Effects of lead and chelators on growth, photosynthetic activity by Hanna Kuzyo

Environmental Pollution 144 (2006) 11e18 www.elsevier.com/locate/envpol

Effects of lead and chelators on growth, photosynthetic activity and Pb uptake in Sesbania drummondii grown in soil Adam T. Ruley a, Nilesh C. Sharma a, Shivendra V. Sahi a,*, Shree R. Singh b, Kenneth S. Sajwan c a

Department of Biology, Western Kentucky University, 1906 College Heights Blvd 11080, Bowling Green, KY 42101-1080, USA b Alabama State University, 915 S. Jackson Street, Montgomery, AL 36104, USA c Department of Natural Sciences and Mathematics, Savannah State University, Savannah, GA 31404, USA Received 1 June 2005; accepted 8 December 2005

Sesbania drummondii tolerates and accumulates high concentrations of Pb. Abstract Effects of lead (Pb) and chelators, such as EDTA, HEDTA, DTPA, NTA and citric acid, were studied to evaluate the growth potential of Sesbania drummondii in soils contaminated with high concentrations of Pb. S. drummondii seedlings were grown in soil containing 7.5 g Pb(NO3)2 and 0e10 mmol chelators/kg soil for a period of 2 and 4 weeks and assessed for growth profile (length of root and shoot), chlorophyll a fluorescence kinetics (Fv/Fm and Fv/Fo) and Pb accumulations in root and shoot. Growth of plants in the presence of Pb þ chelators was significantly higher (P < 0.05) than the controls grown in the presence of Pb alone. Fv/Fm and Fv/Fo values of treated seedlings remained unaffected, indicating normal photosynthetic efficiency and strength of plants in the presence of chelators. On application of chelators, while root uptake of Pb increased four-five folds, shoot accumulations increased up to 40-folds as compared to controls (Pb only) depending on the type of chelator used. Shoot accumulations of Pb varied from 0.1 to 0.42% (dry weight) depending on the concentration of chelators used. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Sesbania drummondii; Pb-remediation; Chelators; Pb accumulation

1. Introduction Lead (Pb) contamination in soil is a widespread phenomenon and originates from automobiles, metal smelting plants, mines, lead-contaminated sewage sludge, industrial wastes, etc. (Zakrzewski, 1991). Pb exposure to plants causes effects such as the disturbance in mitosis (Liu et al., 1994; Wierzbicka, 1994), induction of leaf chlorosis (Johnson and Proctor, 1977), depression of photosynthetic rate, (Bazzaz et al., 1974), inhibition in root and shoot growth (Fargasova, 1994; Liu et al., 1994), and inhibition and activation of enzymatic activities (Van Assche and Cliisters, 1990). Severe Pb

* Corresponding author. Tel.: þ1 270 745 6012; fax: þ1 270 745 6856. E-mail address: shiv.sahi@wku.edu (S.V. Sahi). 0269-7491/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2006.01.016

contamination in soils may lead to a variety of environmental problems e loss of vegetation, ground water contamination, and ultimately Pb toxicity to animals and humans (Body et al., 1991). Thus, there is an urgent need for remediation of contaminated sites using an effective and environmentfriendly technology such as phytoremediation. In recent years, phytoremediation has emerged as a viable biotechnology to decontaminate the heavily polluted sites (Blaylock et al., 1997; Huang et al., 1997; Kirkham, 2000; Sharma et al., 2004). This strategy makes use of hyperaccumulator plants, which have the inherent potential to survive and accumulate excessive amounts of metal ions in their biomass without incurring damage to basic metabolic functions (Cunningham et al., 1997). With successive cropping and harvesting of accumulator crops, the levels of contaminants can be reduced substantially. For a plant species to be efficient

