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Plant Physiol. (1998) 117: 447-453
The Role of EDTA in Lead Transport and Accumulation
by Indian
Mustard1
Andrew D. Vassil,
Yoram Kapulnik,
Ilya Raskin, and
David E. Salt*
AgBiotech Center, Rutgers University, Cook College, New Brunswick,
New Jersey 08903 (A.D.V., I.R.); Agronomy and Natural Resources
Department, The Volcani Center, Bet Dagan, 50250, Israel (Y.K.); and Chemistry Department, Northern Arizona University, Flagstaff,
Arizona 86011 (D.E.S.)
 |
ABSTRACT |
Indian mustard (Brassica
juncea) plants exposed to Pb and EDTA in hydroponic solution
were able to accumulate up to 55 mmol kg 1 Pb in dry shoot
tissue (1.1% [w/w]). This represents a 75-fold concentration of Pb
in shoot tissue over that in solution. A threshold concentration of
EDTA (0.25 mM) was found to be required to stimulate this
dramatic accumulation of both Pb and EDTA in shoots. Below this
threshold concentration, EDTA also accumulated in shoots but at a
reduced rate. Direct measurement of a complex of Pb and EDTA (Pb-EDTA)
in xylem exudate of Indian mustard confirmed that the majority of Pb in
these plants is transported in coordination with EDTA. The accumulation
of EDTA in shoot tissue was also observed to be directly correlated
with the accumulation of Pb. Exposure of Indian mustard to high
concentrations of Pb and EDTA caused reductions in both the
transpiration rate and the shoot water content. The onset of these
symptoms was correlated with the presence of free protonated EDTA
(H-EDTA) in the hydroponic solution, suggesting that free H-EDTA is
more phytotoxic than Pb-EDTA. These studies clearly demonstrate that
coordination of Pb transport by EDTA enhances the mobility within the
plants of this otherwise insoluble metal ion, allowing plants to
accumulate high concentrations of Pb in shoots. The finding that both
H-EDTA and Pb-EDTA are mobile within plants also has important
implications for the use of metal chelates in plant nutritional
research.
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INTRODUCTION |
The synthetic chelate EDTA forms a soluble complex with many
metals, including Pb (Kroschwitz, 1995 ), and can solubilize Pb from
soil particles (Means and Crerar, 1978). Recently, application of EDTA
to Pb-contaminated soils has been shown to induce the uptake of Pb by
plants (Jøgensen, 1993; Huang and Cunningham, 1996; Blaylock et al.,
1997; Huang et al., 1997), causing Pb to accumulate to more than 1%
(w/w) of shoot dry biomass (Huang and Cunningham, 1996; Blaylock et
al., 1997; Huang et al., 1997). For the in situ remediation of
Pb-contaminated soils it appears that this chelate-assisted
phytoextraction strategy (Salt et al., 1998) may be more effective than
a strategy based on the natural ability of certain wild plant species
for metal hyperaccumulation (Chaney, 1983; Baker et al., 1988).
For more than 40 years, synthetic chelates have been used to supply
plants with micronutrients in both soil and hydroponics. Yet the
mechanisms by which chelates enhance metal accumulation are still not
well characterized (Wallace and Wallace, 1992), and what is known
appears contradictory. For example, some evidence suggests that the
Fe-chelate EDTA can be absorbed by plants and translocated to shoots
(Weinstein et al., 1954; Hill-Cottingham and Llyod-Jones, 1961, 1965).
However, Tiffin et al. (1960) concluded that Fe-chelates are excluded
from root tissue, and this was supported by Chaney et al. (1972), who
demonstrated that Fe is taken up by plants only after first being split
from the Fe-chelate complex by the action of a specific plasma
membrane-bound Fe-chelate reductase.
