**7. Harmful elements (Cd, Cr, Hg and Pb) and heavy metal toxicities**

Heavy metals are the intrinsic component of the environment. It is usually accumulated due to unplanned municipal waste disposal, mining and use of extensive pesticides. Other agrochemicals uses as chemical fertilizer are the significant cause of elevation in environment.

Shute et al. (2006) reported results of greenhouse study regarding Cd and Zn accumulation in soybean. The highest dose of Cd (100 mg/kg) reduced plant height and dry weight (down to 40 % and 34 % of control, respectively), while the analogical data for the highest dose of Zn (2000 mg/kg) were 55 % and 70 %, respectively. With both metals present, the plants were approximately the same size as those treated with cadmium only. When both metals were added to the soil, 80-100 % of the cadmium and 46-60 % of the zinc were bioavailable. Concentrations of both metals were highest in root tissue (10-fold higher for Cd and up to 2-fold higher for Zn). Although relatively little Cd was translocated to pods and seeds, the seeds of all plants (including those from control and zinc-treated plants) had concentrations of cadmium 3-4 times above the limit of 0.2 mg/kg set by the Codex Alimentary Commission. This was surprising given that Cd in the soil was only 1 mg /kg well below the maximum allowable amount for agricultural soil.

The heavy metal content of municipal and industrial sewage sludge and swine manure lagoon sludge are quite high in Cu and Zn and cause a buildup of the elements in the soil and for this reason have potential toxicity to the environment. (King, 1986; King & Hajjar 1990). Physiological effects of zinc toxicity in soybean elaborated Fontes (1992) and Fontes & Cox, 1995, 1998). Borkert & Cox (1999) evaluated the effects of high concentrations of both Zn and Cu on soybean status. Miner (1997) looked at soil factors affecting plant concentrations of these elements in sludge-amended soils. When concentrations of heavy metals are high, knowledge of their solubility becomes important.

As soybeans are one of the principle sources of dietary intake in the Japanese population, the Codex Committee on Food Additives and Contaminants has proposed an upper limit of 0.2 mg/kg for cadmium concentration in soybean grain with aim of protection dietary uptake of harmful quantities of Cd (Arao et al., 2003).

Arao et al. (2003) tested Cd uptake and distribution of Cd in 17 soybean varieties grown in pots (three soils: Mid-Cd Soil, High-Cd Soil, Low-Cd Soil) and under field conditions in unpolluted soil (low-Cd field). The sources of cadmium pollution were thought to be mine waste in the case of the Mid-Cd Soil, and refining plant waste in the High-Cd Soil. The seed cadmium concentration was lowest for the *En-b0-1-2* soybean variety, and highest for *Harosoy*. The seed cadmium levels of *Tohoku 128*, a cross between *Enrei* and *Suzuyutaka*, were intermediate between those of the parents (Table 13). For four soil types, containing from 0.2 to 6.5 mg kg−1 extractable cadmium, the ranking of soybean genotypes based on seed cadmium level was similar, indicating that there is a genetic factor involved in the varietal differences in cadmium concentration. The lower levels of cadmium found in the seeds of certain varieties of soybean could be result from the combination of lower initial uptake and retention of higher levels of cadmium in the roots, thus limiting its translocation to the shoot.

Different actions can be undertaken in order to reduce the absorption of Cd by plants. The addition of amendments such as calcium carbonate, zeolite, and manganese oxide can reduce Cd uptake in plants. With that regard, zeolite was more effective in suppressing Cd uptake by plants than calcium carbonate or manganese oxide (Chen et al., 2000; Putwattanaa et al., 2010). Also, organic amendment such as farmyard manure and compost which contains a high proportion of humified organic matter can decrease the bioavailability of Cd and other heavy metals in soil (Li et al. 2006, Pichtel & Bradway, 2008; Tordoff et al., 2000). Shamsi et al. (2010) tested effects of potassium supplementation on alleviation of Cd toxicity in hydroponics experiment. K supplementation at a rate of 380 mg/l in combination either with Cd addition (1 ug Cd) or without Cd. K supplementation alleviated the reduction of growth, photosynthesis and nutrient uptake in Cd-treated soybean plants. It was concluded that Cd toxicity could be alleviated through enhanced K nutrition in soybean.

Soybean cultivars show significant differences in seed cadmium concentrations, primarily because of genetic rather than environmental factors. One-six of the total soybean produced in Japan exceeded 0.2 mg Cd/kg, the international standard proposed by the Codex Alimentarius Commission. Further, the soybean crops had considerably higher Cd contents than other field crops MAFFJ (2002). Sugiyama & Noriharu (2009) investigated the seed Cd concentrations in four soybean cultivars (*Suzuyutaka*, *Hatayutaka*, *Enrei* and *Kantou 100*) in pot experiment on Cd-polluted soil. In *Suzuyutaka*, which had high Cd concentrations in the seeds, the concentrations of Cd distributed from the shoots to the leaves was 67% and that distributed from the shoots to the seeds was 13%. In *Kantou 100* which had low Cd concentrations into seeds, 57% Cd was distributed from the shoots to the leaves and 21% from the shoots to the seeds. These results suggest that cultivars that have a low capacity for Cd accumulation in the roots have a mechanism that prevents Cd accumulation into seeds by promoting its accumulation in the leaves (Sugiyama & Noriharu, 2009).

Chromium (Cr) is a nonessential and toxic element to plants. Chromium interferes with several metabolic processes, causing toxicity to plants as exhibited by reduced seed germination or early seedling development (Sharma et al., 1995), root growth and biomass, chlorosis, photosynthetic impairing and finally, plant death (Scoccianti et al., 2006). Normal range of Cr is from 10 to 50 mg/kg depending on the parental material (Pandey & Pandey, 2008). Researchers have demonstrated experiments with plants associated with high levels of Cr. Thus, 1-5 ppm Cr present in the available form in the soil solution, either as Cr (III) of Cr (VI), is the critical level for a number of plant species. Increased Cr (VI) concentration of 10-800 mg/l in culture medium led to the detection of inhibited growth parameters. There was a reduction in growth, dry weight and vigour index in four soybean genotypes of

polluted soil (low-Cd field). The sources of cadmium pollution were thought to be mine waste in the case of the Mid-Cd Soil, and refining plant waste in the High-Cd Soil. The seed cadmium concentration was lowest for the *En-b0-1-2* soybean variety, and highest for *Harosoy*. The seed cadmium levels of *Tohoku 128*, a cross between *Enrei* and *Suzuyutaka*, were intermediate between those of the parents (Table 13). For four soil types, containing from 0.2 to 6.5 mg kg−1 extractable cadmium, the ranking of soybean genotypes based on seed cadmium level was similar, indicating that there is a genetic factor involved in the varietal differences in cadmium concentration. The lower levels of cadmium found in the seeds of certain varieties of soybean could be result from the combination of lower initial uptake and retention of higher levels of cadmium in the roots, thus limiting its translocation to the

Different actions can be undertaken in order to reduce the absorption of Cd by plants. The addition of amendments such as calcium carbonate, zeolite, and manganese oxide can reduce Cd uptake in plants. With that regard, zeolite was more effective in suppressing Cd uptake by plants than calcium carbonate or manganese oxide (Chen et al., 2000; Putwattanaa et al., 2010). Also, organic amendment such as farmyard manure and compost which contains a high proportion of humified organic matter can decrease the bioavailability of Cd and other heavy metals in soil (Li et al. 2006, Pichtel & Bradway, 2008; Tordoff et al., 2000). Shamsi et al. (2010) tested effects of potassium supplementation on alleviation of Cd toxicity in hydroponics experiment. K supplementation at a rate of 380 mg/l in combination either with Cd addition (1 ug Cd) or without Cd. K supplementation alleviated the reduction of growth, photosynthesis and nutrient uptake in Cd-treated soybean plants. It was concluded

