Mechanisms For The Accumulation Of And The Resistance To Arsenic, Selenium And Antimony In Plants

With the development of industry and agriculture,continuous deposition of heavy metals in the environment is unavoidable,which may result in the aggravation of environmental pollution.Resently,increasing concerns from the scientists and public have been paid for the technology using hyperaccumulating plants to phytoremediate the comtaminated environment.Investigations on the mechanisms for uptake,accumulation and interactions among deleterious elements in plants will contribute to the understandings of tolerance mechanisms,and also benefit to provide more theoretical knowledge in phytoremediation process.In this study,a series of experiments in nutrition culture or in soil culture have been conducted to investigate:1) the mechanisms on the uptake and accumulation of selenium and antimony in plants;2) the interactions among arsenic,selenium and antimony in plants;3) the strategies to alleviate the toxicity of arsenic and antimony to plants using the extra supplementation of selenium to the substrate.In this study,most employed plants were fern plants and at the same time, physiological and chemical determination methods,hydride generation atomic fluorescence spectrometer test technology,inductively coupled plasma optical emission spectrometry test technology and the subcellular fractionation technology were also used.The main results were as follows:1.We examined the selenium(Se) accumulation as well as its related antioxidant responses in Chinese brake fern(Pteris vittata L.),an arsenic hyperaccumulator.The results showed that Se was accumulated more in roots than in fronds,with the highest Se concentration of 1573.3 mg kg^-1.Addtion of Se in the range of 0-2 mg L^-1 exerted a beneficial effect that was indicated by a significantly decreased content of malondialdehyde(MDA) in the fronds of Chinese brake fern.However,the toxicity of Se in Chinese brake fern occurred with Se addition of greater than 5 mg L^-1,which was shown by significant increases in MDA contents,especially at the highest Se addition rate of 20 mg L^-1.The enzymes catalase(CAT),ascorbate peroxidase(APX) and peroxidase (POD) in the fronds of Chinese brake fern were largely induced at low Se addition rates from 0 to 5 mg L^-1 while Se addtion of greater than 5 mg L^-1 reduced their activities.The contents of glutathione(GSH) and activities of GR increased when Se addition was greater than 5 mg L^-1,and a significant increase of superoxide dismutase(SOD) activities was observed at the highest addition rate of Se at 20 mg L^-1.The results suggested that participation of APX and POD in the destruction of H₂O₂ might be promoted by low addition of Se,while SOD,GSH and GR contributed to high Se tolerance through the reduction of superoxide radicals(O₂^-) accumulation. 2.Hydroponic(nutrient solution culture) and pot(soil culture) experiments were simultaneously conducted to investigate the effects of Se on the uptake and distribution of essential elements in Chinese brake fern.Chinese brake fern took up much more Se in nutrient solution culture than that in soil culture.In soil culture,Chinese brake fern also accumulated high contents of Se,with the highest contents of 81 mg kg^-1 and 233 mg kg^-1 in the fronds and roots,respectively.In soil culture,the addition of Se suppressed the uptake of most measured elements,including magnesium(Mg),potassium(K), phosphorus(P),iron(Fe),copper(Cu) and zinc(Zn).In nutrient solution culture,when the Se contents in the tissues of Chinese brake fern were relatively low,the supplementation of Se suppressed the uptake of most essential elements;however,with the increase of Se contents,stimulation effects of Se on the uptake of Ca,Mg,K were observed.An initial decrease followed by a rapid increase of Fe contents in the fronds of Chinese brake fern were found with Se addition and tissue Se contents increasing in nutrient solution culture,suggesting antagonistic and synergic roles of Se on these elements under low to high Se exposure,respectively.The results indicated that Ca,Mg, K might be involved in the tolerance mechanism of Se,and that the regulation of Fe accumulation by Se in the fronds might be partially due to the dual effects of Se on Chinese brake fern.3.The interactive effects of As and Se on their uptake by Chinese brake fern were explored in two hydroponic experiments based on a two-factor,five-level central composite design.At Se levels of less than 2.5 mg L^-1,increasing amounts of As stimulated the uptake of Se in Chinese brake fern roots,possibly because of the beneficial effects of Se.In contrast,at Se concentrations greater than 2.5 mg L^-1,As suppressed the uptake of Se in Chinese brake fern roots.Uptake of As by both fronds and roots of Chinese brake fern was suppressed by the addition of Se,indicating the antagonistic effects of Se on As.In addition,at Se concentrations of less than 2.5 mg L^-1,As stimulated the translocation of Se from roots to fronds;meanwhile,the addition of Se resulted in reduced translocation of As from roots to fronds.These findings demonstrate the interactive effects of As and Se on their uptake by Chinese brake fern.4 Investigation of the potential of antimony(Sb) tolerance and accumulation by plants as well as the antioxidative