Effect of Exposure Levels and Exposure Time on Distribution of Cadmium Species in Indian Mustard (Brassica Juncea)

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.