| Trace level concentrations of heavy metals are found everywhere on earth. However activities such as mining and localized intensive agriculture, have contributed to an undesirable accumulation of toxic metals in many areas worldwide. Current technologies used in the cleaning process of contaminated sites are expensive and frequently not environmentally friendly. Phytoremediation, the use of plants to remove, stabilize, or degrade soil contaminants, is a promising remediation technology, which has several advantages over traditional clean up methodologies. Corn and soybean, planted world-widely, are not only the main sources of food, but also the ideal materials for the production of biomass fuels. And biomass fuels are always the main kinds of energy resources except for coal in China. This investigation demonstrated the capabilities of corn (Zea mays L.) (Shendan No. 2) and soybean (Glycine max L.) to uptake and accumulate cadmium (Cd), zinc (Zn) and lead (Pb) under hydroponic conditions, examined Zn accumulation and competitive ability in corn and soybean fed with different Zn salts. Chelate-enhanced phytoextraction of Cu, Zn, Pb and Cd from a multiply contaminated soil by soybean was also studied.Three studies were done to determine the capabilities of corn and soybean to uptake and accumulate Cd, Zn and Pb under hydroponic conditions.In the study of determining the growth responses, cadmium uptake and accumulation, and the changes of physiological traits (chlorophyll and proline) in corn under eight cadmium supply levels (control, 12.5, 25, 50, 100, 200, 400, 800μmol Cd 1-1), the maximum Cd concentrations in the shoots and roots were 389.5 and 505.5 mg kg-1 (DW), respectively, when the plants were grown at 50μmol Cd 1-1 for 21 days, and the maximum amounts of Cd taken up by the shoots and roots were as high as 105.2 and 126.4μg pot-1, respectively. Shendan No. 2 can be considered as a Cd-hyperaccumulator, according to the current accepted shoot concentration that defines hyperaccumulation as 0.01% (w/w) for cadmium. At the treatment of 800μmol 1-1, the concentration values of chlorophyll a, chlorophyll b and chlorophyll a+b in the leaves were 33.03, 50.67 and 83.70 mg 100g-1 FW, which decreased to 38.9%, 46.0% and 42.9% of the control plants, respectively. Proline concentrations of corn increased when the plants were grown under the external Cd influence.In the study of determining the growth responses, uptake and accumulation of Cd and Zn by corn, and the interactions between Cd and Zn at Cd/Zn combining supply levels in hydroponic experiment, with Cd concentrations ranging from 0 to 400μmol 1-1 and two Zn concentration levels (100 and 1000μmol 1-1), the dry matter yields of shoots and roots decreased slightly with increasing Cd supply at each Zn supply level. The maximum Cd concentration in the shoots and roots was 454 and 775 mg kg-1, respectively, both at the Cd/Zn complex level of 400/100μmol 1-1. At the Zn level of 1000μmol 1-1, shoot and root Zn concentration was reduced by 66.9% and 84.8% when Cd levels increased from 0 to 400μmol 1-1, respectively. Cadmium had a negative effect on Zn concentration in the shoots and roots of corn when the plants were grown at both low Zn supply level (100μmol 1-1) and high Zn supply level (1000μmol 1-1). Zn concentrations in nutrient solutions had a positive effect on Cd concentrations in corn different parts at relatively low Cd supply levels, however a negative effect of Zn on Cd uptake was noticed at relatively high Cd supply levels, and the influence was not always significant (P<0.05).The next experiment determined the phytoextraction ability of soybean under eight different Cd supply levels, the complex Cd/Zn supply levels and the complex Cd/Pb supply levels. According to the thresholds for plant hyperaccumulation (shoot dry weight) which were set at 10000 mg kg-1 (1%) Zn, 100 mg kg-1 (0.01%) Cd and 1000 mg kg-1 (0.1%) Pb, Glycine max L. was not a hyperaccumulator, but it had a certain ability to phytoextract heavy metals. There existed an antagonistic effect between Cd and Zn, in which increasing Cd or Zn content in nutrient solutions may decrease Zn or Cd concentration in soybean different tissues at both Zn supply levels (100 and 1000μmol 1-1) and relatively high Cd supply levels (>25μmol 1-1). The antagonistic activity was also determined between Cd and Pb at all Cd/Pb complex supply levels.There were another two studies, which examined the heavy metal phytoextraction abilities of corn and soybean under soil conditions.Corn and soybean are compared for growth and Zn accumulation in plant in a pot experiment in seven soil conditions with contrasting Zn availability. Corn had higher Zn concentrations in shoot than soybean in all treatments and the difference varied with Zn availability, being largest with Zn-sulphate (spiked as 50 mg Zn kg-1). In the mixed culture, Zn concentrations in the shoot of corn were significantly higher than that of soybean in all treatments except metallic-Zn (500 mg Zn kg-1) and metallic-Zn (50 mg Zn kg-1), where the reverse were true. Soybean could accumulate much more Zn in shoot when it was cocropped with corn than its pure culture in field conditions.The last experiment studied the effects of ethylenediamine tetraacetic acid (EDTA), triethanolamine (TEA) and citric acid application on metal extractability from a multiply metal-contaminated soil, as well as on their uptake and accumulation by soybean. The results showed that EDTA was more effective than any other chelator at increasing the concentration of Cu in soybean. The application of 5 mmol kg-1 soil EDTA to soil significantly increased concentrations of Cu in shoots and roots. Concentrations of Zn, Pb and Cd in shoots or roots were also higher in the soybeans treated with EDTA than in those treated with TEA or citric acid.
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