Elevated CO2 reduces O-3 flux and O-3-induced yield losses in soybeans: Possible implications for elevated CO2 studies

Soybeans were grown for three seasons in open-top field chambers to determine (1) whether elevated CO2 (360 Versus 700 mu mol mol(-1)) alleviates some of the yield loss due to pollutant O-3, (2) whether the partial stomatal closure resulting froth chronic O-3 exposure (charcoal-filtered air versus 1.5 x ambient concentrations) is a cause or result of decreased photosynthesis, and (3) possible implications of CO2/O-3 interactions to climate change studies using elevated CO2. Leaf conductance was reduced by elevated CO2, regardless of O-3 level, or by exposure to O-3 alone. AS a result of these effects on conductance, high CO2 reduced estimated midday O-3 flux into the leaf by an average of 50% in charcoal-filtered air and 35% in the high O-3 treatment. However, while exposure to O-3 reduced seed yields by 41% at ambient CO2 levels, the yield reduction was completely ameliorated by elevated CO2. The threshold midday O-3 flux for yield loss appears to be 20-30 nmol m(-2) s(-1) in this study. Although elevated CO2 increased total biomass production, it did not increase seed yields. A/C-i curves show a large reduction in the stomatal limitation to photosynthesis due to elevated CO2, but no effect of O-3. These data demonstrate that (1) reduced conductance due to O-3 is the result, and not the cause, of reduced photosynthesis, (2) 700 mu mol mol(-1) CO2 can completely ameliorate yield losses due to O-3 within the limits of these experiments, and (3) some reports of increased yields under elevated CO2 treatments may, at least in part, reflect the amelioration of unrecognized suppression of yield by O-3 or other stresses.