| Phytoremediation technology for remediation heavy metal polluted soil has becoming a research focus in recent years. According the investigation of our research group, Phytolacca americana can accumulate high concentrations of Cd and Mn in its shoots and roots. The present study conducted to investigate the physiological response of excessive cadmium (Cd) and manganese (Mn) of P. americana. Experiments were also conducted to analysis the physiological mechanism of accumulation and tolerance of Cd and Mn in P. americana. The results would provide theoretical basis on practical application of phytoremediation.The biomass of leaves and roots decreased after 5μM Cd or 10 mM Mn treatments for 15 days. The roots of P. americana are more sensitive on excessive Cd or Mn treatment than leaves. All Cd treatments resulted in a significant increase of thiobarbituric acid reactive substances (TBARS) in roots, compared to control plants, but leaves treated with 5 or 10μM Cd and all Mn treatments showed no significant increase of TBARS. Chlorophyll a, carotenoids and the value of chlorophyll a/b decreased significantly on Cd treatment, but not on Mn treatment. Treatment with excessive Mn (concentrations below 10 OOOμM) induced thermal energy dissipation. The inhibition of photosynthetic rate by excessive Mn was caused by stomatal factors. The accumulations of Cd and Mn in roots and leaves of P. americana grow on a logarithmic scale with treatment times. As with the cadmium concentrations, the manganese concentrations trend to saturate in roots and leaves, respectively, after 8 or 10 days treatment. After 15 days of exposure to 5-100μM Cd or 2,000-20,000μM Mn, P. americana roots contained concentrations of Cd and Mn that were about 2.5-2.9 and 1.4-1.8 times higher, respectively, than the amounts in leaves. The distribution of the metals in leaves, stems, and roots showed that a larger proportion of total absorbed Cd is retained in the roots, whereas Mn is accumulated primarily in the leaves.This study was also conducted to further investigate the Cd- and Mn-accumulation strategies of this plant. The binding forms of Cd and Mn were analyzed in P. americana, as well as the concentrations of phosphorus, pectin, lignin, phenolic compounds, soluble protein, free amino acids, organic acids and non-protein thiols (NPTs). Concentrations of NPTs and phytochelatins (PCs) increased significantly as the concentration of Cd in solution increased. The molar ratios of PCs:soluble Cd ranged from 1.8 to 3.6 in roots and 8.1 to 31.6 in leaves, suggesting that the cellular response involving PC synthesis was sufficient to complex Cd ions, especially that of leaves. In contrast, excessive Mn treatments did not result in a significant increase in NPTs or PCs concentrations. Oxalic acid concentrations in leaves of plants exposed to 2,000 or 20,000μM Mn was approximately 3.7-to 8.6-fold higher than the Mn level in the 0.6 M HCl extract. Thus, oxalic acid may play an important role in the detoxification of Mn.To explicate exact roles of PCs and oxalic acid in accumulation of cadmium and manganese, respectively, precursors and inhibitors were used in this chapter. Glutathione, precursor of PCs, alleviated growth inhabitation of Cd stress and decreased amounts of Cd transferred form roots to shoots. This result suggested that GSH and PCs might response for absorption, long-distance transportation, and accumulation of Cd. Oxalic acid and one of its precursors, ascorbic acid, enhanced Mn concentrations of roots and reduced them of leaves, while malic acid and critic acid had opposite effects. The result suggested that malic acid and critic acid had import roles in Mn lang-distance transportation, while oxalic acid acted as the "terminal acceptor" of Mn in plant.To tolerant mechanisms of P. americana to Mn, the response of P. americana to manganese excess was investigated, as well as the relationships between lignin deposition in the plant’s leaves, activities of apoplastic peroxidase and laccase, and ascorbic acid metabolism in the apolast. The exceptionally high tolerance of P. americana to Mn, both in solution and in tissue, was confirmed. No visible brown spot was observed in the leaves of plants treated with≤10,000μM Mn for 10 days. Mn treatment significantly increased lignin content and laccase activity in the apoplastic washing fluid (AWF) of P. americana leaves. In contrast, an increase in the Mn supply was paralleled by a significant decrease in the concentration of total phenolic compounds and in water-soluble guaiacol peroxidase (SPOD) activity in leaf AWF. This result suggested that an increase in lignin deposition decreased the concentration of apoplastic phenolics available to act as peroxidase substrates. Consequently, the decrease in both SPOD activity and the concentrations of phenolics may be responsible for depressing the generation of potentially toxic intermediates that induce symptoms of Mn toxicity in plant leaves. The lignin deposition was catalyzed by laccase. Ascorbic acid in apoplast might play a regulatory role on keeping the apoplast in the reduced state on Mn treatment. In conclusion, P. americana may have developed different mechanisms to detoxify Cd and Mn, based on the different preferences of the metals for sulfur and oxygen ligands. And Mn increases the activity of laccases in the leaf AWF and the lignin content in leaf, but decreases SPOD activity and phenolics concentrations in the leaf AWF. Our results suggest that lignin formation by laccases is involved in the Mn tolerance of P. americana. |
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