Elevated atmospheric CO₂ improved Sorghum plant water status by ameliorating the adverse effects of drought

The interactive effects of atmospheric CO₂ concentration and soil-water content on grain sorghum (Sorghum bicolor) are reported here. Sorghum plants were exposed to ambient (control) and free-air CO₂ enrichment (FACE; ambient + 200 μmol mol^-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 (g₅) by 0.17 mol (H₂O) 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 (CO₂) m^-2s^-1 (23% and 9% for dry and wet, respectively), whereas dry decreased A by 10.50 μmol (CO₂) m^-2s^-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 CO₂ improved plant water status, which indirectly caused an increase in carbon gain.