国家自然科学基金面上项目41171226;陕西省自然科学基础研究计划2011JM5011;西北农林科技大学基本业务费专项Z109021514.[National Natural Science Foundation of China,No.41171226;Natural Science Basic Research Program of Shaanxi,No.2011JM5011;Fundamental Research Funds for the Northwest A&F University,No.Z109021514.]