Sensitivity of field-grown soybean to future atmospheric CO2: selection for improved productivity in the 21st century

Although genotypic differences among soybean (Glycine max (L.) Merr.) culti vars in their response to future CO2 partial pressures have been observed i n the glasshouse, it is unclear if similar responses would occur among cult ivars when grown under field conditions at normal stand densities. To deter mine variation in the sensitivity of soybean growth and seed yield to CO2, we grew two contrasting cultivars of the same maturity group, Ripley (semi- dwarf, determinate) and Spencer (standard, indeterminate), to reproductive maturity at ambient and elevated (30 Pa above ambient) CO2 partial pressur es for two field seasons. Spencer had been previously selected in glasshous e trials as responsive to increased CO2. Significant cultivar x CO2 interac tion was observed for both vegetative biomass and seed yield, with Spencer demonstrating a consistently greater yield enhancement at elevated CO2 than Ripley (60 vs 35%, respectively). Differences in CO2 sensitivity between c ultivars were not evident in measurements of single leaf photosynthesis tak en during anthesis, nor early or late pod-fill. Analysis of reproductive ch aracteristics indicated that the sensitivity of the seed yield response to CO2 in Spencer was associated with the ability to form additional seed on a xillary branches in response to elevated CO2. Data from this experiment sug gest that screening of soybean germplasm at the glasshouse level, when comb ined with field trials, may be an effective strategy to begin selecting soy bean lines that will maximize yield in a future, higher CO2 environment.