Photosynthetic responses of aspen clones to simultaneous exposures of ozone and CO2

Current projections indicate steady increases in both trophospheric ozone and carbon dioxide well into the next century with concurrent increases in plant stress. Because information about effects of these interacting stresses on forest trees is limited, we have conducted ozone and carbon dioxide experiments using ozone-tolerant and ozone-sensitive trembling aspen (Populus tremuloides Michx.) clones (clones 216 and 259, respectively). Aspen plants were grown either in pots (square-wave study) or in the ground (episodic study) in open-top chambers. Plants in the square-wave study were exposed for a single growing season to charcoal-filtered air (CF) or to CF plus elevated carbon dioxide (CO2), ozone (O-3), or O-3 plus CO2 (O-3 + CO2). Plants in the episodic study were exposed for three growing seasons to CF, twice simulated ambient (2x) O-3 (2x O-3), or 2x O-3 plus CO2 (2x O-3 + CO2). Photosynthetic measurements were made either in the open-top chambers at treatment CO2 concentrations or in controlled-environment cuvettes with various CO2 concentrations, producing assimilation versus intercellular CO2 concentration (A/C-i) curves. Ozone decreased photosynthetic rate and stomatal conductance and accelerated leaf senescence. Elevated CO2 increased photosynthetic rate and decreased stomatal conductance when measured at treatment CO2 concentrations, and exacerbated the negative effect of O-3 on photosynthesis. For example, for clone 259, photosynthesis decreased 9% for the O-3 treatment compared with the CF treatment, but decreased 24% for the O-3 + CO2 treatment compared with the CO2 treatment. Similar decreases for clone 216 of 2% and 6% for O-3 and O-3 + CO2, respectively, were not significant. A/C-i curves showed that O-3 decreased carboxylation efficiency and maximum photosynthetic rate and that photosynthetic inhibition in response to O-3 was greater with elevated CO2. The simultaneous declines in all factors of photosynthetic gas exchange measurements suggest that the equilibrium between stomatal conductance, carboxylation, and light harvesting systems was not disrupted by O-3 and O-3 x CO2 interactions. Carbon dioxide did not ameliorate the detrimental effects of O-3 on the leaf photosynthetic apparatus. In fact, the O-3-tolerant clone appeared more sensitive to O-3 with elevated CO2.