| Aluminum toxicity in acid soil has become the main limiting factor of the production and quality in rape. As a middle Al-resistant plants. Rape is vulnerable to aluminum toxicity. Currently, the research concentrated on the components, time and sites of organic acids in root exudates under Al stress. It provides a better theoretical basis on explaining the mechanism of aluminum tolerance in plants, but the specific regulatory mechanism is still not clear. In this experiment, we chose Al-resistant variety (Zhongshuang No.7) and Al-sensitive variety (Huayou 2790) of rape as the materials and added citric acid and inhibitors by pre-screened. We studied growth, physiological characteristics and the regulation of root exudates in rape under aluminum stress. We used culture method to find the role and mechanism in growth process to provid a theoretical basis on the security of rape production in acid rain regional, the prevention and treatment mechanisms of aluminum toxicity and tolerance. The results of this study are summarized as follows:1. The selective results of the best concentration of organic acids and inhibitors showed that though combined treatments of 5,10,20μmol/L citric acid or malic acid, A9C or Mersal and 200μmol/L Al3+. We can find that adding organic acid can decrease the inhibition rate of relative root length, and with the increasement of organic acid concentration, the mitigation was better. The effect of citric acid was better than malic acid. Therefore we chose 20μmol/L citric acid as the best concentration of exogenous organic acids. Adding A9C had great harm to rape itself, but adding 5μmol/L Mersal does not inhibit the growth. After adding aluminum, the root length had nearly 50% inhibition rate under high Mersal concentration. Therefore we chose 5μmol/L Mersal as the best concentration of exogenous inhibitor. 2. Al stress decreased the relative root length, total root length and total surface area in two varieties of rape, and the decline deal of 7d was greater than 3d treatment. Al-sensitive variety (Huayou 2790) declined more than Al-resistant variety (Zhongshuang No.7). The inhibition of plant height was less than root length, and only after 7d processing, the leaf area, width, length began to show significant inhibition. Al stress decreased the chlorophyll content of rape, and decreased Fm, Fv/Fo, (?)pPSⅡ, increased Fo, Fv/Fo decline significantly. Low concentration of aluminum stress increased protein content slightly,but under high Al concentration, it decreased significantly. When dealing with 7d, APX, PPO increased, the SOD of Huayou 2790 increased, but Zhongshuang No.7 decreased. The CAT, POD, MDH, PEPCase, APA of both varieties increased firstly and then decreased. ACO activity decreased, but with the increasement of aluminum concentration and processing time, the activities of metabolic enzymes declined. Al stress also increased aluminum contents in roots and shoots. It was about 4 times than shoots. Under low concentrations of Al stress, citric acid, malic acid significantly increased, the total organic acid content also increased, but with the increasement of Al concentration, the organic acid content decreased. Amino acid did not change obviously under aluminum stress. After processing 7d, the soluble sugars in Zhongshuang No.7 increased first and then decreased with the increasement of Al concentration. Under Al stress, pH value in root exudates decreased significantly, and the pH value of Zhongshuang No.7 is greater than Huayou 2790. Low concentrations of aluminum stress increased the conductivity, but with the increasement of aluminum concentration, the conductivity declined. Summary above, the toxicity of Aluminum on the rape had differences in sites, varieties and times. It had a certain extent of effects in the physiological characteristics, and increased aluminum accumulation in roots and shoots of rape. Under low concentration of aluminum stress, rape secreted citric acid, malic acid to protect against Al toxicity, but with the increasement of Al concentration and treatment time, the tolerance mechanism was damaged.3. After adding 20μmol/L citric acid, the morphological indexs of roots and leaves in two varieties of rape were increased to some extent compared with single-aluminum treatment. The chlorophyll content, Fo value was decreased and other fluorescent indicators were increased. Under short term (3d) Aluminum treatment, the protein content, antioxidant enzyme activities were not significant affected. After processing 7d, the protein content of Huayou 2790 significantly decreased, APX, CAT significantly increased. SOD, PPO increased firstly and then decreased, POD did not change. The change of CS after adding organic acid was different in two varieties. ACO decreased with the increasment of Al concentration. MDH, PEPCase increasd firstly and then decreased. After the accession of organic acids, the aluminum contents in roots and shoots decreased significantly. Citric acid content increased significantly, but malic acid changed a little, the total organic acid content also increased. The total amino acid content still had no significant change. The soluble sugar content increased at first and then decreased. pH values decreased, when dealing with 7d, pH value became stable. The conductivity increased. To sum up, adding organic acids can relief Al toxicity in roots and leaves of rape to a certain excent. The plant returned to normal physiology and metabolism level. It also decreased accumulation of aluminum, increased citric acid content in rape. So the plant enhanced aluminum tolerance.4. After adding 5μmol/L Mersal, each index of roots and leaves in two varieties were significantly decreased compared with single-Al treatment, and the aluminum toxicity transferred to shoots. The chlorophyll content, the index of chlorophyll fluorescence parameters did not change significantly. The protein content, SOD, CAT has declined, PPO was slightly less than the single-Al treatment group, and APX, POD increased slightly under low concentration of Al treatments. After adding exogenous inhibitors, the metabolic activity did not change significantly under short-term Al treatment, but with increasement of Al treatment time, CS, PEPCase, APA and other activities were significantly decreased, while the ACO, MDH and other enzymes changed little. After adding Mersal, the accumulation of aluminum contents in roots and leaves increased. The secretion of citric acid in rape roots decreased significantly, malic acid secretion of roots, total organic acids increased, but total amino acid content was no significant change. The soluble sugar content increased. The pH value was bigger than the single Al treatment group. The conductivity increased slightly. Overall, the secretion of organic acids in root mainly affected through the regulation of protein content, SOD, CAT and other antioxidant enzymes, CS, PEPCase, APA and other metabolic enzymes to alleviate Al toxicity. In addition, rape can increase the secretion of malate, soluble sugars, etc. to against low concentration, short period Al stress, but with the increasement of aluminum concentration and treatment time, the aluminum toxicity further deepened. |
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