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Journal of Environmental Quality 30:2091-2098 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
Plant and Environment Interactions

Phytosiderophores Influence on Cadmium Mobilization and Uptake by Wheat and Barley Plants

M. Shenker*,a,c, T.W.-M. Fanb and D.E. Crowleya

a Dep. of Environmental Sciences, University of California, Riverside, CA 92521-0424
b Dep. of Land, Air, and Water Resources, University of California, Davis, CA 95616
c Dep. of Soil and Water Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, Rehovot 76-100, Israel

* Corresponding author (Shenker{at}agri.huji.ac.il)

Received for publication February 14, 2000.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A constant anthropogenic release of cadmium to the environment has resulted in a continuous buildup of Cd in soils. Uptake and accumulation of Cd in plant tissue and in grains may lead to food chain transfer to humans. Application of synthetic chelates was suggested to increase metal mobilization and facilitate phytoextraction as a means for the remediation of metal-polluted soils. However, most of the chelate-extracted metal may be leached rather than mobilized to plant roots. In contrast to the synthetic chelates added to soils, plant-produced chelators called phytosiderophores (PS) are excreted directly to the rhizosphere. Previous studies have shown that PS facilitate uptake of Zn and Fe by graminaceous plants. In this study, a two-step PS mediation of Cd uptake was hypothesized: (i) extraction and chelation in the soil solution, and (ii) delivery of the chelated Cd to the uptake system of the plant. We examined Cd extraction by PS, the synthetic chelate HEDTA [N-(2-hydroxyethyl)-ethylenediamine-triacetic acid], and a fungal siderophore rhizoferrin from solid-phase Cd phosphate at pH 7.3 with and without Fe competition in the presence of Ca and Mg as additional competing metals. While rhizoferrin did not extract Cd, PS and HEDTA did extract Cd even in the presence of Fe. Yet, uptake of Cd by wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) plants was not significantly influenced by Fe stress, but instead was controlled primarily by Cd2+ activity in solution. These results suggest that even though Cd may be mobilized by PS, there is no significant uptake of the Cd–PS complex by the plant roots.

Abbreviations: PS, phytosiderophores


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A CONSTANT anthropogenic release of cadmium to the environment has resulted in a continuous buildup of Cd in soils. Some heavily contaminated soils are confined to areas of nonferrous metal mining and production. Other widespread regions are subject to moderate input of anthropogenic Cd through activities such as waste incineration, sewage sludge applications, and use of phosphate fertilizers (World Health Organization, 1992). A concern was raised that uptake and accumulation of Cd in plant tissues and grains might lead to food chain transfer to humans (Gupta and Gupta, 1998), hence limits for metal addition onto soils were set (e.g., USEPA, 1993). Other investigators are examining the potential use of plants for phytoextraction of metals from soils (e.g., Chaney, 1983; Baker et al., 1994; Cunningham et al., 1995; Salt et al., 1995); however, the efficiency of this strategy for soil remediation is often limited because of low solubility and mobility of metals in the soil (Cunningham and Ow, 1996). In order to facilitate metal solubility and bioavailability, Cunningham and Ow (1996) and Chaney et al. (1997) suggested the use of synthetic chelates such as EDTA (ethylenediaminetetraacetic acid) as soil amendments. Indeed, using this approach, Huang et al. (1997) could increase Pb uptake by 20-fold in corn (Zea mays L.) and pea (Pisum sativum L.) plants, and Blaylock et al. (1997) could demonstrate a dramatic increase in the uptake of Pb, Cd, Cu, Ni, and Zn by Indian mustard [Brassica juncea (L.) Czern.] grown in contaminated soils. However, to achieve this result they had to apply the chelate at an extremely high rate of several thousand kilograms of the chelate per hectare. Since roots occupy only a small portion of the soil volume, usually in the range of 1% (Marschner, 1995), the majority of the applied chelate would be far from the uptake sites. Thus, while this approach might increase plant uptake and translocation of the toxic metal to the shoot, it might also increase the risk of toxic metal leaching out of the contaminated site to neighboring sites and ground water.

Many previous studies have shown that plant-produced metal chelators called phytosiderophores (PS) (Takagi, 1976; Takagi et al., 1984) facilitate not only iron but also Zn uptake by graminaceous plants (Zhang et al., 1991; Römheld, 1991; Hopkins et al., 1998). Since Zn and Cd are both members of the IIb group of the periodic table, PS also have been speculated to mediate Cd uptake in grain crops that accumulate this metal. If PS are efficient Cd-mobilizing agents in soil and if they serve as efficient mediators for Cd uptake by graminaceous plants, they might serve as an effective means to achieve phytoextraction of this metal from contaminated soils. Further, since PS release is confined to the same small portion of the soil where roots are located, the adverse result of enhanced leaching of the pollutant metal by excess chelators is expected to be negligible. The stability constants were determined for Fe3+, Fe2+, Ca, Mn, Ni, Cu, and Zn PS by Murakami et al. (1989). According to this work, PS are highly specific for the ferric iron; however, no data of stability constants of PS with Cd are available to allow a reasonable estimate regarding the efficiency of these natural chelates to complex Cd and thereby facilitate its uptake.

If PS production enhances Cd uptake, this process might be amplified by inducing Fe stress, or by breeding techniques that would introduce PS production efficiency to Cd-accumulator plants. Another promising approach that might facilitate using PS would be by cloning genes encoding PS production in plants of high biomass and high resistance to Cd toxicity. A first step in the latter direction was achieved recently by Higuchi et al. (1999), who had isolated genes encoding nicotianamine synthase, a key enzyme in the PS biosynthetic pathway in barley and rice (Oryza sativa L.) genomes. However, no data are available to indicate whether high concentrations of Cd would impair PS production.

The objective of this study was to examine the effects of PS production on Cd uptake by wheat and barley plants and the effect of Cd on PS production by these plants. We hypothesized a two-step process in PS-mediated Cd uptake by plants from a soil: (i) Cd extraction from a solid phase and chelation in the soil solution, and (ii) delivery of the chelated Cd to the uptake system of the plant.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Cadmium Extraction from Solid Phase Cadmium Phosphate
Different Cd to Fe ratios were prepared by addition of CdCl2 and FeCl3 in the following concentrations of the two metals (M): (Case 1) 0 and 3 x 10-5, (Case 2) 3 x 10-5 and 0, and (Case 3) 3 x 10-5 and 3 x 10-5 of Cd and Fe, respectively, in the presence of other ions in the following final composition (M): 7 x 10-4 K2SO4, 10-4 KCl, 2 x 10-3 Ca(NO3)2, 5 x 10-4 MgSO4, and 10-4 KH2PO4. These same macronutrient concentrations were used for the plant culture experiment described later in this section. The pH was adjusted to 7.3 and buffered by 2 x 10-3 M HEPES [N-(2-hydroxy-ethyl)piperazine-N'-(2-ethanesulfonic acid)] (Sigma Chemical Co., St. Louis, MO). The solutions were left for 24 h to allow precipitation. According to stability constants of Lindsay (1979), 100% of the added Fe (Cases 1 and 3) and 94% of the Cd (Cases 2 and 3) had precipitated as Cd3(PO4)2 and Fe(OH)3 amorph. Then, the tested chelates, detailed hereafter, were added to a final concentration of 3 x 10-5 M. Solutions were shaken gently to assure close contact of the chelate with the solid surface, and left for 3.5 h to extract and complex metals. This step was conducted in an oven at a temperature of 60°C to accelerate the extraction rate and to minimize the possibility of microbial degradation of the chelates. No chelate was added to the control. After cooling, the solutions were filtered through 0.2-µm syringe filters (Nalgene, Rochester, NY) and acidified by 2 M HCl to pH 2.0. Concentrations of Fe and Cd were measured by atomic absorption spectrometer (AAS; PerkinElmer [Norwalk, CT] Zeeman/3030). The tested chelates were: (i) PS, collected from barley plants and purified as described below; (ii) rhizoferrin, a fungal siderophore produced by Rhizopus arrhizus and purified as described by Shenker et al. (1995a); and (iii) HEDTA (Sigma Chemical Co.). The concentration of each chelate stock solution was determined beforehand by ligand exchange assay with Cu–CAS (Shenker et al., 1995b) to ensure accurate final concentration.

Seedling Cultivation
Seeds of Fe-efficient wheat (cv. AgCs; provided by Jan Dvorak, Dep. of Agronomy and Range Science, University of California, Davis) were soaked in 18% H2O2 for 20 min to ensure surface sterilization, washed thoroughly with distilled water, and soaked in saturated CaSO4 solution for 10 min. Thereafter, the seeds were wrapped in germination paper presoaked in saturated CaSO4 solution and kept at 4°C for 24 h to enhance uniform germination. After an additional 2 d at 20°C in the dark, the seedlings were transferred to an aerated saturated CaSO4 solution for another 2 d in a plant growth chamber.

Seeds of barley (cv. CM 72) were soaked in saturated CaSO4 solution for 4 h, wrapped in germination paper presoaked in saturated CaSO4 solution, and kept in the dark for 3 d. The seedlings were transferred to the growth chamber and cultured for 4 d in aerated nutrient solution of the following composition of macroelements (M): 7 x 10-4 K2SO4, 10-4 KCl, 2 x 10-3 Ca(NO3)2, 5 x 10-4 MgSO4, and 10-4 KH2PO4; and microelements (M): 5 x 10-7 ZnSO4·7H2O, 5 x 10-7 MnSO4·H2O, 2 x 10-7 CuSO4·7H2O, 1 x 10-6 H3BO3, and 1 x 10-8 (NH4)6Mo7O24·4H2O. Iron was not included in the nutrient solution.

Seedlings of wheat and barley at this stage were used in all of the experiments described below. Plant growth chamber conditions during seedling culture and thereafter were as follows: photoperiod of 16 h, photosynthetically active radiation of ca. 400 µmol m-2 s-1 at leaf height, 20°C; dark period of 8 h, 14°C; 60% humidity.

Barley Root–Exudate Collection and Purification of Phytosiderophores
Barley seedlings were transferred to 3-L black plastic containers with eight plants per container and were grown in aerated nutrient solutions as described in the previous seedling cultivation section. Nutrient solutions were changed every 7 to 10 d. Collection of root exudates began immediately after first symptoms of Fe deficiency were observed (about 14–18 d after the seedlings were transferred). Root exudates were collected for 2 h in 200 mL double distilled H2O during peak diurnal release of PS, which was 2 to 4 h after light onset (data not shown). The solutions were immediately filtered through glass microfibre filters (GF/C; Whatman, Maidstone, England) to exclude root fragments and much of the microbial population, and then frozen. The PS were purified as the cationic fraction of the root exudates following the procedure of Takagi (1976). The root exudate solution was loaded on a column of Amberlite IR-120 Plus (Sigma Chemical Co.) prewashed with 0.1 M HCl and eluted by 0.5 M NH4OH. Fractions were collected, freeze-dried, and redissolved in 2 mL of double distilled H2O. Fractions with high chelation capacity were identified and quantified by the Cu–CAS method and pooled together. The purified PS were used for the Cd extraction from solid phase. The major component of the collected and purified PS of barley (cv. CM 72) was identified previously by nuclear magnetic resonance (NMR) techniques to be epi-HMA (epi-hydroxymugineic acid) (Fan et al., 1997), while that of the wheat (cv. AgCs) was identified to be 2'-DMA (2'-deoxymugineic acid) (Fan et al., 2001).

Plant Culture in Cadmium- and Iron-Buffered Solutions
Wheat and barley plants were cultured at various Cd and Fe levels to test Cd effects on PS production and the effect of de novo–produced PS on metal uptake by the plants. A chelator-buffered hydroponic solution system was used to assure constant free metal concentration during the growth period. Macroelement composition of all nutrient solutions was as above.

Seedlings of wheat were transferred to 3-L black plastic containers with eight plants per container and were grown in aerated nutrient solutions with microelement composition as described in Table 1 to establish the detailed four Cd–Fe treatments. Nutrient solutions were renewed 11 and 18 d after the transfer. Root exudates of wheat plants were collected from Day 19 to Day 21, 2 h per day, as described above. Barley seedlings were similarly transferred to nutrient solutions for 3 d of pretreatment in nutrient solutions with high or low levels of Fe (see Table 1). Plants of each Fe level were then separated into two groups for two levels of Cd to produce the four Cd–Fe treatments as described in Table 1. Microelement composition of the various solutions is detailed in Table 1 and in the footnote to the table. Nutrient solution was renewed 6 d after initiation of the Cd–Fe treatments and root exudates were collected on the next two days, 2 h per day, as described above. The solution pH of 6.2 for wheat or 6.3 for barley plants was buffered by 2 mM MES (2-[N-morpholino]ethanesulfonic acid; Sigma Chemical Co.) and adjusted daily by few drops of 1 M HCl or NaOH to keep the solution pH within ±0.2 pH units. The chelation capacity of the collected exudates was quantified by ligand exchange assay with Cu–CAS and was related to root dry weight (DW) at the end of the experiment. The chelation capacity was attributed to PS.


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  		Table 1. Composition of buffered solutions and values of pFe and pCd in nutrient solutions.{dagger}

 
Each treatment had three replicates for wheat or four replicates for barley, with each replicate having eight plants. Data were analyzed for Fe and Cd main effects and for Cd x Fe interaction by analysis of variance (ANOVA) using the statistical package of SAS (SAS Institute, 1990). When interaction was significant at a P < 0.05 significance level, data were further analyzed by Duncan's multiple range analysis of variance from the GLM statistical package of SAS at a P < 0.05 significance level.

Double-Chelator Metal-Buffered Nutrient Solutions
To ensure uniform levels of Fe and Cd, as well as those of other microelements, in the nutrient solutions throughout the culture period, metal-buffered nutrient solutions (Parker et al., 1995a) were used. According to this method, high levels of microelements are present to compensate for uptake that occurs during plant growth, while excess chelating agent maintains the free metal activities at low levels that are in the desired biological range. Thus, when a metal M is reacting with the ligand L to form the complex ML:

with the apparent stability constant K0app ML:

where parentheses represent activities, and L and ML represent the sum of all protonated species of the ligand and its metal complex, respectively, the negative logarithm of the metal ion activity, pM, may be expressed in terms of the apparent stability constant by the relationship:

The apparent stability constant for a given metal and a ligand depends on the pH (for details see Shenker et al., 1996). Thus, if the ligand is presented in an appreciable surplus over the metal, almost all metal is complexed by the ligand. Upon removal of free metal ion by plant uptake, the ratio [L]/[ML] changes slightly to retain almost a fixed value of pM. By selecting a proper ligand and setting the amounts of total M and L, one can determine a desirable pM value for a given solution pH (the ionic strength will have a negligible effect).

In this research, it was assumed, as was suggested by Chaney et al. (1989) and Parker et al. (1995a), that the excess chelating agent used in the metal-buffered nutrient solutions method may simulate various solid and soluble ligands that buffer metal free activities in the soil environment. Further it was assumed that Fe is taken by graminaceous plants by means of ligand exchange between the presented Fe complexes and the de novo–produced phytosiderophores, as was shown by Yehuda et al. (1996). The stability constants of the Fe3+–L complexes of most of the commonly used chelates in nutrient cultures are too high to allow such ligand exchange. Hence, HEDTA, a chelate with a lower stability constant with Fe3+, was used as the direct source of Fe to the ligand exchange reaction. The EDTA (Sigma Chemical Co.) was used to buffer the concentration of the complex Fe–HEDTA. Table 2 summarizes the apparent stability constants (Kapp) of the discussed chelates plus EDDHA [ethylenediamine-di(o-hydroxyphenylacetic acid)] with Fe3+, at pH 6.0, 25°C, and ionic strength of 0.1 M, as well as the references for the stability constants used to calculate the Kapp.


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  		Table 2. The apparent stability constants of Fe3+ complexes of the phytosiderophores mugineic acid, 2'-DMA, and epi-HMA, and the synthetic chelates HEDTA, EDTA, and EDDHA at pH 6.0, 25°C, and ionic strength of 0.1 M.

 
Microelement and chelate compositions in the nutrient solutions are summarized in Table 1. The concentrations of CdCl2·2.5H2O, FeCl3·6H2O, HEDTA, and EDTA were varied to set the Cd 2+ and Fe3+ free activities to the desired low and high levels. The pFe (-log free activity Fe3+) was set to 18.2 and 17.0 for the wheat plants, and 18.4 and 17.2 for the barley plants. The pCd (-log free activity Cd2+) was set to 10.0 and 8.8 for both plants. Other microelement concentrations were as follows (µM): 4.0 ZnSO4·7H2O, 2.0 MnSO4·H2O, 1.0 CuSO4·7H2O, 1.0 H3BO3, and 0.01 (NH4)6Mo7O24·4H2O. The pM (-log free metal activity) values were: Zn, 10.2; Mn, 7.2; and Cu, 13.5. All pM values were calculated by GEOCHEM-PC (Parker et al., 1995b) using stability constants of Smith et al. (1997) for EDTA and HEDTA. All reagents used were of analytical grade. Each of the chelate solutions was accurately quantified by the Cu–CAS method.

Plant Analysis
Wheat plants after 18 d and barley plants after 8 d in the Cd- and Fe-buffered solutions were harvested, separated for root and shoot, washed with distilled water, dried at 70°C, weighed, and homogenized. Subsamples of ca. 250 mg were digested in closed pressure-tested teflon vessels by microwave in HNO3–H2O2 solution (2 mL 70% HNO3, 2 mL 30% H2O2, 1 mL H2O) and analyzed for Cd, Fe, Zn, Cu, and Mn by graphite furnace or flame atomic absorption spectrometry. Subsamples (ca. 0.02 g fresh weight) of the youngest mature leaves were analyzed for chlorophyll content by extraction with N,N-dimethylformamide and reading absorbance at 647 and 667 nm as described by Moran (1982).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Cadmium Extraction from Solid Phase Cadmium Phosphate
The concentrations of Fe and Cd extracted by the test chelates from the Fe and Cd solids in the presence of competing ions at pH 7.3 are listed in Table 3. In accordance with the first step hypothesized by us to regulate the efficiency of a chelate to mediate Cd uptake from a soil, PS appeared to be capable of mobilizing Cd in the presence of solid phase Fe(OH)3 amorph and competing ions. This efficiency is comparable with that of HEDTA and much higher than that of the siderophore rhizoferrin.


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  		Table 3. Extraction of Fe and Cd from solid phases at pH 7.3. PS = phytosiderophores.

 
Wheat and Barley Plant Response
Using the double-chelator buffered solutions to manipulate Fe deficiency, the resulting effect on PS production and other physiological parameters of wheat and barley plants was examined.

For the wheat plants at the pFe levels used in this experiment, iron stress led to a threefold elevated release of phytosiderophores and a twofold decrease in chlorophyll content. Shoot dry weights were significantly (at P < 0.05) reduced by the lower Fe level, while root dry weights were not significantly influenced by the Fe stress (Table 4). Shoot and root dry weights were decreased by the higher Cd treatments, for both iron-sufficient and iron-stressed plants. Chlorophyll content decreased under Fe stress and at the higher Fe level by Cd toxicity. Plants grown at the high Cd concentration had visible symptoms of Cd toxicity, irrespective of Fe stress, which was manifested as leaf flagging. The low level of Fe and the high level of Cd also suppressed the shoot to root weight ratio. The quantities of PS released by iron-sufficient and iron-stressed wheat were not affected by the Cd concentration of the nutrient solution (Table 4).


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  		Table 4. Wheat shoot and root dry weights, chlorophyll content, and phytosiderophore release in response to Fe and Cd levels in nutrient solution.

 
For the barley plants at the pFe and pCd levels used in this experiment, the results indicated similar trends in shoot and root dry weights, shoot to root weight ratio, and chlorophyll content, as observed in the wheat plants (Table 5). The relatively less-pronounced effect of the high Cd level on barley plants might result from the shorter period of exposure to the Cd treatments. The PS release was increased threefold by Fe stress, and as in the case of wheat plants, it was not affected by the Cd concentration of the nutrient solution (Table 5).


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  		Table 5. Barley shoot and dry weights, chlorophyll content, and phytosiderophore release in response to Fe and Cd levels in nutrient solution.

 
Metal analyses of shoots and roots of wheat and barley plants are shown in Tables 6 and 7, respectively. Shoot contents of Zn, Cu, and Mn were all elevated in response to Fe stress for both plants, except for that of Cu in barley. Metal uptake, calculated as total metal accumulated in the plant (shoot plus roots) per kilogram final root dry weight, and the percent translocation of each metal are presented in Fig. 1 and 2 for wheat and barley, respectively. As expected from previous research (Zhang et al., 1991; Römheld, 1991; Gries et al., 1995; Hopkins et al., 1998), Fe stress increased Zn uptake by both plants (Fig. 1 and 2), which was probably due to the increase in PS release into the nutrient solution (Tables 4 and 5). The same effect was observed for Cu uptake by both plants. Furthermore, translocation of Fe, Zn, and Mn to the shoot of both plants increased in response to Fe stress (Fig. 1 and 2). However, while Fe stress had increased shoot metal content of Zn, Cu, and Mn, and the uptake of Zn and Cu, and the translocation of Fe, Zn, and Mn, none of these effects were observed consistently for Cd.


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  		Table 6. Iron, cadmium, zinc, copper, and manganese, content in shoots and roots of wheat plants in response to Fe and Cd levels in nutrient solution.{dagger}

 

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  		Table 7. Iron, cadmium, zinc, copper, and manganese, content in shoots and roots of barley plants in response to Fe and Cd levels in nutrient solution.{dagger}

 


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  		Fig. 1. Iron, cadmium, copper, manganese, and zinc uptake (up) and translocation (bottom) by wheat plants. Manganese and iron uptake values were divided by 10 and 2, respectively, to fit into the graph. Values shown are the mean of three replicates with ±standard deviation.

 


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  		Fig. 2. Iron, cadmium, copper, manganese, and zinc uptake (up) and translocation (bottom) by barley plants. Manganese and iron uptake values were divided by 10 and 2, respectively, to fit into the graph. Values shown are the mean of three replicates with ±standard deviation.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
As shown in this and other studies, PS release not only facilitated Fe uptake, but also led to coincidental uptake, translocation, and accumulation of other transition metal ions in the plant shoot. However, these effects were not observed for cadmium. The possibility that the absence of these effects is a result of PS inhibition by high levels of Cd, or that PS are not efficient in Cd extraction from solid phase Cd, was ruled out in this study. Other explanations for this finding include the possibility that the presence of competing metal ions such as Fe, Zn, Cu, and Mn prevented chelation of Cd by PS, or that PS do not form a stable complex with Cd. Unfortunately, no data are available for the stability constants of PS with Cd. Alternatively, the transport system that takes up phytosiderophore-chelated metals may discriminate against the Cd–phytosiderophore complex such that it is not taken up by Fe-stressed plants.

Based on these results, we suggest that plants that are highly efficient in PS release under iron stress, such as barley and wheat, may not necessarily accumulate more Cd from contaminated soils than iron-inefficient species and cultivars. These results further suggest that the selection of grasses that produce high quantities of phytosiderophores may not necessarily provide any advantage for use in phytoextraction of Cd as a method for phytoremediation of Cd-contaminated soils. Our observations are similar to those of Römheld and Awad (2000) and Awad and Römheld (2000), who studied Fe, Zn, Ni, and Cd uptake in soil-grown plants, as opposed to our nutrient solution studies. It should also be noted that in their study as well as in ours the importance of root exudates in the uptake processes is stressed. Further experiments in soil are required to empirically confirm our observations, since concentration of metal ions and other natural chelates such as soil soluble humic substances and microbial siderophores may vary in the rhizosphere in such a way that phytosiderophores might yet influence Cd uptake from soil.


   ACKNOWLEDGMENTS
 
This work was supported in part by U.S. Department of Energy grant number DE-FG07-96ER20255, USEPA grant #R825960010, and USEPA funded (#R819658) Center for Ecological Health Research at Univ. of California-Davis. We also thank Dr. Andrew Lane for his assistance in the NMR measurement.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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