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 C 3 trees: Populus tremuloides Michx., Quercus rubra L.; two C 3 grasses: Agropyron smithii Rybd., Koeleria cristata L.; two C 4 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 CO 2 and 3 or 95 nmol mol −1 O 3 over a 101‐d period. Elevated CO 2 had minimal effect on either photosynthesis or RGR . By contrast, RGR for all six species was lower in high O 3 concentrations at ambient CO 2 , significantly so in A. smithii and P. tremuloides . Five of the six species also exhibited reductions in in situ photosynthesis at ambient CO 2 in high‐O 3 ‐grown compared with low‐O 3 ‐grown plants. For all species, these O 3 ‐induced reductions in RGR and photosynthesis were absent in the elevated CO 2 environment. Root weight ratio was significantly reduced by elevated O 3 in A. smithii and P. tremuloides in ambient but not elevated CO 2 . Species with high stomatal conductance were the most susceptible to oxidant injury, while those with low stomatal conductance, such as the C 4 species and Q. rubra , were not as detrimentally affected by O 3 . Elevated levels of CO 2 will reduce stomatal conductance and O 3 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 O 3 , related to the ratio of photosynthesis to stomatal conductance. In general, results of this study of six functionally diverse plant species suggest that O 3 pollution effects on carbon balance and growth are likely to be ameliorated by elevated concentrations of atmospheric CO 2 .