Reference
Noormets, A., Sober, A., Pell, E.J., Dickson, R.E., Podila, G.K., Sober, J., Isebrands, J.G. and Karnosky, D.F. 2001. Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO2 and O3. Plant, Cell and Environment 24: 327-336.
What was done
The authors grew O3-sensitive and O3-tolerant aspen (Populus tremuloides) clones for approximately six months in 30-m diameter FACE plots located in Wisconsin, USA, at atmospheric CO2 concentrations of 360 and 560 ppm with and without exposure to elevated O3 (1.5 times ambient ozone concentration) to study the interactive effects of these parameters on photosynthesis in this deciduous tree species.
What was learned
Elevated CO2 increased rates of photosynthesis in both clones at all leaf positions. Maximum rates of photosynthesis were increased in the O3-tolerant clones by an average of 33 and 49% due to elevated CO2 alone and in combination with elevated O3, respectively. In comparison, maximum rates of photosynthesis in the O3-sensitive clones were increased by 38% due to elevated CO2 alone and in combination with elevated O3. These results indicate that CO2-induced increases in maximal rates of net photosynthesis were at least maintained and sometimes even increased during simultaneous exposure to elevated O3 concentrations, which often cause reductions in photosynthetic activity.
What it means
As the tropospheric ozone concentration continues to rise, it will likely impose an ever greater stress upon plants that physically damages their tissues and negatively impacts their photosynthetic rates and other physiological processes. Fortunately, because the air's CO2 content is also on a rising trajectory, it should provide plants, including trembling aspen seedlings, with an increasing degree of protection from this harmful aerial pollutant. Indeed, CO2-induced increases in maximal photosynthetic rates were not decreased by elevated O3 concentrations, even in an O3-sensitive clone! Thus, as rising atmospheric CO2 concentrations increase photosynthetic rates in these aspen clones, it is likely the extra CO2 will increase their carbon sequestering capacities by enabling them to produce greater amounts of biomass with the additional carbon they remove from the atmosphere, even in the face of increasing ozone pollution.