Photosynthesis and Transpiration Responses of Soybean Canopies to Short‐and Long‐Term CO2 Treatments1

Abstract

AbstractThe purpose of this investigation was to determine whether long‐term adaptation to CO2 pretreatment affects canopy response to short‐term CO2 treatments. Soybeans [Glycine max (L.) Merr. cv. Bragg] were grown in outdoor controlled environment chambers from seedling onwards at 330 and 800 μmol CO2 per mole air. After 68 days, the CO2 treatment levels were temporarily reversed for 5 days. Before the step change in CO2 concentrations, CER at high light in elevated CO2 treatments was 85% greater than in ambient treatments. After the reversal, canopies in the newly enriched chambers had CO2 fixation rates 75% greater than in the chambers that were switched to ambient CO2 concentrations. Furthermore, the CER responses to PAR were similar whether the canopy was acclimated or newly exposed to the particular CO2 level. Before treatment reversal, transpiration rates were essentially equivalent in the two treatments. Stomatal resistances of leaves were higher in elevated CO2, 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 CO2 increased from 330 to 800 μmol mol−1 and increased by 36% in the canopies with CO2 decreased from 800 to 330 μmol mol−1. It was concluded that short‐term exposure to different CO2 treatments adequately estimated long‐term canopy CER response to CO2 level, but did not adequately predict long‐term canopy transpiration response.