Chromium accumulation and toxicity in aquatic vascular plants.
I. Abstract II. introduction III. Occurrence and States of Chromium IV. Uses and Waste Management V. Chromium Uptake and Distribution in Plants VI. Chromium Phytotoxicity A. Growth and Development B. Biochemical Effects VII. Chromium-Induced Morphological/Ultrastructural Changes VIII. Chromium Bioindicators IX. Acknowledgment X. Literature Cited
Chromium toxicity among workers in tanneries and other chromium-based industries have been known for a long time. The workers are reported to suffer from ulcers, allergic dermatitis, lung cancer, renal insufficiency, and liver necrosis (Sujana & Rao, 1997). In 1960, the famous catastrophic incidence of lung cancer due to inhaling of dust containing Cr (VI) was reported from the Kiryama factory of the Nippon-Denko concern on the island of Hokkaido, Japan (Forstner & Wittmann, 1979). Chromium pollution in groundwater as a result of leaching of Cr (VI) from spent solid and liquid wastes at chromium-based industries has currently become a serious problem at the global level. In August 1975 it was found that underground drinking water in Tokyo near the Cr (VI)-containing spoil heaps contained chromium more than 2000 times the permissible limit. In Ludhiana and Chennai, India, chromium levels in underground waters have been recorded at more than 12 mg/L and 550-1500 ppm Cr/L, respectively (Sujana & Rao, 1997).
The increased usage of chromium has led to great growth In chromium-based industries. This growth, however, has severely affected the ecoenvironment on one hand and has reduced the forest cover considerably on the other. Although there are laws to prevent the discharge of chromium-rich spent wastes into the land, in a country like India, ca. 2000 to 3200 tons of elemental chromium escapes into the environment annually from the tanning industry alone. Also, the concentration of chromium in the effluent is in the range of 2000 to 5000 ppm, against the maximum permissible limit of 2 ppm (Thyagarajan, 1992).
Chromium is considered an essential micronutrient for human health. Of the two species of chromium [Cr (VI) and Cr (III)] commonly occurring in nature, an intake of 50-200 [micro]g chromium as Cr (III) is recommended. It acts as essential micronutrient for physiological activity. The higher intake of chromium as soluble chromates (50mg/kg body wt), however, has been proved to be lethal to human beings. Chromium oxalate complexes, a probable form of chromium, in plants is considered a nutritionally available source of chromium in food crops (Cary et al., 1997).
Aquatic vascular macrophytes in tropical areas constitute the largest single form of biomass in freshwater ecosystems. Some early studies (Hutchinson, 1975; Dykyjova, 1979) reported that concentrations of essential and nonessential trace elements in freshwater plants were substantially higher than in the ambient water. In some cases, tissue levels of toxic metals such as Cd, Pb, and Hg were at least one order of magnitude higher than in ambient water. Other studies (Barsdate et al., 1974; Drifmeyer et al., 1980; Gallagher & Kibby, 1980) showed that aquatic vascular plants constituted the largest biological reservoir of metals. Further study (Wells et al., 1980) on submerged and emergent species in relation to chromium showed that submergents contained more chromium than did emergents. However, the roots of emergents--Phragmitis communis, Scirpus lacustris, Typha angustifolia, Nymphaea gigantea--were found to be important accumulators of chromium.
Despite the large number of previous studies (Guilizzoni, 1975; Baudo & Varini, 1976; Mangi et al., 1978; Stave & Knaus, 1985; Garg & Chandra, 1990; Gerth et al., 1991; Outridge & Noller, 1991; Cary et al., 1997), the present review is much needed in order to synthesize past and recent findings on chromium accumulation and toxicity, as well as physiological, biochemical, and ultrastructural changes induced by tissue chromium concentrations in aquatic vascular macrophytes. This may facilitate the development of macrophyte-based systems for monitoring and abatement of chromium pollution.
