## 3 Results
### The Accumulation and Translocation of Pb, Zn, Cd, and Cu in M. cordata  
After the plants were exposed to a range of concentrations of Pb, Zn, Cd, and Cu treatment for 20 days, the contents of Pb, Zn, Cd, and Cu in the roots, stems, and leaves of _M. cordata_ were, respectively, detected (Table 1). With an increase in the treatment concentration of Pb, Zn, Cd, and Cu, the corresponding metal content in the roots, stems, and leaves of _M. cordata_ increased. Overall, the metal content in the roots was higher than that in the stems and leaves of _M. cordata_. We found that the Pb content in the roots of _M. cordata_ under 500 \(\upmu\)mol\(\cdot\)L\({}^{-1}\) Pb was the highest, while the Cu content in the roots under 1000 \(\upmu\)mol\(\cdot\)L\({}^{-1}\) Cu was the highest. The Zn content in the leaves with 100 and 500 \(\upmu\)mol\(\cdot\)L\({}^{-1}\) Zn was the largest, reaching 698 \(\upmu\)g\({}^{-1}\), while the contents of Pb and Cd in leaves were below 50 \(\upmu\)g\({}^{-1}\). The total content of Zn in the leaves under all Zn concentrations was, respectively, 18.4, 12.6, and 4.8 times higher than that of Pb, Cd, and Cu under corresponding treatments; the Pb content in the stems and leaves was the lowest, and the total Pb content in the stems under all Pb concentrations was, respectively, 4%, 3.8%, and 3.3% that of Zn, Cd, and Cu under corresponding treatments. By analyzing the accumulation of Pb, Zn, Cd, and Cu in the roots and aboveground parts (the sum of the stems and leaves) of a single plant, we can better understand the transport mechanism of Pb, Zn, Cd, and Cu in plants. We found that the accumulation of Zn in the shoots of _M. cordata_ under 100 \(\upmu\)mol\(\cdot\)L\({}^{-1}\) Zn was the largest, reaching 1430 \(\upmu\)g per plant, while the accumulation of Zn in the roots was 508 \(\upmu\)g per plant; therefore, the Zn translocation factor was up to 4.4 (Table 2). The accumulation of Pb in the roots far exceeded that in the shoots, especially under 100 \(\upmu\)mol\(\cdot\)L\({}^{-1}\) Pb; the Pb content per plant was the largest, reaching 738 \(\upmu\)g, whereas the concentration of Pb in the shoots was 60 \(\upmu\)g\(\cdot\)plant\({}^{-1}\) and its TF was the lowest, only 0.08 (Table 2). The accumulation of Cu in the roots and shoots of _M. cordata_ was similar to that of Pb (Table 2). Furthermore, the accumulation of Cd in the aboveground of _M. cordata_ under the 100 \(\upmu\)mol\(\cdot\)L\({}^{-1}\) Cd treatment exceeded that in the roots, and TF was 1.92 (Table 2). The total accumulation of Zn in the leaves of each plant under all Zn treatments was, respectively, 16.1, 3.3, and 8.8 times higher than that of Pb, Cd, and Cu under corresponding treatments. Figure 3: Chlorophyll content in leaves of _M. cordata_ under control (CK) and different concentrations of Cd, Pb, Cu, and Zn after 20 days of exposure. Values are means \(\pm\) SEs (\(n=3\)) of three different experiments. Means denoted by different letters refer to the significant differences (\(p<0.05\), Duncan’s test). In order to further understand metal accumulation in situ, we stained the roots and leaves of _M. cordata_ under 100 umol\(\cdot\)L\({}^{-1}\) Pb, Zn, Cd, and Cu treatment for 20 days with dithizone and determined the contents of Pb, Zn, Cd, and Cu in the roots, stems, and leaves under the same conditions. Dithizone can form colored complexes with Pb, Zn, Cd, and Cu; a darker stain was indicative of the presence of more metals in roots or leaves. We found that the root tips of _M. cordata_ exposed to 100 umol\(\cdot\)L\({}^{-1}\) Pb were stained the deepest, considering the colored complexes on the root-tip surfaces (Figure 4a); accordingly, the total metal content in the roots of _M. cordata_ under the Pb treatment was the highest (Figure 4b). The root-tip staining degree of Pb treatment was successively followed by that of Cu, Cd, and Zn, which were also consistent with the amount of the four metals in the root surfaces of _M. cordata_.