## 4 Discussion
A TF value for Pb, Cu and Cd less than unity (Table 4) in the 4 th leaf stage, for both the varieties, suggests that the HMs are not effectively transferred from the roots to the above-ground parts of the plants [50]. The results obtained in this study concur with those of [51] who showed that with a Pb concentration in the range of 9 to 267 uM for _Arabispaniculata Franch_, the TF ratio was less than unity. A TF value greater than one was reported for _Solanumrigrum_ by [51], suggesting that this plant species can easily transfer lead from the soil to airborne organisms. To reiterate, hyper-accumulators are plants which are effective in moving HMs up from roots to the stems and leaves, and thereby have TF ratios greater than unity [52]. In the vegetative stage, as mentioned earlier too, HMs (lead especially) tend to accumulate in the roots, and thus the two varieties of beans considered in this particular study cannot be hyper-accumulators of lead in this stage of growth. However, when it comes to copper, the TF is close to unity, indicating that copper is transferred from the soil to the roots to the aerial parts. This, as mentioned on a few occasions earlier in this article, is because copper is an essential micronutrient for plant growth, especially in the vegetative stage. The TF values for Cd obtained in this study are lower than those for copper in the vegetative stage. [PERSON] et al. [53], however, have reported higher TF ratios for Cd in the vegetative stage. For a plant to be an effective phytoremediating agent, its BAF must be greater than unity [54]. In this study, the BAFs for Pb, Cu and Cd in the vegetative stage of common bean and faba bean were 0.43, 0.81, 0.12, and 0.15, 0.94, 0.14, respectively. One can conclude from these values that _Vicia fuba_ cannot be an effective phytoremediating agent in the vegetative stage [55]. Most HMs are insoluble in the vascular system of plants, and are thereby less mobile. This is owing to their presence as sulphate, phosphate or carbonates, which immobilises them in the apoplastic (extracellular) and symplastic compartments of the vascular system [56]. The WHO proposes limits of 10 mg/kg biomass and 0.3 mg/kg biomass for Pb and Cd, respectively. As can be seen in Table 4, the Pb and Cd concentrations in the roots, stems, leaves and flowers recorded in this study are above this tolerable upper limit prescribed by the WHO [8]. When it comes to copper, its uptake by plants depends on the ability of plants to take it up through the soil-root interface as a divalent cation or chelated copper, and also on the total amount of Cu present in the soil. Faba bean, as this study showed, is a hyper-accumulator of copper. As reported by [PERSON] et al. [57], if there are free metals available for translocation from roots to shoots, it indicates that the sequestration of these metals in root vacuoles is limited. Copper, though necessary at low doses, is often present in soils at concentrations which may be quite toxic to plants growing there. Values of 0.4 to 45.8 mg Cu per kg of soil have been reported for uncontaminated soils, in [58]. Plants which are tolerant to high concentrations of toxic heavy metals are usually capable of compartmentalizing metal ions, sequestering them in vacuoles and thus excluding them from cell-sites where processes such as cell division and respiration occur [58]. [PERSON] et al. [59] reported that the process of translocating HMs in plant species is a crucial factor in determining the distribution of these elements in different plant tissues. The HMs are mobilized and removed by the root cells, bound by the cell wall and then transported through the plasma membrane, driven by proton pumps dependent on ATP. In addition to cationic nutrients, plant transporters also participate in the migration of potentially toxic cations between plant membranes [60]. This study yielded a TF value of less than unity for lead in the full-flowering stage, indicating a propensity for lead to lodge itself in the roots preferentially, vis-a-vis Cu and Cd.