III. Occurrence and States of Chromium
Chromium is never found in nature as a single metal. It occurs as a compound of chromium, oxygen, and iron known as "chromite" (Fe[Cr.sub.2][O.sub.4]). A French chemist, Louis Nicolas Vauquelin, discovered chromite in 1797 and prepared the free metal "chromium" by itself the following year. The geographical distribution of chromite mines is very uneven. Over 95% of all chromite deposits are in the southern part of Africa, Albania, and the former Soviet Union. Some deposits are also reported from Zimbabwe, the Philippines, and Turkey (Gauglhofer & Bianchi, 1991). In India over 90% of chromite deposits are located in the state of Orissa in Sukinda Valley (Dubey et al., 2001).
Chromium is one of the widely distributed metals, occupying 21st position in the index of most commonly occurring elements in the earth's crust (Bowen, 1979). It occurs in several oxidation states, ranging from [Cr.sup.2+] to [Cr.sup.6+], with trivalent and hexavalent states being the most stable and common in terrestrial environments (Zayed et al., 1998). The average concentration of chromium in nonpolluted soils is ca. 100 mg/kg, whereas contaminated soils may contain up to 7000 mg/kg.
The annual global production of chromium is ca. 9 million tons. It is mainly mined in the former Soviet Union, South Africa, and Albania. Chromium is produced either by reducing chrome ores directly to ferrochrome with coal in an electric oven or oxidized to chromate by oxygen in air in an alkaline melt. This is dissolved in air in aqueous sodium carbonate at 100[degrees]C to produce sodium chromate. Ferrochrome is an iron-chrome alloy containing 60% chromium. In developed countries like Germany, consumption of pure chromium obtained by the reduction of chromium oxide with aluminum is ca. 1000 tons per annum. The recycling of scrap steel provides ca. 10% of chromium required for the industrial consumption.
In the environment this metal is present as Cr (III) and Cr (VI), both forms being completely different chemically (Gerth et al., 1991). Chromium (III) is an acid that forms strong complexes with various ligands (Pawlisz et al., 1997). The principal species of trivalent chromium are [Cr.sup.3-], Cr [(OH).sup.2-], Cr (OH)[O.sub.3], and Cr[(OH).sup.4-] (Rai et al., 1987). Dissolved Cr (III) has an affinity for O-, N-, and S-containing ligands and forms many organic complexes (Nakayama et al., 1981). The solubility of Cr (III) is limited by the formation of highly insoluble oxides, hydroxides, and phosphates. Dissolved Cr (III) is easily adsorbed to surfaces (Cranston & Murray, 1978). It forms hydrolysis products and coprecipitates with ferric iron. Chromium (III) is used for leather tanning because it forms stable complexes with amino groups in organic material.
Hexavalent chromium is the principal species in surface waters and aerobic soils. It forms a number of stable oxyacids and anions, including HCr[O.sub.4] (hydrochromate), [Cr.sub.2][O.sub.7.sup.2] (dichromate), and Cr [O.sub.4.sup.2-] (chromate). The chromate ion has a large ionic potential and tetrahedral coordination and is both a strong acid and an oxidizing agent. Cr (VI) is not readily adsorbed to surfaces.
Most of Cr (VI) salts are soluble in water, and hexavalent chromium is very mobile (Nriagu & Nieboer, 1988). Cr (VI) has a long residence time in surface water and groundwater. The high oxidizing potential, high solubility, and ease of permeation of biological membranes make Cr (VI) more toxic than Cr (III). Sr2, Fe (II), felvic acid, low-molecular-weight organic compound, and proteins (Rai et al., 1987; Saleh et al., 1989) can reduce hexavalent chromium to the trivalent state. Because Cr (VI) hydrolyses extensively, only neutral or anionic species occur in water (Kumaresan & Riyazuddin, 1991). In the presence of excessive oxygen, Cr (III) also oxidizes into Cr (VI) (Gauglhofer & Bianchi, 1991; Gerth et al., 1991). Effluent discharged from chromium-based industries usually contains Cr (VI), which causes high levels of chromium pollution in surface and subsoil waters in the vicinity of industrial units.
