Influence of ozone stress on soybean response to carbon dioxide enrichment: III. Yield and seed quality

Ozone in the troposphere can cause plant stress, whereas elevated CO2 generally causes positive responses. Little is known of how these gases interact to affect plant response. Interactive effects on yield and seed quality of soybean [Glycine max (L.) Merr.] grown in 14-L pots were measured in open-top field chambers. Essex was tested in 1993, and Essex, Holladay, and NK 6955 were tested in 1994. Plants were exposed from emergence to maturity to four CO2 levels (ambient and 1.3, 1.6, and 2.0 times ambient) and three O-3 levels (0.4, 0.9, and 1.5 times ambient) in 12 combinations. Increasing O-3 suppressed growth and yield, whereas CO2 enrichment stimulated growth and yield. Carbon dioxide-induced stimulation was greater for plants stressed by O-3 than for non stressed plants. For example, CO2 at 2.0 times ambient increased 2-yr mean seed yield of Essex by 16, 24, and 81% at O-3 levels of O.4, 0.9, and 1.5 times ambient, respectively. Effects of O-3 and CO2 on seed oil content were variable with numerous cultivar differences. Seed protein content was never affected. Elevated O-3 suppressed oleic acid content in seeds, whereas CO2 increased it; the nature of the O-3 x CO2 interaction for oleic acid was similar to that observed for most yield measures. Carbon dioxide-induced stimulation of plants stressed by O-3 was apparently caused partly by amelioration of O-3 stress. Interactions between O-3 and CO2 must be considered for proper interpretation of cause-effect relationships in CO2 enrichment studies.