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The interactive effects of atmospheric CO[sub 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[sub 2] enrichment (FACE; ambient + 200 pmol mol[sub -1]), under ample (wet, 100% replacement of evapo-transpiration) and reduced (dry, postplanting and mid-season irrigations) water supply over two growing seasons. FACE reduced seasonal average stomatal conductance (g[sub s]) by 0.17 mol (H[sub 2]O) m[sup -2] s[sup -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[sub 2]) m[sup -2] s[sup -1] (23% and 9% for dry and wet, respectively), whereas dry decreased A by 10.50 μmol (CO[sub 2]) m[sup -2] s[sup -1] (26%) compared with wet. Total plant water potential (ψ[sub 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[sub 2] improved plant water status, which indirectly caused an increase in carbon gain. [ABSTRACT FROM AUTHOR]