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Published in J. Environ. Qual. 33:1247-1255 (2004).
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677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Heavy Metals in the Environment

Influence of Nutrient Levels on Uptake and Effects of Mercury, Cadmium, and Lead in Water Spinach

Agneta Göthberga,*, Maria Gregerb, Karin Holma and Bengt-Erik Bengtssona

a Institute of Applied Environmental Research (ITM), Laboratory for Aquatic Environmental Chemistry, SE-106 91 Stockholm, Sweden
b Department of Botany, Stockholm University, SE-106 91 Stockholm, Sweden

* Corresponding author (agneta.goethberg{at}itm.su.se).

Received for publication November 7, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
In Southeast Asia the aquatic macrophyte water spinach (Ipomoea aquatica Forsk.) is a popular vegetable that is cultivated in freshwater courses. These often serve as recipients for domestic and other sorts of wastewater that often contain a variety of pollutants, such as heavy metals. In addition, fertilizers are frequently used where water spinach is cultivated commercially for the food market. To estimate the importance of ambient nutrient concentrations for accumulation of mercury (Hg), cadmium (Cd), and lead (Pb) in water spinach, plants were exposed to nutrient solutions of different strength and with varying metal concentrations. Metal-induced toxic effects, which might possibly affect the yield of the plants, were also studied. The lower the nutrient strength in the medium was, the higher the metal concentrations that accumulated in the different plant parts and the lower the metal concentration in the medium at which metal-induced toxic effects occurred. Accordingly, internal metal concentrations in the plants were correlated to toxic effects. Plants exposed to metals retained a major proportion of the metals in the roots, which had a higher tolerance than shoots for high internal metal concentrations.

Abbreviations: TI, tolerance index


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN SOUTHEAST ASIA, the aquatic macrophyte water spinach is a widely used vegetable. It is cultivated in freshwater courses that often serve as recipients for domestic and other sorts of wastewater and are consequently rich in nutrients and various pollutants. In addition, fertilizers are frequently used where water spinach is cultivated commercially for the food market. The heavy metals Hg, Cd, and Pb have been found in water spinach cultivated in the greater Bangkok region (Thailand) and Hg concentrations were high in plants from some sites (Göthberg et al., 2002). One question is if the ambient nutrient level influences the uptake of these toxic metals by water spinach.

The metal uptake in plants varies. It is well established that metal aqueous chemistry (i.e., metal speciation, free ion concentration, and metal–metal interactions) is critical in controlling trace metal uptake and toxicity to aquatic organisms (Nelson and Donkin, 1985; Campbell, 1995; Sunda and Huntsman, 1998). In nutrient-enriched environments, the bioavailable fraction of metals may be reduced as a result of binding to nutrient anions. The uptake of heavy metals in plants may also be affected by competition, since nutrient cations compete with the metal for uptake sites (Greger, 1999). Thus, the uptake of the studied metal may decrease with increasing external nutrient to toxic heavy metal ratio. On the other hand, a generous availability of nutrients promotes plant growth, which in turn creates an increasing number of uptake sites for metals in the plants. This may increase the uptake (Ekvall and Greger, 2002) and metal concentrations in plants may be expected to either increase, decrease, or stay constant, depending on the relative responses of metal uptake and growth rate.

Studies show that bioaccumulation of toxic metals in plants is influenced by nutrient enrichment. The Cd uptake rate in water hyacinth [Eichhornia crassipes (Mart.) Solms] was much higher in deionized water than in 50% Hoagland nutrient solution (O'Keeffee and Hardy, 1984). Greger et al. (1991) showed that the Cd net uptake in the roots of sugar beet (Beta vulgaris L.) was greater when the nutrient concentration was minimal, rather than optimal. Some studies show that metal accumulation may be depressed by competition with a certain nutrient cation. John (1976) showed that an increasing potassium (K) supply in culture solution caused a significant decrease in Cd absorption by oat (Avena sativa L.) and lettuce (Lactuca sativa L.). Trivedi and Erdei (1992) found that wheat (Triticum aestivum L.) plants with a high K supply accumulated less Cd and Pb than plants with a low K status. Cadmium and calcium (Ca) antagonistically reduced the uptake of each other in the submerged macrophyte coontail (Ceratophyllum demersum L.) (Tripathi et al., 1995). On the other hand, the addition of a separate nutrient may increase the accumulation of heavy metals. Wang and Dei (2001) showed that an increase in ambient nitrogen concentration increased both the Cd uptake rate by cells and the cell growth rate of four species of marine phytoplankton, and that the Cd uptake increased exponentially with increasing cell growth rate. An increased Cd accumulation rate in the green macroalgae Ulva fasciata was seen with increased ambient nitrate concentrations, but not ammonium concentrations (Lee and Wang, 2001). Speciation of metals in solution has not been considered in studies referenced in this paragraph.

