1. Dept. Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, AB, Canada T6G 2P5.
2. China University of Geosciences, Wuhan, PRC 430074.
3. Corresponding author. Phone: +1-780-492-0038. Fax: +1-780-492-4265. E-mail: lingyun.chen@ualberta.ca