Response of Soybean to Air Temperature and Carbon Dioxide Concentration

Documented increases in global atmospheric CO₂ concentration have stimulated interest in the direct effects of CO₂ on plant growth and yield as well as the interactive effects of CO₂ with other major climatic variables. This study was conducted to determine the effects and interactions of CO₂ concentration and air temperature on the development, growth, total nonstructural carbohydrate (TNC), and final seed yield of soybean [Glycine max (L.) Merr., cv. Bragg] grown season-long in naturally lit, controlled-environment chambers. Day/night air temperatures of 26/19, 31/24, and 36/29 °C were maintained in CO₂ treatments of 330 and 660 μmol CO₂ mol⁻¹ air. Both CO₂ enrichment and increasing air temperature decreased main stem plastochron interval, while increasing air temperature increased final main stem node number. Leaf area and above-ground biomass increased with CO₂ enrichment and with temperature from 26/19 °C to 31/24 °C. The nonlinear increase with temperature in leaf area, aboveground biomass, and plastochron interval was attributed to the highest temperature treatment being near or above the optimum for soybean growth and development. Seed yield increased with CO₂ enrichment due mainly to an increase in seed number rather than weight per seed. Individual seed weight decreased, while seed number increased with increasing temperature. Leaflet TNC displayed large diurnal variations, while stem TNC was relatively stable throughout the day. Stem TNC was less affected by CO₂ than by temperature treatment and decreased with increasing temperature. These results indicate that the response of soybean to elevated CO₂ concentration is highly temperature dependent. Contribution by the Inst. of Food and Agricultural Sciences, Univ. of Florida, and the USDA-ARS. Supported in part by the U.S. Dep. of Energy, Carbon Dioxide Res. Div., Interagency Agreement no. DE-AI01-81ER60001, IFAS Journal Series no. 8840.