Effect of Exposure Levels and Exposure Time on Distribution of Cadmium Species in Indian Mustard (Brassica Juncea)
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摘要: 螯合作用是植物对细胞内重金属耐受的主要方式之一，植物中螯合肽(PCs)在植物耐重金属毒害中的作用己有许多报道，但作用程度并未得出一致的结论，关于PCs是在镉刺激下直接合成还是以谷胱甘肽为底物合成同样存在争议。本文研究了胁迫浓度和胁迫时间对超积累植物印度芥菜(Brassica juncea)中镉形态分布的影响。印度芥菜幼苗分别用0.5、1.0、3.0、5.0、10.0 mg/L镉标准溶液胁迫24 h、48 h、72 h、96 h后收获，用体积排阻高效液相色谱-电感耦合等离子体质谱技术测定植物体根部和叶部中镉形态的含量。结果表明，在低胁迫浓度下(≤3.0 mg/L)，植物叶中PCs-Cd的含量与胁迫浓度成正比；在高胁迫浓度下，PCs-Cd含量反而降低，根中PCs-Cd的含量持续增加，但叶中PCs-Cd总量高于根部，说明PCs在植物体内会由根部向叶部转移，从而提高了镉耐受性。在持续长时间胁迫下，PCs-Cd含量也降低，表明PCs在镉解毒机制中仅有短暂的作用；持续高浓度胁迫下，植物会引发其他机制来抵制Cd的毒性。研究认为PCs在镉解毒机制中的作用需要考虑胁迫时间和胁迫浓度这两个重要参数。Abstract: Chelation is one of the main ways to tolerate the heavy metals in the cells of plants. Phytochelatin (PCs) was reported to have detoxification and compartmentalization of heavy metals, but no accordant conclusion for its contribution. Whether the PCs were directly synthetized under the stimulation of Cd or synthetized by glutathione is still an unsolved issue. In this paper, a study is described examining the relationship between exposure levels, exposure time and Cd tolerance. Root and leaf samples were placed in 0.5, 1.0, 3.0, 5.0 and 10.0 mg/L Cd standard solutions and harvested after 24, 48, 72, 96 hours. The Cd species in the root and leaf samples were measured by using Size-Exclusion High-Performance Liquid Chromatography and Inductively Coupled Plasma-Mass Spectrometry. Results indicated that PCs-Cd contents were positively correlated with Cd exposure levels when the lower Cd exposure levels were less than 3.0 mg/L. Under the higher exposure levels, the contents of PCs-Cd were reduced, the PCs-Cd contents in root samples were continually increasing, but lower than those found in the leaf samples, which indicated that the PCs were transported from root to leaf with a higher tolerance of Cd. Increasing exposure time also reduced PCs-Cd production which indicated PCs may only have a temporary role in metal resistance. Under continuous higher exposure, plants may trigger other mechanisms that tolerate heavy metal toxicity. Our results suggest that concentration and time of exposure are important factors that must be taken into consideration when evaluating the true role of PCs in heavy metal detoxification.
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Key words:
- Indian mustard /
- cadmium species /
- exposure levels /
- exposure time /
- tolerance
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