## 3 Results and Discussion
### TE Transfer in the Aerial Plant Parts of M. sylvestris  
Cd, Cu, Pb, and Zn concentrations were evaluated in the three plant compartments (flower, stem, and leaf) composing the shoots of _M. sylvestris_ and are reported in Table 3. This allowed the assessment of several biomass valorization options for these plant parts by taking into account their TE concentrations (Section 3.5). For Cd and Zn, the laboratory measurements showed equal concentrations in the flowers and the stems (\(p>0.05\), Table 3). For these two TE, the concentrations measured in the leaves were above those contained in the flowers and the stems (\(p<0.05\)). These differences could be explained by the role of the TE, essential versus not essential, and by \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline  & & **Cd** & & & **Cu** & & & **Pb** & & **Zn** & \\  & **Flower** & **Stem** & **Leaf** & **Flower** & **Stem** & **Leaf** & **Flower** & **Stem** & **Leaf** & **Flower** & **Stem** & **Leaf** \\ \hline Area & 0.94 & 1.08 & 2.15 & 14.81 & 8.68 & 13.18 & 9.54 & 1.52 & 5.67 & 186.04 & 188.35 & 465.33 \\
1 & (\(\pm\)0.21) & (\(\pm\)0.22) & (\(\pm\)0.41) & (\(\pm\)4.61) & (\(\pm\)1.61) & (\(\pm\)2.85) & (\(\pm\)5.54) & (\(\pm\)1.13) & (\(\pm\)4.33) & (\(\pm\)4.33) & (\(\pm\)4.12) & (\(\pm\)4.91) & (\(\pm\)3.71) & (\(\pm\)5.96) \\ Area & 0.93 & 1.08 & 2.07 & 11.65 & 7.73 & 11.11 & 9.88 & 1.28 & 7.02 & 175.03 & 175.32 & 453.79 \\
2 & (\(\pm\)0.2) & (\(\pm\)0.25) & (\(\pm\)0.24) & (\(\pm\)0.74) & (\(\pm\)0.87) & (\(\pm\)1.50) & (\(\pm\)2.72) & (\(\pm\)4.63) & (\(\pm\)0.34) & (\(\pm\)2.79) & (\(\pm\)4.05) & (\(\pm\)2.97) & (\(\pm\)4.92) & (\(\pm\)49.69) \\ Area & 0.95 & 1.1 & 2.18 & 12.79 & 8.05 & 12.05 & 11.28 & 1.36 & 7.3 & 201.85 & 27.09 & 567.72 \\
3 & (\(\pm\)0.16)\({}^{\rm A}\) & (\(\pm\)0.28)\({}^{\rm A}\) & (\(\pm\)0.45) & (\(\pm\)1.15)\({}^{\rm B}\) & (\(\pm\)4.23) & (\(\pm\)3.38) & (\(\pm\)10.79) & (\(\pm\)0.38) & (\(\pm\)5.18) & (\(\pm\)5.18) & (\(\pm\)6.76) & (\(\pm\)5.44) & (\(\pm\)55.73) \\ \hline \end{tabular}
\end{table} 
Table 3: Concentrations of Zn, Cd, Pb, and Cu (\(\upmu\)g g\({}^{-1}\) of DW) in the flower, stem, and leaf parts of plants (\(n=10\)) collected from the three areas (mean + SD). For each TE and each area, the concentrations between the three parts were compared. Significant differences are indicated by different letters (lowercase, uppercase), numbers (normal, italics) at the level of \(\alpha=0.05\) (Tukey HSD test). the physiological role played by the plant parts. The leaf is the part where photosynthesis takes place, whereas the flower is a reproductive organ that allows seed production. The stem permits the transport of nutriments and TE to all plant parts. Zn is essential because it is involved in several plant metabolisms and functions (e.g., photosynthesis, mitochondrial function) and composes different plant proteins [43; 44]. Nevertheless, at concentrations ranging between 100 and 500 \(\mu\)g g\({}^{-1}\) DW, Zn could be toxic for most of the plants [43]. As our Zn concentrations are in this range (Table 3), our results suggest the Zn tolerance of _M. sylvestris_. In agreement with this hypothesis, the Zn concentrations in the flowers (Table 3) are four times more than the ones measured in the flowers of the control plants (Table 4). On the contrary, as Cd is both not essential and phytotoxic from 10 \(\mu\)g g\({}^{-1}\) DW [43; 45], the plant might protect itself by limiting its accumulation in the flower and by sequestering it in the vacuoles as a main detoxification mechanism [43; 44; 45]. Although the Cd concentration in the flowers of the control plants were two times lower (Table 4) than those measured in the flowers from the contaminated areas (Table 3), our results support this hypothesis. As for the Cd concentrations measured in the three parts, the calculated Cd shoots concentrations of around 1.6 \(\mu\)g g\({}^{-1}\) of DW (Table 4, \(p>0.05\)) were not affected by the area, whereas based on the extractable Cd results, a difference could be expected (Figure 2).