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Acid rain is the greatest ecological environmental disasters in the world, as well as global warming and ozone depletion. Due to the special energy structure and unique geography location, the southwest regions of China had become one of the four greatest acid rain zones. Canna indica is a commonly use plant for park landscaping, which can absorb harmful gas, decontaminate water and cumulate heavy metals reported before. However, the damage and resistance mechanism of Canna indica by the acid rain is not well known exactly at present. In this research, pot experiments were done to study the variation of visible damage of leaves, malondialdehyde (MDA) contents, membrane permeability (MP), chlorophyll (Ch1) contents activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), net photosynthetic rate (Pn), stomatal conductance (Gs) and the intercellular CO2 concentration(Ci) under acid rain stress of artificial stress simulation with pH4.0, pH3.0, pH2.0 in six cycles, while using pH5.6 for control group. This study can provide reference for the option of city landscaping or indicator plants in the acid rain zones.The results showed that:(1) There’s no visible injury spot caused on the Canna indica leaves after six cycles with acid rain stress of pH4.0, but caused irreversible damages with pH2.0 and pH3.0, which the leaf damage index reached III and I respectively.(2) During the acid rain stress, with the decrease of pH, the MDA contents and MP showed an overall increase, which suggested that the treatments with pH<4.0 could cause the large accumulation of the intracellular active oxygen, intensify peroxidation of the membrane lipid and damage the leaf cells.(3) When pH<3.0, the fertilization of nitrogen had a short stimulative effect on the total Ch1 contents. With the increase of the stress cycles, excessive hydrion and sulfion inevitably inhibited the Ch1 synthesis and accelerated the chloroplast decomposition. The decrease of the Ch1 a contents was a main reason for the total chlorophyll. Ch1 b content demonstrated an overall increase with the decrease of Ch1 a/b.(4) When pH≤4.0, the damage on the Canna indica leaves would be alleviated by decreasing the Gs under acid rain stress. Thanks to the low Gs, it caused the largest decrease rate of the Pn in the canna in the pH4.0 group. In the second cycle, with the increase of the Ch1 contents in the pH<3.0 groups, acid rain treatment enhanced the Pn. In the sixth cycle, the Pn decreased owing to the declination of the Gs in pH2.0 group; but the Pn largely increased while the Gs decreased in pH3.0 group, that is needed further research.(5) The antioxidant enzyme system including the activities of SOD, CAT, POD. When Canna indica was stressed under acid rain, POD was the main enzyme removing the active oxygen, but the SOD and CAT were insufficient. The incease of the single enzyme was the main reason for the poor anti-adversity in the Canna indica.Above all, though Canna indica is a sensitive plant to acid rain, it still has a certain ability to resist the acid, positively reacts to the acid stress and withstands the poor living environment. According to the observation of the visible damage on the leaves under acid rain, the threshold of the visible damage of Canna indica was pH2.0～3.0; according to the evaluation of the differences significant test results of the physiological and biochemical indexes, the invisible damage threshold of Canna indica by the acid stress was pH3.0-4.0.
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