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in lead phytoextraction it should accumulate metal concentration >0.1% of shoot dry weight, besides having high biomass productivity (Kirkham, 2000). A balance between metal accumulation and plant biomass productivity is critical for a plantspecies to be used in Pb phytoextraction (Huang and Cunningham, 1996). From this standpoint, plant species such as Indian mustard, pea, and corn were focused recently for Pb phytoremediation research. These species accumulate high amounts of lead, and produce satisfactory biomass (Huang et al., 1997; Blaylock et al., 1997; Epstein et al., 1999). Another interesting Pb accumulator is Sesbania drummondii, a perennial large bushy plant with greater biomass productivity than the above plant species (Ruley, 2004). S. drummondii grows naturally in seasonally wet places of the southern coastal plains of the United States and tolerates high concentrations of soil Pb. It demonstrated a unique potential of Pb accumulation in aerial parts from an aqueous solution (Sahi et al., 2002). To compensate for the relatively low metal accumulation capacities of Indian mustard, corn, pea and other potential plant species, chelates such as ethylenedinitrilotetraacetic acid (EDTA), N-(2-hydroxyethyl)ethylenediaminetriacetic acid (HEDTA), diethylene trinitrilopentaacetic acid (DTPA), trans-1,2-cyclohexylenedinitrilotetraacetic acid (CDTA) and ethylenebis [oxyethylenetrinitrilo] tetraacetic acid (EGTA) were supplemented to the Pb-contaminated soils (Blaylock et al., 1997; Huang et al., 1997; Epstein et al., 1999; Kirkham, 2000; Sarret et al., 2001). Application of chelators induces metal desorption from minerals and boosts translocation of Pb from root to shoot. A chelate-assisted increase of 100 to 200 folds in shoot Pb accumulation was noticed in Indian mustard (Blaylock et al., 1997; Epstein et al., 1999) while several-fold increases were observed in pea and corn (Huang et al., 1997). Kirkham (2000) reported a significant increase in shoot Pb when sunflower plants were grown in soils contaminated with sewage sludge. Chelators not only facilitate Pb uptake and translocation, but also protect plants from oxidative stress that is produced as a result of heavy metal (Pb) exposure, as reported in Sesbania seedlings grown in vitro (Ruley et al., 2004). Studies show how exposure of Pb or other heavy metals affect growth and photosynthetic activities in plants (Xiong, 1997; KrishnaRaj et al., 2000). Chlorophyll a fluorescence, a non-destructive marker of the photosynthetic apparatus, has been used extensively in screening for abiotic stresses, such as heat, chilling, drought, salinity and heavy metal stresses (Becerril et al., 1988; Krause, 1991; KrishnaRaj et al., 2000; MacFarlane, 2003). In the present investigation, we have utilized chlorophyll a fluorescence parameters as a quantitative marker to assess and compare the tolerance of Sesbania sp. when exposed to Pb and different chelators. Therefore, in order to understand the effects of high concentrations of Pb and chelators, this study was focused to determine 1) growth profile, 2) chlorophyll a fluorescence kinetics [Fv/Fm and Fv/Fo], and 3) Pb accumulation in Sesbania drummondii seedlings grown in soils contaminated with a high concentration of Pb in the presence or absence of synthetic chelators, such as EDTA, DTPA, HEDTA, NTA and citric

acid. Comparing the efficacy of different chelators on Pb accumulation by Sesbania was also aimed in this study. 2. Materials and methods 2.1. Preparation of seed bed and pot plants Seeds of Sesbania drummondii were scarified in 85% H2SO4 for 35 min, rinsed for 30 min, sterilized in 0.1% HgCl2, and rinsed for 10 min. After sterilization, seeds were germinated into trays containing peat moss and vermiculite (Sahi et al., 2002). Three week-old seedlings of similar growth (8e10 cm long shoots and 6e10 cm long roots) were selected and transferred to individual pots filled with 2.0 kg of soil (three parts soil and one part sand passed through 2 mm sieve). The soil used in this experiment belonged to Pembroke series e dark brown silt loam and neutral to slightly alkaline e having characteristics of Mollic epipedon (80e100; 700; 180; 15 g/kg sand, silt, clay and organic matter, respectively). The soil was spiked, 6 weeks before planting, with different concentrations of Pb(NO3)2 and chelators as described below. After transplantation into individual pots, plants were maintained in greenhouse under 16 h light/8 h dark regime, and watered as needed until harvest.

2.2. Treatment of Pb Ăž chelators Seedlings were grown in the presence of Pb and the chelators (EDTA, DTPA, HEDTA, NTA, citric acid). Each experimental group consisted of 15 seedlings grown individually in pots containing 7.5 g Pb(NO3)2 and 1.25e 10 mmol chelators/kg soil. Lead nitrate was dissolved in sufficient amount of water and applied in soil 6 weeks prior to planting. Different solutions of chelators were applied to individual pots after a week of planting. For each experimental group, controls were set up without Pb(NO3)2 and containing the same concentration of synthetic chelators. Controls were also set up with 0 or only 7.5 g Pb(NO3)2/kg soil. Plants were harvested after 2 and 4 weeks, separated into roots and shoots, and then measured for growth by means of shoot and root length.

2.3. Estimation of photosynthetic activities Before each harvest, seedlings were analyzed for photosynthetic activities by measuring chlorophyll a fluorescence parameters (Ruley et al., 2004). This was performed using the Handy-PEA instrument (Hansatech Instruments, UK). Plants were dark-adapted for 30 min and then given a 1 s pulse of red light. The following fluorescent parameters were measured: Fo, the minimum chlorophyll a fluorescence after the dark-adaptation, and Fm, the maximum fluorescence after the pulse of red light. From these two measurements the Fv (the variable fluorescence calculated as the difference between the minimal and maximal fluorescence), Fv/Fm (the ratio of variable to maximal fluorescence) and Fv/Fo (the ratio of variable to minimal fluorescence) values were determined.

2.4. Pb analysis Roots and shoots were dried at 60 C (2 d) for Pb analysis by ICP-MS (Sahi et al., 2002). Samples were weighed and placed into a 15 ml screw capped Teflon beaker. Concentrated HNO3 (3 ml) was added to the sample, and the beaker was placed on a hot plate at a temperature of 100 C overnight, and the contents were then evaporated to dryness. Samples were allowed to cool and mixed gravimetrically with 2% HNO3 to a volume of 20 ml. The ICP-MS analysis was carried out using external calibration procedure, and Y (0.1 ppm) was used as an internal standard to correct for drift and matrix effect (Sahi et al., 2002).

2.5. Statistical analysis All statistical analyses were performed using SYSTAT 9 for Windows 95. Growth and photosynthetic measurements were the means of 6 samples taken from 2 experiments; 3 replicates were taken in each experiment. Four samples

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chelators only, or Pb only (Fig. 2A,B). Growth of plant roots in the presence of Pb þ DTPA or HEDTA did not differ significantly from controls (P > 0.05) during both 2- and 4-week treatments. However, plants grown in the presence of Pb þ NTA (2.5 and 10 mmol/kg soil), and citric acid (1.25 and 2.5 mmol/kg soil) had significantly reduced roots (P < 0.05) relative to the controls (Fig. 2A,B).

each treatment were processed for Pb accumulation. The analysis of variance (ANOVA) appropriate for the design was carried out to detect the significance of differences (P < 0.05) among the treatment and control means and Tukey HSD post hoc test was performed to compare among the groups for significant differences.

3. Results 3.1. Effects of chelated Pb on plant growth

3.2. Effects of chelated Pb on photosynthesis Fig. 1 depicts the effect of Pb þ chelators or chelators (only) on plant growth, as shown by the shoot length. For both lengths of time, Pb þ DTPA, Pb þ NTA or Pb þ citric acid treatments resulted in the shoot growth not significantly different (P < 0.05) than controls (grown in the presence of chelators only), with an exception of plants grown at Pb þ 2.5 mmol citric acid/kg soil (Fig. 1A,B). At the same time, plants had longer shoots (P < 0.05) as a result of these treatments, particularly after 4-weeks, relative to the plants grown in the presence of Pb only. However, significantly reduced shoot length (P < 0.05) was observed in case of plants grown in the presence of Pb þ EDTA (10 mmol) or HEDTA. Control plants had also reduced shoots at 10 mmol HEDTA/ kg soil (Fig. 1B). Fig. 2 compares root length of Sesbania plants grown in the presence of Pb þ chelators or chelators only. Roots of plants grown in soil containing Pb þ 5, 10 mmol citric acid or EDTA were significantly greater (P < 0.05) than those of plants grown in the presence of

Fv/Fm ratios of S. drummondii seedlings grown in the presence of Pb þ chelators or Pb alone are depicted in Fig. 3A,B. Fv/Fm ratios of plants exposed to Pb þ chelators were not significantly different (P > 0.05) than controls. The notable exceptions to this pattern occurred at two weeks in plants exposed to Pb þ 10 mmol HEDTA or NTA/kg soil; however, these plant groups showed normal Fv/Fm ratios (>0.8) at four weeks (Fig. 3A). On the other hand, control plants grown in the presence of chelators demonstrated a different pattern of Fv/Fm ratios (Fig. 3B). Plants grown in the presence of EDTA alone were the most severely affected, not surviving until 4 weeks at a concentration of 5 or 10 mmol/kg. Also, plants grown in the presence of 10 mmol HEDTA/kg soil (alone) survived less than 2 weeks (Fig. 3B). Fig. 4A,B illustrate the effect of Pb and synthetic chelators on photosynthetic activities of plants, as measured by Fv/Fo ratios. In most of the treatments, Fv/Fo ratios in Sesbania plants

