1.1 Background of the Study

Plants require minerals and nutrients to grow. Presently, there is a sudden rise in concerns on the levels of absorption of heavy metals by plants and the level that is retained in the different parts of the plant. The retention of the metals taken up by plants leads to bioaccumulation of these metals in plant. The release of trace metals to the surface of the earth’s crust (land, air, water and biota) is the aftermath effect of human activities which include quarrying, manufacturing, manure or fertilizer, pesticide, atmospheric deposition, metropolitan and industrialized dumpsite (Alloway and Ayres, 1993; Hajar et al., 2014). Several investigations on the toxic potentials of heavy metals give consideration to animals and lower plants such as bacteria, while neglecting their effects on higher plants. The buildup of metals in higher plant gives a very high degree of exposure to human (Oliveira, 2012). Due to the fact that plants through the process of transportation and absorption take up metals from the soil and then transfer these metals to herbivorous animals (Al-Farraj and Al-Wabel, 2007). The aftermath of plant eating animals in some cases have been identified to be the major causes of some diseases namely spongiform encephalopathy or prion disease (Rasaq et al., 2015). Metals whether essential to plant and animal life or not has a limit to which the body will not require it. At very high concentrations, they result in either plant or animal toxicity. The condition of excessive accumulation of heavy metals in plants originated from excess concentrations of those metals in the soil. In such situation, they constitute health risk to both plants and animals in the interdependence chain of consumption (food chain) (Singh et al., 2012). The rate of retention of heavy metals in plant parts varies from one plant to the other. Despite the fact that in most cases these heavy metals do not inhibit the growth of these plants, yet crops reaped from such contaminated soil or lands pose health risks to humans. These risks are as a result of high concentrations of the individual contaminants above the required standard for food (Singh et al., 2015). Plant life grows through the supply of nutrients taken up from the environment (soil, water and air). The uptake of plant nutrients is not selectively done to take up only the required minerals but also other chemical components which may be toxic or poisonous found within the environment (Arora et al., 2008). The soil is the final sink for environmental contaminants and as such accumulates pollutants to levels that may be unacceptable and hazardous to the environment (Boke et al., 2015). These chemicals or metals in particular are dispersed through natural means or human factors or activities. Other forms of soil pollution originate from human exploration and exploitation activities of which the petrochemical industry is a major contributor (Nwineewii and Neeka,2017).