随着全球工业化与城市化的发展，人类向土壤排放的污染物种类、数量都在急剧增加，导致多来源、多途径、低剂量、多种类的污染物共存并形成了土壤复合污染，且已成为人类社会可持续发展的重要限制因素。为此，探索对土壤重金属及有机污染物忍耐性和富集性强、生物量大、非食源性植物修复技术，已成为现在国内外环境科学研究领域的前沿议题。在综合分析国内外相关研究的基础上，论文采用田间调查采样与同步盆栽实验的方法，研究了土壤-作物系统中多种重金属元素及其与HCHs复合污染的生物积累和生态效应，综合分析了运用陆地棉（Gossypium hirsutum L.）修复重金属及HCHs污染钙质土壤的可行性。论文的主要研究成果如下：第一，对北京东郊及其与河北省廊坊接壤区的农田土壤-棉花系统中重金属（Cd、Cu、 Zn和Pb）的积累和分布特征研究表明：⑴受研究土壤中重金属Cu、Zn 和 Pb均低于WHO限值，而Cd明显地高于WHO限值。然而，与北京土壤背景值相比，全部样品中的Cu和Cd，大部分样品中的Zn以及小部分Pb都高于它。这表明长期的污灌已经导致明显的Cd、Cu和Zn积累，且土壤已出现潜在的Cd污染现象。另外，分析显示距离灌渠越近的表土层和底土层中，重金属含量越高。⑵土壤中Cu、Zn和Pb主要以残余态和有机结合态存在，其次是铁锰氧化态，很小的比例以碳酸盐结合态和离子交换态存在。在不同样点不同深度中，四种重金属的生物可利用性表现为：Cd>Cu>Pb>Zn。而且，Cd的生物易利用态（碳酸盐结合态和离子交换态）占有相当大的比例。另外，表层土壤中四种重金属生物可利用性高于底土层。且越接近灌渠，表层土壤中Cd生物可利用性越低，而底土层中则有增高趋势。⑶四种重金属在棉花中的积累能力表现为：Zn>Cu>Cd>Pb，这可能与重金属的生物可利用性以及生理作用有关。重金属从棉花根到其茎、叶的转移系数（TC）高于1，表明棉花具有重金属萃取植物的特征。Cd、Cu、Zn和Pb在棉花主要组织（茎）中的富集系数（BCFs值）分别是0.11、0.24、0.34和0.02，基于棉花的生物量（28500 kg hm-2 a-1）和组织中重金属的浓度，估算出的棉花萃取潜力依次为：2.06、165.96、752.94和17.50 g hm-2 a-1。棉花中重金属的积累并不依赖于土壤中重金属的总量，而与其有机结合态和铁锰氧化态等含量密切相关。而且，棉花根、茎和壳中Cd含量，越接近灌区越低，它与Cd在表土层中的有机结合态和碳酸盐结合态分布趋势一致。对北京中心城区、近郊区到远郊区不同断面上的土壤-小麦系统中重金属（Cd、Cu、Zn和Pb）的积累和迁移特征研究表明：⑴城市中心土壤中Pb含量明显高于WHO限值，而Cd、Cu和Zn低于该值。这暗示由交通和工业释放所带来的空气沉降，已导致城市土壤受到了Pb污染。对于近郊农业土壤而言，仅个别样点中的Cu、Zn和Pb平均含量高于限值，而远郊区土壤中所有重金属的平均浓度均在限值以下。这表明长期污灌已导致近郊区部分农业土壤受到重金属污染。然而，与北京土壤背景值相比，城区与近郊所有样点的重金属都出现了明显的积累现象。⑵土壤中Cu、Zn和Pb主要以残余态和有机结合态存在，其次为铁锰氧化态，很小的比例存在于碳酸盐结合态和离子交换态。与近郊农业土壤相比，城市土壤中Cu、Zn和Pb表现出较高的生物可利用性，这与城市土壤中相对高的有机质所导致较高比例的有机结合态有关，特别是对于Pb和Zn来说。而远郊土壤中Cu、Zn和Pb呈现出最低的生物可利用性。然而，对Cd来说，这个顺序正好相反，即远郊>近郊>城区。在郊区土壤中Cd的生物可利用性最高，且其离子交换态和碳酸盐结合态占有相当比例。⑶重金属主要停留在小麦的根部，它们在小麦中的积累能力依次为：Cd>Zn>Cu>Pb（籽例外），进一步证实Cd有较高的生物可利用性。小麦中Cd的BCFs值表现为：近郊>城区>远郊，与土壤中Cd总量和移动性分布相矛盾，而大大依赖于土壤中Cd的铁锰氧化态。Cu在城区小麦根和籽中的BCFs值低于郊区土壤，这可能与Cu的铁锰氧化态和有机结合态的分布有关。Zn在小麦中的BCFs值呈现出：城区>近郊>远郊，这与土壤中Zn的总量和铁锰氧化态含量相关。值得注意的是，尽管城区土壤中Pb含量最高，但与郊区相比，小麦中确有较少的Pb含量。原因可能是，Pb的生物可利用性大大依赖于铁锰氧化态，而不是有机结合态。而且，由于土壤中Pb的离子交换态含量非常少，导致Pb的生物可利用性也最低。⑷在城区和近郊的部分样品中，发现麦粒中Zn和Pb含量高于标准限值。就单一重金属来说，其风险商（HQ）在安全范围内。然而，多种金属的累加潜在风险被发现，特别是对郊区的儿童和成人而言。重金属对于不同暴露人群的HQ从高到低依次为：郊区儿童、郊区成人、城区儿童和城区成人。第二，通过盆栽实验，对土壤-作物（大蒜、土豆）系统中Cd和HCHs单一及复合污染下的积累与生态效应研究表明：大蒜作为可试作物：⑴在单一Cd胁迫下，蒜苗中Cd积累与盆栽土壤中Cd浓度明显正相关。被吸收的Cd主要停留在地下部分，且蒜苗对Cd有较强的吸收能力，在低于5 mg kg-1 Cd的土壤中，蒜苗中的Cd含量远远大于标准限值，易对人类食品安全构成危害。与单一Cd污染相比，在Cd和HCHs的复合污染中，大多数情况下HCHs对Cd在根表面的吸附和根吸收表现出抑制作用（40 mg kg-1 Cd和2 mg kg-1 HCHs例外），然而，Cd和HCHs的螯合作用增强了Cd向植物的地上部分转移。⑵在单一的HCHs胁迫下，蒜苗对HCHs有较强的积累能力（BCFs值能达到12.18），地上部分显著高于地下部分。而且，蒜苗中HCHs含量与土壤中含量呈明显正相关。HCHs从根到茎通过木质部的传输对地上部分HCHs的积累供献重大，这可能与HCHs相对高的水溶性有关。与单一HCHs污染相比，在HCHs和Cd的复合污染中，蒜苗中HCHs的积累相对较低。Cd和HCHs的螯合作用可能是减少HCHs积累的主要原因。⑶在单一的Cd胁迫下，Cd基本没有对叶绿素含量产生明显影响。而在较高浓度的Cd和HCHs处理下，叶绿素含量明显增多。同时，Cd和HCHs的复合处理，与单一Cd胁迫相比具有相对较高的MDA含量。这暗示Cd和HCHs的同时存在可能增强植物的氧化状态。另外，Cd和HCHs的复合污染处理（40 mg kg-1 Cd和2 mg kg-1 HCHs例外），与单一Cd胁迫相比表现出了相对较高的脯胺酸含量，这可能与HCHs吸附在根表面所导致的水缺乏有关。⑷单一的HCHs胁迫没有对叶绿素产生明显影响，而Cd和HCHs复合污染则显示出较高的叶绿素含量。同时，在各种处理中，MDA表现出了或高或低的不规则变化。然而，与单一HCHs胁迫相比，HCHs和Cd的复合处理（40 mg kg-1 Cd和2 mg kg-1 HCHs除外），有相对较高的脯胺酸含量。这可能是Cd2+的直接影响，或者由于Cd存在情况下对根的吸水能力产生了伤害。⑸对蒜苗中Cd的定位分析发现：在蒜瓣中细胞质膜的内表面有明显的Cd沉淀。⑹HCHs在蒜苗地上和地下部分的平均BCFs值约为4.07, 其中δ-HCH在蒜苗地上部分的BCFs值最高，达到12.18。四种同分异构体的BCFs值表现出δ- > γ- > β- > α-HCH的规律，这可能与它们的水溶性和降解性有关。土豆作为可试作物：⑴单一Cd胁迫下，土豆各部分Cd的积累随土壤中Cd含量增加而增加，相应的BCFs值呈下降趋势。各组织中Cd的分布情况为：叶>>根≥茎叶>>块茎。与单一Cd胁迫比，Cd和HCHs的复合污染抑制了Cd的生物积累。它们表现出拮抗效应，原因可能与二者的螯合作用有关。⑵单一HCHs胁迫下，根、茎、叶和块茎中HCHs的BCFs范围分别为：1.19-4.96、0.24-1.00、0.10-2.89和0.07-0.63。叶中HCHs浓度与土壤及其它部位中HCHs的含量无关，这可能是由于叶片吸收了从盆栽土壤挥发至空气中的HCHs（如α-HCH）所导致。同分异构体δ-HCH和γ-HCH优先的被积累在各组织中，而相当的α-HCH存在于叶中，这很大程度上与它们的水溶性和挥发性有关。HCHs与Cd的复合污染在不同浓度处理中对不同组织中HCHs的积累显示出了或协同或拮抗的复杂效应。⑶单一Cd胁迫下，叶绿素a和叶绿素b含量发生下降，但没有随Cd含量的递增而发生变化。同时，除了最高的浓度处理外，叶中脯胺酸的含量也没有随Cd浓度的增加而发生变化。然而，随着土壤中Cd浓度从5到25 mg kg-1的增加，土豆叶中MDA的含量明显增加，暗示由于Cd的胁迫促进了脂质氧化反应。而且，在高浓度Cd和HCHs的复合处理（25 mg kg-1 Cd和1 mg kg-1 HCHs）中，MDA的含量高于中等浓度Cd和HCHs的复合处理（5 mg kg-1 Cd和0.2 mg kg-1 HCHs）。它暗示HCHs的增加可能增加了土豆中脂氧化反应的产物和氧化状态。⑷单一HCHs胁迫下，叶绿素a和叶绿素b随土壤中HCHs增加而减少，而HCHs和Cd复合处理中叶绿素a和叶绿素b表现出了递减效应。同时，与单一HCHs胁迫相比，HCHs和Cd复合处理有明显少的脯胺酸含量，这些可能是由于二者间的螯合作用导致了毒性减少。然而，在5 mg kg-1 Cd和0.2 mg kg-1 HCHs复合处理中，MDA含量低于单一HCHs胁迫。它暗示Cd的增加可能致使土豆中脂氧化产物的减少和氧化状态的降低。⑸对Cd在土豆块茎、地上茎和叶的定位研究发现：在块茎中的皮层薄壁组织细胞、质膜内表面和原生质体中淀粉粒表面有Cd存在。在地上茎中，有相当数量的Cd在皮层中发现，这可能与髓射线的营养通道和贮藏功能有很大关系。