IV. Uses and Waste Management
Chromium was used as an industrial material for the first time in the production of corrosion-resistant metal (stainless steel) and coatings over 100 years ago (Jacobs, 1992). Subsequently, many industrial uses of chromium--namely, in electroplating, the printing industry (photomechanical reproduction process), the oil industries as anticorrosive, fix tell industry chrome dying process), matches, and fireworks (as an additive to an inflammable mixture)--became very popular within a short period of time (Gauglhofer & Bianchi, 1991). One of the most promising discoveries was its use in the dyes and pigments industry. Production of zinc chromate, lead chromate, chrome yellow, and chromium oxide quickly rose to as much as ca. 120,000 tons per year.
Wood impregnation is done by CCF (chromium, copper, fluoride). During impregnation, chromium remains loosely bound to the wood directly after the treatment and can be partially washed out by rainwater, causing chromium pollution in the water resources.
The tanning industry is one of the major users of chromium salts (Walsh & O'Halloran 1966a, 1966b). The process of tanning involves conversion of the putrefactive proteinaceous matter, skin, into a non-putrefactive one. This is done by treating the skin with solutions of chromium (III) salts. In a country like India, it is estimated that ca. 32,000 tons of basic chromium sulphate salts are used annually in the making of leather (Thyagarajan, 1992). The huge quantity of unused solutions of Cr (III) salts discharged into tannery wastes has raised severe ecological concerns (Khasim & Kumar, 1989). The chrome sludge generated by chromium-based industries is usually dumped on the ground. This contains chromium compounds in different valancy states (Walsh & O'Halloran, 1966a, 1966b). Hexavalent chromium formed due to oxidation of Cr (III) compounds percolates down into the soil during the rainy season and pollutes underground water.
Several reductants have been reported that are quite effective in the reduction of chromium concentration in industrial wastes. Ferrous sulphate or ferrous chloride is the most common one. In industrial waste, chromium (III) and iron (III) hydroxide are formed at an appropriate pH. The insoluble hydroxides are separated by the process of flocculation. Sodium sulphite is occasionally used as an reductant. Alternatively, sulphur dioxide or sodium polysulphide can be used. Iron filings--iron containing particles and an electrolytically generated iron (II) ion--serves a similar purpose (Sujana & Rao, 1997).
Kaolinite, chlorite, and alpha-quartz are reported to absorb Cr (VI) from solution. Activated red mud and blast-furnace slag have also shown potential for removing Cr (VI) from effluents. Coal and coke not only are good absorbants but also can reduce Cr (VI) to Cr (III). Inorganic oxides or oxyhydroxides are efficient absorbants of hexavalent chromium. Activated alumina is also a good adsorbent for Cr (VI). Fly-ash china-clay mixture is highly effective in the removal of chromium (VI) from aqueous solutions.
Several aquatic species--namely, Ceratophyllum demersum, Bacopa monnieri. Spirodela polyryhiza, Hydrilla verticillata, and Nymphaea alba--have been reported as potential accumulators of chromium. The common aquatic weed Eichhornia crassipes has been used in China for the removal of chromium (VI).
V. Chromium Uptake and Distribution in Plants
The oxidation state of chromium strongly influences the rate of chromium uptake. Chromium (VI) can easily cross the cell membranes (Riedel, 1989), and the phosphate-sulphate carrier also transports the chromate anions. On the other hand, Cr (III) does not utilize any specific membrane carrier, so it enters the cell through simple diffusion. The diffusion of Cr (III) is possible only when it forms complexes with appropriate lipophilic ligands (Gauglhofer & Bianchi, 1991).
Aquatic vascular plants absorb heavy metals, concentrate them, and play an important role in storing and recycling metals (Boyed, 1970; Frazier, 1972; Cowgill, 1974; Gommes & Muntau, 1975, 1981; Low & Lee, 1981). No generalization, however, is possible due to the fact that different species have different mechanism of absorption, transport, and storage of metals (Antonovics et al., 1971; Baudo & Varini, 1976). For instance, Potamogeton absorbs nutrients through the roots, whereas in Vallisneria (Arisz, 1960), Elodea, and Ceratophyllum (Chandra et al., 1993) the leaves are the main absorbing organs. Absorption can occur in both ways because it depends on the relative availability of the elements in water as well as sediments (Sculthorpe, 1967; Denny, 1972).
