Abstract: The effect of aluminum on some physiological characters such as growth index,membrane lipid peroxidation,activities of antioxidant enzymes,content of proline in mangrove Kandelia obovata seedlings was studied in the present paper.Mature K.obovata hypocotyls with similar length and fresh weight were cultured hydroponically in sand beds under the day/night temperatures of 32/25℃.Hoagland's liquid solution mixed 3‰ NaCl were used to assure salt re-quires physiologically and were renewed every five days.Tap water was added daily to keep the salinity constant. Long-term aluminum treatments were designed to add AlCL3 to liquid solution with low concentrations of 0(as control),0.0625,0.125,0.25,0.5,1 mmol/L Al3+ as soon as hypocotyls were planted.After three-month growth,the height and fresh weight of K.obovata seedlings were measured.No difference was found in each experimental group except that root elongation was restrained by AlCL3. The second experiment was applied aluminum treatment with high concentrations of 10,25,50 mmol/L Al3+ to one-month natural growth of K.obovata seedlings.It was found that seedlings stopped to grow more and even most of them began to withered away under 50 mmol/L Al3+ treatment after seven days later and under 25 mmol/L Al3+ treat-ment after fifteen days later.On the contrary,those seedlings under 10 mmol/L Al3+ treatment could keep growing naturally. Short-term aluminum treatment was designed within one week(7d) by adding 25 mmol/L and 100 mmol/L Al3+ to K.obovata seedlings.The second pair of leaves in each treatment group was collected randomly at the time of 2h,6h,12h,24h,48h(2d),72h(3d),120h(5d) and 168h(7d) to measure the activity of superoxide dismutase(SOD),peroxi-dase(POD),catalase(CAT),and content of malondialdehyde(MDA) and proline.In addition,photosynthesis efficiency was measured at the time of 24h and 48h with PAM-2100 chlorophyll fluoroscope. In the 25 mmol/L Al3+ treatment group,the activity of SOD increased to the maximum at 24h and kept being active in 168h(7d).However in the 100 mmol/L Al3+ treatment group,the activity of SOD increased to the maximum at 48h,and then began to decrease.The CAT activity in the 25 mmol/L Al3+ treatment group increased with the prolongation of stress until 72h,and then the activity decreased to 14.9% of the control.Contrastively in the 100 mmol/L Al3+ treatment group,the CAT activity increased rapidly in 2h and then decreased to 11.5% of the control in 6h.The POD activity in-creased after 6h both in two treatment groups.And with the increase of the activities of antioxidant enzymes,the content of MDA and proline in the leaves increased respectively.In addition,our results also indicated that 100 mmol/L Al3+ treatment made oxygenation damage worse than that of 25 mmol/L Al3+ treatment and even more reduced the photo-synthesis efficiency evidently. In the short-term aluminum treatment experiment,roots were also collected after 72h(3d),120h(5d) and 168h(7d) to measure the activity of SOD,POD,CAT,MDA content and proline content.And the results were noticed that POD activity increased both under 25 mmol/L and 100 mmol/L Al3+ treatment;the activity of SOD and CAT changed indis-tinctively and content of proline increased evidently. All of these results in the study indicated that K.obovata seedlings behaved adaptability to aluminum stress in a certain extent and could keep natural life process under 10 mmol/L Al3+ treatment.Stress of 25-100 mmol/L Al3+ made physiological reaction of K.obovata seedlings more active,such as the membrane lipid oxidation aggravated,the MDA content increased and the activities of antioxidant enzymes changed with the aluminum concentrations.Adding high Al3+ concentration solution to K.obovata seedlings would stimulate excessive free active oxygen species(AOS),such as superoxide radical(O2),hydrogen peroxide(H2O2),and hydroxyl radical(OH) to be formed,and excess production of highly reactive AOS would cause damage to cell membrane system and even cause death of cells.This would induce the K.obovata seedlings to be hurt or even to be dead. Synergistic effect of SOD,CAT and POD in the oxidant enzymes system could maintain the metabolic bal-ance of active oxygen in cells within a certain time.Especially,POD activity was fast stimulated to the highest degree,and kept being at a high level during aluminum treatment.Thus in this present study the POD activity might be considered as a physiological symbol of K.obovata seedlings to aluminum stress.