The role of ozone flux and antioxidants in the suppression of ozone injury by elevated CO2 in soybean

The projected rise in atmospheric CO2 concentration is expected to increase growth and yield of many agricultural crops. The magnitude of this stimulus will partly depend on interactions with other components of the atmosphere such as tropospheric O-3. Elevated CO2 concentrations often lessen the deleterious effects of O-3, but the mechanisms responsible for this response have received little direct examination. Previous studies have indicated that protection against O-3 injury by elevated CO2 can be attributed to reduced O-3 uptake, while other studies suggest that CO2 effects on anti-oxidant metabolism might also be involved. The aim of this experiment was to test further the roles of O-3 flux and antioxidant metabolism in the suppression of O-3 injury by elevated CO2. In a two-year experiment, soybean [Glycine max (L.) Merr.] was exposed from emergence to maturity to charcoal-filtered air or charcoal-filtered air plus a range of O-3 concentrations in combination with ambient or approximately twice-ambient CO2 concentrations in open-top field chambers. Experimental manipulation of O-3 concentrations and estimates of plant O-3 uptake indicated that equivalent O-3 fluxes that suppressed net photosynthesis, growth, and yield at ambient concentrations of CO2 were generally much less detrimental to plants treated concurrently with elevated CO2. These responses appeared unrelated to treatment effects on superoxide dismutase, glutathione reductase, and peroxidase activities and glutathione concentration. Total ascorbic acid concentration increased by 28-72% in lower canopy leaves in response to elevated CO2 and O-3 but not in upper canopy leaves. Increasing concentrations of atmospheric CO2 will likely ameliorate O-3 damage to many crops due to reduced O-3 uptake, increased carbon assimilation, and possibly as yet undetermined additional factors. The results of this study further suggest that elevated CO2 may increase the threshold O-3 flux for biomass and yield loss in soybean.