Ecological Responses Of Exotic Species Spartina Alterniflora And Flaveria Bidentis To Heavy Metal And Saline-alkali Stresses

In recent years, soil heavy metal pollution and salinization have become the research hotspots in the field of environmental pollution control at home and abroad. Heavy metal pollution not only reduces the soil fertility, but also affects plant growth and crop yield, leading to the hazard on human health and environment security through food chains. Soil salinization reduces the soil availability, and causes harm to plant growth. Generally, the injury caused by soil salinization results from salt stress (NaCl and Na2SO4) and alkali stress (Na2CO3and NaHCO3). Compared to native plants, exotic plants have stronger competitiveness, and can endure a series of stress factors, including drought stress, temperature stress, heavy metal stress and salt-alkali stress. The further study of exotic plants to adapt to environmental stresses, can not only reveal the diffusion mechanism of exotic plants in saline and contaminated soil environment, but also has important theoretical and practical significance in prevention of exotic plants and rational utilization of exotic plants for ecological restoration. This paper has chosen exotic plant Spartina alterniflora and Flaveria bidentis as the research object to study the mechanism of adaptation on heavy metal and salt-alkali stresses.In the present study, the effect of S. alterniflora on sediment heavy metal accumulation was investigated in Tianjin coastal, China. Based on the results of the investigation, we studied the effects of heavy metals (Cd, Pb, Cu, and Zn) on plant growth and heavy metal accumulation in S. alterniflora, respectively. Furthermore, the physiological responses and Cd accumulations of S. alterniflora were also detected under salt stress. Finally, the function of carbon nanotubes on heavy metal phytoremediation of S. alterniflora was evaluated. The results were shown as follows:(1) In coastal zones of Tianjin, China, S. alterniflora decreased sediment bulk density and salinity, increased sediment adsorption of Cd, Cu and Pb compared to the mudflat. The heavy metal concentrations in Spartina and mudflat sediments were:Zn> Cu> Pb> Cd. In both Spartina and mudflat sediments, the potential ecological risk (PER) of Cd is higher than other heavy metals including Zn, Cu and Pb. Most of heavy metals exist in the residual form, which were not easy used by plants. In terms of heavy metal available for plants (acid-soluble, oxidable and reducible), heavy metal concentrations in mudflat were lower than Spartina sediments.(2) The heavy metal tolerance of fine roots was significantly higher than leaves, stems and rhizomes. In S. alterniflora, the heavy metal contents (Cd, Pb, Cu and Zn) increased with increasing heavy metal stresses. Continuous increases of Cd and Pb accumulation were also detected with increasing stresses. However, total amounts of Cu and Zn increased firstly, and then reduced. Cd, Pb and Cu were mainly accumulated in fine roots, with most of Zn located in leaves. S. alterniflora produced large amounts of free proline and soluble sugar to take part in osmotic adjustment under heavy metal stresses. Oxalic and citric acids were the two most abundant organic acids in S. alterniflora. Under Cd stress, oxalic and citric acids acted as indicator of Cd uptake and accumulation in fine roots. Under Pb stress, citric acid was positively related with Pb accumulation in fine roots. Under Cu stress, oxalic and citric acids may take part in Cu accumulation in leaves and stems. Under Zn stress, oxalic acid may function in Zn accumulation in leaves.(3) Under different Cd stresses, the effects of NaCl may be varied on physiological responses and heavy metal accumulation. Under moderate Cd stress, NaCl improved the harm of Cd on plant, which can be determined by lower biomass, plant height and chlorophyll a+b contents. Under moderate Cd stress, NaCl increased proline and Ca2+contents, different from unchanged trends under severe Cd stress. Further, NaCl alleviated the oxidative stress under moderate Cd stress by improving the activities of CAT and POD. With NaCl addition, Cd contents in S. alterniflora increased and reduced under moderate and severe Cd stress, respectively. However, biological dilution caused by improved biomass led to increases of total Cd accumulation with increasing NaCl concentration. Under moderate Cd stress, NaCl increased Cd translocation factor (TF); while, phytostabilization of Cd may be probable under severe Cd stress due to the reduced TF.(4) In S. alterniflora, the effects of CNTs on physiological responses and Cd accumulation depend on the degree of Cd stress. CNTs alleviated higher Cd stress because of restored shoot growth reduction, water content and plant height. CNTs alleviated the detrimental effects of Cd stress by increasing K+and Ca2+contents, while reducing Na+/K+and Na+/Ca2+ratios. The proline contents in treatment with only Cd were higher than treatments with both Cd and CNTs, indicating that CNTs reduce production of organic solutes under Cd stress. There were higher Cd accumulation in roots than shoots, and both were improved by CNTs, except for reduction in roots under higher Cd stress. CNTs did not affect the inhibition of Cd on growth of S. alterniflora, but improved Cd accumulation under lower Cd stress. However, under higher Cd stress, CNTs resumed the inhibited plant growth, promoted and reduced the Cd amounts in shoots and roots, respectively.In order to understand the invasion mechanism F. bidentis in saline soil,25different salinity and pH conditions were simulated and seed germination were determined. On the other hand, we study the effects of neutral salt NaCl and alkaline salt Na2CO3on growth of F. bidentis, which is helpful to illustrate the salt-alkali resistance of F. bidentis in the vegetative growth phase. The results showed that:(1) The germination rates of seeds reduced with increasing salinity and pH. Un-germinated seeds germinated well after being transferred to distilled water, demonstrating that parts of F. bidentis seeds are well adapted to salt-alkali stresses due to high capacity for germination recovery. Therefore, the ungerminated seeds under high salt-alkali stresses may be a tolerance mechanism which avoids massive death of plant under severe saline condition. Stepwise regression analysis showed that salinity is the dominant factor affecting seed germination; after seed germination, alkalinity (pH) acted as the main factor which impacted seedling growth. The buffer capacity has some protective effect on seedling growth.(2) The influence of lower NaCl stress on plant growth is not significant, and the inhibitory effect becomes significant with increasing NaCl concentration. Na2CO3reduced relative growth rate, improved leaf electrolyte leakage rate, and increased contents of MDA, proline and soluble sugar. Thus, there were stronger resistance and adaptability of F. bidentis to neutral saline soil compared to alkaline saline soil.