Heavy metals or trace elements are a large group of elements with higher density generally greater than 5gm/cm3. These elements are important both industrially and biologically. Heavy metals occur naturally in earth’s crust and surface soils in varying concentrations (13, 224). Natural Processes like weathering, erosion remove small amounts of metals from the bed rocks and allow them to circulate in water and air. Heavy metals like Zn, Se, Cu, and Fe are essential to maintain the metabolism of the human body and play important role in chemical, biological, biochemical and enzymatic reactions in the cells of plants, animals and human beings (228). Heavy metals are also known as ‘trace inorganics’, ‘micronutrients’, ‘toxic elements’ etc. More than 60 elements in various parts of human body have been detected, but only 17 are available in living cells (206). Heavy metals like Mn, Mo, Fe are more important as micronutrients while Ni, Cu, Co, V, Zn, W and Cr are of lower importance and can be toxic beyond the limits(75). Heavy metals like Cd, Pb, Mg, As, Sb, have no biological functions, but are rather toxic to living organisms (48, 58). Injury to vegetation caused by heavy metals has been well recognized in many botanical and chemical investigations during past years (250). Heavy metals occur in all ecosystems of the world. The total concentration of heavy metals in soil and water however varies from local to regional and further to continental level.
Heavy metals are very harmful in reference to their non biodegradable nature, long biological half lives and their potential to accumulate in different body parts. Most of the heavy metals are extremely toxic because of their solubility in water. Even at low concentrations heavy metals can have damaging effects in human beings and animals as there is no good mechanism for their elimination from the body. The heavy metals are taken up faster than they are metabolized or excreted. Even those heavy metals which are considered to be essential can become toxic in case present in excess. The heavy metals can impair important biochemical processes posing a threat to human health (88, 242).
Heavy metals can be toxic at source level of solubility; however only a few have been observed to cause phyto-toxicity in soils (27, 56). Soils contain these metals in form of inorganic compounds or they may remain bonded with organic matter,
clays or as oxides. Lead and Cadmium are of interest not only because of phytotoxicity but also due to their uptake by the plants and then passing over in the food chain (25, 29, 68).
As the usage of metals increased inexorably, so did the pollution associated with it. Changes in the environment due to anthropogenic activities may have strong impact on the physiology and ecology of the organisms. Human activities and consequent developments have brought about degradation of all facets of the natural environment; physical, chemical, biological and social which are adversely affecting the quality of life (6). Rapid developments, increase in mining, industrial activities, have gradually redistributed many of the toxic metals from the earth crust to the environment, raising the chances of exposure through ingestion, inhalation or on skin contact.
Heavy metals can have different sources or origin e.g. Smelters (52, 118), tannery (169), mines (198), steel mills, coal fired power plants which can lead to metal pollution (266). Other sources of metal pollution are sewage sludge (55, 59), compost refuse, fly ash (129, 225), industrial wastes or effluents (71, 130, 132). Emission of heavy metals as particulate matter and gases from volcanoes, forest fires, crusted materials and continental dust have always been a natural input sources to soils and ecosystems (78, 81, 261, 262).
The spreading of urban waste and sewage sludge in agricultural fields has been a common practice since decades (32, 101, 186, 267). Sewage sludge, live stock manure, waste water irrigation are feasible alternatives(47, 163) for reutilization of residual resource of high nutrient and organic matter contents representing a good fertilizer or soil conditioner for plants and soil (226). Besides agricultural fields, recreational parks, golf courses, home gardens are also irrigated using waste waters from sewage plants. The solids from the sewage plants are processed and sold as soil amendments and low grade fertilizers (43, 155). Reclaimed lands are known to contain significant amount of metals and are being used for growing food crops and vegetables.
Pig and poultry manure generally contains elevated concentrations of Cu and Zn which improve food conversion efficiency. Arsenic was also used for this purpose (38, 172). Beneficial properties of sludge and manures are limited by their contents of potentially harmful substances such as heavy metals and organic micro pollutants (40, 196). Sludge and manure amendments are observed to improve physical properties of soil like soil aeration, water holding capacity and aggregation (239). The slightly alkaline property of the sludge and fly ash works as buffer against the acidity of acidic soils (42, 249). Alkaline pH of the soil may restrict the mobilization of heavy metals in the soil matrix and consequently metal uptake by crop plants, vegetables etc. may be controlled, thus reducing the heavy metal toxicity (82, 180, 241).
Waste water reuse such as industrial waste water, sewage water for applications like irrigating dry and semi dry regions is considered as a useful method to minimize the problem of water shortage (84, 85, 191, 208, 214, 215). The waste water may contain different types of pollutants including heavy metals which may cause heavy metal contamination in the vegetation due to irrigation with this contaminated water (46, 121, 162, 207, 210, 222, 227, 230). Mosleh Yhaia Y I and Omer Abed El-Hakeem Almagrabi (162) investigated the accumulation of heavy metals in some vegetables irrigated with waste water and sludge in three vegetable forms in Jeddah. The trend of heavy metal uptake in fruit type vegetables was Fe ; Zn ; Cd ; Cu ; Pb while Fe ; Cd ; Zn ; Cu ; Pb was the trend found in leafy vegetables. Similar trend was revealed by other researchers (124, 200, 216, 232).
Singh P K et al. (223) studied the effect of sewage and waste water irrigation on crops like wheat, gram, palak, methi and barseem and found that if the land with suitable topography, soil characteristics, proper drainage is available, sewage effluents can be put to good use as a source of irrigation and plant nutrients. Similar studies by Kiran D Ladwani et al. (136) and Sharma R K et al. (212) were also conducted revealing that these effluents contain high organic matter, and various nutrients. The results have shown improvement in the physical, chemical properties of soil, yield of crops, and quality of grains. However it was also found
that the soils are contaminated by the heavy metals because of being irrigated with the waste water (2, 3). Soils may accumulate heavy metals to an extent which may cause clinical problems to animals and human beings because of the change in physico-chemical parameters of the soil and the vegetables grown may be loaded with heavy metals (86, 87, 96).
Bigdeli M and Seilsepour M (31) found the accumulation of heavy metals in vegetables irrigated by waste waters and industrial effluents in the farms of Shahre Rey Iran. Leafy vegetables tend to accumulate relatively higher concentrations of heavy metals in comparison to fruit type (12). They found that Cd, Pb, Zn, Cu etc. moved into the stems and leaves of, celery, coriander, spinach, dill, the most consumed part of the plants and in lower concentration in radish, green chillies and red chillies, tomatoes, egg plants consumed either by human beings or animals as fodder (24, 177, 178).
Food is the major intake source of toxic elements by human beings. Vegetables are used as staple part of food both in cooked and raw form (64). The recommended amount of vegetables in our daily diet is 300-350 gm per person (61). Heavy metal contamination of fruits and vegetables cannot be underestimated as these food stuffs are important components of human diet. It is therefore felt necessary to assess the levels of trace elements concentration in different varieties of fruits and vegetables (60, 174, 188, 275).
Atmospheric emissions are also a matter of great concern (92, 95). Leaded gasoline in vehicles is one of the major sources of Pb pollution in the cities worldwide (49). Luilo G B ; Othman C C (147) reported that only 3% of Pb in the soil is translocated through roots to the shoots and fruits, rest are due to absorption through foliage. Sources like engine oils, corrosion of batteries, wear and tear of tyres, vehicular parts contribute for Cu, Pb etc. (20, 69). Moreover bitumen, mineral filler materials in asphalt road surfaces have also been reported to contain metals like Cu, Zn, Cd ; Pb by Yan X et al. (269).
The heavy metals or trace elements play an important role in the metabolic pathways during the growth and development of plants when available in
required concentrations. In addition to soil, plants function as a sink for atmospheric pollutants because of their capacity to act as efficient interceptions of air borne matter (107, 272).
The plants are widely used as passive bio-monitors in urban environments. There is no doubt that, leafy vegetables grown in the neighborhood of major highways can contain significant traces of Pb and Cd due to air borne metal particulates derived from vehicle emissions (10, 22, 91). The distribution of these metals (Pb and Cd) in the road side soils are strongly but inversely correlated with the distances away from the road side (111). The determination of metal content in vegetables is important from the point of view of crop yield technology, nutrition and health impacts.
Yargholi B et al. (271) investigated the trace metal content in different parts of vegetables. Nabula G et al. (165) also pointed out that leafy vegetables grown in road side areas were considered a potential source of toxic metals to consumers. Pb and Cd accumulation in several crops including horticultural crops as well as in soils and irrigation water in urban areas have been documented (94, 185) and compared with the concentration of Pb in the uncontaminated areas (182, 190).
Zamora P W et al. (276) assessed Pb concentrations in leafy vegetables in markets of Manila, Philippines. Washed and unwashed vegetables were compared for the heavy metal contents. They suggested that when compared with earlier studies (209, 211) results revealed proper washing of vegetables reduce concentrations of heavy metals suggesting that atmospheric deposition may be one of the important reasons for contamination (204, 274). Recently survey for heavy metals in vegetables conducted by Sharma R K et al. (213) presented data on heavy metal (Cu, Zn, Cd ; Pb) concentrations in some key Indian vegetables such as palak (Spinacia oleracea) , lady finger (Abelmoschus esculentus) and cauliflower (Brassica oleracea) grown locally in suburban and rural areas and sold in urban open markets . It was hypothesized that atmospheric depositions in urban areas may increase the levels of heavy metals during transport and marketing, leading to significant contaminations of vegetables at the market sites
than that at the production sites (23, 63, 237). Observed concentrations of Cu, Zn, Cd, Pb in the vegetables were also compared with Prevention of Food Adulteration (PFA) Act (21) and Joint FAO/WHO Food Standards Program (50), which are 30, 50, 1.5, 2.5 mg/g respectively, and as per European Commission (70), a standards of food contamination, it is 0.1 and 0.3 mg/g respectively for Cd and Pb. The contribution of heavy metal contamination through dietary intake of vegetables tested is also assessed on the basis of average daily consumption. Thus appropriate precaution can reduce the elevated levels of heavy metals in the vegetables (219, 240).
International ; national regulations on food quality have lowered the maximum permissible levels of toxic metals in food items due to an increased awareness of the risk of these metals which pose food chain contamination. However, intake of heavy metal contaminated vegetables may pose a threat to human health. Heavy metal contamination of the food items is the one of the most important aspects of food quality assurance (113, 114, 123, 126, 135, 143).
Islam Ejaz-ul et al. (103) revealed the factors affecting the thresholds of dietary toxicity of heavy metals in soil-crop system are soil pH, organic matter content, clay mineral, soil chemical and biological properties, crop species or cultivars. Similarly Tyler L D and Mc Bride M B (243) reported the effect of Ca ion, pH and organic acids on the uptake of Cd in Corn and Snap beans. It was found that increase in Ca ion concentrations of solution depressed the translocation of Cd by roots. Addition of humic acid to soils decreased the Cd activity and subsequent absorption of Cd by corn roots.
Lesser attention has been focused on the possible accumulation of heavy metals in small home gardens especially in rural areas and small towns. Various crops are cultivated using organic, inorganic fertilizers, agrochemicals, pesticides etc for enhancing the yield and quality, which may be the sources of heavy metals (125, 127, 128). Kabata-Pendias also suggested that agro-chemicals, phosphate fertilizers are important sources of heavy metals (117). Super phosphates and air-
pollution can have acidifying effect on the soil hence facilitate the mobilization and uptake of heavy metals specially cadmium by plants (44, 173, 231, 235).
It is not completely possible to avoid exposure to toxic metals because people who are not occupationally exposed carry certain heavy metals in their body due to food, beverages or inhalation of air (23, 150, 151, 152, 153). It is however possible to reduce metal toxicity risk through life style choices that diminish the probability of harmful heavy metals uptake such as dietary measures that may promote safe metabolism or excretion of ingested heavy metals (65).
Food chain contamination by heavy metals has become a blazing issue in recent years because of their accumulation in the bio-system through contaminated water, soil and air. Fertilizers may be responsible for heavy metal addition in very small amounts however on the other hand sewage sludges may add them 100 times more in short duration (164, 199). Toxic metals in the atmosphere also get accumulated in soils through precipitation and fallout. Availability of heavy metals to plants is due to mining activities, industrial exhausts ; effluents, atmospheric depositions, waste disposals, agro-chemicals (11, 19, 160, 161). However availability of heavy metals to plants depends on various physico-chemical properties of soil. Metal toxicity in plants is aggravated at higher temperature and low pH as it facilitates the mobility from roots to shoots. Therefore a better understanding of heavy metal sources, their accumulation in soil and their effect on the ecosystem is an important issue of the present day researches or risk assessment (57, 281).
The Phytotoxicity of heavy metals in plants can be seen, as plants develop some peculiar (229) symptoms or characteristics given as below-
Symptoms/characteristics in plants
Brown margin in leaves, chlorosis, necrosis, curled leaves,
brown, stunted roots, reddish veins and perioles, reduction
in growth, purple coloration.