Photosynthesis and Transpiration Responses of Soybean Canopies to Short‐and Long‐Term CO 2 Treatments 1

The purpose of this investigation was to determine whether long‐term adaptation to CO 2 pretreatment affects canopy response to short‐term CO 2 treatments. Soybeans [ Glycine max (L.) Merr. cv. Bragg] were grown in outdoor controlled environment chambers from seedling onwards at 330 and 800 μmol CO 2 per mole air. After 68 days, the CO 2 treatment levels were temporarily reversed for 5 days. Before the step change in CO 2 concentrations, CER at high light in elevated CO 2 treatments was 85% greater than in ambient treatments. After the reversal, canopies in the newly enriched chambers had CO 2 fixation rates 75% greater than in the chambers that were switched to ambient CO 2 concentrations. Furthermore, the CER responses to PAR were similar whether the canopy was acclimated or newly exposed to the particular CO 2 level. Before treatment reversal, transpiration rates were essentially equivalent in the two treatments. Stomatal resistances of leaves were higher in elevated CO 2 , but the acclimated enriched canopies had developed additional leaf area resulting in similar total canopy resistances to transpiration. After reversal, absolute water use declined by 18% in the canopies with CO 2 increased from 330 to 800 μmol mol −1 and increased by 36% in the canopies with CO 2 decreased from 800 to 330 μmol mol −1 . It was concluded that short‐term exposure to different CO 2 treatments adequately estimated long‐term canopy CER response to CO 2 level, but did not adequately predict long‐term canopy transpiration response.