Rain water in natural state is slightly acidic with a pH value ranging from 5.6 to 7.0 due to the presence of carbon dioxide in the atmosphere. The acid rain means the pH value is lower than 5.6 levels. Nowadays, acid rain has become a global environmental issue, due to inadvertent human interference, such as combustion of fossil fuels and industrial processes 1. Acid rain stress is also a serious problem in China in recent years. Acid rain monitoring data from the late 1970's demonstrate that rain is acidic in most places throughout China, with pH ranging from 2.3 to 4.47 2. As one of the detrimental effects, acid rain may result in various toxicity symptoms in plants, including visible effects of injury (chlorosis and:or necrosis) and invisible effects such as reduced photosynthesis, nutrient loss from leaves, altered water balance and variation of several enzyme activities 3-5. Previous data indicated that acid rain treatment decreased photosynthetic CO2 fixation and photochemical activity of bean plants. Lipid peroxidation and increased level of H2O2 were induced in acid rain-treated bean leaves 6. Shan et al. 7 reported that chlorolhyll content of Armand pine increased but net photosynthetic rate (Pn) on a chlorolhyll content basis decreased with increasing acidity of rain. In addition, acid rain significantly increased the activities of guaiacol