Effects of drought stress on photosystem Ⅱ and photosynthesis in Beta vulgaris during the vegetative development period

• LI Sizhong, 
• GAO Weishi, 
• ZHANG Liming, 
• BAI Xiaoshan, 
• LIU Jun, 
• DONG Xinjiu, 
• YANG Hongze, 
• SHA Hong, 
• GAO Yan