SHEN Ying-bai Key Laboratory for Genetics and Breeding of Forest Trees and Ornamental Plants (Ministry of Education), College of Biological Sciences and Technology, Beijing Forestry University, Beijing 100083, R R. China. Email: huwudi@bifu.edu.cn WANG Wen-he College of Gardening, Beijing University of Agriculture, Beijing 102206, P. R. China. E-mail: wwhals@163.com