This study was conducted to determine the response in leaf growth and gas exchange of soybean (Glycine max Merr.) to the combined effects of water deficits and carbon dioxide (CO2) enrichment. Plants grown in pots were allowed to develop initially in a glasshouse under ambient CO2 and well-watered conditions. Four-week old plants were transferred into two different glasshouses with either ambient (360 μmol mol-1) or elevated (700 μmol mol-1) CO2. Following a 2-day acclimation period, the soil of the drought-stressed pots was allowed to dry slowly over a 2-week period. The stressed pots were watered daily so that the soil dried at an equivalent rate under the two CO2 levels. Elevated [CO2] decreased water loss rate and increased leaf area development and photosynthetic rate under both well-watered and drought-stressed conditions. There was, however, no significant effect of [CO2] in the response relative to soil water content of normalized leaf gas exchange and leaf area. The drought response based on soil water content for transpiration, leaf area, and photosynthesis provide an effective method for describing the responses of soybean physiological processes to the available soil water, independent of [CO2].
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