1. Assistant professor, Department of Crop Agriculture and Life Science, Chonbuk National University, Jeonju, 561-756, Korea.
2. Associate professor, Department of Crop and Soil Sciences, Washington State University, Pullman, WA 99164-6376.
3. Corresponding author. Phone: (509) 335-8230. Fax: (509) 335-8674. E-mail: bbaik@wsu.edu