在叶中，Cd主要位于从维管柱到表皮细胞的水移动通道中，如栅栏细胞、海绵细胞和气孔，这与Cd在蒸腾作用下的被动吸收相一致。显然，叶和地上茎是Cd的主要储存场所和吸收动力机制。第三，将陆地棉作为试验植物，通过盆栽实验研究了在不同浓度处理下，棉花对重金属与HCHs的植物可利用性和萃取潜力。结果表明：⑴在不同浓度处理下，棉花的生长没有发生明显变化。棉花中Cd积累随土壤中Cd含量升高而升高。各组织中Cd的BCFs值从高到低依次为：叶、根、茎、壳、籽和棉。花期前Cd的吸收对棉花中总Cd的积累作了很大贡献。但EDTA的添加（0.8 mmol kg-1，分四次）没有对Cd的萃取起明显作用；棉花中Zn积累与土壤Zn含量不相关。Zn在棉花的生长后期强烈积累。棉花各组织中Zn的BCFs值从高到低依次为：籽、根、叶（或茎）、壳和纤维。EDTA的添加促进了棉根和棉茎中的Zn积累；棉根中Cu积累与土壤中Cu含量明显相关，各组织中Cu的BCFs值从高到低依次为：根、茎、籽、壳、叶和纤维。棉花生长后期对Cu的吸收较显著。EDTA的投加有助于棉花对Cu的吸收。棉花根中Ag的浓度与土壤中明显正相关，但壳中Ag的积累呈负相关。棉花组织中Ag的BCFs从大到小依次为：根（或叶）、壳、茎、籽和纤维。一定程度上，EDTA的添加对棉花中Ag的积累表现为负影响。相关分析表明：根和叶中Cu、Zn、Cd和Ag含量彼此相关，Zn和Cu在茎和籽中也表现出明显的相关性。另外，它被发现Cd与茎中的Ag和壳中的Zn含量相关。从低浓度到高浓度处理，在棉花的主要组织（茎）中Cd、Cu、Zn和Ag的BCFs值范围分别是：0.34-0.1、0.57-0.02、0.50-0.04和1.23-0.01。基于棉花的生物量和组织中重金属的积累浓度，相应的棉花对Cd、Cu、Zn和Ag的萃取潜力分别为：6.17-72.44、324.04-545.66、757.50-943.38和2.88-9.94 g hm-2 a-1。其萃取能力高于前人对木本植物的研究结果；对Cd、Cu和Zn的年萃取率分别为：17.75%-6.36%、27.97%-1.26%和22.35%-2.06%，在不同含量条件下，其萃取效率差异巨大。可见，棉花能够弥补超积累草本植物以及树木的不足，成为一种更实用的植物修复技术。⑵对棉花根、茎和叶中Cd、Zn、Cu和Ag定位分析表明：①在根中，重金属总是以白色沉淀颗粒形式存在于周皮和次生韧皮部，以及周皮的外表面。同时，位于中柱体的导管及其邻近处也经常有重金属被检测到。结果表明：根表皮的吸附-金属离子的渗透与木质部导管应该是Cu、Zn、Cd和Ag进入植物的两个重要路径。然而，较少的Cu存在于次生韧皮部，又暗示了皮层及其细胞壁对不同重金属的选择渗透性和障碍性。②在茎中，有相当数量的重金属以白色颗粒形式存在于髓部的细胞质或细胞壁中，这可能由其特别的贮藏功能所导致。而棉茎木质部中重金属的存在表明木质部导管对重金属离子发挥了重要的运输功能。另外，重金属在茎表皮和皮层也频繁出现，这很大程度上是髓射线的横向运输所导致。Cd和Ag，以及Zn和Cu经常伴随出现在茎的一定部位，与相关分析结果一致。③在叶中，重金属主要存在于包括栅栏组织、海绵组织、表皮毛，特别是主脉的木质部在内的一系列叶组织中。例外地是，Cu没有在表皮毛的脱落物或分泌物中检测到，海绵组织中也没有检测到Cd。但总得说来，重金属位于从维管柱到表皮的水移动通道上。上述结果表明：根表皮吸附与根吸收、木质部的纵向运输与髓射线的横向输导以及叶的蒸腾作用是棉花中重金属转移和贮藏的关键要素。⑶单一HCHs胁迫下，HCHs优先积累在棉叶中，然后是根和茎，壳和纤维中含量很少，棉籽中基本检测不到。在较低浓度的HCHs处理（0.016和0.04 mg kg-1）中，棉花根、茎和叶的BCFs值分别达到10.03-1.53、11.21-1.49和31.95-3.99之间。即使在较高浓度处理（0.2, 0.4和0.8 mg kg-1）中，它们的BCFs值也能够达到0.86-0.52、0.49-0.28和0.93-0.56这样的范围。可见，棉花对HCHs有较强的萃取能力。且在不同浓度处理下，棉花的生长没有发生明显变化。基于棉花年生物量和棉花各组织中HCHs含量，对六种不同处理情况估算棉花对HCHs的萃取能力分别为：1.24、2.51、1.85、2.83、3.68和10.99 g hm-2 a-1。棉花根、茎和叶中HCHs浓度与土壤中HCHs含量呈明显正相关，表明HCHs在棉花中积累的重要机制是：根吸附并吸收HCHs，然后通过木质部迁移至地上部分。研究还发现，δ-, γ-和β-HCH均具有很高的BCFs值，而α-HCH的BCFs值最低，这可能与它们的水溶性和降解性有关。另外，初步研究发现四种重金属（Cd、Cu、Zn和Ag）与HCHs共存时，它们对棉花中HCHs的积累影响是复杂的，需要进一步的研究去解释。基于上述研究结果，以强适应性、广泛种植、高生物量和非食源性为特征的经济作物——棉花，对受污染土壤中重金属Cd、Cu和Zn及HCHs具有较好的萃取潜力与忍耐性。因此，种植棉花是一种绿色、经济、可行性强的植物修复技术。

With the development of global urbanization and industrialization, the rapidly increasing types and quantities of the contaminants are releasing into soils as a result of human activities, which leads to multi-sources, multi-routes, low-dose and multi-types contaminants coexisted in local soils, and comes into being soil combined pollution. As a result, eco-environment quality, health of agricultural land and food security resulted from soil pollution, as well as phytoremediation of the polluted soils have been extensively and increasingly concerned. The present study, based on field investigation and pot experiments, research the bioaccumulation and ecological effects in soil-crop system under singly and combined existence of heavy metals (HMs) and HCHs, and analyze the potential of Gossypium hirsutum L. (cotton) for phytoremediation of HMs or HCHs polluted calcareous soils. The main results of this research are as follows:Firstly, a field investigation was conducted to research the bioaccumulation and translocation of HMs (Cd, Cu, Zn and Pb) in soil-cotton system, effects of HMs chemical forms in soils on their uptake by cotton organs, and interactions among HMs in various compartments in soil-cotton system located in the eastern suburb of Beijing City and the connection region between Beijing and Langfang Town of Hebei Province. The feasibility of extracting HMs from contaminated soils using cotton plants was conducted a preliminary assessment. The results showed that 1) For all sites of the study area, the contents of Cu, Zn and Pb in soils in different depths were below the WHO limits, while Cd was obviously above the limit. However, compared with soil background values of Beijing, all samples of Cu and Cd, most samples of Zn, and very small portion of Pb exhibited values beyond the background values. This indicated that long-term sewage irrigation had led to remarkable accumulation of Cd, Cu and Zn, and had potential risk on soil contamination with Cd. Generally, the contents of HMs in topsoils and subsoils showed an increasing trend with the decreasing of the distance from the irrigation channel. With the increment of soil depth, HMs contents presented firstly decreasing, and then increasing trends. 2) Cu, Zn and Pb were predominantly associated with residual and organic fraction, followed by Fe/Mn oxide, and very small proportion of carbonate and exchangeable fraction. On average, the mobility and bioavailability of the four metals in different sites and different depths was in the sequence: Cd>Cu>Pb>Zn. Besides, the Cd Carb. and Exch. fractions, as easily bioavailable forms, accounted for considerable proportion, compared with other metals. Moreover , in topsoils, the mobility and bioavailability of the four metals were higher than that of subsoils. In addition, the farmlands close to the main channel showed the lower mobility and bioavailability of Cd in topsoils, while the higher bioavailability in subsoils. 3) Overall, the total metal accumulation ability in cotton organs, was in the order of Zn>Cu>Cd>Pb, which might be explained by combining bioavailability and physiological function of HMs. Although the bioconcentration factors (BCFs) of HMs in cotton plants were no more than 1, their translocation ratios from root to upper parts were often above than 1. This result meant that cotton was a phyto-extraction plant rather than phyto-stabilization plant. Additionally, the contents of HMs in cotton organs were not dependent on their contents in soils, whereas, might related to HMs Orga., Fe/Mn ox. and Carb. fractions. Besides, Cd contents in roots, stems and shells of cotton were decreasing with nearer to the irrigation channel, which was in good agreement with the distribution of Cd Orga. and Carb. fractions in topsoils of different distance from the irrigation channel. Also, Cu contents in leaves exhibited a slight decreasing trend from far to near, which was in accordance with the distribution of Cu Carb. fractions in soils. Furthermore, the BCF values of Cd, Cu, Zn and Pb in main organs of cotton (stems) were 0.11, 0.24, 0.34 and 0.02, respectively. Based on the cotton biomass (28500 kg hm-2 a-1) and HMs contents in cotton organs, the estimated potential of cotton for phytoextraction of Cd, Cu, Zn and Pb in field condition were 2.06, 165.96, 752.94 and 17.50 g hm-2 a-1, respectively.Another field investigation was also conducted to compare and analyze the accumulation, chemical forms, translocation and interaction of HMs (Cd, Cu, Zn and Pb) in different soil-wheat system located at the central city, suburb and exurb of Beijing. In addition, health risk of HMs contents in wheat grains was assessed. The results showed that 1) For NKY site in the central city, Pb content in soils was obviously above the WHO limits, while Cd, Cu and Zn were under the limit. This implied that urban soils were contaminated by Pb, possibly due to aerial deposition resulted from traffic and industrial emission. As for the suburban agricultural soils, only the average Cu and Zn contents of NDH site, as well as Pb of ZJW exceeded the limits, whereas, for DX site in the exurb, the average concentrations of all HMs in soils were below the limit. It indicated that the suburban agricultural soils was partly contaminated by HMs, possibly as a result of long-term sewage irrigation and application of pesticide and fertilizer. However, compared with soil background values of Beijing, remarkable accumulation of HMs was observed in all sites with the exception of the exurb. 2) Cu, Zn and Pb were predominantly associated with residual and organic fraction, followed by Fe/Mn oxide, and very small proportion of carbonate and exchangeable fraction. Compared with the suburban agricultural soils(S), Cu, Zn and Pb in urban agricultural soils(U) showed higher mobility and bioavailability, which was mainly contributed by higher organic fractions as a result of relatively high organic matter contents, particularly Pb and Zn, whereas, the exurban agricultural soils(E) presented the lowest bioavailability. However, for Cd, the order was contrary (E>S>U). Besides, Cd showed the highest bioavailability among the four HMs in the suburban and exurban agricultural soils, meanwhile, its exchangeable and carbonate fractions occupied considerable proportion. 3) The contents of the four metals were found significantly higher in roots than those in the aerial parts of wheat (stalks, hulls and grains). Overall, the total metal accumulation in wheat organs, except for grain, was in the order of Cd>Zn>Cu>Pb, further confirming that Cd had the strongest bioavailability. The BCF values of Cd in wheat organs in various sites followed the sequence: S>U>E, which were contradicting with Cd concentrations and mobility in various soils, while depended largely on Cd Fe/Mn ox. fractions in soils. For Cu, the BCF values in roots and seeds of NKY site were lower than those of the suburban agricultural soils, possibly resulted from the variation of Cu Fe/Mn ox. and Orga. fractions. As for Zn, the order is U>S>E, which might be determined by total Zn concentrations and Zn Fe/Mn ox. fraction in various soils. In despite of the highest soil Pb content in NKY site, the less Pb was uptake in wheat plants, compared with the suburban agricultural soils. The reason might be that Pb phytoavailability was largely dependent on Fe/Mn ox., not Orga. fraction. Besides, Pb bioavailability was low, largely due to very low percent of exchangeable Pb in the soils. 4) In most samples of NKY and NDH, as well as part samples of FT, Zn and Pb concentrations in wheat seeds were above standard limits, respectively. Hazard Quotient (HQ) of individual metal presented values inside the safe interval. However, the potential risk due to the added risk of all HMs through the consumption of wheat grain was found, particularly for suburban children and suburban adults. Consequences of HQs of every metal for different exposure populations were in the same order of: Country children>Country adults>Urban children>Urban adults. It was suggested to pay more attention on the potential added threat of HMs to the health of suburban inhabitants (both children and adults) through consumption of wheat in Beijing.Secondly, pot experiments were conducted to study the mechanisms and ecological effects under singly and combined stress of Cd and HCHs on soil-crop system: garlic and potato as studies plants. The pot soils were spiked with HCHs or Cd to achieve the soil concentrations of different treatments. Under different concentration treatments of HCHs and Cd, the growth of crops, physiological indexes (MDA, proline and leaf pigments), as well as contents of Cd and HCHs are investigated. Particularly, EMAX method was used to perform qualitative analysis on Cd localization in plant tissues or cell compartments. Possible food security risk was considered. The results showed thatFor garlic as studied plants: 1) Under the single Cd stress, the accumulation of Cd in garlic organs depended largely on Cd concentration applied in pot soils. Most of the metal taken up was retained in the underground parts, and garlic seedlings had stronger absorption ability of Cd, which might easily pose harm on human health. Under the combined stress of Cd and HCHs, for underground parts of garlic seedlings, it was only found that there was an obvious increase under 40 mg kg-1 Cd and 2 mg kg-1 HCHs treatment, whereas, in other coexistence treatments, Cd concentrations showed decline or no obvious rise in comparison with the single Cd stress. As for aboveground parts, the trend was contrary. The results implied that in most cases, HCHs might inhibit Cd adsorption on root surface and uptake by roots. However, possible complexation of HCHs and Cd could help to enhance Cd translocation to aboveground parts of garlic seedlings. 2) Under the single HCHs stress, it was found that Allium sativum could effectively accumulate HCHs in their tissues, with significantly higher accumulation in aboveground parts than in underground parts. Besides, HCHs concentrations in plant tissues were largely dependent on HCHs application rate. It was implied that the transport of HCHs from roots to shoots through xylem had a major contribution to the accumulation of HCHs by shoots, which might be attributed to relatively high water solubility of HCHs compared with DDT and PAHs. However, HCHs concentrations in garlic seedlings grown in soil spiked with Cd were relatively lower than that of single HCHs treatment. The complexation of Cd and HCHs might be the main mechanisms of decreasing HCHs bioaccumulation. 3) Under the single Cd stress, it was found that Cd did not have significant effects on the content of leaf pigments in garlic except for 5 mg kg-1 soil Cd treatment, while under the higher concentration Cd and HCHs treatments, leaf pigments exhibited a clearly higher contents. Also, under the single Cd stress, MDA contents in garlic leaves had negative relativity with Cd concentration in soils, whereas, most combined treatments of Cd and HCHs showed relatively higher MDA contents in comparison with the single Cd stress, which indicated that the coexistence of Cd and HCHs might enhance oxidative stress for plant growth. In addition, under the single Cd stress, the proline content in garlic leaves exhibited a slightly decreasing trend with the increase of Cd concentration. However, under combined existence of Cd and HCHs, except for the treatment of 40 mg kg-1 Cd and 2 mg kg-1 HCHs, other combined treatments had relatively higher proline contents in comparison with the single Cd stress, which might be explained by water deficiency resulted from HCHs adsorption on root surface. 4) The single HCHs stress had no obvious effects on the content of leaf pigments in garlic, while under the combined stress of Cd and HCHs treatments, leaf pigments exhibited a clearly higher contents in almost all cases. Also, under the single HCHs stress, MDA contents in garlic leaves presented relatively lower than that of the control, whereas, most combined treatments of Cd and HCHs showed irregular changes, either higher or lower contents happened in comparison with the single HCHs stress, which indicated that the single HCHs and combined effects of Cd and HCHs on plant growth should be complicated. In addition, under the single HCHs stress, the proline content in garlic leaves exhibited a slightly increasing trend with the increase of HCHs concentration, while the content was relatively lower than that of the control. However, under combined existence of Cd and HCHs, except for the treatment of 40 mg kg-1 Cd and 2 mg kg-1 HCHs, other combined treatments had relatively higher proline contents in comparison with the single Cd stress, which might be explained by a direct effect of Cd ions, or damage to the water-absorbing capacity of roots under the conditions of Cd stress. 5) Under the single stress of 40 mg kg-1 soil Cd, localization of Cd confirmed that Cd deposition was in interior surface of the plasmalemma in the protoplast of garlic cloves. 6) The mean BCFs of HCH isomers in both aboveground and underground parts was equal to 4.07, and the BCFs of δ-HCH in aboveground parts of garlic seedling was the highest with a maximum value of 12.18. The BCFs of the four HCH isomers were in the following sequence: δ- >γ- ≥β- >α-HCH, which might be correlated with their water solubility and resistibility to degradation. These results might have important implications for possible food security risk of Allium sativum grown in even low-level contaminated soils with HCHs or Cd.For potato as studied plants: 1) Cd accumulation in the different organs of potato increased with an increase in Cd concentration applied in soils, while corresponding BCFs presented a decline trend. The order of Cd concentration in each potato organ was leaves>>roots≥shoots>>tubers. On average, combined existence of Cd and HCHs inhibited Cd bioaccumulation compared with the situation when Cd was present singly. They presented the antagonistic interactions, which might be partly due to the formation of Cd-HCHs complex. These phenomena need to be explained by further research. 