CO₂ enrichment increases water-use efficiency in sorghum

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.