1. Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, OK 74078, USA
2. Current address: Department of Agronomy and Horticulture, University of Nebraska‐Lincoln, P.O. Box 830915, Lincoln, NE 68583, USA
3. E‐mail
4. United States Environmental Protection Agency, Office of Research and Development, National Risk Management Research Laboratory, P.O. Box 1198, Ada, OK 74820, USA; E‐mail