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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.
Wall et al. (Thu,) studied this question.