There is also evidence that the toxic effects of metals are influenced by ambient nutrient levels. Plant growth is promoted by high nutrient levels and metal-induced toxic effects may be depressed because of the "dilution" of toxic metal concentrations in plant tissue (Greger et al., 1991). Competition between nutrient and toxic metal cations in the plant for binding sites in different apartments, such as cell walls, plasma membranes, and inside the cells may influence the distribution of toxic metals. This may affect the rate of interference with sensitive metabolic reactions, with resultant effects on, for example, growth (Maserti et al., 1998; Gupta et al., 1995, 1998; Gupta and Chandra, 1998; Haag-Kerwer et al., 1999). Increased access to a separate nutrient element may increase the toxicity of a metal. Rijstenbil et al. (1998) showed copper (Cu)-induced reduction of the cell division rate of the planktonic diatom Thalassiosira pseudonana at high, but not low, nitrogen concentrations in spite of similar cellular Cu concentrations. Breitburg et al. (1999) found that trace-metal-induced reductions in the production, abundance, and growth of many organisms in an experimental estuarine food web were often greater when nitrogen and phosphorus were added.

The aim of this study was to investigate the importance of nutrient concentrations in the external medium for the accumulation and toxic effects of the heavy metals Hg, Cd, and Pb in the aquatic tropical macrophyte water spinach. The hypothesis was that accumulation and toxic effects of the heavy metals are reduced when nutrient concentrations in the external medium are increased.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Plant Material
Water spinach plants were originally collected in the Chao Praya River in Bangkok, Thailand, and transported to Stockholm, Sweden. They were maintained in a climate chamber at 27°C and at a photon flux density of 100 to 110 µmol m–2 s–1 in a 14/10 h light and dark cycle. The plants grew in commercial plant soil that was flooded with full-strength Hoagland nutrient solution (Table 1; Eliasson, 1978) made up with Stockholm city tap water. The soil and nutrient medium were renewed every second month, while water loss due to transpiration and evaporation was compensated for by adding tap water.


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  		Table 1. Composition of 100% Hoagland nutrient solution.

 
Experiment
One week before the start of the experiment, about 25-cm-long shoots were cut off and placed in tap water to produce new root growth.

The experiments were performed in a climate chamber at 27 ± 1°C. Hoagland nutrient solutions (made with deionized water) of different strength and with varying metal concentrations were prepared (Table 2). Mercury, Cd, and Pb were added as HgCl2, CdSO4·8H2O, and Pb(NO3)2, respectively.


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  		Table 2. Nominal metal concentrations and percent Hoagland nutrient medium in the external solution in tests with water spinach.

 
A number of 1.2-L black plastic pots were each filled with 1 L of test solution. Each plant was carefully wiped, measured, and weighed and mounted in a dark gray styrofoam plate that floated on the solution surface to keep the plant upright and the roots and the lower part of the stem beneath the surface. There were five replicates (four in tests with Cd) for each experimental condition investigated. The pH was 6.0 ± 0.2 and the plants were illuminated for 14 h per day at a photon flux density of 100 to 110 µmol m–2 s–1.

The plants were harvested after 7 d and wiped, weighed, and measured. Thereafter, they were rinsed twice in deionized water for 2 min, in 20 mM ethylene diamine tetraacetic acid (EDTA) solution (a metal chelating agent) for 5 min, and twice in deionized water for 2 min (Naraho and Gaur, 1995). The leaves (with petioles), stem above the surface, stem beneath the surface, and roots were separated and dried at 45°C for 7 d.

