1. State Key Laboratory of Crop Stress Biology for Arid Areas; College of Horticulture; Northwest A&F University; Yangling 712100 China
2. Biosystematics Group; Wageningen University; 6708 PB Wageningen the Netherlands
3. College of Information Engineering; Northwest A&F University; Yangling 712100 China
4. Department of Plant Science and Landscape Architecture; University of Maryland; College Park MD 20742 USA
5. Department of Horticulture; Michigan State University; 1066 Bogue St East Lansing MI 48824 USA