Study On Absorption, Accumulation And Tolerance Mechanisms Of Cadmium In Hyperaccumulator Solanum Nigrum L.

Heavy metal contamination has becoming one of the most severe environmental problems in world and attracting more and more attention of people on polluted treating. Phytoremediation is a technology that using plants which has the hyperaccumulation ability to heavy metals to remove contamination from soil or water. Phytoremediation technology has becoming a research hotspot owing to its low-investment, high efficiency and environmental friendly manner. The purpose of this study is to testify the plants in Xiangxi Area which had investigated of our research group weather has the hyperaccumulation ability to heavy metals. Via solution culture in greenhouse, to research the mechanism of tolerance or hyperaccumulation to heavy metals, and the results would be helpful in determining if those plant species are suitable for ecosystem recovery on mining areas and phytoremediation of metal-contaminated soils.In the present study, we sampled four solanaceae plants and soils from Xiangxi Area, Changsha, Zhuzhou, Xiangtan and Yongzhou of Hunan province, also in Nanjing, Jiangsu province of China. The contents and relationship of heavy metals such as copper (Cu), cadmium (Cd), lead (Pb), manganese (Mn) and Zinc (Zn) in plants and soils were determined and analyzed. At the same time, the absorption of Cd and other mineral elements in Solanum nigrum L., Cd-induced oxidated stress and its response mechanism of antioxidative system and phenol metabolism to Cd stress under solution culture had studied. The mainly results as follows:Five heavy metals in plants and soils sampled from Datianwan, Zhenxing and Tuanjie in Xiangxi Area were analyzed in 2006. The result showed that soil of sampled area was contaminated with different heavy metals. As for contents of Cd, Pb and Mn in four plants leaf, S. nigrum possibly has the high accumulation ability to three elements, which reached 145.9,391.2 and 5239 mg kg-1 in leaves, respectively. Lycopersicon esculentum has tolerance ability to Cd and Pb, which may inhibit and exclude them from absorption. Solanum indicum has the ability of transition and detoxification of Cd and the top content of its leaf reached 46.62 mg kg*1. Moreover, a great lot of Pb was enriched in its roots and the maximum value was 207.1 mg kg-1. We also found that considerable Cd accumulated in shoots of Capsicum annuum, which reached 42.71 mg kg-1. From the results we found that S. nigrum may have the potential ability of hyperaccumulation to Cd and other heavy metals. S. indicum may have the ability of repellency and detoxification of Pb and Cd. They may use for phytoremediation research on heavy metal contaminated soil.Because of the potential hyperaccumulation ability to Cd in S. nigrum plants, the plants and soils were sampled again in other place in 2007. The results showed that Cd in soils of five areas exceeded the critical value of III kind soil of China (Cd?1.0 mg kg-1) and the soils were severe contaminated with heavy metals, especially in Changsha and Zhuzhou areas. The Cu contents in Nanjing, Mn contents in Xiangtan and Yongzhou higher than other areas, and the maximum value reached 1008,17032 and 22786 mg kg-1 respectively. We found that Cd contents reached 176.9,197.4 and 187.2 mg kg-1 in S. nigrum root, stem and leaf, respectively, which illuminated high ability of Cd accumulating and transferring of plants. From the data of S. nigrum and soils sampled in two years, we found that there were best remarkable pertinence of Cd content between plants leaves and soils, as well as the Cd content and Pb, Zn content in S. nigrum leaves.Under solution culture with Cd addition, the root elongation of S. nigrum was inhibited, water quantity and dry weight also decreased remarkably after Cd treatments for 15 days. The contents of Cd in every parts of S. nigrum were increased with the increasing of Cd treatment concentrations. With 500 ?M Cd treatments, the contents reached 8729, 1059 and 472.8 mg kg-1 in root, stem and leaves, respectively, which showing high hyperaccumulation ability of Cd in plants. The ratio of Cd contents in organelle fraction of roots and leaves were higher than other fractions (>50%), indicated the important strategy of Cd detoxification in plant may organelle sequestration. Cd treatment had influenced the absorption and accumulation of other elements. The absorption of Mn and Mg of S. nigrum were inhibited with Cd addition. Lower Cd concentration promoted Cu, Fe and Ca absorption, however, high Cd concentration had opposite effects on these elements. Zn contents had no remarkable change under lower Cd treatments, but decreased in root, increased in stem and leaf with higher Cd concentration treatments.It was found that Cd treatments decreased the chlorophyll a, b and carotenoids contents remarkably, also the root vigor with 200 ?M Cd treatments. However, proline contents and reactive oxygen species (ROS) accumulated significantly in plants. On the other hand, the growth and differentiation of xylem and phloem in root or leaf were also restrained with Cd addition. Total ascorbic acid content increased significantly in root of S. nigrum with 200 ?M Cd treatment, which increased about 43%, as compare to control plant. But, the ratio of AsA and DHA decreased form 5.787 to 1.634, which suggested the active response of plant to Cd-induced oxidated stress.After Cd treatments for 3 days, a marked increase of thiobarbituric acid reactive substances (TBARS), hydrogen peroxide (H2O2) and superoxide anion (O2·-) levels in plants were found. Activities of antioxidative enzymes, such as catalase (CAT), glutathione peroxidase (GSH-Px) and superoxide dismutase (SOD) were increasing obviously in plant roots, in the presence of Cd. The experiments with inhibitors showed that the major source of ROS may nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and POD, and GSH-Px, CAT, SOD may have important role in ROS scavenging. Under Cd treatments, total thiols, including protein thiols (PT) and nonprotein thiols (NPT), were increased obviously on plant roots, but reduced glutathione (GSH) and cysteine (Cys) contents decreased. PCs content had increased form 38.19 of control plant to 197.5 (n mol SH g"1 FW) with 200?M treatment. The increasing of phytochelatins (PCs) and other NPT materials may the important strategy for Cd detoxification in plants.N-acetyl-L-cysteine (NAC, an important precursor of GSH) addition alleviated Cd-induced growth inhibition and oxidated stress in S. nigrum plants. NAC strongly increased the Cd-induced up-regulation of antioxidant enzyme activities, thus ROS level and TBARS degree were decreased. After absorption NAC deacetylated rapidly to provide cysteine for cellular GSH synthesis, as well as NPT contents, which increased about 35% to control plant. Meanwhile, PCs content reached 260.4 nmol SH g-1 FW. The results of gel filtration showed that Cd mainly binding to nonprotein small molecular weight organic substance, and the addition of NAC increased the ultraviolet absorption of protein peak especially nonprotein small molecular weight peak of elution liquid, i.e. increased the NPT contents. These results suggest that exogenous NAC can increase antioxidative enzymes activities and PCs contents to scavenge ROS and detoxify Cd in S. nigrum roots, which may the important means in alleviation of Cd toxicity.With Cd treatments, the contents of total soluble phenol, flavonoids, lignin, shikimic acid and free amino acid in S. nigrum roots increased remarkably. The activities of glucose-6-phosphate dehydrogenase, cinnamate-4-hydroxylase, cinnamyl alcohol dehydrogenase, Laccase and ?-Glucosidase of shikimic acid and phenylpropanoids metabolism include anabolic process of lignin in S.nigrum root increased notably compared to control plants. Using high performance liquid chromatography we found that the contents of gallic acid, salicylic acid, vanillic acid, caffeic acid, ?-coumaric acid and ferulic acid were increased remarkably with Cd treatments, and the maximum reached 1.4 fold,1.6 fold,1.7 fold,2.1 fold,1.7 fold and 1.9 fold to control plant, respectively. The increasing of phenol substance contents include soluble phenol, flavonoids, lignin and phenolic acids which may have anti-oxidated and chelation function to heavy metal, indicated the actively response to Cd oxidated stress in S. nigrum, and the possible mechanism were free radical scavenging and chelating function on Cd.