The uptake of chromium by roots takes place in the form of chromium (VI). However, the accumulation is not correlated with the tissue of macrophytes and could be absorbed by chance (Cowgill, 1974; Baudo & Varini, 1976) The uptake of chromium has been supported by the fact that, during oxidation of Cr (III) in chrome leather waste, up to 100 mg/kg hexavalent chromium was found. Merian (1991) also reported that hexavalent chromium ions remain free in the soil and are available to plants.
Table I shows variations in chromium concentrations recorded by Bowen (1979) and Outridge and Noller (1991) in maerophytes of uncontaminated and contaminated water bodies. Submerged macrophytes showed higher accumulations of chromium than do floating and emergent ones. The concentration of chromium in these plants was recorded in the following order: Elodea canadensis (20.7 ppm) > Lagarosiphon major (12.4 ppm) > Potamogeton crispes (11.4 ppm) > Trapa natans (4.98 ppm) > Phragmitis communis (2.01 ppm) (Cowgill, 1974; Baudo & Varini, 1976). Substantial accumulation was also found in the plants of lpomoea aquatica, Marsilea minuta, Nelumbo nucifera, and Ceratophyllum demersum occurring in polluted waters (Chandra et al., 1993; Rai et al., 1996). Under controlled conditions, C. demersum, Hydrilla verticillata, Chara carollina, and Hydrodictyon reticulatum showed lower levels of chromium accumulation (Garg & Chandra, 1990; Rai et al., 1995a, 1995b). The plants of channel grass (Vallisneria spiralis) showed maximum accumulation in the roots (1050 [micro]g g-1 dw), followed by leaves (697 [micro] g [g.sup.-1] dw), and the least accumulation in rhizomes (437 [micro]g [g.sup.-1] dw) when cultured in nutrient a solution containing 10 [micro]g [ml.sup.-1] Cr after 72 hours (Vajpayee et al., 2001). Bioconcentration factor values were very high in C. demersum (15,330-31,400) and H. reticulatum (11,394).
Duckweeds are fast-growing species, adapt easily to various aquatic conditions, and play an important role in the extraction and accumulation of metals from water. They bioconcentrate heavy metals such as Fe and Cu up to 78 times in wastewater (Jain et al., 1989). Duckweeds have shown potential to accumulate chromium substantially (Bassi et al., 1990; Tripathi & Chandra, 1991; Zaranyika & Ndapwadza, 1995). Chromium accumulations of 0.002 and 0.4 mM have been recorded, related to ambient chromium concentrations (Stave & Knaus, 1985). Although the duckweed Lemna attained higher chromium concentrations in its tissues compared with other aquatic macrophytes, its bioconcentration factor (BCF) value was much lower than those reported in other aquatic species (Zayed et al., 1998). The floating species water hyacinth (Eichhornia crassipes) is commonly reported to be quite effective in controlling water pollution and a promising chromium-accumulating plant (Wolverton, 1981; Jana, 1988). The roots of E. crassipes showed as high as 18.92 [micro]mol (g dry tissue [wt.sup.-1]) Cr accumulation.
Emergent species--Scirpus validatus, Cyperus esculentus--showed a moderate accumulation, 0.55 [kg.sup.-1] and 0.73 [kg.sup.-1] Cr, respectively (Lee et al., 1981). In Bacopa monnieri and Scirpus lacustris an accumulation of 1600 and 739 [micro]g [g.sup.-1] dw Cr, respectively, have been reported when exposed to 5 mg [L.sup.-1] for 168 L in a solution culture. The accumulation of chromium was greater in roots than in shoots (Gupta et al., 1994). Higher accumulations of chromium in roots and lowest accumulations in shoots of emergent species have also been recorded (Sinha & Chandra, 1990; Smith et al., 1989; Qian et al., 1999). Nymphaea alba, Bacopa monnieri, and Hydrilla verticillata grown in tannery effluent in monoculture brought down the level of chromium more than 5% (Vajpayee et al., 1995; Sinha, 1999).
VI. Chromium Phytotoxicity
The effects of toxic substances in an aquatic ecosystem can be assessed by changes in the community structure, physiological activities, and ultrastructural components of the macrophytes (Schmidt et al., 1978; Guilizzoni, 1991). However, to date, ecotoxicologists are of the view that the effects of the toxicants should be considered at the species level (Stanley, 1974; Hutchinson & Czyrska, 1975; Labus et al., 1977; Filbin & Hough, 1979; Baudo et al., 1981). Also, while considering the toxicity of heavy metals, a distinction should be made between elements essential to plants and metals, which have no proven beneficial biochemical effects. For example, increased levels of chromium may actually stimulate growth without being essential for any metabolic process (Gardner, 1980; Guilizzoni et al., 1984).
A. GROWTH AND DEVELOPMENT
The phytotoxicity of chromium in aquatic environments has not been explored adequately. Although some information is available on the uptake aspects, the mechanism of injury in terms of ultrastructural organization, biochemical changes, and metabolic regulation have not been elucidated. These changes may be used as pollution indicators (Ray & White, 1976; Wolverton & McDonald, 1978).
A study by Baszynski et al. (1981) revealed that although chromium is not known to play any role in metabolic process, it is absorbed by plants and affects their metabolism. Also, chromium at low levels in a liquid medium was not required for normal growth; however, at higher concentrations it has caused a reduction of growth (Huffmann & Allaway, 1973).
Similarly, in Myriophyllum spicarum maximum increases in shoot length found at a concentration of 50 [micro]g [l.sup.-1] Cr and higher concentrations up to 1000 [micro]g [l.sup.-1] caused an almost linear reduction in both shoot weight and length (Guilizzoni et al., 1984). They also showed 50% inhibition of root weight at concentrations of 1.0 ppm chromate and 9.9 ppm [Cr.sup.3+] (Stanley, 1974).
The Lemnaceae are relatively tolerant to chromium (Landolt & Kandeler, 1987). However, inhibition of growth in Spirodela polyrrhiza and Lemna minor was found at 0.02 mM and 0.00002 mM Cr concentrations, respectively (Mangi et al., 1978). Mortality in the ease of Lemna aequinoctialis was observed at 0.005 mM Cr concentration (Clark et al., 1981).
The growth in Spirodela polyrrhiza, S. punctata, and Lemna gibba was greatly inhibited at 0.02 mM when plants were treated with 51 Cr. Between 0.002 mM and 0.4 mM the uptake of chromium was directly related to ambient chromium concentrations (Stave 1980; Stave and Knaus, 1985). In contrast, hydrophytes--Nasturtium officinale, Apium nodiflorum, Veronica beccabunga, V. anagallis-aquatica, V. lysimachioides, V. anagalloides, Mentha longifolia, M. aquatica, M. puleguim, M. sylvestris, Potentiella reptens, and Cardamine uliginosa--grown in the presence of chromium showed no visible symptoms of toxicity. In all of the above plants except R reptens the growth rate was unaffected and compared well with those of fast-growing plants such as Spirodela sp. (Leopold & Kriederman, 1975; Zurayk et al., 2001).
In duckweeds at higher metal concentrations, chlorosis has been observed. Surprisingly, the toxicity effect was in descending order of damage, Cu > Se > Pb > Cd > Ni > Cr. Chromium caused very little damage to duckweed (Zayed et al., 1998). A reduction on the frond counts of Spirodela polyrrhiza was observed as a result of chromium toxicity (King & Coley, 1984). In contrast, Outridge and Noller (1991) reported that chromium was 10 times more toxic to freshwater plants than was Pb or Zn. The effective chromium concentrations (EC-.50) for some of the freshwater plants are as follows: Lemna minor, 5.0 mg [L.sup.-1] 14 d EC (Mangi et al., 1978); Myriophyllum spicatum, 1.9 mg [L.sup.-1] 32 d EC 50 (Stanley, 1974); and Spirodela polyrrhiza, 50 mg [L.sup.-1] 14 d EC (Mangi et al., 1978).
