1. Gansu Provincial Key Lab. of Aridland Crop Science Gansu Agricultural Univ. Lanzhou 730070 China
2. College of Agronomy, Gansu Agricultural Univ. Lanzhou 730070 China
3. College of Life Science Northwest Normal University Lanzhou 730070 China
4. Dep. of Agronomy and Plant Genetics Univ. of Minnesota St. Paul MN 55108 USA
5. College of Resource and Environment, Gansu Agricultural Univ. Lanzhou 730070 China