To optimize the process of chelate-assisted phytoextraction, it is
important to understand the biological mechanisms responsible for this
process. Because of the stimulatory role of chelate application in the
uptake of Pb and other metals by plants, we have investigated the role
of EDTA in Pb accumulation in plants. In this study we have
demonstrated that the previously described EDTA-enhanced Pb
accumulation in Indian mustard (Brassica juncea) is based on the ability of EDTA to chelate and transport Pb from soil into shoot
tissue.
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MATERIALS AND METHODS |
Roots of Indian mustard (Brassica juncea [L.] Czern.
var 426308) (Kumar et al., 1995) were grown under microbiologically
controlled conditions as follows. Seeds were surface sterilized in
2.6% (w/v) bleach for 30 min, rinsed four times in autoclaved
deionized water, and transferred onto sterile 1.2% (w/v) agar plates
containing 3.0% (w/v) Suc. Plates were held vertically and the seeds
allowed to germinate and grow in the dark at 22°C for 72 h.
Etiolated seedlings that did not show microbial contamination on the
agar plates were transferred individually into small glass vials
(29 × 65 mm) containing 23 mL of sterile nutrient solution. Soft
Styrofoam stoppers used to cap the vials were incised radially to
provide support for the hypocotyls. The nutrient solution contained 0.7 mmol L1 Ca2+, 1.5 mmol
L1 K+, 0.5 mmol
L1 Mg2+, 0.25 mmol
L1 NH4+,
2.9 mmol L1
NO3, 0.25 mmol
L1
H2PO4,
0.5 mmol L1
SO42, 4.75 µmol
L1 ferric tartrate, 0.075 µmol
L1 Cu2+, 0.2 µmol
L1 Zn2+, 1.25 µmol
L1 Mn2+, 11.5 µmol
L1
H3BO3, and 0.025 µmol
L1 MoO3 at pH 6.0. Vials
were agitated on an orbital shaker (Labline Instruments, Inc., Melrose
Park, IL) at 60 rpm to provide aeration and mixing, and the nutrient
solution was replaced weekly. Plants were cultivated for 11 d in a
growth chamber with a 10-h light period, with light provided by
fluorescent and incandescent lamps at a light intensity of 17,200 lux.
All plants were maintained at day/night temperatures of 22/22°C and a
constant humidity of 50%.
For the experiments plants were transferred under microbiologically
controlled conditions to new vials containing 23 mL of nutrient
solution supplemented with various concentrations of Pb and EDTA.
Plants were exposed to the treatments for 48 h at room temperature
under constant light (6700 lux) and with orbital shaking (60 rpm). To
measure transpiration rates, water loss from vials containing solution
but no plants was subtracted from the water loss from vials containing
both plants and solution (Krämer et al., 1997). Plants treated
for xylem sap collection were transferred under microbiologically
controlled conditions to vials containing 10 mL of nutrient solution
supplemented with Pb and EDTA. Filter-sterilized Pb(NO3)2 and
K2EDTA were added to autoclaved nutrient
solution, and autoclaved ferric tartrate was added to the nutrient
solution just before the addition of EDTA and Pb.
The Effect of EDTA on Shoot Pb Accumulation
Two-week-old Indian mustard seedlings were exposed to 0.5 mM Pb(NO3)2 and 0.01 to 2.5 mM K2EDTA in the nutrient solution, with a final pH adjusted to 4.0 with 1 N KOH. This was
performed to approximate the pH optima for Pb accumulation from
hydroponic solution by Indian mustard (Blaylock et al., 1997). After
48 h of exposure, shoot tissue was harvested from each of three
replicate treatments (three plants per replicate were pooled) and oven
dried at 80°C for 3 d. Dried plant material was digested at
180°C for 1.5 h in 5 mL of concentrated
HNO3 and cooled to room temperature, and 1 mL of
30% H2O2 was added.
Samples were then heated at 180°C for 20 min and cooled, and
deionized water was added to a final volume of 12.5 mL. Pb
concentrations were determined using a flame-atomic-absorption spectrometer (model 3110, Perkin-Elmer). The speciation of Pb and EDTA
in the hydroponic solution was predicted using GEOCHEM-PC version 2.0 (Parker et al., 1995).