Test solutions were repeated without plants. These solutions were sampled after 3 d and analyzed for Hg, Cd, Pb, phosphate, and sulfate. Lead was also analyzed in samples taken after 12 d.

Analyses
Test solutions without plants were analyzed for Hg by flameless atomic absorption spectrometry according to the Swedish standard method (Swedish Standardization Organization, 1989) and for Cd and Pb by atomic absorption spectrometry with a flame (Model 3100; PerkinElmer, Wellesley, MA). These test solutions were also analyzed for phosphate phosphorus with an autoanalyzer and a modified method (Swedish Standardization Organization, 1984). Sulfate was analyzed in solutions with Pb with a spectrophotometric method modified from Vogel (1961) and in solutions with Hg and Cd with ion chromatography (Swedish Standardization Organization, 1995). Nutrient solutions without added metal were analyzed for Cd by inductively coupled plasma mass spectrometry (ICP–MS) on a PerkinElmer Sciex ELAN 5000 (Douglas and Houk, 1985; Jarvis et al., 1992).

Before analysis, the plant samples were ground in an agate mortar and digested at 110°C in 65% HNO3 (pro analysi quality) that had been purified by sub-boiling distillation in a quartz apparatus. Cadmium and Pb were analyzed with ICP–MS on a PerkinElmer Sciex ELAN 5000 (Douglas and Houk, 1985; Jarvis et al., 1992). Cadmium was measured with the isotope 114Cd corrected for 114Sn, and Pb was measured with the isotopes 206Pb + 207Pb + 208Pb. Indium (In) was used as an internal standard. Mercury was analyzed by flameless atomic absorption spectrometry according to the Swedish standard method (Swedish Standardization Organization, 1989) on a PerkinElmer 305B with a Tecator (Höganäs, Sweden) flow injection analyzer and a homemade reaction vessel, using tin chloride (SnCl2) as the reducing agent. The analytical methods are accredited by the Swedish Board for Accreditation and Conformity Assessment and analyses were performed by continuously monitoring the standard reference material SRM 1571 (National Institute of Standards and Technology, Gaithersburg, MD). The result was 0.13 ± 0.02 mg kg–1 for Cd (certified value = 0.11 ± 0.01), 41.5 ± 1.3 mg kg–1 for Pb (certified value = 45 ± 3), and 0.159 ± 0.013 mg kg–1 for Hg (certified value = 0.155 ± 0.015).

Calculations
To get an idea of the metal speciation in the different nutrient solutions, a chemical equilibrium program (Puigdomenech, 1983) was used.

Metal concentrations in the shoot were calculated as: (concentration in leaves x weight of leaves) + (concentration in stem above surface x weight of stem above surface)/(weight of leaves + weight of stem above surface).

The tolerance index (TI) for the weight increase of the plants was calculated as the increase in the fresh weight of the whole metal-treated plant divided by the mean value for the increase in the fresh weight of the whole untreated plants. The dry weight as a percentage of the fresh weight of the whole plant, and root dry weight as a percentage of the whole-plant dry weight, were also calculated.

Statistics
Differences in the weight increase of the reference plants grown at different nutrient concentrations were calculated using analysis of variance (one-way ANOVA) and the Tukey honestly significant difference post hoc test.

Differences in toxic effects and metal concentrations in plants exposed to different nutrient concentrations were calculated using the t test.

