Influence of Elevated Carbon Dioxide on Water Relations of Soybeans

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Abstract
Soybean (Glycine max L. Merrill cv `Bragg9) plants were grown in pots at six elevated atmospheric CO2 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 CO2 enrichment and water stress conditions. Groups of plants at each CO2 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 CO2 concentration. Therefore, although leaf area per plant was greater in the high CO2 treatments, the rate of water loss per plant decreased with CO2 enrichment. After 4 days without irrigation, plants in lower CO2 treatments showed greater leaf tissue damage, lower leaf water potential, and higher stomatal resistance than high CO2 plants. Stomatal closure occurred at lower leaf water potentials for the low CO2 grown plants than the high CO2 grown plants. Significantly greater starch concentrations were found in leaves of high CO2 plants, and the reductions in leaf starch and increases in soluble sugars due to water stress were greater for low CO2 plants. The results showed that even though greater growth was observed at high atmospheric CO2 concentrations, lower rates of water use delayed and, thereby, prevented the onset of severe water stress under conditions of low moisture availability.