Elevated atmospheric CO2 alleviates drought stress in wheat

Rising levels of atmospheric CO2 affect transpiration and water absorption processes that influence total leaf water potential (psiW) of wheat (Triticum aestivum L.). The objective of this case study was to determine the effect of a rise in atmospheric CO2 concentration on predawn, midday, and sunset psiW as soil matric potential (psiM) ranged from -0.03 to -1.50 MPa. 'Yecora Rojo' was sown on 15 December 1992 (130 plants m(-2)) and 7-8 December 1993 (180 plants m(-2)) in an open field at Maricopa, AZ (33.1 degrees N, 112.0 degrees W), in air enriched for 24 h per day to a atmospheric CO2 concentration of approximately 200 micromol mol(-1) (550 micromol mol(-1)) above ambient (370 micromol mol(-1)) using a free-air CO2 enrichment (FACE) apparatus (main-plot) in four replicates. A sub-surface drip-tape irrigation system provided two soil-water content treatments: 50% (dry) and 100% (wet) replacement of potential evapotranspiration (split-plot). Treatments were control-dry (CD), FACE-dry (FD), control-wet (CW) and FACE-wet (FW). A pressure chamber was used to measure psiW on 3-5 upper-canopy fully-expanded sunlit leaves at predawn, midday, and sunset between tillering and hard dough during 1993 and 1994. The psiW for the dry plots was regressed on psiM from field capacity (-0.03 MPa) to the permanent wilting point (-1.50 MPa). Slopes (standard error) were derived for CD (psiW[CD] (MPa) PsiM[CD] (MPa)-1) compared with FD (psiW[FD] (MPa) psiM[FD] (MPa)-1) at predawn (5.1 (1.2) compared with 1.1(0.2)), midday (4.3 (1.8) compared with 0.7 (0.3)), and sunset (5.9 (1.5) compared with 1.0 (0.9)). Regardless of time of day, more negative psiW for a given psiM occurred in control compared with FACE (P < 0.01). As psiM ranged from -0.03 to -1.50 MPa, a approximately 200 micromol mol(-1) rise in atmospheric CO2 concentration resulted in a curvilinearly increase in drought stress alleviation from 0 to 77% at predawn, 0 to 67% at midday, and 0 to 79% at sunset. Hence, as the CO2 concentration of the atmosphere rises, both drought avoidance and tolerance mechanisms will be enhanced resulting in improved water relations in wheat.