B. BIOCHEMICAL EFFECTS
Toxicity of chromium in relation to biochemical parameters--chlorophyll, carotenoids, aminoacids, DNA, RNA, and photosynthesis--has been worked out by several researchers (Baszynski et al., 1981; Roy & Mukherjee, 1982; Pillord et al., 1987; Sen et al., 1987; Jana 1988). In Limnanthemum cristatum Griseb., chromium caused a slight reduction in chlorophyll and almost no change in carotenoid (Chandra & Garg, 1992). Ceratophyllum demersum, Najas indica, Vallisneria spiralis, and Alternanthera sessilis showed decreases in chlorophyll content as well as carotenoid and protein with increased Cr concentrations (Garg & Chandra, 1990; Sinha et al., 2002).
At 0.1 mg/L K2 Cr[O.sup.4], reduction in growth, chlorophyll content, and wilting were observed in Lemna minor and Pistia stratiotes (Bassi et al., 1990). Hydrilla verticillata showed decreases of 26% in chlorophyll content at 1.0 mg/L (Sinha et al., 1993) and 52% at 0.25 mg/L [K.sub.2] Cr[O.sup.4] (Rai et al., 1996). Jana (1988) reported decrease in hill activity, chlorophyll content, and protein at 19.2-6M Na[Cr.sup.04] in H. verticillata and Eichhornia crassipes. Chara carollina also showed a decrease of 42% in chlorophyll content when treated with 0.25 mg/L [K.sub.2] Cr[O.sup.4] (Rai et al., 1995b). Decreases in sugar and protein contents at 10 mg/L [K.sub.2] Cr[O.sup.4] have also been recorded in E. crassipes and P. stratiotes (Satyakala & Jamil, 1992).
In chlorococcalean green alga (Glaucocystis nosto-chinearum Itzigsohn.), chromium toxicity in growth, photosynthetic pigments, photosynthesis, in vivo nitrate reductase activity, and protein content was quite marked (Rai et al., 1992). Chromium toxicity varied in Pistia stratiotes at different levels of chromium concentrations. Reduction in chlorophyll a, b (at 1 mg/L Cr) and protein (at 10 mg/L Cr) in shoots (not in roots) and no effect on DNA, protease, catalase, and perioxidase was found at 5 mg/L Cr ambient concentration. However, at 20 mg/L Cr, plants of P. stratiotes showed 100% death after three days (Sen et al., 1987). In Nelumbo nucifera. higher chromium concentrations reduced chlorophyll and protein contents (78.49% in roots, 74.74% in leaves, 56.99% in rhizomes) and in vitro nitrate reductase activity (Vajpayee et al., 1999).
VII. Chromium-Induced Morphological/Ultrastructural Changes
When large quantities of chromium are absorbed by plants it may cause various types of damage at the morphological and ultrastructural levels (Heumann, 1987). Rebechini and Hanzely (1974) reported changes in chloroplast fine structure due to lead toxicity in Ceratophyllum demersum. Corradi and Bassi (1987) reported disorganized thylakoids with loss of grain and formation of many vesicles in the chloroplast of Lemna minor and Pistia stratiotes. In Limnanthemum cristatum, when treated with 1 ppm chromium (VI), significant morphological change was the production of stunted, brown roots by chromium-treated excised leaves in contrast to the control, in which thin, elongated, green roots were formed. During chromium treatment, juvenile leaves produced on the excised leaves showed brown coloration in the hydathodes at 226 [micro]g/g Cr dry wt leaf-tissue concentration. However, no coloration was found in the juvenile leaves produced either in the cultures containing metals other than chromium and in the control. The characteristic ultrastructural changes reported in the juvenile leaves of L. cristatum at 1 ppm Cr are: heavily packed starch grains, irregular nuclei in the root, disorganized plastids, retracted plasma membranes, and thickening in the tracheids near the hydathodes (Garg et al., 1994). Marked changes in physiological, ultrastructural, and mineral element composition have been found in bush beans as a result of chromium toxicity (Barcelo et al., 1985, 1986; Vazquez et al., 1987).
In Salvia sclarea, no change in chloroplast structure has been reported; the only ultrastructural alteration found was a partial detachment of the plasma membrane from the cell wall as a result of chromium toxicity (Corradi et al., 1993). Structural changes--namely, increases in numbers of hairs and closures of vascular bundles--were observed in chromium-treated Scirpus lacustris (Suseela et al., 2002). In Spirodela polyrrhiza, decreases in frond numbers (Mangiet al., 1978) and reductions in root phytomass were found as a result of chromium (VI) toxicity (Gaur et al., 1994; Tripathi & Smith, 1996). Petiole length in Nelumbo lutea was decreased considerably when the plants were treated with different concentrations of [Na.sub.2] Cr[O.sup.4] at pHs ranging from 5.6 to 8.2 (Francko et al., 1993).
VIII. Chromium Bioindicators
Vascular plants and algae have been found to be highly effective for recognizing and predicting metal stress in aquatic environments. By their ability to accumulate toxic substances, they indicate their presence in the environment even in very low concentrations (Muntau, 1981). The bioindicator species are very specific, but there are exceptions in which the response to different compounds is identical. The specific accumulation of metals like Hg and Zn has been shown in Salvinia natans and Spirodela polyrrhiza, respectively (Sen et al., 1987); Salvinia molesta removed Cr and Ni more effectively than did Spirodela polyrrhiza.
Limnanthemum cristatum, an emergent species, has shown great potential for chromium absorption. Juvenile leaves grown in a nutrient solution containing 1 ppm chromium produced brown coloration in the hydathodes. Since this coloration is produced only with the metal chromium, this could be used for indicating presence of chromium in aquatic environments (Garg et al., 1994).
The rootless duckweed Wolffia globosa showed substantial accumulations of chromium as well as a very high concentration factor (5616) at 0.05 ppm. The high metal enrichment at a very low ambient concentration may be useful in defecting chromium in water (Garg & Chandra, 1994).
In several aquatic and terrestrial species chromium (VI) induced morphological alterations, which could serve as bioindicators (Corradi & Gorbi, 1993; Rauser, 1987; Vazquez et al., 1987). Chromium caused reductions in root systems in Phaseolus and produced reddish brown coloration in the petioles and leaf veins of white beans. Corradi and Gorbi (1993) observed that chromium-treated cells of Scenedesmus acutus became uniformly dark blue and that the number of cells increased with increasing chromium concentrations in the culture medium.
Table I Variations in chromium concentrations recorded by Bowen (1979) and Outridge and Noller (1991) in macrophytes of uncontaminated and contaminated water bodies Uncontaminated Contaminated Species ([micro]g dry ([micro]g dry w[t.sup.-1]) w[t.sup.-1]) Blyxa eichnosperma 4.0-21 -- Ceratophyllum demersum 0.67, 5.7 -- Eliocharis dulcis 5.7 -- Elodea canadensis 6.4 3.5, 6.0 Hydrilla verticillata 4.9 -- Lemna perpusilla -- 5.2, 6.5 Limnophylla sp. 21-24 -- Myriophyllum spicatum -- 2.5-5.5 Nymphaea odorata 2.7, 0.7, 0.60 3.3 Nymphoides indica 10 -- Phragmitis australis -- 0.8 Phragmitis communis 4.8 -- Polygonum natans -- 4.7 Pontederia cordata 0.60 2.6-3.0 Potamogeton crispus 0.56 -- Potamogeton pectinatus -- 2.8-7.4 Sagittaria latifolia -- 2.1 Scirpus lacustris 6.0 -- Typha angustifolia 4.3 3-4.5 Typha latifolia 1.2 1.3-4.0 Vallisneria americana 1.3-2.5 2.9
The authors are deeply indebted to Dr. P. Pushpangadan, director, National Botanical Research Institute, Lucknow, India, for his constant encouragement and keen interest in this work.
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Head, Aquatic Botany Laboratory (ret.)
National Botanical Research Institute
Lucknow 226001. Uttar Pradesh, India
Head, Eco-Education Division
National Botanical Research Institute
Lucknow 226001, Uttar Pradesh, India
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|Author:||Chandra, Prakash; Kulshreshtha, Kamla|
|Publication:||The Botanical Review|
|Date:||Jul 1, 2004|
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