Subcellular Distribution, Morphology And Toxicological Effects Of Yttrium (Y) In Higher Aquatic Plants

In the present study, heavy rare earth elements(HREEs)-yttrium(Y) was chosen as the stress factor, the aquatic plants-Elodea Canadensis and Nymphoides peltata were chosen as the experimental materials. The accumulation, subcellular distribution, combine forms and toxic-effects of various concentrations of rare earths(REEs) in aquatic plants were investigated by applying technology of plant physiology, claser scanning confocal microscope(CLSM), Inductively Coupled Plasma Atomic Emission Spectrometry(ICP-AES) and biochemistry, combined with transmission electron microscopy technology in pot experiment. The results are as follows:(1) Bioaccumulation, subcellular distribution and toxicity of Y were evaluated in E. Canadensis. The effects of Y concentrations of50-200μmol·L-1applied for7d were assessed by measuring changes in photosynthetic pigments, nutrient contents, enzymatic and non-enzymatic antioxidants and ultrastructure. The highest concentration of Y was found in the cell wall (68-76%), followed by the cell organelles (16-20%) and then the soluble fraction (7-12%). The accumulation of Y in biological molecules decreased in the order of Cellulose and pectin> protein> polysaccharides> lipid, much more Y was located in cell wall. The content of some mineral elements (Mg, Ca, Mn and K) declined in E. canadensis, Fe levels reached a peak value at150μmol·L-1. A sharp reduction was noted in the levels of chlorophyll a (Chi a), chlorophyll b (Chi b) and carotenoids (Car) in the Y-treated leaves of E. canadensis compared with the controls. Chloroplast spontaneous fluorescence intensity is abate, analysis of the proportion of leaf area damaged (PLAD) found that the leaf damage degree increases with the increase of Y treatment concentration, the data was as high as98%at200μmol·L-1Y. Meanwhile, The activities of peroxidase (POD), catalase (CAT), glutathione reductase (GR) decreased significantly for all Y concentrations. In contrast, the contents of the antioxidants ascorbate (AsA) and glutathione (GSH) were induced, while initial rises in superoxide dismutase (SOD) activity was followed by subsequent declines. Y caused significant changes in the activities of antioxidative enzymes, but malondialdehyde (MDA), hydrogen peroxide (H2O2) and superoxide anion (O2·-) content rose progressively with increasing Y level. Morphological symptoms of senescence, such as chlorosis and damage to chloroplasts and mitochondria were observed even at the lowest Y concentration. Pigment content decreased as the Y concentration rose and the calculated EC50and MPC of Y for E. canadensis were107μmol·L-1and10.7μmol·L-1after7d of exposure, respectively. The results showed that exogenous Y was highly available in water and that its high concentration in water bodies might produce harmful effects on aquatic organisms in E. Canadensis.(2) After exposing Nymphoides peltata to various concentrations of Y (0,10,20,40,60μmol·L-1) for4days, the subcellular fractions, chemical form, physiology and ultrastructure were investigated. In this study, the accumulation of Y in subcellular fractions decreased in the order of cell wall> organelle> soluble fraction. A large amount of Y was located in cellulose and pectin and was much higher than in other biomacromolecules. H2O2content and O2-generating rate rose progressively with increasing Y level, the behavior caused the decrease in heavy metals the contents of unsaturated fatty acids and then the increase in the peroxidation of membrane lipid, thereby the structure of N. peltata cell was damaged. The content of some mineral elements (Mg, Ca, Fe, Mn and Mo) also increased in N. peltata as the concentration of Y increased, but there was an opposite effect on the content of other mineral elements (P and K). Meanwhile, the content of AsA and CAT, APX activity decreased significantly as the concentration of Y rose. In contrast, POD activity was induced, while initial rises in the activities of SOD, GR and the content of GSH were followed by subsequent declines. Morphological symptoms of senescence, such as chlorosis and damage to chloroplasts and mitochondria were observed even at the lowest Y concentration. Pigment content decreased as the Y concentration rose and the calculated50%effective concentration (EC50) and the maximum permissible concentration (MPC) of Y for N. peltata was26μmol·L-1and2.6μmol·L-1after4days of exposure, respectively. Elevated Y levels may represent a potential risk for aquatic ecosystems.(3) Similar to N. peltata, Y is mainly isolated in the cell wall, with cellulose and pectin as the main storage form in E. Canadensis, but have some Y can enter into cell cause this resulted in considerable physiological and ultrastructural injury, These results indicate that a high level of REEs may exert an adverse effect on aquatic ecosystems.