Abstract: A pot experiment was conducted with 1-year-old Melia azedarach as material. Simulated acid rain of four pH levels (5.6 (CK), 4.5, 3.5 and 2.5) was sprayed on leaves every two days, and the chlorophyll (Chl) content, soluble protein content, cell membrane permeability, MDA content, antioxidant enzyme activity of leaves were investigated after 20, 40 and 60 days of treatment. The results showed that with the extension of treatment time, the soluble protein content decreased firstly and increased afterward at pH 4.5 with a greatest increase of 10.3%, while it decreased at pH 3.5 and pH 2.5 with a sharpest drop of 42.9%. The plasma membrane permeability and MDA content significantly increased up to 84.6% and 145.5%, respectively. SOD activity increased at pH 4.5, while it first increased and then decreased at pH 3.5 and decreased at pH 2.5, with a greatest increase of 47.9% and a sharpest drop of 20.9%. CAT activity appeared in a dropping trend after an initial increase at pH 4.5, and it decreased at pH 3.5 and pH 2.5, with a greatest increase of 23.0% and a sharpest drop of 27.0%. POD activity increased at pH 4.5 and pH 3.5, and decreased at pH 2.5, with a greatest increase of 47.1% and a sharpest drop of 17.6%. Chla, Chlb and Chl decreased with sharpest drops of 39.0%, 37.0% and 38.3%, respectively. Combining the findings on leaf shape, abscission rate and plant height increment, it was found that leaves turned yellow and abscission rate increased significantly at pH 3.5 (P<0.05), while leaves deformed severely and plant height decreased significantly at pH 2.5 (P<0.05). Thus, the threshold of M. azedarach to resist acid rain is about pH 3.5.

Key words: nonstructural carbohydrate, fine root respiration, soil warming, respiration acclimation