| In the present study, the aquatic plant Spirodela polyrrhiza was chosen as the experimental materials, the rare earth elements praseodymium (Pr) and ytterbium (Yb) were chosen as the stress factors. The stress effects of Pr and Yb in S. polyrrhiza were investigated by applying technology of plant physiology and biochemistry, high performance liquid chromatography (HPLC), tansmission electron microscopy (TEM) and fourier transform infrared spectroscopy (FTIR), combined with Western-blot technology.(1) The distribution of Pr and its influences on proportion of leaf area damaged (PLAD), fatty acids, antioxidant system, photosystem, FTIR, isozyme expression and the chloroplast ultrastructure of S. polyrrhiza exposed to various concentration of Pr (0,20,40, 60 μmol·L-1) for 20 d were analyzed. The results showed that Pr was mainly bound with cellulose and pectin. The Pr contents in different biological molecules decreased in the following order of cellulose and pectin (65%-69%), protein (18%-25%), polysaccharide (6%-10%), lipid (3%-4%). Significant increase in PLAD, MDA, cell death and decrease in soluble protein, photosynthetic pigment, Fv/Fm, Fv/Fo, oleic acid (18:1), linoleic acid (18:2), linolenic acid (18:3) and arachidonic acid (20:4) were observed in Pr-treated leaves of S. polyrrhiza. Pr caused significant effects on antioxidant system and osmotic adjustment substances. The activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) were inhibited apparently. In contrast, peroxidase (POD) activity showed increasing trend. The isoenzymic spectrum analysis demonstrated that SOD, POD and APX isoenzymic band number did not change. Meanwhile, SOD and APX isozyme expression quantity declined, POD expression quantity increased with increasing Pr level. The contents of ascorbate (AsA), glutathione (GSH), free amino acid and soluble sugars increase under Pr stress. Pr imposed serious injury on chloroplast ultrastructure of S. polyrrhiza, such as chloroplast deformation, osmiophilic granules increment, disorder of grana lamellae and disintegration of chloroplast membrane. The results of Western-blot showed that D1 and D2 protein degraded gradually as the increase of Pr concentration. The absorbance of bands near 1068, 1649,2924 and 3421 cm-1 elevated, indicating that organic substances (carbohydrate, amino acids, carboxylic acid and so on) serve as osmotic contents to enhance Pr stress.(2) After exposing S. polyrrhiza to various concentration of Yb (0,20,40,60 μmol·L-1) for 15 days, the distribution of Yb was investigated, together with the influences of Yb on surface morphology, the levels of nutrient elements, membrane system, AsA-GSH cycle, photosystem, FTIR, abscisic acid (ABA), osmolytes and isozyme expression. Our study demonstrated that the highest concentration of Yb (50%-53%) was combined in cellulose and pectin and lowest in lipid (5%) of S. polyrrhiza. Aggravated chlorosis symptoms were observed under Yb stress. The photosynthetic pigment, Fv/Fm and Fv/Fo decreased significantly, accompanied by MDA and cell death rising up after treatment of Yb. Yb stress also induced imblance of nutrient elements, such as inhibiting the contents of K, Ca and Mg, promoting the absorption of Cu. Whereas the contents of Zn, Fe and Mn increased first, followed by a decline at higher concentration. The results showed that unsaturated fatty acid content decreased significantly after Yb treatment and the IUFA in the highest concentration was 68% compared with the control. The change of ABA was measured by liquid chromatography/mass spectrometry. The research results demonstrated that ABA content in S. polyrrhiza increased first and then decreased with the increase of Yb concentration. These show that S. polyrrhiza has some adaptable protection to low concentration of Yb stress. The activities of key enzymes (APX, GR and DHAR) decreased and the contents of AsA and GSH increased in AsA-GSH cycle under Yb treatment. Meanwhile, the activities of SOD and CAT declined significantly together with POD activity was induced as the concentration of Yb rose. The proline and soluble sugars contents increased gradually with the increasing concentration of Yb. The SOD and APX isozyme expression quantity reduced, while POD expression quantity presented a trend of escalation under Yb stress. Western-blot analysis showed that the reaction centre protein D1 and D2 of photosystem Ⅱ (PS Ⅱ) degraded gradually with the increase of Yb concentration. The FTIR analysis showed that the absorption peak shape had no change, but the absorbance changed markedly, indicating that Yb changed the content instead of changing the material composition in S. polyrrhiza. |
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