| Arsenic(As) is a main pollutant and widely exists in ecosystem. There are various forms of As translocation, accumulation and transformation in plants. Organic acids are widely distributde in both plants and their surrounding environment. As one kind of the chelants of arsenic, organic acids an play an important role in arsenic tolerance and detoxificative process in plants. A hydroponic culture method, together with the gas chromatography-mass spectrometry (GC-MS), was used to evaluate the contents of seven organic acids (oxalic acid, malonic acid, amber acid, malic acid, palmitic acid, citric acid, and linolenic acid) which are in plants and root exudant of Hydrilla verticillata (arsenic hyperaccumulator) and Potamogeton malaianus (non-hyperaccumulator) as affected by different of arsenic species [As(Ⅲ), As(Ⅴ), dimethylarsinate (DMA)]. Furthermore, the organic acids with high contents were selected. And the selected organic acid with different concentrations (0-2000μg·L-1) were externally added in culture solution to investigate the effects of exogenous organic acid on plant arsenic uptake and physiologic and biochemical indexes [activities of antioxidative enzymes, contents of photosynthetic pigments, soluble sugar, malondialdehyde (MDA), and proline]. By doing so, the adaptation mechanisms of plants which were exposed to exogenous organic acid to different arsenic species were understood. The main results were listed as folows:1. The natural contents of oxalic, malonic, citric, malic and palmitic acids in H. verticillata were significantly higher than those of P. malaianus (P<0.05). In the As(Ⅲ) treatments, the contents of citric, malic, palmitic, linolenic and total organic acids significantly decreased in H. verticillata, and the similar decreasing trend was also observed for palmitic acid content in P. malaianus. In the As(Ⅴ) treatments, a significant decrease was observed for the contents of palmitic acid and total organic acids in H. verticillata and palmitic acid in P. malaianus. In the DMA treantments, the contents of oxalic, citric and malic acids significantly decreased in H. verticillata, and the decreasing trend was also observed for the contents of palmitic, linolenic and total organic acids in P. malaianus. The contents of palmitic acid, linolenic acid, or total organic acid was negatively correlated to arsenic concentrations in H. verticillata in As(Ⅲ) treantments, and the negative correlation was also found between oxalic acid and arsenic concentration in As(Ⅴ) treantments (P<0.05). A significantly negative correlation was also noted between the contents of palmitic acid, linolenic acid, or total organic acid and concentrations of arsenic in P. malaianus in As(Ⅲ) or DMA treantments. Citric acid content also negatively correlated to arsenic uptake in DMA treantment(P<0.05).2. Oxalic, malonic, succinic and palmitic acids were determined in the root exudation of two tested plants, and the exudated oxalic of H. verticillata was markedly increased by adding different concentrations of As (Ⅲ); the exudated oxalic acid of H. verticillata was markedly increased by adding As (Ⅴ) ranged from3.0to5.0mg·L-1. But the content of exudated oxalic acid was not remarkably increased for P. malaianus by adding As (Ⅲ) or As (Ⅴ); the exudated oxalic acid of H. verticillata showed little variation by adding different concentrations of DMA, but that of P. malaianus was significantly increased (P<0.05) in the concentrations of5.0mg·L-1. The exudated oxalic acid of H. verticillata was significantly decreased by adding As (Ⅴ) with1.0mg·L-1, compared with the As (Ⅲ) or DMA treatments. The exudated oxalic acid of P. malaianus was also remarkably decreased by adding As (Ⅲ) or As (Ⅴ) with5.0mg·L-1compared with the DMA treatment. It was indicated that a significantly positive correlation was observed between the concentration of oxalic exudation and that of arsenic in H. verticillata in As(Ⅲ) or As(Ⅴ) treantments. This result concludes that root exudation of oxalic is one of impotrant mechanisms in arsenic hyperaccumulation by H. verticillata in As(Ⅲ) or As(Ⅴ) treantment.3. The results of exogenous addition of oxalic acid showed that the As content of H. verticillata markedly increased by adding oxalic acid with a concentration of200-2000μg·L/-1when the plant were exposed to3.0mg·L-1As (Ⅲ) or As (Ⅴ). But it showed little variation in the DMA treatment. Therefore, it can be concluded that the addition of oxalic acid could promote the uptake of As (Ⅲ) and As (Ⅴ) by H. verticillata.4. The effects of oxalic acid addition on the physiologic and biochemical indexes of H, verticillata varied. The activities of peroxidase(POD)and catalase(CAT), as well as the contents of photosynthetic pigments, soluble sugar, MDA and proline significantly increased with increasing concentrations of oxalic acid when the plant was treated by As (Ⅲ). But when the plant was treated by As(Ⅴ), the activity of superoxide dismutase(SOD) and proline content remarkably increased. The contents of chlorophyll a and total chlorophyll significantly lower than the control when the concentration of oxalic acid was1000μg·L-1. However, in the DMA treantment, the content of proline was notably higher than the control only when the concentration of oxalic acid was500μg·L-1. |
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