Bivariate correlations between internal metal concentrations in the plants and metal-induced toxic effects were calculated using Pearson's correlation coefficient.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
In nutrient solutions without plants where Hg had been added, the analyzed concentrations of Hg were lower than the nominal concentrations (Table 3). According to the equilibrium program (Puigdomenech, 1983), all added Hg was present in solution mainly as EDTA complexes and, at a much lower concentration, as Hg hydroxide, the concentration of which increased at low nutrient strength. Cadmium concentrations that were analyzed in solutions with added Cd were in agreement with the nominal concentrations (Table 3). All added Cd occurred in solution as ions and in complexes with phosphate and EDTA (Puigdomenech, 1983), but the concentration of Cd ions was about twice as high at the low nutrient level than at the high (Puigdomenech, 1983). In solutions without added Cd, the measured Cd concentration at 100% Hoagland was three times higher than at 25% Hoagland (Table 3). In solutions with added Pb, the analyzed Pb concentrations were much lower than nominal (Table 3). They were in the same range that, according to the equilibrium program (Puigdomenech, 1983), should occur in solution, and mainly in the form of EDTA complexes, but the concentration decreased with decreasing nutrient strength while the concentration of Pb ions increased. The absolute major part of the Pb was precipitated as phosphate complexes (Puigdomenech, 1983). The precipitation was easily seen and became more obvious the more Pb that was added.


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  		Table 3. Measured concentrations of Hg, Cd, Pb, phosphate, and sulfate 3 d after preparation in test solutions of various nutrient strength and without plants.{dagger}

 
The weight increase of plants that were not treated with metals did not differ between nutrient levels of 10, 25, 50, and 100% Hoagland, while at 1% Hoagland it was significantly lower when compared with 25 and 100% Hoagland (Fig. 1).



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  		Fig. 1. Weight increase (g fresh wt.) of untreated water spinach plants after 7 d at different nutrient levels. Values are means ± standard errors.

 
In untreated plants, the Hg concentrations in the leaves were significantly higher than in the stems, but lower than in the roots (Table 4). In the Hg-treated plants, the roots always contained higher Hg concentrations than the other plant parts. Furthermore, the distribution of Hg (% of total amount in the plant) to the shoots (leaves and stems) was much higher in the untreated plants than in those treated with Hg and decreased with increasing external Hg concentration (Table 4). The Hg concentration in the leaves and stems, as well as the distribution of Hg to the shoots increased with decreasing ambient nutrient strength (Table 4). The different strength of nutrient solutions did not, however, influence the Hg concentrations in the plant roots (Table 4).


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  		Table 4. Mercury concentrations and distribution in water spinach leaves, stems (above surface of nutrient medium), and roots after cultivation in 1, 25, and 100% Hoagland solution in the absence or presence of 0.05, 0.10, or 0.20 µM Hg.{dagger}

 
Toxic effects on water spinach by increasing Hg concentrations in the medium were found at the two lowest nutrient levels. An increased dry weight to fresh weight ratio was found at 0.10 and 0.20 µM Hg at 1 and 25% Hoagland, respectively (Fig. 2). There was, however, no significant effects on the tolerance index (TI) and the root to whole-plant ratio.



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  		Fig. 2. Effects of combinations of nutrient strength (1, 25, and 100% Hoagland) and Hg concentrations on water spinach determined as (A) tolerance index (TI) for the weight increase (fresh weight) of the whole plant, (B) dry weight as percent of fresh weight of the whole plant (DW:FW), and (C) root dry weight as percent of whole-plant dry weight (R:WP). Values are means ± standard errors (n = 5), * = significant at the 0.05 probability level.

 
In both untreated and Cd-treated plants, the Cd concentrations were lowest in the leaves and highest in the roots (Table 5). Furthermore, the Cd concentrations in the leaves, stems, and roots of untreated plants were lower at the low (25% Hoagland) rather than at the high (100% Hoagland) nutrient strength (Table 5). The opposite was found at 9 µM Cd, where Cd concentrations in the leaves, stems, and roots were higher at the low rather than at the high nutrient level. At even higher external Cd concentrations, Cd concentrations in the roots were still highest at the low nutrient level while the opposite was found in the leaves and stems. At the two highest Cd concentrations and the low nutrient level in the medium, necrosis of leaves and parts of stems was observed (not shown). The internal Cd concentration in these cases decreased with increasing external Cd concentration (Table 5). The distribution of Cd to the shoots (leaves and stems) was higher in untreated plants than in those treated with Cd (Table 5). The distribution to the leaves decreased with increasing Cd concentration, to the highest degree at the low nutrient level.


