La And Ce Study In Vivo Accumulation Elodea And Toxicological Effects

In the present study, the aquatic plant——Elodea Canadensis was chosen as the experimental material, the most abundant rare earth elements in the earth’s crust——lanthanum(La) and cerium(Ce) were chosen as the stress factors. The accumulation, subcellular distribution, combine forms and toxic-effects of various concentrations of rare earths in aquatic plants were investigated by applying technology of plant physiology and biochemistry, combined with transmission electron microscopy technology in pot experiment. The results are as follows:(1) The subcellular distribution, chemical forms of La and its influences on mineral elements, photosynthetic pigment, malondialdehyde (MDA), reactive oxygen species (ROS) generation, antioxidative systems (antioxidant enzymes and micromolecule antioxidants) and the ultrastructure of E. Canadensis exposed to various concentration of La (0,5,10,15,20mg·L-1) for7d were analyzed. Our results demonstrated that the accumulation of La was found to increase in a concentration-dependent manner and La content in E. canadensis under La stress for7d was up to1956.9±57.2μg·g-1FW. Accumulation of La in subcellular fractions was highest in cell walls (83.8%-86.0%), intermediate in organelles (8.2%-9.3%), and lowest in soluble cellular fraction(5.4%-7.4%), which indicated that the cell wall was the most important site of La storage in E. Canadensis. Levels of La in different chemical forms were:cellulose and pectin> protein> polysaccharides> lipid. A remarkable imbalance of nutrient elements was occured in La-treated plants, mainly including decreased absorption of Ca、K、Mg and Mn. Aggravated chlorosis symptoms were observed with increasing concentrations of exogenous La. The content of photosynthetic pigment (including chlorophyll and carotenoids) decreased significantly with increased La concentrations, accompanied by substantial accumulation of MDA and ROS in La-treated plants. La stress caused various effects on antioxidant system. The peroxidase (POD) activity declined first and then increased, which was higher than control level at20mg·L-1La. Whereas the activities of superoxide dismutase (SOD)、catalase (CAT)、ascorbate peroxidase (APX) and glutathione reductase (GR) decreased gradually. The contents of the antioxidants [ascorbate (AsA)、glutathione (GSH)、non-protein thiols (NP-SH)、phytochelatins (PCs)] increased under La treatment in lower concentration and then declined at higher concentration, which were still higher than the control level at the highest La concentration. Electron microscopy observation indicated that La imposed serious injury on ultrastructure (especially chloroplast and mitochondria) of E. Canadensis, such as swelling of chloroplast and mitochondria, vacuolation of thylakoid and disorderly arrangement of cristae under lower La level. The extent of ultrastuctural damage was much more serious with the increasing of La concentration:disappearing of the outer membranes of chloroplast, vacuolization and further disintegration of mitochondria and chloroplast. The50%effective concentration (EC50) and the maximum permissible concentration (MPC) of La for a7d exposure period for E. canadensis was3.1and0.31mg·L-1, respectively. The data presented in this paper have demonstrated that La is mainly isolated in the cell wall, with cellulose and pectin as the main storage form of La in E. canadensis. Physiological changes that accompanied Ce accumulation in cells included disruption of nutrient accumulation, decreased content of chlorophyll, lipid peroxidation, ROS production, and damage of the antioxidant system and ultrastructure.(2) After exposing E. canadensis to various concentrations of Ce (0,5,10,15,20mg·L-1) for7days, the subcellular distribution and chemical forms of Ce were investigated, together with the influences of Ce on the levels of nutrient elements, photosynthetic pigments, MDA, ROS and the activities of antioxidative systems. The results showed that Ce content in E. Canadensis augmented obviously with the increase of concentration of Ce added in culture medium, which was up to2008.7±6.7μg·g-1FW at20mg·L-1Ce. Accumulation of Ce in subcellular fractions decreased in the order of cell wall (75.4%-81.4%)> organelle (13.0%-20.4%)> soluble fraction (3.7%-5.6%). The large amount of Ce was located in cellulose and pectin (84.1%-97.6%) of E. Canadensis and lowest in lipid (0.6%-1.0%), which was consistent with the distribution patterns of Ce in the subcellular components. Ce stress induced remarkable disorder of nutrient elements, such as inhibiting the absorption of Ca and Mg significantly. Whereas the content of Fe、K and Mn declined progressively first, followed by an increase at higher concentration. As Ce concentration increased, the contents of chlorophylls declined distinctly. At20mg·L-1Ce, the total chlorophyll, chlorophyll a and b decreased by75.7%,78.2%and68.5%as compared to control, respectively. MDA and hydrogen peroxide (H2O2) content as well as superoxide anion (O2-) generating rate rose progressively with increasing Ce level. At the same time, Ce caused significant changes in the activities of antioxidative enzymes. SOD、APX and GR activities were inhibited apparently. In contrast, POD and CAT activity showed increasing trend. The content of GSH、AsA、NP-SH and PCs shared the same variation trend, with enhanced levels at exogenous Ce concentrations of10-15mg·L-1, but with lower levels at the highest Ce concentration that were still higher than the control level. The50%effective concentration (EC50) of Ce to E. Canadensis for7d of exposure was3.6mg·L-1and the critical concentration (MPC) of Ce in aquatic environment was0.36mg·L-1.(3) Similar to La, Ce is mainly isolated in the cell wall, with cellulose and pectin as the main storage form in E. Canadensis, which resulted in considerable physiological and ultrastructural injury. These results indicate that a high level of rare earths may exert an adverse effect on aquatic ecosystems.