Effects of three Trichoderm aasperellum strains on the growth and photosynthetic characteristics of tissue-cultured Populus davidiana×P. alba var. pyramidalis seedlings

• Chuan-ying Jiang, 
• Guo-dong Zhu, 
• Zhi-hong Yao, 
• Xing-tang Yang, 
• Zhi-hua Liu, 
• Rong-shu Zhang