Elevated CO₂ significantly delays reproductive development of soybean under Free-Air Concentration Enrichment (FACE)

The effect of rising atmospheric concentration of carbon dioxide [CO2] on the reproductive development of soybean (Glycine max. Merr) has not been evaluated under open-air field conditions. Soybeans grown under Free-Air CO2 Enrichment (FACE) exhibit warmer canopies due to decreased latent heat loss because of decreased stomatal conductance. According to development models based on accumulated thermal time, or growing degree days (°Cd), increased canopy temperature should accelerate development. The SoyFACE research facility (Champaign, Illinois, USA) was used to test the hypothesis that development is accelerated in soybean when grown in [CO2] elevated to 548 μmol mol ^-1. Canopy temperature was measured continuously with infrared thermometry, and used in turn to calculate GDD. Opposite to expectation, elevated [CO2], while increasing canopy temperature, delayed reproductive development by up to 3 days (P <0.05). Soybean grown in elevated [CO2] required ∼49 °Cd more GDD (P <0.05) to complete full bloom stage (R2) and ∼52 °Cd more GDD (P <0.05) to complete the beginning seed (R5) stage, but needed ∼46 °Cd fewer GDD (P <0.05) to complete seed filling (R6). Soybeans grown in elevated [CO2] produced significantly more nodes (P <0.01) on the main stem than those grown under current [CO2]. This may explain the delay in completion of reproductive development and final maturation of the crop under elevated [CO2]. These results show a direct effect of rising [CO2] on plant development that will affect both projections of grain supply and may be significant to other species including those in natural communities. The Author [2009]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved.