Natural and induced cadmium-accumulation in poplar and willow: Implications for phytoremediation杨树和柳树对镉的自然累积与诱导累积：对植物修复的意义

Brett H. Robinson^1,3, Tessa M. Mills¹, Daniel Petit², Lindsay E. Fung¹, Steve R. Green¹ & Brent E. Clothier¹

¹Horticulture and Food Research Institute of New Zealand, Palmerston North, New Zealand. ²Laboratoire de G´en´etique et Evoloution des Plantes, CNRS, Universit´e de Lille 1, Villeneuve d’Ascq, 59655 France. ³Corresponding author^∗

Received 30 December 1999. Accepted in revised form 23 August 2000

Key words: bioaccumulation coefficient, cadmium, phytoremediation, poplar, willow

关键词：生物富集系数，镉，植物修复，杨树，柳树

1Abstract摘要

Potentially poplars and willows may be used for the in situ decontamination of soils polluted with Cd, such as pasturelands fertilised with Cd-rich superphosphate fertiliser. Poplar (Kawa and Argyle) and willow (Tangoio) clones were grown in soils containing a range (0.6–60.6 µg g⁻¹ dry soil) of Cd concentrations. The willow clone accumulated significantly more Cd (9–167 µg g⁻¹ dry matter) than the two poplar clones (6–75 µg g⁻¹), which themselves were not significantly different. Poplar trees (Beaupré) sampled in situ from a contaminated site near the town of Auby, Northern France, were also found to accumulate significant quantities (up to 209 µg g⁻¹) of Cd. The addition of chelating agents (0.5 and 2 g kg⁻¹ EDTA, 0.5 g kg⁻¹ DTPA and 0.5 g kg⁻¹NTA) to poplar (Kawa) clones caused a temporary increase in uptake of Cd. However, two of the chelating agents (2 g kg⁻¹ EDTA and 0.5 g kg⁻¹ NTA) also resulted in a significant reduction in growth, as well as abscission of leaves. If the results obtained in these pot experiments can be realised in the field, then a single crop of willows could remove over 100 years worth of fertiliser-induced Cd contamination from pasturelands.

杨树和柳树有望用于Cd污染土壤的原位净化，例如施用富Cd过磷酸钙肥料的牧场。将杨树（Kawa和Argyle）和柳树（Tangoio）无性系种植于Cd浓度范围为0.6–60.6 µg g⁻¹（干土）的土壤中。柳树无性系的Cd累积量（9–167 µg g⁻¹干物质）显著高于两个杨树无性系（6–75 µg g⁻¹），而两个杨树无性系之间无显著差异。在法国北部Auby镇附近一处污染场地原位采集的杨树（Beaupré）样品同样累积了大量Cd（最高达209 µg g⁻¹）。向杨树（Kawa）无性系添加螯合剂（0.5和2 g kg⁻¹ EDTA、0.5 g kg⁻¹ DTPA以及0.5 g kg⁻¹NTA）使其对Cd的吸收出现暂时性增加。然而，其中两种螯合剂处理（2 g kg⁻¹ EDTA和0.5 g kg⁻¹ NTA）也导致植株生长显著降低以及叶片脱落。若本盆栽试验所得结果能够在田间实现，则种植一茬柳树即可去除牧场土壤中超过100年施肥所引入的Cd污染。

2Introduction引言

Elevated Cd concentrations in soils are of concern because of their risks to human health and the productivity of plants and animals (Underwood and Suttle, 1999). They could even be used as a non-tariff trade barrier to exports. Many years of fertilisation with Cd-rich superphosphate has increased concentrations of Cd in New Zealand soils, in some cases to levels above a self-imposed limit of 3 µg g⁻¹ in dry soil (Zanders, 1998).

土壤中Cd浓度升高对人类健康以及植物和动物的生产力构成风险，因而受到关注（Underwood and Suttle, 1999）。Cd甚至可能被用作限制出口的非关税贸易壁垒。多年施用富Cd过磷酸钙已使新西兰土壤中的Cd浓度升高，部分土壤甚至超过了自行设定的干土3 µg g⁻¹的限值（Zanders, 1998）。

Conventional treatments for Cd-contaminated soils fall into three general categories: Isolation, cleansing and ‘inerting’. Isolation may involve removal of the topsoil, covering with concrete or non-contaminated soil, or hydraulic isolation from surrounding areas. Cleansing involves the leaching of pollutants with acids. ‘Inerting’ is the addition of other chemicals to the soil that render the pollutants into a nontoxic form. Conventional procedures cost between US$ 100 000 and 1 000 000 per hectare (Russel et al., 1991). Furthermore, these methods may leave the soil infertile, cause further pollution by leaching or only be a temporary solution.

Cd污染土壤的常规处理方法大体可分为三类：隔离、清洗和“惰性化”。隔离可包括移除表土、以混凝土或未污染土壤覆盖，或与周边区域进行水力隔离。清洗是指用酸淋溶去除污染物。“惰性化”是指向土壤中添加其他化学物质，使污染物转化为无毒形态。常规处理方法的费用为每公顷10万至100万美元（Russel et al., 1991）。此外，这些方法可能导致土壤丧失肥力，因淋溶造成进一步污染，或仅为临时性解决方案。

∗ FAX No: +64(6) 3546731. E-mail: brobinson@hort.cri.nz

Chaney (1983) suggested that some heavy-metal contaminated soils may be cleaned up by growing a crop of plants which accumulate the pollutants, then harvesting the plants and disposing of them in a ‘safe area’. This process was termed phytoremediation. The first feasibility experiments using ‘hyperaccumulator’ plants (see below) were conducted by McGrath et al. (1993), although Huiyi et al. (1991) had already shown that some forest species, including poplar, could be used to remove Cd from polluted soils. The cost of these operations is estimated to be US$ 60 000 to 100 000 per hectare (Salt et al., 1995) considerably less than the cost of conventional operations. Unlike some conventional operations the solution is permanent, there is a low risk of pollution due to leaching, and the soil is left fertile.

Chaney（1983）提出，某些重金属污染土壤可通过种植能够累积污染物的植物，随后收获植物并将其处置于“安全区域”的方式加以清除。这一过程被称为植物修复。首批利用“超富集植物”（见下文）开展的可行性试验由McGrath et al.（1993）完成，但Huiyi et al.（1991）此前已表明，包括杨树在内的一些森林树种可用于去除污染土壤中的Cd。据估计，此类操作的成本为每公顷6万至10万美元（Salt et al., 1995），远低于常规处理方法的成本。与某些常规方法不同，该方法可永久解决问题，因淋溶导致污染的风险较低，且土壤肥力得以保持。

Plants used for phytoremediation should be fastgrowing, deep-rooted (in the case of immobile contaminants at depth), easily propagated and accumulate the target metal. Accumulation of the target metal is of paramount importance, ideally the species used should have a high bioaccumulation coefficient which is defined as the plant/soil metal concentration quotient. Poplars and willows exhibit the first three of the aforementioned properties making them good candidates for phytoremediation, providing they have a high Cd bioaccumulation coefficient. Plant-metal uptake is usually a function of the ‘bioavailability’ of the metal in the soil, the transpiration rate of the plant and the selective uptake of that metal by the plant (Chardonnens et al., 1998). Certain plants naturally accumulate inordinately high concentrations of heavy metals in their aerial portions. Such plants, termed ‘hyperaccumulators’ (see Brooks, 1998) are able to sequester heavy metals in the rhizosphere, then transport them and store them in the above ground portions. Poplars and willows have not been reported to exhibit this phenomenon, however, their water use is high relative to other plants so their relative uptake of Cd may also be high (Hinchman et al., 1996). Göransson and Philippot (1994) and Landberg and Greger (1994) reported high Cd accumulation by willow and suggested they could be used for Cd phytoextraction. Greger et al. (1997) discussed the potential of willows for the decontamination of Cd-contaminated soils, reporting that 12 years of growth would be required to remove the Cd accumulated in Swedish soils during the last century. Felix (1997) calculated that a hybrid willow could remove 0.222 kg of Cd per hectare per annum from a soil containing 6.6 µg g⁻¹ Cd. It would take 77 years to reduce the soil Cd burden to 0.8 µg g⁻¹.

用于植物修复的植物应生长迅速、根系深（针对深层不可移动的污染物而言）、易于繁殖，并能累积目标金属。其中，目标金属的累积最为重要，理想情况下所用物种应具有较高的生物富集系数，即植物与土壤金属浓度之比。杨树和柳树具备上述前三项特性，只要其Cd生物富集系数较高，便是植物修复的良好候选植物。植物对金属的吸收通常取决于土壤中该金属的“生物有效性”、植物的蒸腾速率以及植物对该金属的选择性吸收（Chardonnens et al., 1998）。某些植物能够在其地上部分自然累积极高浓度的重金属。这类植物被称为“超富集植物”（见Brooks, 1998），能够在根际中螯合重金属，继而将其转运并储存于地上部分。尚无报道表明杨树和柳树具有这一现象，但与其他植物相比，其耗水量较高，因此其对Cd的相对吸收量也可能较高（Hinchman et al., 1996）。Göransson and Philippot（1994）以及Landberg and Greger（1994）报道了柳树对Cd的高累积，并提出柳树可用于Cd的植物提取。Greger et al.（1997）探讨了柳树净化Cd污染土壤的潜力，报道称需生长12年才能去除瑞典土壤在上个世纪累积的Cd。Felix（1997）计算得出，在含Cd 6.6 µg g⁻¹的土壤上，一种杂交柳树每年每公顷可去除0.222 kg Cd，需要77年才能将土壤Cd负荷降至0.8 µg g⁻¹。

Huang and Cunningham (1996) reported that the addition of chelates such as EDTA to Pb-contaminated soils greatly increased plant-uptake. In this way, highbiomass non-hyperaccumulator plants such as Zea mays could be used to remove Pb from contaminated soils. The effect of chelating agents on plant-Cd uptake is unknown. Any chelating agent used in a phytoremediationoperation should be cheap and, most importantly, have a low environmental impact. It is imperative that the chelating treatments do not lead to leaching of the mobilised heavy metal. Means et al. (1980) reported that the degradation rates of the chelating agents used in this study are 10–40 days, however, more recent studies (Hong et al., 1999) indicate that EDTA can persist for extended periods in soils. The addition of EDTA may also lead to the loss (either by leaching or plant accumulation) of essential plant micro-nutrients such as Cu and Zn.

Huang and Cunningham（1996）报道，向Pb污染土壤中添加EDTA等螯合剂可大幅提高植物吸收。借此，Zea mays等高生物量的非超富集植物即可用于去除污染土壤中的Pb。螯合剂对植物吸收Cd的影响尚不清楚。植物修复操作中所用的螯合剂应价格低廉，最重要的是对环境影响小。螯合处理绝不能导致被活化的重金属发生淋溶。Means et al.（1980）报道，本研究所用螯合剂的降解时间为10–40天，但较新的研究（Hong et al., 1999）表明，EDTA可在土壤中长期残留。添加EDTA还可能导致Cu、Zn等植物必需微量营养元素的损失（通过淋溶或植物累积）。

The aim of this study was to determine the tolerance and accumulation of Cd by poplar and willow clones grown in soil containing a range of Cd concentrations and investigate the effect of chelating agent addition on Cd uptake. We also sought to investigate the accumulation of Cd by poplars growing in situ in contaminated soil. These results would thus establish the potential of poplar and willow for the phytoremediation of Cd-contaminated soils.

本研究旨在测定种植于不同Cd浓度土壤中的杨树和柳树无性系对Cd的耐受性和累积量，并探讨添加螯合剂对Cd吸收的影响。我们还研究了原位生长于污染土壤中的杨树对Cd的累积。上述结果将由此确立杨树和柳树用于Cd污染土壤植物修复的潜力。

3Materials and methods材料与方法

Shade house experiments using potted trees

盆栽树木荫棚试验

Shade house experiments were performed at Hort- Research, Palmerston North, New Zealand (latitude 40.2 ^◦ S, longitude 175.4 ^◦ E). Average monthly vapour pressure deficits (in Pascals) were: Oct. 408, Nov. 503, Dec. 638, Jan. 857, Feb. 907. Poplar and willow clones were grown in 20 l buckets with varying Cd additions as described in Table 1. The soil used was Manawatu silt loam which has a Kd value for Cd of 640 (soil/solution cadmium concentration quotient) over the concentration ranges used. The pH of the soil was 5.7, total organic carbon 6.3% and an exchange capacity of 13.4 cmol(+) kg⁻¹ Soils were prepared by adding varying amounts of a 1% Cd solution to 20kg of soil and mixed using a concrete mixer. The Cd was added as the nitrate.

荫棚试验在新西兰Palmerston North的HortResearch进行（南纬40.2 ^◦ S，东经175.4 ^◦ E）。月平均水汽压亏缺（单位：帕斯卡）为：10月408，11月503，12月638，1月857，2月907。杨树和柳树无性系种植于20 l桶中，Cd添加量各异，详见表1。供试土壤为Manawatu粉砂壤土，在所用浓度范围内，其对Cd的Kd值（土壤/溶液镉浓度比）为640。土壤pH为5.7，总有机碳6.3%，交换容量为13.4 cmol(+) kg⁻¹。将不同量的1% Cd溶液加入20kg土壤中，并用混凝土搅拌机混匀以制备供试土壤。Cd以硝酸盐形式添加。

The varieties of poplar used were Kawa (Populus deltoides × P. yunnanensis NZ 5006) and Argyle (P. deltoides × P. nigra NZ 5015). The willow variety was Tangoio (Salix matsudana × S. alba NZ 1040). Kawa was chosen for the soil-amendment experiment because this variety has one of the highest rates of water use amongst poplars. Soils were fertilised with an Osmocote^{T M} slow release fertiliser at a rate recommended by the manufacturer. Planting occurred at the end of September, 1998, some 2 months after preparation of the pots. This delay was to allow time for the added Cd to come to equilibrium with the soil. Trees were grown from c.a. 0.2 m cuttings. Buckets were arranged in a randomised block design within the shade house. The chelating agents NTA (nitrilotriacetic acid)

所用杨树品种为Kawa（Populus deltoides × P. yunnanensis NZ 5006）和Argyle（P. deltoides × P. nigra NZ 5015）。柳树品种为Tangoio（Salix matsudana × S. alba NZ 1040）。选择Kawa用于土壤改良试验，是因为该品种是杨树中耗水速率最高的品种之一。按生产商推荐用量向土壤施用Osmocote^{T M}缓释肥。种植于1998年9月底进行，即盆钵制备约2个月之后。此延迟旨在使添加的Cd与土壤达到平衡。树木由约0.2 m长的插条培育而成。桶在荫棚内按随机区组设计排列。螯合剂NTA（次氮基三乙酸）

EDTA (ethylenediaminetetraacetic acid), and DTPA (diethylenetriaminepentaacetic acid) were added on the 19/1/1999. Leaf samples (first mature leaf from apical bud) were taken on the 12/12/98, 26/1/99 and 3/2/99. Periodic measurements of transpiration rates were made using porometery (LiCor LI1600 steady state porometer) to indicate the water use of the trees. Plants were harvested on 5/3/99.

、EDTA（乙二胺四乙酸）和DTPA（二乙烯三胺五乙酸）于1999年1月19日添加。分别于1998年12月12日、1999年1月26日和1999年2月3日采集叶片样品（顶芽下第一片成熟叶）。采用气孔计（LiCor LI1600稳态气孔计）定期测定蒸腾速率，以反映树木耗水量。植株于1999年3月5日收获。

Collection of poplar samples from Auby, Northern France法国北部Auby杨树样品的采集

The accumulation of Cd in situ was investigated by taking leaf, stem and root samples as well as root zone soil samples from poplar trees (Beaupré: P. tricocapa × P. deltoides) growing near the town of Auby, Northern France. The site was chosen because the levels of Cd pollution vary from relatively unpolluted (<1 µg g⁻¹) to highly polluted (>300 µg g⁻¹). This is due to the presence of a nearby base metal smelter. Beaupré has been used in many revegetation and beautification projects in the area.

通过采集生长于法国北部Auby镇附近的杨树（Beaupré：P. tricocapa × P. deltoides）的叶、茎、根样品以及根区土壤样品，研究Cd的原位累积。选择该场地是因为其Cd污染水平从相对未污染（<1 µg g⁻¹）到重度污染（>300 µg g⁻¹）不等，这是由于附近存在一座贱金属冶炼厂。Beaupré已被用于该地区的许多植被恢复和美化项目。

Sample preparation and Cd determination样品制备与Cd测定

Leaves, stems and roots were separated and placed in a drying cabinet at 80 ^◦C until a constant weight was reached. Samples were weighed, and ground. Approximately 0.2 g of material from each sample was accurately weighed into 50 mL Erlenmeyer flasks. Concentrated nitric acid (10 mL) was added to each tube and the mixtures heated on a heating block until a final volume of ca. 3 mL was reached. The samples were then diluted to 10 mL using deionized water and stored in polythene containers.

将叶、茎和根分开，置于干燥箱中于80 ^◦C烘至恒重。样品称重后粉碎。从每个样品中准确称取约0.2 g材料置于50 mL锥形瓶中。向每管加入浓硝酸（10 mL），在加热块上加热混合物至最终体积约为3 mL。随后用去离子水将样品稀释至10 mL，并储存于聚乙烯容器中。

∗Significantly different (p=0.05) to 0.6 µg g−1 plant. n.d. = not determined.

∗与0.6 µg g−1处理植株相比差异显著（p=0.05）。n.d. = 未测定。

Soil samples were dried at 80 ^◦C and sieved to <1 mm size using a nylon sieve. About 0.2 g quantities of sieved soil were accurately weighed into boiling tubes. Ten mL of concentrated nitric acid was then added and the mixtures boiled until a final volume of 3 mL was reached. A further 10 mL of concentrated hydrochloric acid was then added and the mixtures again evaporated to 3 mL. After filtration, the solutions were diluted to 10 mL with deionized water.

土壤样品于80 ^◦C烘干，并用尼龙筛过<1 mm筛。准确称取约0.2 g过筛土壤置于消煮管中，加入10 mL浓硝酸，煮沸混合物至最终体积为3 mL。再加入10 mL浓盐酸，将混合物再次蒸发至3 mL。过滤后，用去离子水将溶液稀释至10 mL。

Chemical analyses on the plant and soil solutions were performed using a GBC 904 atomic absorption spectrometer. Differences between means were determined using ANOVA on Microsoft Excel.

植物和土壤溶液的化学分析采用GBC 904原子吸收光谱仪进行。均值间差异采用Microsoft Excel中的ANOVA确定。

4Results and discussion结果与讨论

Effect of Cd concentration on plant growthCd浓度对植物生长的影响

Table 2 shows the biomass production of the clones for each Cd concentration. Growth of Kawa was significantly reduced in the two highest Cd concentrations. Argyle and Tangoio also showed growth reduction at 20.6 µg g⁻¹ and above, but these decreases were not significant. The visible effects of high soil Cd concentrations were shortened shoots, increased branching, and chlorosis of the leaves. Growth was unaffected at levels below 5.6 µg g⁻¹, the range where the plants may be used for phytoremediation.表2列出了各无性系在不同Cd浓度下的生物量。在两个最高Cd浓度下，Kawa的生长显著降低。Argyle和Tangoio在20.6 µg g⁻¹及以上浓度时生长亦有所降低，但降幅不显著。土壤高Cd浓度的可见影响包括枝条缩短、分枝增加以及叶片失绿。在低于5.6 µg g⁻¹的水平下生长未受影响，该浓度范围即植物可用于植物修复的范围。

The accumulation of Cd by poplar and willow杨树和柳树对Cd的累积

Shade house experiments荫棚试验

Figure 1 shows the Cd accumulation of all trees tested. The level of Cd in the plants always exceeded the respective soil levels. There was a general decrease in the bioaccumulation coefficient with increasing soil图1显示了所有供试树木的Cd累积情况。植物中的Cd含量始终高于相应的土壤Cd含量。生物富集系数总体上随土壤

Figure 1. Above-ground Cd concentrations (dry stems and leaves) of poplar and willow clones as a function of the Cd concentration in the substrate. Vertical bars indicate standard error of the mean.图1. 杨树和柳树无性系地上部Cd浓度（干茎和叶）随基质Cd浓度的变化。竖线表示均值的标准误。

Cd concentrations. There was no significant difference between the varieties of poplar (at the p=0.05 level), but the willow clone accumulated significantly more (p<0.01) Cd than poplars in all, but the highest soil Cd-concentration. The anomalous high accumulation of Cd by Kawa clones grown in soil containing 60.6 µg g⁻¹ Cd may be explained by a lower ‘dilution by growth’ due to the significantly reduced growth at this soil concentration (Table 2). The higher bioaccumulation factor of willows make them more suitable than poplars for Cd phytoremediation in high-rainfall areas.

Cd浓度的增加而降低。杨树品种之间无显著差异（p=0.05水平），但除最高土壤Cd浓度外，柳树无性系在所有处理中的Cd累积量均显著高于杨树（p<0.01）。Kawa无性系在含Cd 60.6 µg g⁻¹土壤中异常高的Cd累积，可能是由于该土壤浓度下生长显著降低（表2），从而“生长稀释”作用减弱所致。柳树的生物富集系数较高，因而在高降雨地区比杨树更适合用于Cd的植物修复。

Leaves contained on average 1.6 times the Cd concentration of the stems. Leaves were typically 49% of the total above-ground biomass of the plant. Root Cd concentrations were highly variable and their measurement was unreliable due to soil contamination. On average they were 0.9 times the concentration of the shoots, in contrast to the findings of Dickinson et al. (1994) who reported that most Cd accumulated by willows grown in solution culture was in the roots. The biomass of the roots was not determined.

叶片中的Cd浓度平均为茎的1.6倍。叶片通常占植株地上部总生物量的49%。根系Cd浓度变异很大，且由于土壤污染，其测定结果不可靠。根系Cd浓度平均为地上部的0.9倍，这与Dickinson et al.（1994）的研究结果相反，后者报道溶液培养的柳树所累积的Cd大部分位于根系中。根系生物量未测定。

Leaf Cd-concentrations increased over the course of the experiment. Table 3 shows the concentration of Cd in Kawa leaves on four different occasions. If Cd uptake is a function of plant water use, then the leaf- Cd increase over time may be explained by cumulative water use.

叶片Cd浓度在试验期间不断增加。表3列出了Kawa叶片在四个不同时间的Cd浓度。若Cd吸收取决于植物耗水量，则叶片Cd随时间的增加可由累积耗水量来解释。

Figure 2. Cadmium accumulation by poplar (var. Beaupr´e) from Auby, Northern France, as a function of the soil Cd concentration.图2. 法国北部Auby杨树（品种Beaupr´e）对Cd的累积随土壤Cd浓度的变化。

Poplars growing in situ on contaminated soils of the CdO reacting with the soil to render the Cd in a more plant-available form.

原位生长于污染土壤中的杨树CdO与土壤发生反应，使Cd转化为更易被植物利用的形态。

Figure 2 (A, B) show the Cd concentrations in Beaupré clones growing around the town of Auby, Northern France. In soils that were heavily polluted with Cd, leaf Cd concentrations reached values in excess of 200 µg g⁻¹ (Figure 2B). These results indicate that poplar growing in situ will accumulate Cd to a similar degree to plants grown in our controlled studies. It should be noted, however, that within the window of 0.6–60.6 µg g⁻¹ Cd in the soil (the range used in the pot trials), there was no significant increase in plant Cd. Smelter emissions were stopped in 1997, so it is unlikely that the high Cd values reported here are the result of air-borne contamination. Most soil Cd was deposited as the insoluble oxide in the top ten centimetres of soil (Deram, 1999). The high accumulation of Cd by poplars growing in this soil may be the result图2（A、B）显示了法国北部Auby镇周边生长的Beaupré无性系中的Cd浓度。在Cd重度污染的土壤中，叶片Cd浓度超过200 µg g⁻¹（图2B）。这些结果表明，原位生长的杨树对Cd的累积程度与我们控制试验中种植的植物相近。但应注意，在土壤Cd为0.6–60.6 µg g⁻¹的范围内（即盆栽试验所用范围），植物Cd并无显著增加。冶炼厂排放已于1997年停止，因此本文报道的高Cd值不太可能源于大气沉降污染。土壤中的Cd大部分以不溶性氧化物形式沉积于表层10 cm土壤中（Deram, 1999）。生长于该土壤中的杨树对Cd的高累积可能是由于

∗Significantly different (p=0.05) to controls.

∗与对照相比差异显著（p=0.05）。

The effect of chelating agents on Cd accumulation

螯合剂对Cd累积的影响

Table 3 shows leaf concentrations of Cd before and 1, 2 and 6 weeks after the addition of chelating agents to the soil. Two weeks after the addition of the chelating agents, all treatments had significantly higher leaf Cd concentrations than the control. At the end of the experiment, however, there was no significant difference between the controls and any of the treatments, both for leaves and stems. Treatments with 2 g kg⁻¹ EDTA and 0.5 g kg⁻¹ NTA caused necrosis and abscission of most of the leaves by 1 week after treatment. The plants slowly recovered and had significantly lower biomass production at the end of the experiment relative to the controls. Plants treated with 0.5 g kg⁻¹ EDTA showed reduced growth and leaf discoloration. There was no reduction in growth for the 0.5 g kg⁻¹ DTPA treatment. Porometory measurements showed a reduction in the stomatal conductance of the DTPA and 0.5 g kg⁻¹ EDTA treatments, indicating reduced plant water-use due to a loss of leaf functioning. This may have adversely affected metal uptake over time.

表3列出了向土壤添加螯合剂之前以及添加后第1、2和6周的叶片Cd浓度。添加螯合剂2周后，所有处理的叶片Cd浓度均显著高于对照。然而，在试验结束时，无论叶片还是茎，对照与各处理之间均无显著差异。2 g kg⁻¹ EDTA和0.5 g kg⁻¹ NTA处理在处理后1周内导致大部分叶片坏死和脱落。这些植株缓慢恢复，但试验结束时其生物量显著低于对照。0.5 g kg⁻¹ EDTA处理的植株表现出生长减缓和叶片变色。0.5 g kg⁻¹ DTPA处理未导致生长降低。气孔计测定结果显示，DTPA和0.5 g kg⁻¹ EDTA处理的气孔导度降低，表明叶片功能丧失导致植物耗水量减少。这可能随时间推移对金属吸收产生不利影响。

These results indicate that if chelating agents are to be used to enhance plant Cd uptake in a phytoremediation operation, then the plants should be harvested shortly after their addition to take immediate advantage of the surge in uptake. The 0.5 g kg⁻¹ NTA and 2 g kg⁻¹ EDTA treatments should not be used as they caused leaf abscission.

上述结果表明，若在植物修复操作中使用螯合剂来增强植物对Cd的吸收，则应在添加螯合剂后不久即收获植物，以即时利用吸收量的激增。0.5 g kg⁻¹ NTA和2 g kg⁻¹ EDTA处理会导致叶片脱落，不应采用。

The potential of poplar and willow for phytoremediation杨树和柳树的植物修复潜力

Results indicate that poplar and willows have excellent potential for Cd phytoremediation because of their high biomass production and high Cd bioaccumulation coefficient. If the plant-uptake in field conditions is similar to that which has been achieved in these experiments then poplars could be used to decontaminate soils weakly contaminated with Cd. The use of poplar or willow for phytoremediation would require annual harvesting to avoid the recycling of leaf-bound Cd in autumn. Both poplars and willows coppice readily.

结果表明，杨树和柳树生物量高且Cd生物富集系数高，因而在Cd植物修复方面具有极大潜力。若田间条件下的植物吸收与本试验相近，则杨树可用于净化轻度Cd污染土壤。利用杨树或柳树进行植物修复需每年收获，以避免叶片中结合的Cd在秋季重新进入循环。杨树和柳树均易于萌生（平茬更新）。

An annual application of superphosphate fertiliser at a rate of 300 kg ha⁻¹ adds about 10g of Cd per hectare (Loganathan and Hedley, 1997). On a soil containing 5 µg g⁻¹ Cd, poplars harvested shortly after a pulse of EDTA, or willows, would have a Cd concentration in the dry above-ground biomass of 53 µg g⁻¹. The biomass production of poplars and willows under optimal conditions is 30 tonnes per hectare per annum. If we adopt a conservative approach and use a figure of 20 t ha⁻¹ per year, a single crop would remove 1.06 kg of Cd per hectare. This equates to 106 years of fertiliser addition. A soil containing 5 µg g⁻¹ Cd in the top 100 mm of soil contains 6 kg of Cd per hectare, assuming a bulk density of 1.2. Four crops of willows or treated poplars would reduce the soil Cd burden to 2.35 µg g⁻¹, well below the threshold of 3 µg g⁻¹. These calculations make the assumption that the results from the pot trials can be realised under field conditions.

每年以300 kg ha⁻¹的用量施用过磷酸钙肥料，每公顷约带入10g Cd（Loganathan and Hedley, 1997）。在含Cd 5 µg g⁻¹的土壤上，脉冲式添加EDTA后不久收获的杨树或柳树，其地上部干生物量中的Cd浓度可达53 µg g⁻¹。最适条件下杨树和柳树的生物量产量为每年每公顷30吨。若采取保守估计，按每年20 t ha⁻¹计算，一茬作物即可每公顷去除1.06 kg Cd，相当于106年的施肥输入量。假设容重为1.2，表层100 mm土壤中含Cd 5 µg g⁻¹的土壤每公顷含Cd 6 kg。种植四茬柳树或经处理的杨树即可将土壤Cd负荷降至2.35 µg g⁻¹，远低于3 µg g⁻¹的阈值。上述计算假设盆栽试验结果能够在田间条件下实现。

5Conclusions结论

Cadmium levels in poplars growing in situ on contaminated soils indicate that bioaccumulation factors under field conditions are the same, or better, than those obtained in this shade-house study. To verify the link between pots and the field, future work will involve small-scale plot trials and different regimes of chelating agent addition to maximise Cd uptake. Poplars and willows have the potential to provide a cheap method of cleaning up Cd-contaminated soils in New Zealand.

原位生长于污染土壤中的杨树的Cd含量表明，田间条件下的生物富集系数与本荫棚研究所得结果相同甚至更高。为验证盆栽与田间结果之间的联系，今后的工作将开展小规模小区试验，并采用不同的螯合剂添加方案，以最大限度提高Cd吸收。杨树和柳树有望为新西兰Cd污染土壤的清除提供一种廉价的方法。

6Acknowledgements致谢

Thanks to Antonio Diaz and Maria-Jose Palomo for their involvement in the shade-house experiment and to Nihal de Silva for statistical advise. We acknowledge AGMARDT for providing the funding for this project. Markus Deurer and Monika Walter are thanked for a constructive review of the manuscript.

感谢Antonio Diaz和Maria-Jose Palomo参与荫棚试验，感谢Nihal de Silva在统计方面提供建议。感谢AGMARDT为本项目提供资助。感谢Markus Deurer和Monika Walter对本文稿提出的建设性审阅意见。

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