Abstract
This field study was conducted to determine the effect of soil moisture on net carbon dioxide exchange (NCE) and transpiration rates of sorghum ( Sorghum bicolor (L.) Moench) grown under well-watered and water-stressed conditions. Components of the energy balance, soil moisture, leaf and air temperatures, stomatal resistance, and leaf-water potential were measured. A field growth-chamber system was used to determine NCE and transpiration rates. Analysis of the data suggested that transpiration and NCE rates were reduced when available soil moisture was ⩽35% of the maximum. Above that moisture level, transpiration was primarily energy-dependent and the rate was enhanced by air temperatures higher than 33°C. Photosynthetic rates declined when leaf temperatures exceeded 33°C. At and below the critical soil-moisture level, leaf-water potential decreased and the resistance to vapor transport increased; that reduced transpiration and NCE rates.
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