## 4 Discussion  
The tolerance index is determined by the plant growth parameters, including the root length, shoot length, root biomass, and shoot biomass; in particular, root elongation is the most sensitive index of plants to heavy metal stress [28]. In present studies, the  
Figure 6: Content ratios of Cd (**a**), Pb (**b**), Zn (**c**), and Cu (**d**) in roots, stems, and leaves of _M. cordata_ under compound treatments of 100 Cd, 500 Pb, 100 Cu, and 500 Zn. PZCC means the mixed treatment of 500 Pb, 500 Zn, 100 Cd, and 100 Cu. Values are means \(\pm\) SEs (\(n=3\)) of three different experiments. If the mean is greater than 1, it means upregulation by compound treatment; otherwise, it means downregulation. Means denoted by different letters refer to the significant differences (\(p<0.05\), Duncan’s test). Pb and Zn treatments little inhibited root elongation under 100 and 500 \(\upmu\)mol\(\cdot\)L\({}^{-1}\) Pb or 100 \(\upmu\)mol\(\cdot\)L\({}^{-1}\) Zn treatment, and when using 100 \(\upmu\)mol\(\cdot\)L\({}^{-1}\) Zn, it even increased the root biomass (Figures 1 and 2a). The shoot biomass of _M. cordata_ from all the Pb treatments and the 100 \(\upmu\)mol\(\cdot\)L\({}^{-1}\) Zn treatment did not decrease when compared with the control (Figure 2b). All the Cd and Cu treatments decreased root elongation, as well as the root and shoot dry weights (Figures 1 and 2), which shows that _M. cordata_ has more tolerance to Pb and Zn than to Cd and Cu treatments. Metal-induced changes in chlorophyll contents in the leaves of the plant were similar to the result of the shoot biomass of _M. cordata_ (Figure 3). The chlorophyll content can affect the plant's photosynthetic yield and biomass. The greater the biomass of the plant, the greater the capacity of the plant to stabilize and accumulate heavy metals. From this, it can be deduced that _M. cordata_ is more suitable as a phytoremediator of the soils contaminated with Pb and Zn. After being exposed to a range of concentrations of Pb, Zn, Cd, and Cu for 20 days, _M. cordata_ accumulated the highest concentrations of Zn and Cd, respectively, in the leaves and stems and accumulated high contents of Pb and Cu in the roots (Table 1). As shown in Table 2,the amount of heavy metal accumulated per plant was in the order of Zn > Pb > Cd > Cu, and per shoot was in the order of Zn > Cd > Cu > Pb. TF was used to evaluate the plants' phytoremediation efficiency; the TF of Zn in _M. cordata_ exceeded 4, and the maximum Zn content in the shoots exceeded 1 mg for a single plant in the present study (Table 2). The maximum Cd concentration of the shoots (1133.3 mg\(\cdot\)kg\({}^{-1}\)) in the present study exceeded the criterion necessary for a Cd hyperaccumulator, which was also higher than that reported (91.93 mg\(\cdot\)kg\({}^{-1}\)) in the stems of the _M. cordata_ cultivated in the soil by [PERSON] et al. [20]. Even the Cd accumulations in the roots and shoots were more than those observed in the Cd hyperaccumulator _Solanum nigrum_ under the same treatment conditions [29]. However, _M. cordata_ is still not a hyperaccumulator of Pb, Zn, Cd, and Cu, according to the international standards of hyperaccumulators [2]. The results showed that _M. cordata_ is a suitable plant for the phytoremediation of multiple metals, especially in the lead-zinc-contaminated soil. [PERSON] et al. [17] found that the TF of Pb in _M. cordata_ treated with the soils from lead-zinc tailing was 0.5, which is higher than that of the present treatment (0.08-0.31), and the TF of Zn in the _M. cordata_ treated with lead-zinc tailing was about 1.3, which is lower than that of the present treatment (2.84-4.44). This suggests that the solution culture increased the Zn transport but decreased the Pb transport in _M. cordata_ from the roots to the shoots of the plants. In order to investigate the distribution of metals in the roots and leaves of _M. cordata_, we analyzed the accumulation of metals in situ in the roots and leaves of _M.