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.) cultivars in their response to future CO2 partial pressures have been observed in the glasshouse, it is unclear if similar responses would occur among cultivars when grown under field conditions at normal stand densities. To determine 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 pressures for two field seasons. Spencer had been previously selected in glasshouse trials as responsive to increased CO2. Significant cultivar x CO2 interaction 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 cultivars were not evident in measurements of single leaf photosynthesis taken during anthesis, nor early or late pod-fill. Analysis of reproductive characteristics indicated that the sensitivity of the seed yield response to CO2 in Spencer was associated with the ability to form additional seed on axillary branches in response to elevated CO2. Data from this experiment suggest that screening of soybean germplasm at the glasshouse level, when combined with field trials, may be an effective strategy to begin selecting soybean lines that will maximize yield in a future, higher CO2 environment.