Abstract
Rising levels of atmospheric CO 2 affect transpiration and water absorption processes that influence total leaf water potential ( Ψ W) of wheat ( Triticum aestivum L.). The objective of this case study was to determine the effect of a rise in atmospheric CO 2 concentration on predawn, midday, and sunset Ψ W as soil matric potential ( Ψ M) 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°N, 112.0°W), in air enriched for 24 h per day to a atmospheric CO 2 concentration of ∼200 μmol mol −1 (550 μmol mol −1) above ambient (370 μmol mol −1) using a free-air CO 2 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 Ψ W on 3–5 upper-canopy fully-expanded sunlit leaves at predawn, midday, and sunset between tillering and hard dough during 1993 and 1994. The Ψ W for the dry plots was regressed on Ψ M from field capacity (−0.03 MPa) to the permanent wilting point (−1.50 MPa). Slopes (standard error) were derived for CD ( Ψ W[CD] (MPa) Ψ M[CD] (MPa) −1) compared with FD ( Ψ W[FD] (MPa) Ψ M[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 Ψ W for a given Ψ M occurred in control compared with FACE ( P<0.01). As Ψ M ranged from −0.03 to −1.50 MPa, a ∼200 μmol mol −1 rise in atmospheric CO 2 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 CO 2 concentration of the atmosphere rises, both drought avoidance and tolerance mechanisms will be enhanced resulting in improved water relations in wheat.
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