Abstract
Summary • Sorghum (Sorghum bicolor) was grown for two consecutive seasons at Maricopa, AZ, USA, using the free-air CO2 enrichment (FACE) approach to investigate evapotranspiration of this C4 plant at ample and limited water supplies. • Crop evapotranspiration (ET) was measured using two CO2 concentrations (control, c. 370 µmol mol−1; FACE, ambient +200 µmol mol−1) and two irrigation treatments (well watered and water-limited). Volumetric soil water content was measured before and after each irrigation using neutron scattering techniques. • Averaged over both years, elevated CO2 reduced cumulative ET by 10% when plants were given ample water and by 4% under severe drought stress. Water-use efficiency based on grain yield (WUE-G) increased, due to CO2 enrichment, by 9% and 19% in wet and dry plots, respectively; based on total biomass, water-use efficiency (WUE-B) increased by 16% and 17% in wet and dry plots, respectively. • These data suggest that in the future high-CO2 environment, water requirements for irrigated sorghum will be lower than at present, while dry-land productivity will increase, provided global warming is minimal.
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