responses to Sb in four fern plants were carried out.The biomass of fern PCA(Pteris Cretica 'Albo-lineata') remained constant with Sb addition, whereas the biomass of ferns CYF(Cyrtomium Fortunei),MH(Microlepia Hancei) and CYD(Cyclosorus Dentatus) at the high Sb rate exposure decreased by 12.5%,35.0%and 38.3%,respectively as compared with their controls.This suggested a high to low Sb tolerance order for these four fern plants.For all of these fern plants,more Sb was accumulated in the roots than in the fronds.Antimony concentration in the roots at the high rate of Sb addition was recorded,on average,as 358 mg kg^-1 for fern PCA,224 mg kg^-1 for fern CYF,124 mg kg^-1 for fern CYD and 123 mg kg^-1 for fern MH.A high rate of addition of Sb increased the contents of malondialdehyde(MDA) by 41.3%and 171.6% for ferns MH and CYD,respectively,as compared with their controls.No changes for MDA contents were observed in ferns PCA and CYF with Sb addition,indicating no lipid peroxidation reaction in these two plants.At a medium rate of Sb addition,the activities of peroxidase,catalase and ascorbate peroxidase in fern PCA were much higher than those in ferns CYF,CYD and MH,demonstrating the important role of these three enzymes in resisting Sb toxicity.The consistency in unchanged biomass,high accumulation of Sb in roots,lower MDA contents,as well as high enzyme production in fronds,indicated that fem PCA was more tolerant to Sb than the other three fern plants. Antioxidative enzymes(peroxidase catalase and ascorbate peroxidase) might be involved in Sb toxicity resistance of fern PCA.5 Pteris Cretica might be a potential antimony hyperaccumulator.Without As and Sb addition,most As and Sb absorbed by this plant were stored in the cytoplasmic supematant fraction in the fronds and stems,while the cellwall and cytoplastic organelles only sequestrated relatively few of As and Sb.In the presence of antimony alone in the solution,with Sb treatment concentrations increasing,the contents of Sb in all tissues of Pteris Cretica and in all subcellular fractions from leaves,stems and roots were all enhanced;However,the percentages of Sb distribution in the cytoplastic supernatant fraction in all tissues dereased,accompanied with the increases of that in the fraction of cellwall.When both As and Sb were present in the solution,the uptake of Sb by this fern plant were stimulated by the increasing As treatment concentrations,however,more efficient stimulation of Sb uptake by relatively low As treatment concentrations than high As treatment concentrations was also observed.The increased As treatment concentrations induced the more translocation of Sb to the cytoplastic supernatant fraction, along with the decrease in the percentage of As distribution in this fraction.At low As treatment concentrations,the supply of Sb slightly enhanced the uptake of As,but this operation simultaneously reduced the accumulation potential of As in Pteris cretica, reflecting on the decreased percentage of As in the fraction of cytoplastic suprnatant. However,at high As treatment concentrations,the uptake of As by this plant was suppressed by the addition of Sb.Significant decreases in the stem As contents and in the percentages of As in the cytoplastic supematant fractions of both leaves and stems were observed along with the supply of Sb.6.The effects of Sb on the growth of paddy-rice(WeiYouâ…¡416) and the supposition of detoxifying Sb by the addition of selenium to plants were explored.Aider 14 days exposure,the single addition of 5 mg L^-1 Sb slightly reduced the biomass of paddy-rice and increased the leaf malondialdehyde(MDA) contents,and higher Sb addition as 50-100 mg L^-1 Sb totally resulted in the death of paddy-rice,suggesting the toxicity of Sb to paddy-rice.Without Sb addition,0.1 mg L^-1 Se remarkably subdued the formation of MDA in the leaves accompanied with an unaffected biomass,but≥1 mg L^-1 Se evidently decreased the biomass and enhanced the leaf MDA contents,suggesting antioxidant and pro-oxidant roles of Se to paddy-rice under different dosages of Se.The increasing Se treatment concentrations enhanced the synthesis of soluble protein in the leaves of paddy-rice,however,with 5 mg L^-1 Sb in the solution,increasing Se treatment concentrations reversely suppressed it.With both Se and Sb in the solution,the increasing Se levels depressed the uptake of Sb in all tissues of paddy-rice,indicating an antagonistic effect of Se on the uptake of Sb.Moreover,as compared with the single Sb level of 5 mg L^-1,1 mg L^-1 Se addition reversely decreased the leaf MDA contents and remarkably increased the biomass even being higher than that of the control,suggesting an alleviated process of Sb toxicity by Se.Interestingly,the addition of 5 mg L^-1 Sb also ameliorate the toxicity of 1 or 5mg L^-1 Se,reflecting on more or less increased biomass,decreased leaf MDA contents,and reduced above ground Se contents.Unexpected,when Se was imposed at the concentrations of 1 or 5 mg L^-1,more Se were accumulated in the root of paddy-rice with increasing Sb treatment concentrations.The results of this study suggested a mutual detoxification process between Se and Sb in paddy-rice,and the detoxification of Sb by Se might be related with the antagonistic effects of Se on the uptake of Sb,partially with the antioxidant role of Se.