Water Relations of Differentially Irrigated Cotton Exposed to Ozone
This field study was conducted to test the hypothesis that plants chronically exposed to O3 may be more susceptible to drought kcause OM3 typically inhibits root growth and increases shoot-root ratios in plants. Cotton (Gossypium hirsutum L. cv. Acala SJ-2) was grown in open-top chambers on Hanford coarse sandy loam (coarseloamy, mixed, non-acid, thermic, Typic Xerorthents) in Riverside, CA. Plants were grown under three irrigation regimes: optimum water for lint production (OW), suboptimum or moderate drought stress (SO), and severely drought stressed (SS) and were exposed to seasonal 12 h (0800–2000) O3 concentrations of 0.015, 0.074, 0.094, or 0.111 μL L−1. Leaf xylem pressure potentials (Ψ1) and soil water content (Ψv) were measured weekly from June to October. Mean seasonal Ψ1 increased from −1.89 MPa to −1.72 MPa in low to high O3 treatments, averaged across soil water regimes. Ozone had no effect on seasonal water use of cotton, but water use efficiency was significantly reduced by O3 in OW and SO, but noit in SS treatments. Drought-stressed plants extracted proportionally greater amounts of water from deeper in the soil profile than OW cotton, and O3 had no apparent effect on this redistribution of roots in the soil. Since O3 had no apparent effect on the ability of droughtstressed cotton to maintain Ψ1 and to increase root growth relative to shoot growth, this suggests that O3 may have little or no effect: on the potential of cotton to adapt to or tolerate drought.
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