Influence of ozone stress on soybean response to carbon dioxide enrichment: I. Foliar properties

Tropospheric O-3 can cause foliar injury, decreased growth, and decreased yield, whereas CO2 enrichment generally causes opposite effects. Little is known about plant response to mixtures of O-3 and CO2. Open-top field chambers were ere used to determine if foliar responses of soybean [Glycine max (L.) Merr.] to CO2 enrichment are affected by O-3 stress and vice versa. Plants were grown in 14-L pots and exposed to four CO2 and three O-3 concentrations in 12 combinations. The CO2 treatments were ambient (366 mu L L-1) and three treatments with CO2 added for 24 h d(-1) at approximately 1.3, 1.6, and 2.0 times ambient. The O-3 treatments were charcoal-filtered air (CF), nonfiltered air (NF), and NF with O-3 added for 12 h d(-1) (NF+), resulting in seasonal concentrations of approximately 20, 46, and 75 nL L-1. Foliar effects of CO2 enrichment were dependent on the amount of stress caused by O-3 In the CF treatment, plants were not stressed by O-3, and CO2 enrichment caused chlorosis and decreased chlorophyll. In the IVF and NF+ treatments, plants were stressed by O-3, and CO2 enrichment suppressed chlorosis and increased chlorophyll. Ozone decreased specific leaf weight, increased foliar N and C, and decreased C/N ratios, whereas CO2 caused opposite responses for these measures. Ozone increased foliar S and B but did not affect P or K concentrations. Conversely, CO2 enrichment suppressed foliar S, B, P, and K concentrations. These interactions between O-3 and CO2 emphasize a need to consider the amount of plant stress caused by O-3 in studies to measure effects of CO2 enrichment.