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Chelator (mmol/kg soil) Fig. 1. Effects of 7.5 g Pb(NO3)2 þ 0e10 mmol/kg EDTA (E), DTPA (D), HEDTA (H), NTA (N) and citric acid (C) on Sesbania drummondii shoot length: (A) Shoot length of plants grown in the presence of Pb þ chelators for 2 and 4 weeks. (B) Shoot length of control plants grown in the presence of chelators (alone) for 2 and 4 weeks. Values represents mean S.E., where n ¼ 6.

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Chelator (mmol/kg soil) Fig. 2. Effects of 7.5 g Pb(NO3)2 þ 0e10 mmol/kg EDTA (E), DTPA (D), HEDTA (H), NTA (N) and citric acid (C) on Sesbania drummondii root length: (A) Root length of plants grown in the presence of Pb þ chelators for 2 and 4 weeks. (B) Root length of control plants grown in the presence of chelators (alone) for 2 and 4 weeks. Values represents mean S.E., where n ¼ 6.

were at or above 4.0; the only group of plants that demonstrated a differential pattern was those grown at 10 mmol HEDTA/kg soil (two weeks), though these plants also recovered showing Fv/Fo ratios at 5.0 by week four (Fig. 4A). However, Fv/Fo ratios in control plants (grown in the presence of chelators alone) had a different pattern (Fig. 4B), with plants grown in the presence of 5 or 10 mmol EDTA/kg soil not surviving until 4 weeks and those grown in the presence of 10 mmol HEDTA/kg soil surviving less than 2 weeks. 3.3. Uptake of chelated Pb Fig. 5A compares Pb concentrations in Sesbania shoots grown in the presence of Pb þ chelators or Pb alone. Application of a chelator, at any concentration, resulted in a rapid increase in shoot Pb, as compared to the shoot Pb of plants grown in the presence of Pb alone. The effect of chelators on shoot accumulations of Pb was concentration-dependent, except in citric acid treatments, where shoot Pb was maximum at the lowest concentration of the chelate (Fig. 5A). The type of chelator had also a pronounced effect on Pb accumulation in shoots. It was observed that chelators increased Pb uptake in the order EDTA > HEDTA > DTPA > NTA > citric acid (Fig. 5A). Fig. 5B shows root Pb concentrations in plants grown in the presence of Pb þ synthetic chelators or Pb alone. It was observed that in the presence of any of the chelators tested, at any concentration, Pb absorption in roots was significantly higher than in plants grown in the presence of Pb without chelators (Fig. 5B). It was also noticed that in respect of

HEDTA and NTA treatments, the concentration of Pb in roots increased with an increase in chelator concentration. 4. Discussion 4.1. Growth Results show that growth of Sesbania plants in the presence of Pb þ chelators was either significantly higher (P < 0.05) than the plants grown in Pb-contaminated soils or not significantly different (P > 0.05) than controls, grown in the presence of chelators only. Growth in the presence of Pb þ chelators resulted in a significantly decreased (P < 0.05) shoot length only in the case of 5 or 10 mmol HEDTA/kg soil. At the same time, it is also apparent that 10 mmol HEDTA/kg soil (alone) resulted in the significantly reduced (P < 0.05) shoot length relative to normal plants, grown without Pb or chelator (Fig. 1B), proving the point that higher concentration of HEDTA was itself toxic to Sesbania plants. A seemingly different trend emerged in plant roots, where the root length was affected by Pb þ lower concentrations of citric acid (1.25 or 2.5 mmol/kg) and Pb þ higher concentrations of DTPA or HEDTA (Fig. 2A,B). This is interesting to note that higher concentrations of citric acid (5, 10 mmol/kg) favor plant growth, particularly root growth, while lower concentrations affect root growth. As binding affinity of citric acid is low for Pb, only the high concentrations of this chelator will effectively bind and form a complex with Pbþþ, reducing the toxic effects on plants. Measurements of plant biomass

