## 1 Introduction  
Heavy metals and metalloids (HMs) such as cadmium (Cd), arsenic (As), chromium (Cr), copper (Cu), mercury (Hg), nickel (Ni), lead (Pb) and zinc (Zn) are dispersed in the environmental media [1; 2; 3], and are persistent pollutants which may be transported from one environmental medium to another, or may be absorbed by the apoplast of plant roots on the pedosphere. They went their way thereto the edible/non-edible parts of the plants, enter the trophic chain, and imperial human health [4; 5]. Human beings are commonly exposed to Pb, Cd, Cu, As and Cr through the ingestion of contaminated food items [6; 7]. For lead, cadmium and copper--the three metals of interest to the authors of this study--the Food and Agricultural Organisation and the World Health Organisation [8] have set the tolerable upper limits in food commodities as 10, 0.3 and 5 ppm respectively. Recently-published papers have documented the occurrence of toxic HMs in both cultivated [9] and non-cultivated soils [10; 11] in many regions of the world. Some heavy metals are essential micronutrients for plant growth (Cu, Fe, Zn etc.) and are indispensable for some of the metabolic processes (enzymatic activity, electron transfer, redox catalysis etc.) associated with plant growth [3; 4]. On the other hand, As, Pb, Cd and Hg are detrimental to plant metabolism and are among the top 20 toxic substances, as per United States Environmental Protection Agency (USEPA) and the Agency for Toxic Substances and Disease Registry (ATSDR) [5; 6]. In the arable pedosphere, the high concentrations of HMs and their subsequent bioavailability can be attributed to anthropogenic agricultural routines, which include the inefficient use of synthetic fertilizers and pesticides [7]. In many arid and semi-arid regions of the world, wastewater (partially treated) is used for irrigation, and this is a conduit for HMs to reach the soil [8]. Sewage sludge with varying concentrations of HMs is considered as a fertilizer in some parts of the world, and while being a source of essential macronutrients, it also introduces HMs to the soil. Nowadays, phytoremediation (cleaning up contaminated soils with the aid of HM-absorbing plants) is a green strategy that comprises phytoextraction, rhizo-filtration, phytodegradation, phytostabilization, and phytovolatilization for HMs in soil-food crop subsystems [9; 10]. Heavy metals are significant environmental pollutants, and their toxicity is a problem of increasing significance for ecological, evolutionary, nutritional and environmental reasons. The environment is defined as the totality of circumstances surrounding an organism or group of organisms, especially the combination of external physical conditions that affect and influence the growth, development and survival of organisms [11]. A pollutant is any substance in the environment which causes objectionable effects, impairing the welfare of the environment, reducing the quality of life and which may eventually cause death. Such a substance has to be present in the environment beyond a set or tolerance limit. Hence, environmental pollution is the presence of a pollutant in the environment in air, water or soil, which may be poisonous or toxic and will cause harm to living things in the polluted environment. The regulatory limit of cadmium (Cd) in agricultural soil is 100 mg/kg soil. Plants grown in soil containing high levels of Cd show visible symptoms of injury reflected in terms of chlorosis, growth inhibition, browning of root tips and finally death [11]. Cd has been shown to interfere with the uptake, transport and use of several elements (Ca, Mg, P and K) and water by plants [12]. Cd also reduced the absorption of nitrate and its transport from roots to shoots, by inhibiting the nitrate reductase activity in the shoots [13]. Copper (Cu) is considered as a micronutrient for plants and plays an important role in CO\({}_{2}\) assimilation and ATP synthesis.