| Among common pollutants that affect plants, lead is toxic and frequently encountered. Currently, lead (Pb) was reported as being the second most hazardous substance, after arsenic, based on the frequency of occurrence, toxicity, and the potential for human exposure by the Agency for Toxic Substances and Disease Registry (ATSDR2003). Therefore, a comprehensive understanding of phytotoxic and plant resistance mechanisms exposed to various concentrations of lead (Pb) was required. In the present study, steriled seedlings of Nymphoides peltata and Potamogeton crispus were chosed as the experimental materials, aiming to study the effects of Pb contamination on physiological and biochemical parameters, and explore possible strategies adopted by plants under Pb stress. The results are as follow:(1) The effects of increasing concentrations of lead on Pb accumulation and its influence on nutrient elements, malondialdehyde (MDA) content, generation of superoxide anion (O2·-), hydrogen peroxide (H2O2) content, antioxidant enzymes activities, soluble protein and photosynthetic pigment, as well as chloroplast ultrastructure in steriled seedlings of Nymphoides peltata (S. G. Gmel.) Kuntze were investigated in order to understand Pb-induced toxicity. The accumulation of Pb was found to increase in a concentration-dependent manner. Nutrient elements (Ca, K, Fe, Mn and Mo) were also affected. MDA content and O2generation rate increased progressively, while H2O2content first boosted up at a low Pb concentration of12.5μM but then declined. Guaiacol peroxides (POD) and catalase (CAT) activities increased alternately, while superoxide dismutase (SOD) activity gradually fell. Negative correlations were found between Pb and soluble protein and photosynthetic pigment. Moreover, Pb exposure resulted in significant damage of chloroplasts. Taken together, these findings supported the hypothesis that N. peltatum underwent oxidative stress induced by Pb. In addition, both the disorder of nutrient elements and the damage to the ultrastructure of chloroplasts were indicative of general disarray in the cellular functions exerted by Pb.(2) In order to study potential antioxidant defense mechanisms, the effects of increasing concentrations of lead (Pb) on polyamines (PAs), various thiols, vitamins C and E and proline contents in sterilized seedlings of Nymphoides peltata (S.G. mel.) Kuntze were investigated after5days exposure. The levels of total putrescine (Put), spermidine (Spd), and spermine (Spm) decreased significantly, while the ratio of (Spd+Spm)/Put first increased but then declined as the concentration of Pb increased. The trends for free, perchloric acid soluble conjugated (PS-conjugated) and perchloric acid insoluble bound (PIS-bound) PAs were similar to the trend seen for total PAs. Moreover, reduced glutathione (GSH), non-protein thiols (NP-SH), phytochelatins (PCs) and vitamin C were induced at high Pb concentrations. No significant change was observed in vitamin E. An initial decline in proline content was followed by an increase as the Pb concentration rose. The reduced level of Put and elevated contents of GSH, NP-SH, PCs, vitamin C and proline were found to be associated with antioxidant efficiency, which supports the hypothesis that they could play a significant role in the adaptation mechanisms of N. peltatum under Pb stress.(3) To study antioxidant intervention compensates oxidative stress in sterilized seedlings of Potamogeton crispus L. exposed to lead (Pb) stress, the effects of increasing Pb concentration on generation of superoxide anion (O2·-), hydrogen peroxide (H2O2), photosynthetic pigment, malondialdehyde (MDA) content, soluble protein, antioxidant enzymes activities, various thiols, vitamins C and E as well as total antioxidant capacity (T-AOC) were investigated. The oxidative stress and toxic effect caused by Pb were evident by the production of excess O2·-and H2O2, the reduction in photosynthetic pigments and slightly increase in malondialddehyde (MDA) and soluble protein contents. Catalase (CAT) activity increased progressively while Guaiacol peroxidase (POD) activity gradually fell. Non-protein thiols (NP-SH) and phytochelatins (PCs) were induced at lower concentration followed by decline, while a mild increase was observed in reduced glutathione (GSH) content. Vitamin C was synthesized to significant levels as the concentration of Pb increased. However, no significant changes were observed in superoxide dismutase (SOD) activity and vitamin E. Thus, T-AOC rapidly declined after a transitory increase at moderate exposure conditions (25and50uM). Our results demonstrated that antioxidant intervention (CAT, GSH, NP-SH, PCs and vitamin C) was able to confer a protective effect against the Pb-induced oxidative stress in sterilized seedlings of P. crispus.(4) In order to study potential tolerance mechanisms, the effects of increasing concentrations of lead (Pb) on Pb accumulation and nutrients at the subcellular level, polyamines (PAs) and proline metabolism in sterilized seedlings of Potamogeton crispus L. were investigated after5days exposure. The accumulation of Pb was found to increase in a concentration-dependent manner. Subcellular distribution demonstrated that most of Pb was stored in the cell wall, followed the descending order of cell wall> soluble fraction>organelle fraction. Analysis of nutrient elements showed that the levels of total P, Mg, Na and Zn rose and the level of total Fe fell; total Ca increased initially only to decline later. Moreover, the increased nutrients in cell wall fraction were responsible for enhancement of nutrients absorption whereas those in soluble and organelle fraction declined. Pb stress also induced a considerable decrease in total putrescine (Put), while the levels of total spermidine (Spd), spermine (Spm) and (Spd+Spm)/Put ratio increased progressively but then declined. The trends for free, perchloric acid soluble conjugated (PS-conjugated) and perchloric acid insoluble bound (PIS-bound) PAs were similar to the trend seen for total PAs, except that PIS-bound Spm increased significantly. The activities of arginine decarboxylase (ADC) and ornithine decarboxylase (ODC) rose gradually with an increase in Pb concentration, while diamine oxidase (DAO) initially increased but then declined. In addition, an initial increase in proline content was followed by a decline as the Pb concentration rose, due to the increase of r-glutamyl kinase (GK) activity. Meanwhile, ornithine-d-aminotransferase (OAT) activity gradually reduced, while no significant change was observed in proline dehydrogenase (PDH) activity. Therefore, our results indicated that the tolerance of P. crispus to Pb stress was based on cell wall compartmentalization combined with promotion of nutrients, alterations of PAs and induction of proline.In summary, this paper studied from three angles (bioaccumulation, toxic effects and tolerance mechanisms of lead), and the experiments involved multi-changes in the overall Pb content, subcellular distribution of Pb, Pb positioned in chloroplast, levels of reactive oxygen species (ROS), decomposition of chlorophyll and destruction of chloroplast structure, peroxidation of membrane and soluble proteins, disorders of mineral elements, regionalization of cell wall, antioxidant enzyme, thiol, vitamins, ions competition, proline and PAs contents. On one hand, sensitive monitoring indicators could serve as useful and indicative biomarkers in toxicological tests with Pb in aquatic environment. On the other hand, our results explored the intrinsic resistance mechanism of antioxidants to Pb, providing valuable theoretical basis for a comprehensive understanding of plant response, tolerance systems and restoration of Pb-contaminated water environment. |
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