Effect of different growing substrates on heavy metals accumulation in Pinto bean (Phaseolus vulgaris)

Introduction Contamination of soils with heavy metals is often resulted from human activities and phytoremediation is an effective and economic strategy to remove toxic metals from soils. Heavy metals are significant environmental pollutants, and their toxicity is a problem of increasing significance for ecological, evolutionary, nutritional and environmental reasons. The term ‘‘heavy metals’’ refers to any metallic element that has a relatively high density greater than 4 g/cm3, or 5 times or more, greater than water and is toxic or poisonous even at low concentration. However, chemical properties of the heavy metals are the most influencing factors compared to their density. Heavy metals include lead (Pb), cadmium (Cd), nickel (Ni), cobalt (Co), iron (Fe), zinc (Zn), chromium (Cr), iron (Fe), arsenic (As), silver (Ag) and the platinum group elements. Plants experience oxidative stress upon exposure to heavy metals that leads to cellular damage. In addition, plants accumulate metal ions that disturb cellular ionic homeostasis. To minimize the detrimental effects of heavy metal exposure and their accumulation, plants have evolved detoxification mechanisms. Such mechanisms are mainly based on chelation and subcellular compartmentalization. Chelation of heavy metals is a ubiquitous detoxification strategy described in wide variety of plants. The aim of this study was to investigate the effect of different compounds to reduce the toxicity and accumulation of heavy metals in the planting bed is pinto beans.   Materials & Methods In order to evaluate the effects of different growing substrates on reducing toxicity and phytoremediation bean (cv. Sadri) under heavy metals, a greenhouse experiment was conducted as a factorial for four heavy metals (Cd(NO3)2, Pb(NO3)2, Ni(NO3)2 and CuSO4) seperately, based on CRD design with three replications in Yasouj University, 2013. The first factor included of four levels of different growing substrates (control, compost, vermicompost and Populus sawdust) and the second factor included of two levels of heavy metals (heavy metals with 50 mg kg-1 soil concentration and control).   Results & Discussion Analysis of variance showed that the effect of organic compounds and cadmium nitrate in the soil and their interactions on amounts of cadmium accumulation in roots, shoot and grain was statistically significant. Compost and vermicompost significantly increased cadmium. Vermicompost can be used to remove metals from contaminated soils used because it is linked with metals and increases nutrient uptake by providing. A cadmium concentration in shoot tissue was more than adequate at all levels of the leveas (0.2-0.05 mg/kg dry weight). Lead levels in the tissues of roots, shoots and seeds in all treatments were the detection limit. It seems that due to the low accumulation of lead in beans can be attributed to the low mobility of lead in soil and plant. Means comparison showed the highest accumulation of nickel in the root of the control (normal soil) with an average of 14.45 mg/kg dry weight of roots and the lowest value of this attribute in use of poplar sawdust with an average of 11.42 mg/kg root dry weight that using compost and vermicompost was not significantly different. Analysis of variance showed that the effect of organic compounds in soil, copper sulfate and their interactions on the amount of copper metal roots was significant. When the low copper concentration in soil, compost and vermicompost with the stabilization of copper available and disabling decrease copper absorption and accumulation by the plants, but the high concentration of copper in the soil, add compost, vermicompost and poplar sawdust will not be enough to stabilize all accessible copper. In this study, the use of poplar sawdust to accumulate the highest amount of copper in copper sulphate roots in levels zero and 50 mg per kg dry weight soil.   Conclusion The results showed that the use of compost and vermicompost increased amounts of cadmium accumulation in shoot and root. On the other side, compost and Populus sawdust decreases the amount of nickel in the bean shoot and root. Also, the highest accumulation of copper in roots of beans was achived by application of the Populus sawdust and compost. Accumulation of heavy metals in roots far more than shoots and seeds. In general, the results of this study showed that due to low accumulation of heavy metals in the seed of bean and high absorption by the root and shoot, this plant is suitable for cultivation in contaminated areas and if possible leaving the roots and shoots is also appropriate for the purification.