Phenotypic Plasticity Conditions the Response of Soybean Seed Yield to Elevated Atmospheric CO₂ Concentration

Selection for cultivars with superior responsiveness to elevated atmospheric CO₂ concentrations (eCO₂) is a powerful option for boosting crop productivity under future eCO₂. However, neither criteria for eCO₂ responsiveness nor prescreening methods have been established. The purpose of this study was to identify traits responsible for eCO₂ responsiveness of soybean (Glycine max). We grew 12 Japanese and U.S. soybean cultivars that differed in their maturity group and determinacy under ambient CO₂ and eCO₂ for 2 years in temperature gradient chambers. CO₂ elevation significantly increased seed yield per plant, and the magnitude varied widely among the cultivars (from 0% to 62%). The yield increase was best explained by increased aboveground biomass and pod number per plant. These results suggest that the plasticity of pod production under eCO₂ results from biomass enhancement, and would therefore be a key factor in the yield response to eCO₂, a resource-rich environment. To test this hypothesis, we grew the same cultivars at low planting density, a resource-rich environment that improved the light and nutrient supplies by minimizing competition. Low planting density significantly increased seed yield per plant, and the magnitude ranged from 5% to 105% among the cultivars owing to increased biomass and pod number per plant. The yield increase due to low-density planting was significantly positively correlated with the eCO₂ response in both years. These results confirm our hypothesis and suggest that high plasticity of biomass and pod production at a low planting density reveals suitable parameters for breeding to maximize soybean yield under eCO₂.