## 3 Results and Discussion
### Trace Elements Concentrations in Plant Species
\begin{table}
\begin{tabular}{l c c c c c c c c c c c c c c c c c} \hline \multirow{2}{*}{**Location**} & \multirow{2}{*}{**pH**} & \multirow{2}{*}{**Organic Matter (\%)**} & \multirow{2}{*}{**C.E.C \(\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{ \boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{ \boldsymbol{ \boldsymbol{ \boldsymbol{ }}}}}}}}}}}}}\) & **C.E \(\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{ \boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{ \boldsymbol{\boldsymbol{ \boldsymbol{ \boldsymbol{ }}}}}}}}}}}}}\) & **C** & **Mn** & **N** & **Cu** & **Za** & **As** & **Cd** & **Pb** & **Sb** & **V** & **Co** \\ \cline{1-1} \cline{3-13}
**LM** & 5.3 & 0.34 & 2.88 & 82/3/15 & 22.6 & 817.3 & 9.7 & 465.5 & 123.6 & 16.6 & 1.81 & 13.9 & 9.6 & 109.8 & 10.8 \\
**LG** & 5.0 & 1.40 & 4.96 & 83/2/15 & 18.2 & 335.6 & 7.2 & 685.9 & 239.7 & 49.0 & 1.78 & 42.0 & 7.0 & 104.1 & 6.4 \\
**VA** & 6.0 & 7.73 & 14.70 & 72/5/23 & 38.7 & 1888.2 & 16.0 & 224.1 & 166.6 & 10.9 & 1.06 & 21.8 & 4.3 & 121.0 & 26.6 \\
**CH** & 7.3 & 5.65 & 6.88 & 67/8/25 & 32.0 & 977.5 & 11.9 & 52.2 & 144.7 & 18.9 & 0.64 & 19.9 & 1.0 & 107.8 & 12.9 \\ \hline \multicolumn{13}{c}{**Reference Soil Quality Standards**} \\ \hline  & & **Canada**[33] & & _87_ & - & 50 & 91 & 360 & - & 22 & 600 & - & 130 & - \\  & & **Australia**[36] & & & - & 60 & 100 & 200 & 20 & 3 & 1500 & - & 50 & - \\  & & **Netherlands**[37] & & & 380 & - & 210 & 190 & 720 & 55 & 12 & 530 & 15 & 0 & 240 \\ \hline \multicolumn{13}{c}{\({}^{\star}\)C.E.C = Cation exchange capacity} \\ \end{tabular}
\end{table} 
Table 3: Physicochemical characteristics and trace element levels of the composite soil samples collected at the sampling locations Los Malitenes (LM), La Genda (LG), Valle Alegre (VA), the and reference sampling point at Cachagua (CH). Reference soil quality standards guidelines are also given. Comparing the trace element concentrations in the different structural parts of OP species, the levels found in the leaves were around 2-3 times lower than those measured in flower and leaves mixtures (Table 2, LG and LG leaves) in the same location, except for some elements. In this sense, Cd, Mn, and Zn concentrations were similar in both kinds of samples, which may suggest that these elements could be distributed more uniformly through the plant when their internal concentration is higher. As a comparison with our study, we can mention the work in a Pb-Zn mining area in Qingchengzi, China [38], where _Oenothera biennis_ could simultaneously exclude the uptake of Cd and Cu, having maximum plant concentration levels of 2.9 (mg kg\({}^{-1}\)) for Cu, 151.8 (mg kg\({}^{-1}\)) for Zn, 0.79 (mg kg\({}^{-1}\)) for Cd, and 42.4 (mg kg\({}^{-1}\)) for Pb. In a more recent study, plants of _Oenothera_ sp. grown in a copper mining tailing area in Camaqua, Brazil [39], showed high ability to uptake Cu and Cr (196.2 and 15.9 mg kg\({}^{-1}\), respectively) and this species was also highlighted for having good potential for phytoextraction of Pb. Concerning the _S. velutina_ (SV) species, critical copper concentrations were also found in this plant in LM, whereas chromium, nickel, and arsenic were found at toxic levels. Meanwhile, _A. subfusiformis_ (AS) was the plant species with the lowest measured concentrations in LM, reaching concentration levels 80% lower (except for Zn) than those found in OP species at the same location. Moreover, this plant species presented higher concentrations for chromium, manganese, and nickel in VA, reaching critical levels for Cr and toxic levels for Ni. These results are probably due to the higher concentration levels of these metals in VA in conjunction with the soil characteristics (Table 3).