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  		Table 5. Cadmium concentrations and distribution in water spinach leaves, stems (above surface of nutrient medium), and roots after cultivation in 25 and 100% Hoagland solution in the absence or presence of 0.9, 9, 27, or 45 µM Cd.{dagger}

 
Toxic effects on water spinach by increasing external Cd concentrations appeared at lower external Cd concentrations at the low, rather than the high, nutrient level (Fig. 3). At 25% Hoagland, the TI and root to whole-plant ratio decreased and the dry weight to fresh weight ratio increased at 9 µM Cd. At 100% Hoagland, the TI was reduced at 27 and the dry weight to fresh weight ratio was increased at 45 µM Cd. There was no significant effect on the root to whole-plant ratio by Cd at 100% Hoagland.



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  		Fig. 3. Effects of combinations of nutrient strength (25 and 100% Hoagland) and Cd concentrations on water spinach determined as (A) tolerance index (TI) for the weight increase (fresh weight) of the whole plant, (B) dry weight as percent of fresh weight of the whole plant (DW:FW), and (C) root dry weight as percent of whole-plant dry weight (R:WP). Values are means ± standard errors (n = 4), * = significant at the 0.05 probability level.

 
In both untreated and Pb-treated plants, internal Pb concentrations were highest in the roots. Lead concentrations in the leaves, stems, and roots increased with decreasing ambient nutrient strength (Table 6). There was no clear difference in the distribution of Pb to the shoots (leaves and stems) between the untreated and treated plants or between nutrient levels (Table 6). However, within the shoot, there was a higher distribution of Pb to the leaves of the untreated plants compared with the treated plants. Furthermore, the Pb distribution to the leaves decreased with decreasing nutrient strength.


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  		Table 6. Lead concentrations and distribution in water spinach leaves, stems (above surface of nutrient medium), and roots after cultivation in 10, 50, and 100% Hoagland solution in the absence or presence of 24, 72, 120, 360, or 600 µM Pb.{dagger}

 
Toxic effects in water spinach were found at a lower external Pb concentration the lower the nutrient level was (Fig. 4). Decreased TI, increased dry weight to fresh weight ratio, and decreased root to whole-plant ratio were found for 72, 120, and 360 µM Pb at 10, 25, and 50% Hoagland, respectively. At 100% Hoagland only the root to whole-plant ratio was affected, and decreased from 360 µM.



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  		Fig. 4. Effects of combinations of nutrient strength (10, 25, 50, and 100% Hoagland) and Pb concentrations on water spinach determined as (A) tolerance index (TI) for the weight increase (fresh weight) of the whole plant, (B) dry weight as percent of fresh weight of the whole plant (DW:FW), and (C) root dry weight as percent of whole-plant dry weight (R:WP). Values are means ± standard errors (n = 5), * = significant at the 0.05 probability level.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
The results of this investigation show that the lower the strength of the nutrient solution, the higher were the concentrations of Hg, Cd, and Pb in the leaves and stems, and of Cd and Pb in the roots of untreated and metal-treated water spinach plants (Tables 4, 5, and 6).

Measured concentrations of Hg in nutrient solutions were less than nominal (Table 3). Mercury is very volatile and some of it may have evaporated to the air. Uptake of Hg from the air via the stomata of untreated plants is a possible reason for the relatively high Hg concentrations in these plants (Table 4) in relation to plants from unpolluted water areas (Suckcharoen, 1978; Göthberg et al., 2002). Uptake of gaseous Hg via the stomata has been shown in laboratory studies (Browne and Fang, 1978; Du and Fang, 1983; Cavallini et al., 1999). According to the equilibrium program (Puigdomenech, 1983), Hg was present in solution mainly as Hg–EDTA complexes and at a much lower concentration as Hg hydroxide, the concentration of which increased with decreased nutrient strength. Hence, the reason for increased Hg uptake and accumulation at low nutrient strength (Table 4) may be a higher bioavailability of the Hg hydroxide than of Hg–EDTA, or less competition with nutrient cations, or a combination of both. An additional reason for the high Hg concentrations in leaves and stems at 1% Hoagland nutrient solution may be the reduced growth, which hampers "dilution" of the metal (Fig. 1). Reduced growth is also indicated by a high dry weight to fresh weight ratio in all plants at 1% Hoagland (Fig. 2). According to Mukherji and Mukherji (1979), an increased dry weight to fresh weight ratio may be a sign of reduced water uptake, which in turn causes inhibited elongation and enlargement of cells (i.e., reduced growth). In contrast to Cd and Pb, which were given in much higher concentrations and accumulated to higher concentrations in the roots, Hg concentrations in roots were not influenced by the degree of nutrient dilution in the external medium (Table 4). It is likely that water spinach has the ability to accumulate higher concentrations of Hg in its roots, like for instance the water hyacinth (Lenka et al., 1990). At higher Hg additions than in this investigation, there would possibly be a negative correlation between nutrient strength and Hg concentrations in the roots too because there would be a shortage of binding sites in the roots and competition would arise between Hg– and nutrient–cations.

