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 diffe rentially 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 M ichx., Quercus rubra L.; two C-3 grasses: Agropyron smithii Rybd., Koe leria cristata L.; two C-4 grasses: Bouteloua curtipendula Michx., Sch izachyrium scoparium Michx.) differed in growth form, stomatal conduct ance and photosynthetic pathway. In situ photosynthesis, relative grow th rate (RGR) and its determinants (leaf area ratio, specific leaf are a, leaf weight ratio and root weight ratio) were determined via sequen tial harvests of seedlings that were grown in all combinations of 366 or 672 mu mol mol(-1) CO2 and 3 or 95 nmol mol(-1) O-3 over a 101-d pe riod. Elevated CO2 had minimal effect on either photosynthesis or RGR. By contrast, RGR for all six species was lower in high O-3 concentrat ions at ambient CO2, significantly so in A. smithii and P. tremuloides . Five of the six species also exhibited reductions in in situ photosy nthesis at ambient CO2 in high-O-3-grown compared with low-O-3-grown p lants. For all species, these O-3-induced reductions in RGR and photos ynthesis were absent in the elevated CO2 environment. Root weight rati o was significantly reduced by elevated O-3 in A. smithii and P. tremu loides in ambient but not elevated CO2. Species with high stomatal con ductance 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 CO2 will redu ce stomatal conductance and O-3 uptake, and might therefore reduce the potential for oxidant damage. However, there was a stronger relations hip of the percent reduction in whole-plant mass due to O-3, related t o the ratio of photosynthesis to stomatal conductance. In general, res ults of this study of six functionally diverse plant species suggest t hat O-3 pollution effects on carbon balance and growth are likely to b e ameliorated by elevated concentrations of atmospheric CO2.