1. College of Agronomy; Northwest A&F Univ.; Yangling Shaanxi 712100 China
2. National Key Facility for Crop Gene Resources and Genetic Improvement; Institute of Crop Science; Chinese Academy of Agricultural Sciences; Beijing 100081 China
3. National Key Facility for Crop Gene Resources and Genetic Improvement; Institute of Crop Science; Academy of Agricultural Sciences; Beijing 100081 China