Elevated carbon dioxide ameliorates the effects of ozone on photosynthesis and growth: Species respond similarly regardless of photosynthetic pathway or plant functional group

Due to their different physiological effects, elevated carbon dioxide and elevated ozone might have interactive impacts on plants, and differentially so on plants differing in photosynthetic pathway and growth rate. To test several hypotheses related to these issues, we examined the physiological, morphological and growth responses of six perennial species grown at various atmospheric concentrations of carbon dioxide and ozone. The species involved (two C₃ trees: Populus tremuloides Michx., Quercus rubra L.; two C₃ grasses: Agropyron smithii Rybd., Koeleria cristata L.; two C₄ grasses: Bouteloua curtipendula Michx., Schizachyrium scoparium Michx.) differed in growth form, stomatal conductance and photosynthetic pathway. In situ photosynthesis, relative growth rate (RGR) and its determinants (leaf area ratio, specific leaf area, leaf weight ratio and root weight ratio) were determined via sequential harvests of seedlings that were grown in all combinations of 366 or 672 μmol mol^-1 CO₂ and 3 or 95 nmol mol^-1 O₃ over a 101-d period. Elevated CO₂ had minimal effect on either photosynthesis or RGR. By contrast, RGR for all six species was lower in high O₃ concentrations at ambient CO₂, significantly so in A. smithii and P. tremuloides. Five of the six species also exhibited reductions in in situ photosynthesis at ambient CO₂ in high-O₃-grown compared with low-O₃-grown plants. For all species, these O₃-induced reductions in sea and photosynthesis were absent in the elevated CO₂ environment. Root weight ratio was significantly reduced elevated O₃ in A. smithii and P. tremuloides in ambient but not elevated CO₂. Species with high stomatal conductance were the most susceptible to oxidant injury, while those with low stomatal conductance, such as the C₄ species and Q. rubra, were not as detrimentally affected O₃. Elevated levels of CO₂ will reduce stomatal conductance and O₃ uptake, and might therefore reduce the potential for oxidant damage. However, there was a stronger relationship of the percent reduction in whole-plant mass due to O₃, related to the ratio of photosynthesis to stomatal conductance. In general, results of this study of six functionally diverse plant species suggest that O₃ pollution effects on carbon balance and growth are likely to be ameliorated elevated concentrations of atmospheric CO₂.