EDTA Accumulation in Shoots of Indian Mustard
Two-week-old Indian mustard seedlings were exposed to 1.5 mM K2EDTA (containing 0.015 µCi
µmol1 [14C]EDTA) in
the nutrient solution at pH 4.0. After 48 h, the shoot tissue was
harvested, frozen at  80°C, and lyophilized. Lyophilized material
was ground to a powder and extracted with 1 mL of 50% (v/v) ethanol,
heated at 80°C for 10 min, and centrifuged at 1550g for 20 min at room temperature, and the supernatant was removed. Successive
extractions were performed by adding 1 mL of 50% (v/v) ethanol to the
remaining pellet, and repeating the above extraction process.
Supernatants from the first three extractions were pooled; supernatants
from the fourth, fifth, and sixth extractions were analyzed separately
for [14C]EDTA using a liquid-scintillation
counter (model LS5000, Beckman). The 50% (v/v) ethanol-extracted
pellet was digested for 5 d at 80°C with 1 mL of BTS-450
tissue-solubilizing solution (Beckman), and the
[14C]EDTA in the solubilized pellet was
determined using a liquid-scintillation counter. The efficiency of EDTA
extraction was expressed as the amount of 14C
extracted in 50% ethanol relative to the total tissue
14C. This extraction efficiency was later used to
adjust the measured tissue EDTA concentrations to account for
losses during the extraction procedure.
To determine the relationship between EDTA exposure and accumulation in
shoots of Indian mustard, 2-week-old seedlings were exposed to 0.01 to
2.5 mM K2EDTA (containing 0.01-8.7
µCi µmol1
[14C]EDTA) in the nutrient solution at pH 4.0. After 48 h, the shoot tissue was harvested and extracted three
times with 50% (v/v) ethanol, as described above. Supernatants from
all three extractions were pooled, dried in a Speed-Vac (model SVC200,
Savant, Farmingdale, NY), and resuspended in 500 µL of deionized
water. These samples were later analyzed by HPLC for EDTA (see below).
Time Course of Pb and EDTA Accumulation in Shoots
Two-week-old Indian mustard seedlings were treated with 0.5 mM Pb(NO3)2 and 1 mM K2EDTA in the nutrient solution at
pH 4.0. Shoot tissue was harvested periodically during a 48-h period, frozen at 80°C, lyophilized, and ground to a powder. One-third of
the powdered tissue was extracted in 50% (v/v) ethanol (see extraction
protocol above), and the remaining tissue was digested with
HNO3/H2O2
(see digestion protocol above). The digested materials were analyzed
for Pb using a flame-atomic-absorption spectrometer, and the extracted
material was analyzed by HPLC for total EDTA.
HPLC Analysis of Total EDTA
Concentrated 50% ethanol plant extracts were centrifuged at
10,000g for 10 min and prepared for RP HPLC (5000 LC,
Varian, Sunnyvale, CA) as follows. To each 100-µL sample of the plant extract, 400 µL of 6.47 mM iron(III) chloride (in 7.1 M acetic acid) and 500 µL of deionized water were added.
The sample was filtered through a 0.45-µm nylon membrane spin-prep
filter and 100 µL was injected onto an RP C18
column (60 Å, 8 µm, Dynamax, Rainin, Walnut Creek, CA). The column
was run isocratically at 1 mL min1 with 8 mM tetrabutyl ammonium hydroxide in 30 mM
sodium acetate-acetic acid buffer (pH 4). EDTA was detected at 254 nm
using a UV spectrophotometer (Spectroflow 783, Kratos Analytical,
Hamilton, OH) according to the method of Bergers and DeGroot (1994). To
validate the use of UV detection as a quantitative measure of EDTA, we
also collected fractions (1 mL) of eluent from the HPLC column and
measured the [14C]EDTA they contained using a
liquid-scintillation counter.
Analysis of Xylem Exudate for the Presence of Pb Complexed to EDTA
Two-week-old Indian mustard seedlings were exposed to 0.5 mM Pb(NO3)2 (traced with
931.6 µCi mmol1 210Pb) and 1 mM
K2EDTA in the nutrient solution at pH 4.0. After 1 h, plants were decapitated and xylem exudate was collected
during a 12-h period. Samples of the xylem exudate from 20 plants were pooled and a one-tenth dilution of the sap was analyzed by RP HPLC for
the presence of Pb-EDTA (Buchberger et al., 1991). One hundred
microliters of diluted sample was injected onto a RP
C18 column and run isocratically at 1 mL/min with
27.4 mM hexadecyltrimethyl ammonium bromide in 0.7 mM KH2PO4 (pH
7.0) and 65% (v/v) acetonitrile. Standard Pb-EDTA was prepared by
mixing equimolar solutions of K2EDTA and
Pb(NO3)2. Metal-EDTA
complexes were detected at 250 nm using a UV spectrophotometer (Kratos
Analytical) after the postcolumn addition of 0.1 mM copper
sulfate in 0.3 M acetic acid at 0.9 mL/min via a 10-µL
mixer (The Lee Company, Westbrook, CT). Before peak detection, the
column eluent and the added CuSO4 solution were allowed to
react for 2 min in a delay loop. Postcolumn addition of
CuSO4 was necessary to convert all metal-EDTA complexes to Cu-EDTA for efficient detection at 250 nm. Twenty fractions (1 mL) of
eluent were collected and analyzed for 210Pb by
liquid-scintillation counting.
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RESULTS |
Stimulation of Shoot Pb Accumulation by EDTA
Indian mustard plants were grown hydroponically and roots were
maintained under microbiologically controlled conditions to prevent the
biodegradation of EDTA by rhizobacteria. The hydroponic system was
monitored for the presence of bacteria by periodically streaking
samples of the nutrient solution onto agar plates. In addition, care
was taken to eliminate bacterial contamination by screening seedlings
as they germinated.
Treatment of Indian mustard with EDTA concentrations ranging from 0.1 to 2.5 mM in the presence of 0.5 mM
Pb(NO3)2 caused enhanced shoot Pb
accumulation (Fig. 1A). Above 0.25 mM EDTA in solution, shoot Pb accumulation increased
rapidly, reaching a maximum Pb concentration of 56 mmol
kg1 dry weight at 0.75 mM EDTA.
This represents a 400-fold increase in Pb accumulation compared with
that in plants exposed to Pb in the absence of EDTA, and a 75-fold
bioaccumulation of Pb from solution. At EDTA concentrations greater
than 0.75 mM shoot Pb concentrations declined slowly.
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| Figure 1.
Pb accumulation (A) and water content
(B) of shoots of Indian mustard exposed to 0.5 mM
Pb(NO3)2 and various K2EDTA concentrations for 48 h. Values represent the mean ± SE of three replicate samples, and the data presented are
from one experiment representative of three independent experiments.
DW, Dry weight.
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Changes in shoot-tissue water content, measured by subtracting the dry
weight of the shoot tissue from its fresh weight, were used as a
measure of phytotoxicity (Fig. 1B). Application of EDTA at
concentrations greater than 0.5 mM was associated with
significant water loss from the shoot tissue, which in severe cases
resulted in visible drying of the tissue and formation of necrotic
lesions. At an EDTA concentration of 0.5 mM, plants
accumulated high quantities of Pb without apparent phytotoxicity.
As shown in Figure 2, EDTA speciation in
the hydroponic culture solution was predicted using GEOCHEM-PC. In the
absence of EDTA, the majority of Pb was predicted to be insoluble;
however, as EDTA concentrations increased, soluble Pb-EDTA was
predicted to be formed, reaching a maximum concentration of 0.5 mM in the presence of 0.5 mM total EDTA. The
concentration of free protonated EDTA was also predicated to increase
rapidly above 0.5 mM total EDTA.
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| Figure 2.
The concentration of major soluble Pb and EDTA
species in the hydroponic nutrient solution (as described in
``Materials and Methods'') containing 0.5 mM
Pb(NO3)2 and 0.01 to 2.5 mM
K2EDTA, pH 4.0. Values were derived by modeling using
GEOCHEM-PC version 2.0.  , Pb-EDTA;  , Pb2+; and  ,
protonated EDTA.
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Detection and Accumulation of EDTA in Shoots
Different extractants, including water, 50% (v/v) methanol, and
50% (v/v) ethanol, were screened for their relative abilities to
extract 14C from shoots of plants treated with
14C-labeled EDTA. Of the three extractants, 50%
(v/v) ethanol proved to be the most efficient and was found to remove
91% of the total shoot 14C after three
successive extractions. Three further 50% (v/v) ethanol extractions
removed almost no additional 14C from the shoot
tissue (data not shown). Based on this information, three ethanol
extractions were used for the determination of total shoot EDTA
concentrations. Shoot EDTA extraction efficiencies were not affected by
exposure of plants to different EDTA concentrations in solution (data
not shown).
To establish the validity of using HPLC as an assay for total EDTA
extracted from shoot tissue, EDTA (traced with
[14C]EDTA) was monitored by both UV absorption
at 254 nm and liquid-scintillation counting. Approximately 98% of the
14C injected onto the HPLC column was recovered
in the eluent. A distinct peak in chromatograms of shoot extracts,
detected by UV and liquid-scintillation counting, was found to coelute
with an authentic EDTA standard. Concentrations of EDTA in shoot
extracts, determined by UV absorbance and liquid-scintillation
counting, correlated linearly with an r2
value of 0.97. Based on this correlation, we routinely quantified EDTA
by monitoring UV absorption at 254 nm.
EDTA accumulation in shoots of Indian mustard increased significantly
at EDTA concentrations greater than 0.5 mM in solution (Fig. 3A). A maximum shoot accumulation
of 52 mmol EDTA kg1 dry weight was observed
after treatment of plants with 1.5 mM EDTA in solution.
This represents a 35-fold higher concentration of EDTA in shoot tissue
compared with that in the nutrient solution. Solution EDTA
concentrations greater than 0.5 mM caused a large decrease
in the water content of shoot tissue (Fig. 3B), and this was associated
with tissue necrosis and drying at the highest EDTA concentrations.
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| Figure 3.
EDTA accumulation (A) and water content (B) of
shoots of Indian mustard exposed to various K2EDTA
concentrations for 48 h. Values represent the mean ± SE of three replicate samples, and the data presented are
from one experiment representative of three independent experiments.
DW, Dry weight.
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Time Course of Shoot Accumulation of Pb and EDTA
The rates of EDTA and Pb accumulation in shoots of Indian mustard
were essentially linear during a 48-h period in plants exposed to 0.5 mM Pb(NO3)2 and 1.5 mM EDTA in hydroponic solution (Fig. 4). It is possible that the slight
reduction in accumulation rates of Pb and EDTA, observed between 13 and
22 h, was caused by the corresponding reduction in transpiration
rates associated with the night period (Fig.
5). Accumulation of Pb in shoots of
Indian mustard was tightly correlated with accumulation of EDTA (Fig. 6), with an accumulation ratio of 1:0.67
(EDTA:Pb).
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| Figure 4.
Time course of the shoot accumulation of Pb ()
and EDTA () in Indian mustard plants exposed to
Pb(NO3)2 (0.5 mM) and
K2EDTA (1 mM). Values represent the mean ± SE of three replicate samples each composed of three
individual plants. DW, Dry weight.
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| Figure 5.
Time course of water-loss rates from shoots of
Indian mustard exposed to 0.5 mM
Pb(NO3)2 and 1 mM
K2EDTA (), 0.5 mM
Pb(NO3)2 alone (), or of unexposed control
plants (). Values represent the mean ± SE of three
replicate samples each composed of three individual plants. dry wt.,
Dry weight.
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| Figure 6.
A correlation between Pb and EDTA accumulated in
shoots of Indian mustard exposed to Pb(NO3)2
(0.5 mM) and K2EDTA (1 mM). Values
represent the mean ± SE of three replicate samples
each composed of three individual plants.
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Identification of Pb-EDTA Complex in Xylem Exudate
Xylem exudate collected from Indian mustard plants exposed to 0.5 mM Pb(NO3)2 (traced with 100 µCi mmol1 210Pb) and
1.0 mM K2EDTA were analyzed for the
presence of a Pb-EDTA complex by HPLC. Analysis of xylem exudate showed
the presence of a distinct UV and
210Pb-containing peak, with the same retention
time (10.75 min) as an authentic Pb-EDTA standard (Fig.
7).
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| Figure 7.
HPLC profile of xylem sap collected from Indian
mustard plants exposed to 0.5 mM
Pb(NO3)2 (traced with 931.6 µCi mmol1
210Pb) and 1 mM K2EDTA. The
arrow represents the elution time of standard Pb-EDTA.
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Effect of Pb and EDTA on Plant Water Loss through Transpiration
As expected, rates of water loss from Indian mustard followed a
diurnal rhythm (Fig. 5). Rates of water loss declined from the start of
the experiment (0 h), reaching a minimum after the onset of the dark
period (10 h). After the onset of the light period (24 h),
transpiration increased to a maximum and declined again, reaching a
minimum at the onset of the next dark period. Addition of Pb and EDTA
(0.5 mM Pb(NO3)2, 1.0 mM K2EDTA) caused an immediate
decline in transpiration relative to the control. Transpiration reached
a minimum at the onset of the dark period, and did not recover after
the onset of the light period (Fig. 5). After 48 h of exposure,
transpiration in Pb-EDTA-exposed plants was reduced by 80% relative to
the control. Exposure to Pb alone also reduced transpiration relative
to control plants, but only after approximately 25 h of exposure.
This inhibition resulted in a 55% reduction in the transpiration of
Pb-exposed plants after 48 h (Fig. 5).
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DISCUSSION |
EDTA appears to chelate Pb outside of the plant, then the soluble
Pb-EDTA complex is transported through the plant, via the xylem, and
accumulates in the leaves. This is supported by our findings that
Pb-EDTA is the major transported form of Pb in Pb- and EDTA-exposed
plants (Fig. 7). Also, Pb accumulation in shoots is correlated with
formation of soluble Pb-EDTA in the hydroponic solution (Figs. 1A and
2) and the accumulation of EDTA in shoots (Fig. 6).
The kinetics of Pb and EDTA accumulation were found to be
biphasic (Figs. 1A and 3A), suggesting that a threshold concentration of EDTA is required to "induce" the accumulation of high
concentrations of EDTA or Pb-EDTA in shoots. This has also been
observed for chelated Fe uptake. Jeffreys and Wallace (1968) showed
that application of a threshold concentration of 1 mM
Fe-N,N -ethylenebis(2-[2-hydroxyphenyl]-glycine) (EDDHA)
induced the rapid accumulation of the red-colored Fe-EDDHA in shoots of
several different plant species. The physiological basis of this
induction of chelate uptake and accumulation is unknown. However, we
speculate that at these threshold concentrations, synthetic chelates
including EDTA destroy the physiological barrier(s) in roots that
normally function to control uptake and translocation of solutes. The
plasma membrane surrounding root cells is thought to play a major role
in forming this barrier. Both Zn2+ and Ca2+
ions are involved in stabilizing plasma membranes (Pasternak, 1987,
1988; Kaszuba and Hunt, 1990). Therefore, synthetic chelates may induce
metal-chelate uptake and accumulation by removal of stabilizing
Zn2+ and Ca2+ from the
plasma membrane. This scenario would lead to the rapid equilibration of
hydroponic or soil solution with the xylem sap. Once in the xylem,
solutes such as Pb-EDTA would follow the transpiration stream and
accumulate to a high concentration in shoots.