2) Under the single HCHs stress, the greatest amounts of total HCHs were usually accumulated in roots or leaves of potato, then stems, while the least quantities were in tubers. The BCFs ranges in the roots, stems, leaves and tubers were: 1.19-4.96, 0.24-1.00, 0.10-2.89 and 0.07-0.63, respectively. Furthermore, the HCHs concentrations of the leaves were not correlated to those of soils and other organs, which clearly suggested that HCHs (such as α-HCH ) in ambient air volatilized from the pot soils might be absorbed by plant through the leaves of potato. Among four isomers, δ-HCH and γ-HCH were preferentially accumulated in all organs, notably quite a few α-HCH existed in the leaves, which might be mainly determined by their aqueous solubility and volatility. In addition, the combination of HCHs-Cd pollution showed synergistic or antagonistic effects on HCHs bioaccumulation with the differences of the organs and treatment concentrations. 3) Under the single Cd stress, the production of chlorophyll a and chlorophyll b declined, yet not obviously changed with further Cd supplement. The content of proline in leaves almost had no obvious change with the increase of Cd concentration with the exception of the highest concentration treatment. However, Cd applied from 5 to 25 mg kg-1 apparently increased MDA levels in potato leaves, suggesting a higher degree of lipid peroxidation due to Cd stress. Moreover, the MDA contents under the high concentration Cd and HCHs treatments (25 mg kg-1 Cd and 1 mg kg-1 HCHs ) were evidently higher than that exposed to the moderate concentration treatments (5 mg kg-1 soil Cd and 0.2 mg kg-1 HCHs). It suggested that supplemented HCHs concentration might lead to an augment of lipid peroxidative products and oxidative stress in potato. 4) Under the single HCHs stress, contents of Chlorophyll a (Ca) and chlorophyll b (Cb) in potato leaves decreased with the increment of HCHs concentrations in soils, and the contents of total carotenoid (Cx•c) decreased, then increased and finally dropped again. It indicated that HCHs stress might lead to the decline of Ca and Cb contents in potato leaves. Under combined pollution of Cd-HCHs, Ca and Cb contents exhibited progressive decrease in comparison with the single HCHs stress. However, the MDA contents under the combined pollution of 5 mg kg-1 Cd and 0.2 mg kg-1 HCHs were lower than that exposed to single 0.2 mg kg-1 HCHs. It suggested that supplemented Cd concentration might lead to the reduce of lipid peroxidative products and oxidative stress in potato. The proline content in the leaves of potato under single HCHs stress exhibited progressive increase with the augment of HCHs concentrations in soils, which might be related to water deficiency as a result of HCHs adsorption on root surface. However, under combined existence of Cd and HCHs, there was remarkably less than that of the single HCHs stress, possibly due to the complexation of Cd and HCHs. 5) Cd localization in tubers, stems and leaves of potato at the tissue and cellular were examined using scanning electron microscopy and energy dispersive X-ray analysis. In tubers, Cd was determined in cortex parenchyma cells, the interior surface of plasmalemma and the surface of starch granule in the protoplast. In stems, it was found that there were considerable quantities of Cd in the cortex, possibly by the action of pith ray. In leaves, Cd was found in cells lying on the way of water migration from the vascular cylinder to epidermal cells, which was in line with passive Cd transport by the transpiration stream, including palisade mesophyll cells, spongy mesophyll cells and stoma. Evidently, leaves and stems were the main compartment of Cd storage and uptake dynamical mechanisms.Thirdly, Gossypium hirsutum L. (cotton) as a trial plant was conducted pot experiments under different concentrations treatments of HMs and HCHs. This experiment aimed to further investigate the phytoavailability of