| In the present study, floating plants (Spirodela polyrrhiza) were chosen as the experimental materials, Pb and Cd, which are non-essential elements for growth and development of plants, were chosen as the stress factors. The accumulation, subcellular distribution, chemical forms, the physiological and biochemical effect of Pb and Cd in S. polyrrhiza were investigated to explore the toxicological effects of heavy metals in plants. Meanwhile, some theoretical supports for the resistance mechanism of plants were provided. The results were as follows:1 The effects of Pb concentration (0,20,40,60,80 μmol·L-1) applied for 7 d were assessed by measuring changes in subcellular distribution, nutrient contents, chemical forms, photosynthetic systems, antioxidant system and chloroplast ultrastructure. The highest concentration of Pb was found in the cell wall (70%-79%), followed by the soluble fraction (11%-16%) and the cell organelles (9%-15%). In Pb chemical forms, NaCl extractable Pb (48%) were predominated, followed by HAc and HC1 extractable Pb (22% and 14%). Ethanol extractable Pb and d-H2O extractable Pb (7% and 6%) were least. Pb stress led to nutrient imbalance in S. polyrrhiza, the content of some mineral elements (P and Mg) declined. The contents of K and Fe elevated. Ca and Zn levels reached a peak value at 20 μmol·L-1. The contents of Mn and K changed little. As the increase of Pb concentration, the color of the leaves gradually faded and stomata gradually closed. Analysis of the proportion of leaf area damaged (PLAD) found that the leaf damage degree increased to promote cell death. A sharp reduction was noted in the levels of chlorophyll a (Chl a), chlorophyll b (Chl b) and carotenoids (Car) in the Pb-treated leaves of S. polyrrhiza compared with the control. Chlorophyll fluorescence parameters (Fm, Fv, Fv/Fm and Fv/Fo) gradually declined and Fo rose with the increase of Pb concentration. It has been found that the chloroplast structure appeared to be in different degree damage by TEM. The chloroplast grana lamellae were severely damaged, material content lossed, and osmium autophagy particles increased. Pb with granular aggregation deposited in the cell wall. Pb stress induced plant to produce reactive oxygen species, leading to membrane lipid peroxidation. MDA, which is membrane lipid peroxidation product, its content increased significantly. Analysis of fatty acid content found that unsaturated fatty acid content decreased and saturated fatty acid content increased. The activities of superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR) decreased significantly for all Pb concentrations. In contrast, the contents of the antioxidants ascorbate (AsA) and glutathione (GSH) were induced, while peroxidase (POD) and ascorbic acid peroxidase (APx) activities initially rose, and declined subsequently. Pb treatment affected the change of osmotic regulation substances in S. polyrrhiza. The content of soluble protein declined the soluble sugar content and proline content increased. The results of SDS-PAGE showed that polypepitide components degrade gradually with the increase of Pb concentration. Western-blot analysis indicated that D1, D2 and CP47 had a tendency to reduce.2 The effects of Cd concentration (0,2.5,5,7.5,10 μmol·L-1) applied for 4 d were assessed by measuring changes in Cd accumulation, subcellular distribution, nutrient contents, fatty acids content, physiological and biochemical function of Cd in S. polyrrhiza. In this study, Cd content increased with the increased concentration of Cd treatment, the accumulation of Cd in subcellular fractions decreased in the order of cell wall (52%-61%)> soluble fraction (39%-46%)> organelle (2%-5%). Under Cd treatment, nutrient element content showed different changes in S. polyrrhiza. Cd stress promoted the absorption of Ca and Fe elements and inhibited the absorption of P and Mg element. The content of zinc increased initially and declined subsequently, the content at 10 μmol·L-1 Cd is higher than the control. K content declined inicially and increased subsequently, however, the content in all Cd concentrations were lower than the control. FTIR analysis showed that the synthesis and secretion of carbohydrates, proteins, carboxylic acids and phenols have been affected under high concentration of Cd stress. Meanwhile, reactive oxygen rose progressively. Increasing Cd level caused membrane lipid peroxidation, increased MDA content and decreased the index of unsaturated fatty acid (IUFA). Thereby the structure of S. polyrrhiza cell was damaged. Also, SOD, CAT and GR activities decreased significantly as the concentration of Cd rose. In contrast, AsA, GSH and NP-SH contents were induced, while initial rises in the activities of POD and APx were followed by subsequent declines. Even at the lowest concentration of Cd, processed plants had some damage, it characterized by leaf etiolation, lowering chlorophyll content and soluble protein content, increasing soluble sugar and proline content. Cd stress led to cell death aggravation, chloroplast protein D1 and D2 protein degradation. The results showed that high concentration of Cd stress can cause harmful effects to S. polyrrhiza.In summary, the accumulation and toxicity effects of Pb and Cd were studied in this paper, the response of aquatic plants on Pb, Cd and Pb, Cd tolerance mechanism were explored through all kinds of monitoring indicators in toxicological tests with Pb and Cd in aquatic environment. |
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