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 CO, 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 mu mol mol(-1); FACE, ambient +200 mu mol 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.