Summary• The interactive effects of atmospheric CO 2 concentration and soil-water content on grain sorghum ( Sorghum bicolor ) are reported here.• Sorghum plants were exposed to ambient (control) and free-air CO 2 enrichment (FACE; ambient + 200 µmol mol -1 ), under ample (wet, 100% replacement of evapotranspiration) and reduced (dry, postplanting and mid-season irrigations) water supply over two growing seasons.• FACE reduced seasonal average stomatal conductance ( g s ) by 0.17 mol (H 2 O) m -2 s -1 (32% and 37% for dry and wet, respectively) compared with control; this was similar to the difference between dry and wet treatments. FACE increased net assimilation rate ( A ) by 4.77 µmol (CO 2 ) m -2 s -1 (23% and 9% for dry and wet, respectively), whereas dry decreased A by 10.50 µmol (CO 2 ) m -2 s -1 (26%) compared with wet. Total plant water potential ( ψ w ) was 0.16 MPa (9%) and 0.04 MPa (3%) less negative in FACE than in the control treatment for dry and wet, respectively. Under dry, FACE stimulated final shoot biomass by 15%.• By ameliorating the adverse effects of drought, elevated atmospheric CO 2 improved plant water status, which indirectly caused an increase in carbon gain.Key words: carbon dioxide, global change, stomatal conductance, net assimilation rate, water relations, free-air CO 2 enrichment (FACE). AbbreviationsA, instantaneous leaf net assimilation rate (µmol (CO 2 ) m -2 s -1 ); B, final shoot biomass (g m -2 ground area); B a , average accumulated shoot biomass (g m -2 ground area); CD, control-dry; CW, control-wet; C, carbon dioxide effect in ANOVA; C a , atmospheric CO 2 concentration (µmol (CO 2 ) mol -1 ); C i , intercellular CO 2 concentration (µmol (CO 2 ) mol -1 ); C i : C a , ratio of C i to C a ; C m , internal CO 2 concentration in mesophyll (µmol (CO 2 ) mol -1 ); C m : C a , ratio of C m to C a ; D, soil dehydration cycle effect in ANOVA; DAP, day after planting; D r , maximum depth of root penetration (m); ET, evapotranspiration rate (mm d -1 ); ET c , cumulative seasonal evapotranspiration (mm); e a , atmospheric water vapor pressure at T a (Pa); e s , atmospheric saturation water vapor pressure at T a (Pa); e*, atmospheric water vapor pressure deficit ( i.e. , e* = e s -e a ) at T a (Pa); FD, FACE-dry; FW, FACE-wet; G, growth stage effect in ANOVA; g s , stomatal conductance (mol (H 2 O) m -2 s -1 ); HI: harvest index (grain yield divided by total shoot biomass); I, irrigation effect in ANOVA; IWUE: A/g s , intrinsic water use efficiency (µmol (CO 2 ) mol (H 2 O)

Summary• Sorghum ( Sorghum bicolor ) was grown for two consecutive seasons at Maricopa, AZ, USA, using the free-air CO 2 enrichment (FACE) approach to investigate evapotranspiration of this C4 plant at ample and limited water supplies.• Crop evapotranspiration (ET) was measured using two CO 2 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 CO 2 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 CO 2 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-CO 2 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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