Elevated CO2 increases water use efficiency by sustaining photosynthesis of water-limited maize and sorghum

Maize and grain sorghum seeds were sown in pots and grown for 39 days in sunlit controlled-environment chambers at 360 (ambient) and 720 (double-ambient, elevated) μmol mol−1 carbon dioxide concentrations [CO2]. Canopy net photosynthesis (PS) and evapotranspiration (TR) was measured throughout and summarized daily from 08:00 to 17:00 h Eastern Standard Time. Irrigation was withheld from matched pairs of treatments starting on 26 days after sowing (DAS). By 35 DAS, cumulative PS of drought-stress maize, compared to well-watered plants, was 41% lower under ambient [CO2] but only 13% lower under elevated [CO2]. In contrast, by 35 DAS, cumulative PS of drought-stress grain sorghum, compared to well-watered plants, was only 9% lower under ambient [CO2] and 7% lower under elevated [CO2]. During the 27–35 DAS drought period, water use efficiency (WUE, mol CO2 Kmol−1 H2O), was 3.99, 3.88, 5.50, and 8.65 for maize and 3.75, 4.43, 5.26, and 9.94 for grain sorghum, for ambient-[CO2] well-watered, ambient-[CO2] stressed, elevated-[CO2] well-watered and elevated-[CO2] stressed plants, respectively. Young plants of maize and sorghum used water more efficiently at elevated [CO2] than at ambient [CO2], especially under drought. Reductions in biomass by drought for young maize and grain sorghum plants were 42 and 36% at ambient [CO2], compared to 18 and 14% at elevated [CO2], respectively. Results of our water stress experiment demonstrated that maintenance of relatively high canopy photosynthetic rates in the face of decreased transpiration rates enhanced WUE in plants grown at elevated [CO2]. This confirms experimental evidence and conceptual models that suggest that an increase of intercellular [CO2] (or a sustained intercellular [CO2]) in the face of decreased stomatal conductance results in relative increases of growth of C4 plants. In short, drought stress in C4 crop plants can be ameliorated at elevated [CO2] as a result of lower stomatal conductance and sustaining intercellular [CO2]. Furthermore, less water might be required for C4 crops in future higher CO2 atmospheres, assuming weather and climate similar to present conditions.