## 4 Discussion  
In this experimental analysis, the authors focused on two varieties of bean in Tunisia--the common bean and the faba bean, and embarked on investigating the effect of three HMs--Pb, Cu and Cd--on a range of plant-growth aspects, at different stages of plant growth. The addition of HMs to the soil had a detrimental effect on plant growth, with the RFW and SFW being lower vis-a-vis the uncontaminated control samples, in accordance with the results published in [34] for the effects of toxic metals on Arabidopsis. [PERSON] et al. (2016) have noted that the root growth of common bean significantly decreased (\(p<0.05\)) with the increase in soil Pb concentration. In this study, the authors considered two other HMs--Cd and Cu--and showed that the adverse growth-inhibitory effects varied among the HMs [34] (Figures 1 and 2). Cadmium turned out to have the most negative effect and copper the least. The negative effect could be due to the suppression of theelongation growth rate of cells, resulting in an irreversible inhibition exerted by Cd on the proton pump responsible for the process [35]. Additionally, the exposure of the two varieties of beans to these three HMs resulted in a reduction in chlorophyll and carotenoid contents (Table 2). [PERSON] et al. [34] have observed that abiotic stresses--which includes inhibition by toxic metals--on plants tend to affect their chlorophyll biosynthesis capability. Cd and Pb tend to inhibit enzymatic activity of 5-aminolevulinic acid dehydratase and protochlorophyllic reductase associated with chlorophyll biosynthesis. Copper, being an essential micronutrient, was found to be less toxic than the other two HMs, as expected [36]. If one would compare common bean with faba bean, the former was affected more than the latter [37]. However, [PERSON] et al. [38] have reported that HMs can either enhance or decrease carotenoid production, and that would depend on the type of HM. Carotenoids protect chlorophyll pigments under heavy-metal-induced stress conditions [39; 40], and thus if their biosynthesis is disturbed, it would directly hurt the chlorophyll content in the leaves of plants [41; 42]. Bioaccumulation and translocation of the HMs were also studied in this experiment. In addition to the quantification of the accumulation of these HMs on roots, shoots, flowers and pods, the BAFs and TFs were also calculated. Roots accumulate a lot of HMs (in this case, Cd and Pb especially), when the plants are in the vegetative stages of growth (4 th mature leaf stage) [43]. While the point of entry for the HMs--from the soil to the roots--is the apoplast of the root-cortex and the endoderm [44; 45], the upward transport from thereto the stem, branches, leaves, flowers and fruits is termed as translocation. This may or may not require complexing agents such as organic acids, amino acids and peptides [46]. [PERSON] et al. [47] are of the view that metallic trace elements often preferentially accumulate in roots (as Pb does in the case of this study). The low TF values for lead are corroborated by what has been reported by [48; 49]. Lead is tightly retained by the root tissues. Faba bean, which has shown higher TF values in general for all the three HMs studied, can be posited as an effective phytoextracting agent in rhizofiltration facilities [8]. However, post-extraction, they will not be available as food commodities. While HMs prefer in general to make the roots their permanent abodes', [PERSON] et al. [50] have reported that they prefer the leaves next and the stem thereafter. The variation of the HM concentrations for common bean and faba bean at the 4 th leaf stage can be explained by the sorption phenomenon exhibited by the HMs in the substrate. The exchanges between the liquid and solid phases of the soil depend on the sorption mechanism referred to. After being adsorbed on the surface of soil particles, the HMs can be absorbed by the mineral particles.