Influence of ozone stress on soybean response to carbon dioxide enrichment: II. Biomass and development

Previous research has shown that elevated CO2 concentrations can increase plant growth, whereas the air pollutant O-3 is phytotoxic. Because elevated concentrations of these gases will co-occur, the objective of our experiment was to determine if estimates of plant growth response to future levels of CO2 and O-3 require experiments to test the gases in combination. Soybean plants [Glycine max (L.) Merr. cv. Essex) were exposed in open-top chambers to combinations of O-3 and CO2 from plant emergence through physiological maturity. Ozone treatments were charcoal-filtered air (CF), nonfiltered sir (NF), and NF with O-3 added for 12 h d(-1) (NF+) (seasonal mean 12 h d(-1) O-3 concentrations of 20, 50, or 79 nL L-1, respectively). Carbon dioxide exposures were for 24 h d(-1) giving seasonal mean 12 h d(-1) concentrations of 370, 482, 599, or 713 mu L L-1 Over the season, elevated CO2 usually stimulated growth and O-3 suppressed growth. Elevated CO2 usually increased partitioning of biomass to branches, decreased partitioning to pods, increased specific leaf weight, and decreased leaf area ratio. Ozone suppressed leaf and root weight ratios, increased pod weight ratios, and decreased specific leaf weight. Toward the end of the season, both O-3 and CO2 accelerated reproductive development. Elevated CO2 moderated suppression of growth by O-3, and the highest CO2 concentration completely ameliorated O-3 effects on main stem biomass, root biomass, and leaf area. Ozone, however, limited some positive growth responses to CO2, especially at less than a doubling of CO2 concentrations. These results indicate that in order to understand the future impacts of atmospheric gases such as elevated CO2 and O-3 On crop growth, their combined effects should be determined.