Soybean Canopy Growth, Photosynthesis, and Transpiration Responses to Whole‐Season Carbon Dioxide Enrichment1

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Abstract
Soybeans [Glycine max (L.) Merr. cv. Bragg] were grown throughout their life cycle in six controlled‐environment chambers that maintained CO2 concentrations at 330,450,600, and 800 µmol CO2 mol−1 air. Air temperatures were controlled, and solar radiation and dewpoint temperatures were measured continually for each chamber. Net photosynthesis and transpiration were computed on 5‐min intervals. The CO2 enrichment had no apparent effect on the rate of mainstem node development, but leaf area increased at a faster rate proportional to the CO2 concentration. Daytime CO2 exchange rate (CER) increased with increasing CO2 and followed the same trend as final yield. Midday CER was typically about 90 µmol CO2 m−1 of ground area s−1 for 800 µmol mol−1 and 60 µmol CO2 m−2 ground area per second for 330 µmol mol−1 chambers. Canopy transpiration rates, leaf area index and vapor pressure deficits were used to compare bulk canopy resistances among CO2 treatments. Bulk canopy resistance (stomatal and boundary‐layer) of the 800 µmol mol−1 treatment was about 1.6 times that of the 330 µmol mol−1 CO2 treatment. These results imply that the expected continued increase in global atmospheric CO2 will increase both agricultural productivity and water use efficiency.