| To reveal the mechanism of phytoremediation of Malva sinensis Cavan. This research used M. Sinensis in Kangding county, Sichuan province, as the test material. The change characteristic of rhizosphere environment and root physiological response of M. sinensis under cadimum stress were conductd by soil culture and hydroponics experiments. Below are the main results.?1? In the soil culture experiment, when the Cd concentration is 5 mg·kg-1, the height,the biomass of underground and the biomass of aboveground were observed no significant difference in M. sinensis. The growth of M. sinensis was inhibited when the Cd concentration is equal or greater than 25 mg·kg-1. With the increase of Cd concentration in the soil, the Cd content of aboveground and underground of M. sinensis increased. When the Cd concentration was up to 200 mg·kg-1, the Cd content of aboveground and underground of M. sinensis increased up to the maximum respectively (153.11 mg·kg-1 and 120.07 mg·kg-1). Under each treatment, the plant's Cd coefficient of concentration and transport coefficient are all over 1. These results showed that M. sinensis had strong tolerance and enrichment ability to Cd and was able to transport Cd to the aboveground parts. The pH of rhizosphere soil was lower than non-rhizosphere in each treatment. With the increase of Cd concentration, the value of pH of decreased. When Cd concentration was 200 mg·kg-1, the pH of rhizosphere soil was 6.86. Each component of rhizosphere soil organic carbon was greater than it in non-rhizosphere. With the increase of the concentration of Cd, soil dissolved organic carbon ?DOC? showed a trend of first increasing and then decreasing and water soluble organic carbon ?WSOC? showed a trend of decreasing, but the trend of readily oxidation carbon ?ROC? was opposite to WSOC. The increasing of DOC in rhizosphere soil promoted Cd transferred into exchangeable form from weak oxidizable and residual form. This suggested that M. sinensis was able to promote activation of Cd by aciding rhizosphere soil. The increase of ROC content showed that M. sinensis had the capability of keeping the quality of soil under the Cd stress. The value of rhizosphere soil microbial biomass carbon, soil foundation breathing and soil bacterium, fungus and actinomycetes were higher than them in non-rhizosphere soil, and the value of them showed a trend of increasing and then decreasing. When the concentration of Cd was 5 mg·kg-1, rhizosphere soil catalase and phosphatase activity reached to the maximum value ?0.36 and 1.19 units?. When the concentration of Cd was 50 mg·kg-1, the urease activity increased by 72.33%. The soil cuture experiment showed that M. sinensis can not only promote the activation of Cd by increasing microbial activity which has the function of activating heavy metal, but also relieve the harm of Cd from M. sinensis by using three enzymes to increase microbial adsorption fixed ability.?2? In hydroponic experiment, Cd had a certain effect on the root of length, total surface area, total volume and average root diameter. When the Cd concentration in the nutrient solution was 5 ?mol·L-1, the each index of root morphology of M. sinensis index did not decrease significantly compared with the control ?P<0.05?. The roots number and crossing number of M. sinensis were not significantly inhibitted under the higher Cd concentration. The trend of root activity of M. sinensis was similar to the trend of their root morphology. M. sinensis root morphology and root characteristics were able to be regarded as the performance of strong tolerence to Cd. Root H2O2 and malondialdehyde ?MDA? content of M. sinensis increased with the increasing of Cd concentration, and the maximum values were 1.16 times and 1.39 times respectively compared with control, but root H2O2 and malondialdehyde ?MDA? content of M. sinensis didn't change significantly at low Cd concentration. Wth the Cd concentration increased, Glutathione ?GSH? and ascorbic peroxidase ?APX? increased first and then decreased. And when the Cd concentration is 5 and 10 ?mol·L-1, them reached the maximum value of 310.28?g·g-1 and 32.4?g·g-1 respectively. The trend of Proline ?Pro? content was similar to the trend of GSH. With the Cd concentration increased, the soluble sugar and soluble protein content were still rising. And the maximum value of them were respectively increased by 15.29% and 2.21%. Meanwhile, with the increase of Cd concentration, the organic acid content exuded by M. sinensis root increased. These results showed that M. sinensis roots utilized its own antioxidant defense system and osmotic adjustment mechanism to relief the oxidative damage and lipid peroxidation caused by Cd stress.In conclusion, when the M. sinensis suffered from Cd stress, it can alleviate Cd poison by increasing rhizosphere soil enzyme activities and heavy metal adsorption capacity of fixed-microbial activity and also can use its own antioxidant defense system and the osmotic adjustment mechanism to cope with the stress of Cd. The M. Sinensis can adjust the pH of rhizosphere soil, DOC content, phosphatase and part of microbials with activated ability to promote plant absorption of the Cd. |
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