,5-bisphosphate c arboxylase/oxygenase (Rubisco) in the leaves of two 2-year-old aspen (Popul us tremuloides Michx.) clones (no. 216, ozone tolerant and no. 259, ozone s ensitive) were determined to estimate the relative stomatal and mesophyll l imitations to photosynthesis and to determine how these limitations were al tered by exposure to elevated CO2 and/or O-3. The plants were exposed eithe r 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 photos ynthesis and stomatal conductance were measured in all leaves of the curren t terminal and of two lateral branches (one from the upper and one from the lower canopy) to detect possible age-related variation in relative stomata l limitation (leaf age is described as a function of leaf plastochron index ). Photosynthesis was increased by elevated CO2 and decreased by O-3 at bot h control and elevated CO2. The relative stomatal limitation to photosynthe sis (l(s)) was in both clones about 10% under control and elevated O-3. Exp osure 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 R ubisco 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 trigge red by altered mesophyll processes in the two aspen clones of contrasting O -3 tolerance. The changes in stomatal conductance seem to be a secondary re sponse, maintaining stable C-i under the given treatment, that indicates cl ose coupling between stomatal and mesophyll processes.