| Acid aluminum is an important limited factor of rape production and quality, as a middle Al-resistant plants, rape has a lower resistance to aluminum toxicity compared with soybean, rice and other crops, therefore it's vulnerable to aluminum toxicity. In recent years, the effect of root exudates on the plant external resistance to aluminum toxicity has become a research hotspot. In this study, we chose Al-resistant variety of Zhongshuang No.7 and Al-sensitive variety of Huayou 2790 during the 16 different varieties of rape as experiment material. We used hydroponic and soil culture method, collected root exudates through the solution collection method, made a study on the changes characteristics of the main contents of rape root exudates under aluminum toxicity. We studied the effects of rape root exudates on soil nutrient availability, the numbers of microorganisms, soil biochemical strength, soil enzyme activities and soil aluminum forms under aluminum toxicity, we also verified the study through simulation experiment on the soil group treated with root exudates. We are supposed to reveal the role and mechanism of the process of rape root exudates allievating aluminum toxicity, and provide a theoretical basis for further research. The results of this study are summarized as following:1. The selective results of Al-resistant varieties of rape showed that the root relative elongation of Zhongshuang No.7 and Xiangyouza No.2 were 98.6% and 92.2% respectively, which suggusted that their root relative elongation were significantly higher than other varieties of rape, and they had a higher tolerance to Al toxicity. The root relative elongation of Huayou 2790 and Zheshuang 758 were 46% and 50% respectively, which suggested that they had a lower root relative elongation and were sensitive to Al toxicity. So we chose the most Al-resistant variety of Zhongshuang No.7 and the most Al-sensitive variety of Huayou 2790 as the materials of our experiment.2. The experiment on the characteristics of rape root exudates under Al stress showed that the Al toxicity promoted the exudation contents of citric acid, malic acid, amino acids, soluble sugar and total phenolic of rape root exudates, increased acid phosphatase activity and root membrane permeability and reduced the pH of root exudates. The root exudates components and enzyme activity of Al-resistant variety of Zhongshuang No.7 were higher than Al-sensitive variety of Huayou 2790, while the root membrane permeability and root exudates pH were lower than Huayou 2790. The exudation contents, enzyme activity and root membrane permeability were increased and pH was significantly reduced under short time Al stress (2 h,4 h,8 h), but the exudation contents and enzyme activity increased significantly after collected for 6 h, and after 12 h the exudation contents and enzyme activity of each group had a slow increase or decrease. To sum up, all the components of rape root exudates under aluminum toxicity responded to aluminum toxicity, and had a certain correlation with the remission and resistance to Al toxicity.3. The results of the effects on soil nutrient availability of rape root exudates under aluminum stress showed that after the rape was treated for 7 d with low and middle Al concentration (less than 0.2 g/kg), organic acids and amino acids of rape root exudates showed an increase trend, and both of them were inhibited significantly under high Al concentration (0.4 g/kg), while the soluble sugar contents were increased continuously in both low and high Al concentration. With the extension of processing time, under the each Al concentration (except 0 g/kg Al concentration), the contents of organic acids and amino acids of rape root exudates were reduced significantly compared with 7 d, while the soluble sugar content remained increased with a small margin. Rape root exudates had a certain activation on soil phosphorus and kalium, and the activation will be inhibited when the Al concentration was increased and the processing time was prolonged. The contents of root exudates organic acids and amino acids of Zhongshuang No.7 were higher than Huayou 2790 under Al toxicity, and Zhongshuang NO.7 also had a higher activation capacity on soil nutrient availability than Huayou 2790. 4. The study on the effects of rape root exudates on soil microecology under aluminum toxicity showed that along with the aluminum concentration increased, the numbers of soil microorganisms, soil biochemical strength and enzyme activities of control group and non-rhizosphere soil group would decrease, which indicated the effects of soil aluminum toxicity. When the soil was treated with the rape root exudates, the toxicity effects were alleviated. Along with the increase of Al concentration, the indexes showed a downward at first and then an increased trend, and reached the maximum value when the aluminum concentration was 0.1 or 0.2 g/kg. With the extension of processing time, the capacity of root exude was weakened, so that the numbers of soil microorganisms, soil biochemical strength and enzyme activity of each group were decreased. Overall, rape root exudates enable the indicators to show a significant rhizosphere effect, and Zhongshuang No.7 was more significant than Huayou 2790.5. The experiment on the effects of rape root exudates under aluminum stress on soil aluminum forms showed that under the same processing time and Al concentration, the aluminum forms of non-rhizosphere soil group had no significant change compared with the control group. The contents of soluble aluminum, exchangeable aluminum(Al3+), monomer hydroxy aluminum[Al(OH)2+, Al (OH)2+)] and acid-soluble inorganic aluminum [Al(OH)3] of both the rhizosphere soil group and the exogenous root exudates soil group were decreased significantly, and the contents of various soil aluminum forms were decreased more markedly under the Zhongshuang No.7 root exudates, while the contents of humic acid aluminum(HA-Al) were increased significantly. With the increase of Al concentration, The contents of various aluminum forms also increased and reached a significant level; After Al treatment for 14 d, the monomer hydroxy aluminum [Al(OH)2+, Al(OH)2+] content increased slightly, humic acid aluminum(HA-Al) content increased significantly, while the exchangeable aluminum (Al3+), soluble aluminum and acid soluble inorganic aluminum [Al(OH)3] content were reduced significantly. The determination results of soil pH showed that, under the influence of rape root exudates and 0 g/kg aluminum treatment, the soil pH of various soil group was lower than the control group. Along with the increase of treatment concentration, the soil pH of rhizosphere soil group and exogenous root exudates soil group were higher than the control group and with a moderate change. This showed that the root exudates had a certain degree of acidification effect on the rhizosphere soil. Rape root exudates could alleviate aluminum toxicity to some extent, and the effect on Al-resistant variety was more prominent than Al-sensitive variety, while the soil pH had a direct effect on aluminum forms changes, the root exudates and aluminum forms had an indirect link.
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