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Chelator (mmol) Fig. 3. Effects of 7.5 g Pb(NO3)2 þ 0e10 mmol/kg EDTA (E), DTPA (D), HEDTA (H), NTA (N) and citric acid (C) on chlorophyll a fluorescence kinetics (Fv/Fm values) of Sesbania drummondii: (A) Fv/Fm of plants grown in the presence of Pb þ chelators for 2 and 4 weeks. (B) Fv/Fm of control plants grown in the presence of chelators (alone) for 2 and 4 weeks. Values represents mean S.E., where n ¼ 6.

reflected the same trend. Plant shoot and root weights, in most of the Pb þ chelator treatments, were significantly higher than those of plants grown in Pb alone or chelators alone (data not presented). These observations suggest a protective role for chelating agents against Pb toxicity in S. drummondii. Studies on mechanisms of Pb toxicity suggest that Pb2þ binds to nucleic acids and causes aggregation and condensation of chromatin, as well as stabilization of DNA double helix inhibiting the processes of replication, transcription and ultimately the cell division and plant growth (Johnson, 1998). Chelators applied to the Pb-contaminated soil may form complexes with Pb2þ thus inactivating and minimizing the cytological impacts of free metal ions. Heavy metal toxicity in plants also occurs with the induction of oxidative stress at cellular level following production of reactive oxygen species (Dixit et al., 2001; Geebelen et al., 2002). Application of chelators has been reported to mitigate Pb-induced oxidative stress by modulating antioxidative enzyme activities in Sesbania seedlings (Ruley et al., 2004), and this may also be one of the reasons for better growth of Sesbania plants in the presence of a chelator. In this study, Sesbania seedlings were grown in the soil contaminated with 7.5 g Pb(NO3)2/kg soil, in the presence or absence of chelators, as these plants were observed to grow

healthier at this concentration of soil Pb in a preliminary study. Though plants grew even at the higher concentrations of Pb, but stunting or dwarfing of shoots was a marked feature, particularly, at a concentration of 10 g/kg soil, and those grown in the presence of 15 g Pb/kg soil could not survive until 4 weeks (data not shown). 4.2. Photosynthetic activity The Fv/Fm value is an indicator of the photosynthetic efficiency of plants, while Fv/Fo value indicates the size and number of active photosynthetic centers in the chloroplast, and thus the photosynthetic strength of the plant. An Fv/Fm value of 0.8 or higher in all the treatments (Fig. 3A) similar to controls (Fig. 3B) indicates that the plant is healthy and not suffering photosynthetic stress as a result of Pb uptake. Exception to this was found only in the plants grown in Pb þ 10 mmol HEDTA/kg soil, where Fv/Fm value was around 0.7 at the second week, the value picking up to a level of 0.8 in the fourth week. Notably, plant growth was also affected when plants were grown at this treatment (Figs. 1A,B and 2A,B). The another notable feature was recorded in the controls grown in the presence of 5 or 10 mmol EDTA/kg soil (alone), where Fv/Fm values showed a sharp decline in

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Chelator (mmol/kg soil) Fig. 4. Effects of 7.5 g Pb(NO3)2 þ 0e10 mmol/kg EDTA (E), DTPA (D), HEDTA (H), NTA (N) and citric acid (C) on chlorophyll a fluorescence kinetics (Fv/Fo values) of Sesbania drummondii: (A) Fv/Fo of plants grown in the presence of Pb þ chelators for 2 and 4 weeks. (B) Fv/Fo of control plants grown in the presence of chelators (alone) for 2 and 4 weeks. Values represents mean S.E., where n ¼ 6.

the second week, plants not surviving until the fourth week, while those grown in Pb þ EDTA exhibited a normal trend. A similar trend of depressed photosynthetic activity was reported when Sesbania seedlings were grown in the solution culture containing EDTA or HEDTA alone (Ruley et al., 2004). The Fv/Fo values of all the treatments were also normal, 4 or greater, except at Pb þ 10 mmol HEDTA/kg soil (Fig. 4A). At the same time, controls grown in the presence of 10 mmol HEDTA/kg soil (alone) died before 2 weeks. Similar to Fv/Fm values, Fv/Fo values were significantly reduced in controls (5 or 10 mmol EDTA/kg soil, alone). Growth of these controls was also severely affected as seen earlier. Another interesting observation that was recorded in this study was the occurrence of similar normal Fv/Fm or Fv/Fo values for both the groups of plants grown in the presence or absence of Pb (Figs. 3A,B and 4A,B). It is therefore reasonable to conclude that exposure to Pb, either alone or in combination with chelators, does not affect the photosynthetic machinery of Sesbania drummondii. These results are in agreement with the report on Pelargonium sp., where Fv/Fm and Fv/Fo values were not significantly affected by Pb accumulation (KrishnaRaj et al., 2000). However, observations in Sesbania sp. differed from those in Avicennia marina, where heavy metal (Zn) caused depression of photosynthetic activity in a dose-dependent