Soybean cultivars show significant differences in seed cadmium concentrations, primarily because of genetic rather than environmental factors. One-six of the total soybean produced in Japan exceeded 0.2 mg Cd/kg, the international standard proposed by the Codex Alimentarius Commission. Further, the soybean crops had considerably higher Cd contents than other field crops MAFFJ (2002). Sugiyama & Noriharu (2009) investigated the seed Cd concentrations in four soybean cultivars (*Suzuyutaka*, *Hatayutaka*, *Enrei* and *Kantou 100*) in pot experiment on Cd-polluted soil. In *Suzuyutaka*, which had high Cd concentrations in the seeds, the concentrations of Cd distributed from the shoots to the leaves was 67% and that distributed from the shoots to the seeds was 13%. In *Kantou 100* which had low Cd concentrations into seeds, 57% Cd was distributed from the shoots to the leaves and 21% from the shoots to the seeds. These results suggest that cultivars that have a low capacity for Cd accumulation in the roots have a mechanism that prevents Cd accumulation into

that Cd toxicity could be alleviated through enhanced K nutrition in soybean.

seeds by promoting its accumulation in the leaves (Sugiyama & Noriharu, 2009).

Chromium (Cr) is a nonessential and toxic element to plants. Chromium interferes with several metabolic processes, causing toxicity to plants as exhibited by reduced seed germination or early seedling development (Sharma et al., 1995), root growth and biomass, chlorosis, photosynthetic impairing and finally, plant death (Scoccianti et al., 2006). Normal range of Cr is from 10 to 50 mg/kg depending on the parental material (Pandey & Pandey, 2008). Researchers have demonstrated experiments with plants associated with high levels of Cr. Thus, 1-5 ppm Cr present in the available form in the soil solution, either as Cr (III) of Cr (VI), is the critical level for a number of plant species. Increased Cr (VI) concentration of 10-800 mg/l in culture medium led to the detection of inhibited growth parameters. There was a reduction in growth, dry weight and vigour index in four soybean genotypes of

shoot.


soybean at 5 -200 mgl-1 concentrations of chromium, according to control application (Ganesh et al., 2009).

Table 13. Seed Cd concentrations of soybean varieties grown in pots (choice of High-Cd soil and Mid-Cd soil) and under field conditions (Arao et al., 2003)

Mercury (Hg) and his compounds are among the strongest phytotoxic substances and are also extremely dangerous to human and animals. It is a constituent of many crop protection agents. Non-contaminated soils contain only 0.003 to 0.03 mg Hg/kg. Hg levels of about 0.04 mg/kg in dry matter can be considered normal in plants. Maximum tolerance limit of 0.05 mg/kg in fresh matter is proposed for foodstuffs. Mercury uptake in plants is very slight because it is strongly sorbet in the soil, mainly by complexation with organic matter. Apart from growth inhibition, the symptoms of Hg toxicity include chlorosis, necrotic lesions and death. These are mainly results of severe root damage and the consequent inhibition of nutrient and water uptake. Since little Hg is translocated out of the root, there is a little danger of its entering to the food chain through the soil. The mobility of Hg and its uptake by plants can be greatly reduced by liming (Bergmann, 1992).

Lead (Pb) is major chemical pollutant of the environment, and is highly toxic for man. The major source of Pb pollution arises from petrol combustion. This source accounts for about 80% of the total Pb in the atmosphere. Pb is toxic because it mimics many aspects of the metabolic behavior of Ca and inhibits many enzyme systems. There is evidence that Pb pollution can induce brain damage in man and aggressive behavior in animals. Pb toxicity interferes with Fe metabolism and the formation of haem. The total Pb concentrations of agricultural soils lie between 2 to 200 mg/kg soil. Pb contamination very clearly follows the motorway areas. Vegetation at the side of the road may have levels of 50 mg/kg dry matter but in distance of only 150 m away from the motorway the level is normally about 2 to 3 mg/kg. Contamination occurs only on the outer part of plant seed or leaves and stem, and high proportion can be removed by washing (Mengel and Kirkby, 2001).