Pb is known to be effective at displacing various cationic metals from
roots (Harrison et al., 1979), suggesting that Pb may also play a role
in destabilizing the physiological barrier to solute movement in roots.
This is supported by the observation that 0.5 mM Pb lowers
the concentration of EDTA required to induce uptake from 1.5 to 0.5 mM (Figs. 1A and 3A). Also supporting this is the
observation that high concentrations of Pb alone appear to be capable
of inducing Pb accumulation in Indian mustard (Kumar et al., 1995).
Shoot EDTA accumulation, however, does not always appear to be related
to physiological stress. At a low EDTA concentration (10 µM) and neutral pH (pH 6.0), we observed the accumulation of 0.1 ± 0.02 (SE) mmol EDTA
kg1 dry weight in shoots of Indian mustard
during 48 h. This suggests that EDTA, even when present at low
concentrations, is accumulated by plants. The potential for
metal-chelate accumulation should be considered when
metal-chelate-buffered solutions are used to study metal uptake and
nutritional requirements in plants.
The phytotoxicity of EDTA treatment was quantified by monitoring shoot
desiccation. Generally, increasing concentrations of EDTA (Figs. 1B and
3B) caused significant reductions in shoot water content. An
interesting exception to this, however, was treatment with equimolar Pb
and EDTA. Under these conditions, no phytotoxicity was observed (Fig.
1B), even though maximum Pb accumulation was achieved (Fig. 1A). This
suggests that EDTA-induced foliar necrosis may be attributable to the
presence of free protonated EDTA in leaves. We predict that
uncoordinated EDTA would be available to bind various essential
divalent cations, including Fe2+,
Zn2+, and Cu2+, disrupting the
biochemistry of the leaf cells and ultimately causing cell death. This
is supported by the finding that toxicity symptoms in Indian mustard
exposed to Pb and EDTA are strongly correlated with the presence of
free protonated EDTA in solution (Figs. 1B and 2). This toxicity is
also reflected in a reduction in the rate of water loss from shoots
(Fig. 5), demonstrating that increased transpiration is not the
mechanism through which EDTA exerts its influence on Pb accumulation.
Mass flow of water through the xylem to the shoots is required for Cd
accumulation in Indian mustard (Salt et al., 1995), and we would expect
the same to be true for Pb, suggesting that if rates of water loss from
plants exposed to Pb-EDTA could be maintained at control levels, Pb
accumulation rates could be enhanced. To achieve this, plants would
need to be able to tolerate the toxic effects of high concentrations of
EDTA. This may be achieved by applying EDTA to soils at rates that
minimize the availability of free chelate.
Our data on the transport of chelated metals in plants not only advance
our understanding of the role of metal chelates in plant nutrition, but
also point to potential areas of improvement for the development of
chelate-assisted phytoextraction. These improvements should enhance Pb
accumulation and ultimately allow more effective remediation of
Pb-contaminated sites.
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FOOTNOTES |
1
This research was supported by grants from the
U.S. Department of Agriculture (no. 96-35102-3838 to D.E.S.) and
Phytotech, Inc., to I.R.
*
Corresponding author; e-mail david.salt{at}nau.edu; fax
1-520-523-8111.
Received November 10, 1997;
accepted March 1, 1998.
| |
ABBREVIATIONS |
Abbreviation:
RP, reversed-phase.
| |
ACKNOWLEDGMENTS |
We thank Dr. Robert Smith for his ideas and contributions to
discussions and Dr. Ute Krämer for her help with GEOCHEM-PC. Special thanks to Robin Torquatti for her support. A.D.V. was a George
H. Cook honors undergraduate scholar.
| |
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Top
Abstract
Introduction