| Atrazine is one of the triazine herbicides used in many countries all over the world. Due to a high usage of atrazine herbicide in the black soil region of northeast China, this has resulted in a high level of toxicity, an uneasily degradable structural stability, occurrences of soil, water and underground water contaminations; thus resulting into a number of ecological problems which have drawn the attention of researchers and scholars. Bioremediation has been considered as the most effective technology to solve the problem mentioned above. Some relative researches proved that P. americanum could show better resistance to atrazine and has the potential as a remediation plant to repair atrazine contaminated soil. However, related reports on the tolerance limit to atrazine and typical physiological characteristics of resistant plant response to atrazine stress are unclear. Aiming at this problem, this study chose the black soil which is suitable for agriculture as the test soil, and selected P. americanum, which is resistant to atrazine, as the test plant. A series of experiments was conducted to monitor changes in the antioxidant enzyme activities and other indicators under atrazine stress. Furthermore, metabonomic techniques were used to investigate the in-depth root secretion response to atrazine stress in P. americanum seedlings. In order to monitor response effects in P. americanum from the seed germination to growth, physiological and metabolic stages to atrazine stress, P. americanum was treated with various atrazine concentrations. P. americanum tolerance related mechanisms of atrazine were investigated from the aspects of the antioxidant enzymes and the secretion of root exudates. The results obtained in this study could provide theoretical basis for subsequent P. americanum repair of atrazine contaminated soil.The germination characteristics of P. americanum seeds indicated that the germination rate of P. americanum seeds treated with low atrazine concentrations (0、10、20、50mg·L-1) showed no significant difference with the control plants; germination potential was inhibited obviously at high concentration (200 mg-L-1) of atrazine stress; while germination index showed no significant difference with the control plants at the same atrazine concentration.Results of growth of P. americanum seedling under atrazine stress showed that low concentration (≤50 mg·kg-1,≤ 20 mg·kg-1) did not arouse obvious inhibition of plant height and root length. Atrazine applied at 100 mg·kg-1 and 200mg·kg-1 significantly reduced plant biomass by 54.94% and 70.30%, respectively, compared to the control treatment. Atrazine at 200 mg·kg-1 obviously reduced root/shoot ratio by 41.61%. Atrazine accumulation in roots and leaves reached the maximum at 50 mg·kg-1g atrazine, and atrazine accumulation in the root was significantly higher than in the leaf. The ultrastructure of root was damaged obviously at a lower atrazine exposure level (50 mg·kg-1) than was observed in the leaf (200 mg·kg-1).Results of oxidative stress showed that the test plant did not suffer obvious lipid membrane peroxidation, this could be further confirmed by the result that the MDA content in the root and the leaf of the test plant, which did not significantly increase when treated with increasing concentrations of atrazine. The activity of the well-known antioxidants, such as superoxide dismutase(SOD), ascorbate peroxidase(APX), catalase(CAT) and peroxidase(POD) were triggered when the plants were exposure to moderate concentrations of atrazine (20 mg·kg-1 or below). The increased and more stable SOD activity in the leaf compared to the root indicated that the leaf exhibited more pronounced superoxide radical scavenging ability than the root. CAT and APX played an important role in eliminating excess H2O2 in the leaf and root of P. americanum, respectively. Furthermore, correlation analysis showed that the studied antioxidants were positively correlated with the exposure time, suggesting that the antioxidant defense in P. americanum seedlings might become stronger as the plant matures.Using metabolomics approach, the effect of different concentrations of atrazine on the condition of P. americanum root exudates composition and changes in exudate concentrations with time were studied. The SOD activity in leaf was higher than other treatments when atrazine concentration was 10 mg·L-1 and 20 mg·L-1 from 4th day to 8th day. The APX activity in leaf showed significantly positive correlation with atrazine dose from 6th day to 8th day. The content of exudates (Hexadecanedioic acid, Ethyl 2-benzylacetoacetate, N2,N2-Dimethylguanosine, N6,N6,N6-Trimethyl-L-lysine, Behenoylglycine, Sulfaphenazole) in 20 mg·L-1 atrazine treatment was significantly higher than in the control treatment under similar condition after 2 days. With the extension in processing time, the amount of root exudates secreted was restrained. High concentration of atrazine stress (100 mg·L-1 ) caused significant cell damage and inhibited the growth of aboveground part of the test plant which caused oxidative damage by O2.-, which was higher than H2O2. |
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