HMs and HCHs, effects of HMs on HCHs uptake in cotton organs, as well as the endurance of cotton plant on HMs and HCHs, and assessed the potential for phytoextraction of HMs and HCHs by cotton plant. Localization of HMs in cotton tissues was also performed by Scanning Electron Microscope equipped with an energy dispersive X-ray microanalyser. In addition, effects of EDTA on HMs uptake in cotton plants were given preliminary research. The results showed that1) Under different concentration treatments of HMs, the accumulation of studied metals in cotton plants varied with the difference of cotton organs, growth stages and the addition of EDTA. For Cd, its concentrations in cotton organs increased significantly with the increment of Cd concentrations in pot soils. On average, the BCFs of Cd in cotton organs was in the following order: leaves>roots>stems>shells>seeds>fibers. Besides, the Cd uptake before the florescence gave a major contribution to the total Cd accumulation in cotton plants. But the addition of EDTA (0.8 mmol kg-1) played no important role in Cd phytoextraction by cotton plants. As for Zn, there were no statistical correlation between Zn concentrations in various cotton organs and those in soils treated. For cotton seedlings, the largest Zn concentration was detected in cotton leaves, subsequently roots and stems, while in the terms of mature cotton plants, Zn was predominantly in seeds, then roots, only minor concentrations existed in shells, and the least was in fibers. Besides, almost all of Zn average contents in roots and stems of the mature cottons were more than those of the seedlings. On average, the BCFs of Zn in cotton organs in sequence from high to low were seeds, roots, leaves or stems, shells and fibers. It was found, to most extent, that the addition of EDTA produced positive effects on Zn accumulation in cotton roots and stems, however, opposite effects were found in other organs, suggesting that the addition of EDTA played no obvious effects as a whole. In terms of Cu, only its concentrations in roots were statistically correlated to those of corresponding pot soils. The BCFs of Cu in cotton organs followed the sequence: roots>stems>seeds>shells>leaves>fibers. Cu accumulation efficiency also varied with the different growth stages, the average Cu concentrations in roots and stems of cotton seedlings were lower than those of mature cotton plants. Here, it was found that the addition of EDTA could help to enhance the phytoextraction of Cu by cotton plants. For Ag, its concentrations in roots were statistically positive correlated to those of corresponding pot soils, but there was a negative correlation between the Ag concentrations in shells and those in soils treated. On average, the BCFs of Ag in cotton organs followed the sequence: roots, leaves>shells> stems>seeds>fibers. In addition, to some extent, the EDTA addition presented negative effects on Ag accumulation in cotton plants. Correlation analysis on HMs concentrations in cotton organs showed that there were statistical correlation among Cu, Zn, Cd and Ag in roots and leaves, and Zn and Cu presented significant correlations in stems and seeds. In addition, it was found that Cd was related to Ag in stems, and to Zn in shells. In conclusion, the BCF ranges of Cd, Cu, Zn and Ag in main organs of cotton (stems) from low to high concentration treatments were 0.34-0.11, 0.57-0.02, 0.50-0.04 and 1.23-0.01, respectively. Based on the cotton biomasses and HMs contents in cotton organs, the estimated potential of cotton for phytoextraction of Cd, Cu, Zn and Ag from low to high concentration treatments were 6.17-72.44, 324.04-545.66, 757.50-943.38 and 2.88-9.94 g hm-2 a-1, respectively. And the annual extraction rate of Cd, Cu and Zn were 17.75%-6.36%，27.97%-1.26% and 22.35%-2.06%, respectively. Besides, their extraction rate varied greatly with different concentration treatments. However, the estimated phytoextraction ability of cotton plants was higher than the previous research on trees.2) The analysis on localization of Cd, Zn, Cu and Ag in roots, stems and leaves cotton showed that ① in roots, HMs were always detected in white deposits in periderm and secondary phloem, together with on the external surface of periderm. At the same time, another HMs were detected on the walls or its neighborhood of the vessel located in the vascular cylinder. The obtained results showed that the vessel located in the xylem, as well as the adsorption of root periderm and the infiltration of metal ion should be two important routeway to enter in the plants for Cu, Zn, Cd and Ag. However, there was lesser Cu than other metals in the cytoplasm of secondary phloem, which might be explained by the filtration selectivity and the barrier of cortex and its cell walls on different metals. ② in stems, considerable quantities of HMs were found in white particles in the cytoplasm or cell walls of pith, possibly resulted from the storage function of pith. The presence of HMs in the xylem of cotton stem suggested the vessels in the xylem exert important transportation for metal ions. In addition, frequent existence of HMs in epidermis and cortex might be largely explained by transverse transportation of pith ray. The concomitancy of Cd and Ag, together with Cu and Zn was observed. ③ in leaves, HMs were mainly present in different leaf tissues-epidermis hair, spongy tissue, especially palisade tissue and xylem of main vein. Exceptionally, no Cu was detected in the cast of cell wall or secretion of epidermis hair, likewise, no detected Cd was in spongy tissue. However, altogether, specific HMs localized in mesophyll cells lying on the way of water migration from vascular cylinder to epidermis, which distinctly indicated involvement of transpiration in metal translocation in the leaves. In conclusion, the above mentioned results showed that HMs adsorption on the root surface and absorption in roots, HMs translocation from roots to shoots via the xylem loading process, as well as leaf transpiration were critical factors for transportation and storage of HMs in cotton plants.3) The greatest amounts of total HCHs were usually accumulated in leaves of cotton, then roots and stems, while very minor amounts were in shells and fibers, and no detected HCH isomers existed in seeds. Under the lower concentrations HCHs treatments (0, 0.016 and 0.04 mg kg-1), the ranges of BCFs in roots, stems and leaves reached 10.03-1.53, 11.21-1.49 and 31.95-3.99, respectively. Even under the higher concentrations HCHs treatments (0.2, 0.4 and 0.8 mg kg-1), their BCFs also had the higher values, and in the range of 0.86-0.52, 0.49-0.28 and 0.93-0.56, respectively. Based on the cotton biomasses and HCHs contents in cotton organs, the estimated potential of cotton for phytoextraction of HCHs in six different treatments were 1.24, 2.51, 1.85, 2.83, 3.68 and 10.99 g hm-2 a-1, respectively. Besides, HCHs concentrations in roots, stems and leaves were positively correlated with HCHs application rates. It indicated the most likely mechanism of HCHs accumulation in these plants was sorption and uptake of soil HCHs by roots, together with the transport of HCHs from roots to shoots through xylem. Furthermore, in cotton organs, δ-, γ- and β-HCH revealed the highest BCFs values in different organs, while α-HCH showed the lowest BCFs values, which might be correlated with their water solubility and resistibility to degradation. Additionally, the effects of four HMs on bioaccumulation of HCHs in cotton organs were complicated and need further research to explain. The results help to understand the distribution of HCHs in the soil-plant system and allow consideration of the use of phytoextraction in the decontamination of lightly contaminated soils.In conclusion, Gossypium hirsutum L., characterized by strong adaptability, being grown extensively, high biomass and non-edible economic crop, exhibited stronger endurance and extracting ability for HMs and HCHs. Therefore, it was an effective, practical and green remediation technology to remediate contaminated soils throught planting cotton.