| Chinese fir(Cunninghamia lanceolata) is one of the most important timber tree species in the southern China . However for long period production practice indicated continuous planting on the plot leads to productivity decline and soil degradation. Aluminum toxicity on chinese fir was considered that an important reason why continuous planting on the plot leads to soil degradation. Since the majority of phosphorus in the soil exist in the form of insoluble phosphorus, phosphorus deficiency is an important restrict factor in the growth of Chinese fir in acid soil.Low phosphorus and aluminum toxicity are the two obstacle factors limit the growth of Chinese fir. According to this background, we used 1 year old Chinese fir seedlings as experimental materials, cultivated them in Hoagland nutrient solution , analysised the influence of physiological indexes such as SOD,POD,MDA,root activity and chlorophyll fluorescence parameters and studied the tolerance mechanism under the stress of Low phosphorus and aluminum toxicity in this experiment.We expect this research offer some theoretic foundation and experimental basis to cultivate low phosphorus and aluminum toxicity tolerant varieties and some theoretic foundation to the resolution of the barriers of continuous planting. The major findings are as follows:1. Influence of low phosphorus and aluminum toxicity on lipid membranes permeability and active oxygen metabolism of Chinese fir seedlingsLow phosphorus and aluminum toxicity stress significantly affected lipid membranes permeability and active oxygen metabolism of Chinese fir seedlings. In different aluminum concentrations under low P along the extension of stress time, under the stress of low Al the activity of SOD,POD,CAT in Chinese fir's seedlings first increased and then decreased, but the rate was different,which indicated Chinese fir seedlings made adaptive response to avoid being hurt and started own protective enzyme system and reduced the stress by the physiological protective mechanism .However the activity of SOD,POD,CAT in Chinese fir's seedlings declined and the rate of decrease was bigger unden the stress of medium Al and high Al, which indicated Chinese fir seedlings had strong response and seriously affected the physiological responses. The content of chinese fir seedlings'MDA increased as time extended ,the higher the Al concentration was, the higher it increased.The result indicated that under low phosphorus conditions certain degree of aluminum stress can contribute to active the protective enzyme of Chinese fir seedlings,but the level of Al stress too deep will lead to a sharp increase in the number of free radical, inhibit protective enzyme system activity and at last lead to membrane lipid peroxidation aggravation, MDA accumulation.In different phosphorus concentrations under high aluminum along the extension of stress time, under the condition of low P and medium P the activity of SOD in Chinese fir's seedlings decreased,but unden the condition of high P the activity of SOD in Chinese fir's seedlings first increased and then decreased. Under the condition of low P the activity of POD in Chinese fir's seedlings decreased, but unden the condition of medium P and high P the activity of SOD in Chinese fir's seedlings first increased and then decreased. Under the condition of low P and medium P the activity of CAT in Chinese fir's seedlings decreased,but unden the condition of high P the activity of CAT in Chinese fir's seedlings first increased and then decreased. The content of chinese fir seedlings'MDA increased as time extended, the higher the P concentration was, the lower it increased. The result indicated that under high Al conditions high P can contribute to increase active the protective enzyme active of Chinese fir seedlings,decrease MDA accumulation ,but the level of Al stress too deep lead to a sharp increase in the number of free radical, inhibit protective enzyme system activity and at last lead to membrane lipid peroxidation aggravation, MDA accumulation.2. Influence of low phosphorus and aluminum toxicity on root activity of Chinese fir seedlingsLow phosphorus and aluminum toxicity stress significantly affected root activity of Chinese fir seedlings. Under low P, the root activity of Chinese fir seedlings inhibited 7 days later under medium Al and high Al. Under high Al, the root activity of Chinese fir seedlings inhibited 7 days later under free P and low P. In different aluminum concentrations under low P along the extension of stress time, under the condition of free Al the root activity of Chinese fir seedlings first increased and then decreased ,but under the condition of low Al,medium Al and high Al the root activity of Chinese fir seedlings decreased.In different phosphorus concentrations under high aluminum along the extension of stress time, under the condition of free P,low P and medium P the root activity of Chinese fir seedlings decreased,but unden the condition of high P the root activity of Chinese fir seedlings first increased and then decreased. The result indicated that high P can alleviate Aluminum toxicity on chinese fir to some extent, but has certain limit3. Influence of low phosphorus and aluminum toxicity on chlorophyll fluorescence parameters of Chinese fir seedlingsIn different aluminum concentrations under low P along the extension of stress time, under the stress of freeAl, low Al,medium Al and high Al Fo,qN increased,but Fv/Fm,qP decreased in Chinese fir's seedlings. Compared with the control, the decrease rate of Fv/Fm was lower in 0-7d than 7-14 and 14-21d,and the rate was lower under low Al,but higher under medium Al and high Al. In different phosphorus concentrations under high aluminum along the extension of stress time, under the condition of free P,low P, medium P and high P Fo,qN increased,but Fv/Fm,qP decreased in Chinese fir's seedlings. Analysis of variance showed that Compared with the control treatments Fo, qN, Fv / Fm, qP parameters changes were significant or extremely significant.
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