国家高技术研究发展计划863计划项目2013AA102904;西北农林科技大学基本科研业务费专项资金2014YB023;西北农林科技大学本科优质课程建设项目资助. This study was supported by the National High Technology Research and Development Program of China2013AA102904;the Special Funds for Research Activities in Northwest A&F University2014YB023;the Quality Curriculum project in Northwest A&F University