Contradictory to the main findings in this investigation, Cd concentrations in the leaves, stems, and roots of untreated plants were higher at the high rather than the low strength of nutrient solution (Table 5). At the low nutrient strength, the Cd concentrations in untreated plants were within the range of concentrations in aquatic plants from unpolluted sites (Fayed and El-Shafty, 1985; Biney, 1994; Göthberg et al., 2002). The measured Cd concentration in nutrient solutions without added Cd was three times higher at 100% than at 25% Hoagland (Table 3), which is in agreement with the fact that untreated plants at 100% Hoagland accumulated more Cd than plants at 25% Hoagland. A possible reason for this may be trace amounts of Cd in the chemicals used in the preparation of the nutrient medium, most likely the zinc compound. At increased Cd concentrations (i.e., 9 µM Cd), the situation was inversed, which means higher concentrations at the low rather than the high strength of nutrient solution (Table 5). At the two highest Cd concentrations in the medium and the low nutrient strength (25% Hoagland), Cd concentrations in the leaves and stems decreased with increasing external Cd concentrations and were lower than at the high nutrient strength (100% Hoagland) (Table 5). This was probably because of the necrosis that occurred at those experimental conditions, which caused loss of weight and is shown by negative TI values (Fig. 3). Since the concentration of Cd ions in solution was about twice as high at 25 than at 100% Hoagland (Puigdomenech, 1983), the high Cd accumulation in leaves, stems (until causing necrosis), and roots at the low nutrient strength is probably mainly a result of freely available Cd ions.

The concentrations of Pb in untreated plants (Table 6) at all strengths of nutrient solution were within the concentration range for tropical aquatic plants from unpolluted areas (Fayed and El-Shafty, 1985; Gonzales et al., 1989) in spite of higher tissue concentrations at low nutrient strength. According to Puigdomenech (1983), the small fraction of added Pb that was not precipitated occurred in solution mainly as Pb–EDTA complexes, the concentration of which decreased with decreasing nutrient strength, while the concentration of Pb ions increased. As for Cd, a greater access to free ions in solution the lower the nutrient strength is probably an important reason for the increased uptake and accumulation of Pb in the leaves, stems, and roots (Table 6).

It seems that binding of the toxic metal ions to EDTA is vital for the lower uptake at high nutrient strengths in this investigation. Earlier unpublished data by the authors, however, show the same effect in spite of no EDTA in the nutrient solution.

The three metals demonstrate different patterns of distribution (% of total amount in plants) to the shoots and leaves in relation to the strength of the nutrient solution. With decreasing nutrient strength, the distribution of Hg to the shoots in treated plants increased (Table 4). This indicates that a good availability of nutrients reduces the transport of Hg to the shoot, perhaps as a result of competition with nutrient ions in connection to the translocation to the shoot. On the other hand, the transport of Pb to the leaves decreased in both treated and untreated plants with decreasing strength of the nutrient solution (Table 6). The facilitated translocation of Pb to the leaves at a high strength of nutrient solution may be caused by binding to a solute, for example amino acid or organic acid (Baker and Brooks, 1989) or EDTA (Vassil et al., 1998), and subsequent xylem translocation as an uncharged complex in the transpiration stream. Distribution of Cd showed no relation to nutrient strength (Table 5). In the xylem Cd is probably translocated as a divalent ion (White et al., 1981; Hardiman and Jacoby, 1984) by cation exchange due to the negative charges of the xylem vessel walls (Van de Geijn and Petit, 1979), which impedes Cd transport from the transport of uncharged molecules in the transpiration stream.

