| With the rapid increase of the number of automobiles, automobile exhaust has become the first air pollution source in our country cities. The pollution matter produced by automobile exhaust has profound effect on the growth, development and propagation of greening tree species in cities. It is important to study the effect of automobile exhaust on the greening tree species and the tolerance of plants to automobile exhaust for the beautifying of the city environment. The effects of automobile exhaust on injury symptom, pH value of leaf cell juice, peroxidation of membrance lipid, free radical scavenging system and photosynthetic system of four greening tree species, Acer mono Maxim, Mains bacata Borkh, Pyrus ussuriensis Maxim, sad Acer ginnala Maxim were studied by means of fumigation in open top chamber. During the fumigation test, the four plants were exposed to exhaust gas of the same concentration (25 ug.m-3) for different time span (1, 3, 5,7d) and same time span(2h) but different concentration (40, 60. 80, 100 ug.m-3) , the results showed:1 Under the stress of automobile exhaust, apparent injury symptoms and the time series of appearance of injury symptoms were different in different tree species. Under the high concentration treatment, the order of appearance of injury symptoms in four trees was: Mains bacata Borkh, Acer mono Maxim, Pyrus usswiensis Maxim, Acer ginnala Maxim. The order of complete withering of leaves was: Malus bacata Borkh, Pyrus ussuriensis Maxim, Acer mono Maxim, Acer ginnala Maxim.2 Under the stress of automobile exhaust, pH value of leaf cell juice of four plants declined, but the variety was significantly less than other indexes. The variety range of Acer ginnala Maxim was least, the variety was decreased 15.2% than that of CK after seven days, under the stress of 100 u g .m-3, the variety range was decreased 2.72% than that of CK. The variety of Pyrus ussuriensis Maxim was highest, the variety range was decreased 9A2% than that of CK after seven days, under the stress of 100ug .m-3, the variety range was decreased 13.89% than that of CK. The variety range of four plants followed the order of Acer ginnala Maxim< Malus bacata Borkh and Acer mono Maxim< Pyrus ussuriensis Maxim.3 Under the stress of automobile exhaust, peroxidation of membrance Lipid were affected, relative conductivity and malondialdehyde(MDA) content were increased. With the increase of fumigation time, the variety range of relative conductivity andmalondialdehyde(MDA) content of Acer mono Maxim was highest, followed by that oiMalus bacata Borkh, that of Acer ginnala Maxim was least With the increase of treatment concentration, that was enhanced 99. 75% and 52. 52% than that of CK, the variety range of Malus bacata Borkh was highest, that of Pyrus ussuriensis Maxim was in second place, that of Acer mono Maxim and Acer ginnala Maxim were least4 Under the stress of automobile exhaust, the free radical scavenging system was seriously affected. Superoxide dismutase(SOD)activiry, peroxidase(POD) activity of four plants were increased except Malus bacata Borkh in the high concentrate treatment The variation of SOD activity of Acer mono Maxim and Malus bacata Borkh were higher than that of Acer ginnala Maxim and Pyrus ussuriensis Maxim. From the variation of POD activity due to the basic POD activity values of Acer mono Maxim and Acer ginnala Maxim were far discrepant from that of Malus bacata Borkh and Pyrus ussuriensis Maxim, the relative variation of POD activity of Acer mono Maxim and Acer ginnala Maxim were far higher than that oiMalus bacata Borkh and Pyrus ussuriensis Maxim, while the absolute variation of Acer mono Maxim and Acer ginnala Maxim were far lower than that of Malus bacata Borkh and Pyrus ussuriensis Maxim.The ASA contents of four plants were declined, the variation of Malus bacata Borkh and Pyrus ussuriensis Maxim were higher than that of Acer mono Maxim and Acer ginnala Maxim .5 Under the stress of automobile exhaust, photosynthetic systems were damaged. Chlorophy...
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