Influence of elevated carbon dioxide on water relations of soybeans.

Soybean (Glycine max L. Merrill cv ;Bragg') plants were grown in pots at six elevated atmospheric CO(2) concentrations and two watering regimes in open top field chambers to characterize leaf xylem potential, stomatal resistance and conductance, transpiration, and carbohydrate contents of the leaves in response to CO(2) enrichment and water stress conditions. Groups of plants at each CO(2) concentration were subjected to water stress by withholding irrigation for 4 days during the pod-filling stage.Under well watered conditions, the stomatal conductance of the plants decreased with increasing CO(2) concentration. Therefore, although leaf area per plant was greater in the high CO(2) treatments, the rate of water loss per plant decreased with CO(2) enrichment. After 4 days without irrigation, plants in lower CO(2) treatments showed greater leaf tissue damage, lower leaf water potential, and higher stomatal resistance than high CO(2) plants. Stomatal closure occurred at lower leaf water potentials for the low CO(2) grown plants than the high CO(2) grown plants. Significantly greater starch concentrations were found in leaves of high CO(2) plants, and the reductions in leaf starch and increases in soluble sugars due to water stress were greater for low CO(2) plants. The results showed that even though greater growth was observed at high atmospheric CO(2) concentrations, lower rates of water use delayed and, thereby, prevented the onset of severe water stress under conditions of low moisture availability.