manner (MacFarlane, 2003). Lead affects chlorophyll synthesis through inhibition of d-aminolevulinic acid dehydratase, which, in turn, depresses photosynthetic activity of plants through a reduction in chlorophyll content (Geebelen et al., 2002). It is believed that a Pb accumulating plant if maintains photosynthetic activity while accumulating Pb, it will survive and tolerate toxic concentrations of Pb (KrishnaRaj et al., 2000). 4.3. Pb accumulation Results show that application of a chelator increased concentrations of Pb in roots as well as shoots of Sesbania by several folds, relative to the plants grown in Pb only. While root uptake of Pb increased four-five folds, shoot accumulations increased up to 40-folds as compared to controls (Pb only) depending on the type of chelator used. Shoot accumulations of Pb varied from 0.1 to 0.42% (dry weight) depending on the concentration of chelators (EDTA, DTPA, HEDTA) used. The type of chelator also influenced Pb accumulation in Sesbania shoots significantly. It was noticed that chelators increased Pb transport in Sesbania in the order EDTA > HEDTA > DTPA > NTA > citric acid. This pattern of effectiveness of chelators (in Sesbania) is consistent with the earlier report on maize (Huang et al., 1997). The most likely

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explanation for chelate-stimulated Pb transport lies in the enhanced solubility of Pb in soil on application of a chelator like EDTA (Huang et al., 1997; Epstein et al., 1999). It has also been shown in a number of studies that application of chelating agents results in the stimulated translocation of Pb from plant roots to shoots (Blaylock et al., 1997; Huang et al., 1997; Epstein et al., 1999). EDTA and HEDTA were shown to have increased Pb translocation from root to shoot by 200-folds in maize and pea, and translocation was highly specific to the plant species and genotype (Huang and Cunningham, 1996; Huang et al., 1997). In Indian mustard, several fold-increase in shoot concentrations of Pb and other heavy metals was recorded as a result of EDTA application in Pb-contaminated soils (Blaylock et al., 1997). Recent findings suggest that Pb is transported in the plant shoot as the Pb-EDTA complex and thus increasing the concentration of Pb þ chelate may result in maximizing Pb accumulation in shoots (Epstein et al., 1999; Sarret et al., 2001). It was also noted during this study that plants in some control groups exposed only to chelators (EDTA, HEDTA) experienced chlorosis and distortion of leaves, stunted growth, and decreased survival. These symptoms are consistent with deficiencies of magnesium, copper and possibly molybdenum (Hopkins, 1999). Furthermore, as effects of chelators (controls) were dose-dependent and time-dependent, it may be inferred that application of chelators alone resulted in the removal of essential metal nutrients from soil, leading to deficiencies in the plants. Geebelen et al. (2002) observed a similar effect.

The findings related to the above physiological parameters indicate that Sesbania drummondii may be a probable candidate for its use in phytoremediation of Pb. However, its Pb removal capability needs to be tested in the real soil conditions of Pb contamination, as Pb availability may be generally high in spiked soils. When the feasibility of Sesbania-mediated remediation remains unclear at this stage, what makes this plant attractive is its seemingly unaffected growth in the presence of high concentrations of Pb and chelators. The significance of this species is further enhanced by the large biomass that this plant, being a bushy shrub, can generate in natural conditions. On the other hand, maize, Indian mustard or pea though accumulate greater concentrations of Pb, but have potential disadvantage of being crop species. Due to centuries of selective breeding, crop plants have been developed that are not only easy for humans to consume, but are also easier for other animal species to consume. Once Pb enters the food chain, this can be a serious environmental concern (Robinson et al., 2003). This concern may be minimal by incorporating Sesbania drummondii in a Pb remediation strategy, as this taxon is not a food crop and, in its natural state, is toxic to a variety of animal species (Banton et al., 1989). 5. Conclusions Results demonstrate that Sesbania drummondii thrives on a high concentration of Pb (7.5 g/kg soil) in the presence of different concentrations of chelators such as EDTA, HEDTA,

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