title: Elevated CO2 increases water use efficiency by sustaining photosynthesis of water-limited maize and sorghum.
abstract: 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 [CO(2)]. Canopy net photosynthesis (PS) and evapotranspiration (TR) was measured throughout and summarized daily from 08:00 to 17:00h 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 [CO(2)] but only 13% lower under elevated [CO(2)]. In contrast, by 35 DAS, cumulative PS of drought-stress grain sorghum, compared to well-watered plants, was only 9% lower under ambient [CO(2)] and 7% lower under elevated [CO(2)]. During the 27-35 DAS drought period, water use efficiency (WUE, mol CO(2)Kmol(-1)H(2)O), 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-[CO(2)] well-watered, ambient-[CO(2)] stressed, elevated-[CO(2)] well-watered and elevated-[CO(2)] stressed plants, respectively. Young plants of maize and sorghum used water more efficiently at elevated [CO(2)] than at ambient [CO(2)], especially under drought. Reductions in biomass by drought for young maize and grain sorghum plants were 42 and 36% at ambient [CO(2)], compared to 18 and 14% at elevated [CO(2)], 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 [CO(2)]. This confirms experimental evidence and conceptual models that suggest that an increase of intercellular [CO(2)] (or a sustained intercellular [CO(2)]) in the face of decreased stomatal conductance results in relative increases of growth of C(4) plants. In short, drought stress in C(4) crop plants can be ameliorated at elevated [CO(2)] as a result of lower stomatal conductance and sustaining intercellular [CO(2)]. Furthermore, less water might be required for C(4) crops in future higher CO(2) atmospheres, assuming weather and climate similar to present conditions.
authors.affiliation: United States Department of Agriculture - Agricultural Research service, Center for Medical, Agricultural and Veterinary Entomology, Chemistry Research Unit, 1600/1700 SW 23rd Drive, Gainesville, FL 32608-1069, USA. Hartwell.Allen@ars.usda.gov
authors.forename: Leon Hartwell
authors.initials: LH
authors.lastname: Allen
authors.forename: Vijaya Gopal
authors.initials: VG
authors.lastname: Kakani
authors.forename: Joseph C V
authors.initials: JC
authors.lastname: Vu
authors.forename: Kenneth J
authors.initials: KJ
authors.lastname: Boote
mesh_terms: D000222:Adaptation, Physiological; D001692:Biological Transport / Q000187:drug effects; D018533:Biomass; D002245:Carbon Dioxide / Q000494:pharmacology*; D055864:Droughts; D008027:Light; D008291:Malate Dehydrogenase / Q000378:metabolism; D008954:Models, Biological; D010788:Photosynthesis / Q000502:physiology* / Q000528:radiation effects; D018515:Plant Leaves / Q000187:drug effects / Q000201:enzymology / Q000502:physiology / Q000528:radiation effects; D054046:Plant Stomata / Q000187:drug effects; D018526:Plant Transpiration / Q000187:drug effects; D045868:Sorghum / Q000201:enzymology / Q000378:metabolism / Q000502:physiology* / Q000528:radiation effects; D013312:Stress, Physiological; D013997:Time Factors; D014867:Water / Q000494:pharmacology*; D003313:Zea mays / Q000187:drug effects / Q000201:enzymology / Q000502:physiology* / Q000528:radiation effects
chemical_list: D014867:Water; D002245:Carbon Dioxide; D008291:Malate Dehydrogenase; C031917:malate dehydrogenase (oxaloacetate-decarboxylating) (NADP+)