School of Life Science, Northwest A&F University, Yangling, Shaanxi, China. hongxiapang@yahoo.com|School of Life Science, Northwest A&F University, Yangling, Shaanxi, China. hongxiapang@yahoo.com|School of Life Science, Northwest A&F University, Yangling, Shaanxi, China. hongxiapang@yahoo.com|School of Life Science, Northwest A&F University, Yangling, Shaanxi, China. hongxiapang@yahoo.com