The distribution (% of total amount in the whole plant) of Hg and Cd to the shoot (leaves and stems) and of Pb to the leaves was lower in the metal-treated than in the untreated plants (Tables 4, 5, and 6). This mechanism was most pronounced for Hg. Thus, with increasing additions of metal, a larger proportion of the metals was retained in the roots and thereby prevented from interfering with sensitive metabolic reactions in the shoots. This is probably an internal mechanism to avoid toxic metal concentrations in the shoot, which has also been observed elsewhere (Coughtrey and Martin, 1978; Rosas et al., 1984; Jana, 1987; Ernst et al., 1992; Landberg and Greger, 1996). The higher accumulation of metals in the roots than in the shoots is possible because of a greater tolerance to toxic metals in the roots than in the shoots. The roots of water spinach accumulated much higher concentrations of the three investigated metals before toxic effects occurred (Table 7; Fig. 2, 3, and 4). The mechanism behind the higher tolerance of the roots may include binding of the positively charged toxic metal ions to negative charges in the cell walls (Beauford et al., 1977; Wierzbicka, 1998). Part of the metal that has been taken up into the root may be transported further through the plasma membrane and bind to macromolecules, organic acids, or sulfur-rich polypeptides, like phytochelatins, and accumulate in the cytoplasm or the vacuole and thereby be detoxified (Mathys, 1977; Grill et al., 1985; Steffens, 1990; Harmens et al., 1994).


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  		Table 7. Concentrations of Hg, Cd, and Pb in shoots (leaves and stems) and roots of water spinach when metal-induced toxic effects occured in the plant.{dagger}

 
Metal-induced toxic effects occurred in water spinach at lower external metal concentrations the lower the nutrient strength was (Fig. 2, 3, and 4). As the metal concentrations in water spinach increased the lower the strength of the nutrient solution was, one would expect toxic effects to be correlated to internal metal concentrations in the plants. Correlation analysis of Hg, Cd, and Pb concentrations in the shoot and root, respectively, in relation to TI, dry weight to fresh weight ratio, and root to whole-plant ratio, showed mostly significant correlations (Table 8), with the exception of Hg concentrations in roots. Metal-induced toxic effects that are correlated to internal metal concentrations in plants have been shown by some investigators (Heubert and Shay, 1991; Heubert et al., 1993), while others have found no such correlation (Rijstenbil et al., 1998; Österås et al., 2000).


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  		Table 8. Correlation coefficients between internal metal concentrations in roots or shoots and toxic effects given as tolerance index (TI) for the weight increase (fresh weight) of the whole plant, dry weight as percent of fresh weight of the whole plant (DW:FW), and root dry weight as percent of the whole-plant dry weight (R:WP) in water spinach plants.{dagger}

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
The study revealed that the strength of the external nutrient solution is of importance for the accumulation and toxicity of heavy metals in water spinach. The lower the nutrient strength was, the higher the tissue metal concentrations that accumulated and the bigger the metal-induced toxic effects on the plants. Conditions were compared when the addition of all nutrient elements were equally variated. With that prerequisite, the use of fertilizers could have been favorable for the use of water spinach as a frequently consumed vegetable as it reduces the risk for increased metal concentrations in water spinach and for negatively affected crop yields.


   ACKNOWLEDGMENTS
 
We thank Bo Lagerman, Institute of Applied Environmental Research, and Tommy Landberg, Department of Botany, Stockholm University, for valuable discussions. We also thank Eva Stoltz, Department of Botany, Ann-Marie Johansson, Pia Kärrhage, Bertil Nilsson, and Jörgen Ek, Institute of Applied Environmental Research, Stockholm University, for help with chemical analyses. The study was supported by Swedish International Development Cooperation Agency.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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Right arrow Nutrient Management
Right arrow Water Pollution

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The SCI Journals Agronomy Journal Crop Science
Vadose Zone Journal Journal of Plant Registrations
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal