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Interaction of Soil Moisture Stress and Ambient Ozone on Growth and Yields of Soybeans. H. E. Heggestad, Plant pathologist, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705; T. J. Gish(2), E. H. Lee(3), J. H. Bennett(4), and L. W. Douglass(5). (2)(3)(4)(5)Soil physicist, plant physiologists, and statistical consultant, respectively, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705. Phytopathology 75:472-477. Accepted for publication 6 November 1984. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1985. DOI: 10.1094/Phyto-75-472.

A field experiment was conducted in open-top chambers to determine how interactions of soil moisture stress and exposure to ozone may affect soybean yields. Cultivars Williams and Forrest were grown in 1982 and Williams and Corsoy in 1983. Five levels of O₃—including charcoal-filtered (CF) and nonfiltered (NF) air—and two soil moisture regimes (averaging - 0.05 MPa, and about - 0.40 MPa at depths of 0.25 and 0.45 m) were employed. This report describes a significant interaction involving soil moisture stress (SMS) and the CF and NF air treatments. Compared to the no-stress treatment, the combination of SMS and ambient O₃ in 2 yr of experiments reduced yields by 25%. Exposure to ambient O₃ without SMS reduced yields 5%. In CF air, SMS lowered yields 4%. Knowledge of this interaction increases the significance of O₃ as an environmental phytotoxicant. Although cultivar differences existed, the soybean plants acclimated to SMS in CF air but not in NF air. These results provide the first experimental evidence that exposure to elevated ambient O₃ may cause plants to lose tolerance to SMS.