## 3 Results and Discussion
### Trace Element Contents in Brassicaceae Species: Conventional Versus Organic
For this latter element, we found statistically significant differences (\(p<0.05\)) for Ni content in turnip tops (_Brassica rapa_) grown by organic agriculture (0.85 ug/g d.w.) on Farm II and turnip tops grown under conventional agriculture (1.02 ug/g d.w.) on the same Farm, with the lowest concentrations in organic ones (see Table 4). Similarly, [PERSON] et al. [19] have also reported a lower Ni content in organic carrots (0.79 ug/g) than in conventional ones (1.58 ug/g). Other vegetables such as tomato and onion have also shown lower Ni contents when they are grown by organic systems. Conversely, other vegetable foodstuffs such as organic wheat (semolina samples) have shown higher Ni content than conventional wheat. Ni values for conventional turnip tops grown on Farm I were (0.81 ug/g d.w.) and Farm II (1.02 ug/g d.w.) with statistically significant differences (\(p<0.05\)) between them. Trace element content in organic _Sinapis alba_ was below the quantification limit (LOQ < 0.18 ug/g) whereas Ni values for _Eruca vesicularia_ ranged between 1.76-3.99 ug/g d.w without significant differences between conventional and organic system. These results are in agreement with those found in a previous study [3] for this latter _Brassica_ speci \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Cd (\(\mu\)g/g)** & **Ni (\(\mu\)g/g)** & **Pb (\(\mu\)g/g)** \\ \hline \hline
**Conventional** & & & \\ \hline
**Farm I** & & & \\ \hline _Brassica rapa_ (turnip greens) & 0.28 \(\pm\) 0.16 & 1.56 \(\pm\) 1.11 & 0.99 \(\pm\) 0.80 \\ _Brassica rapa_ (turnip tops) & 0.12 \(\pm\) 0.03 & 0.81 \(\pm\) 0.32 & 0.90 \(\pm\) 0.88 \\ _Eruca vesicularia_ & 0.72 \(\pm\) 0.10 & 3.99 \(\pm\) 1.47 & 3.40 \(\pm\) 0.61 \\ \hline
**Farm II** & & & \\ _Brassica rapa_ (turnip greens) & 0.19 \(\pm\) 0.06 & 1.39 \(\pm\) 0.91 & 0.62 \(\pm\) 0.37 \\ _Brassica rapa_ (turnip tops) & 0.13 \(\pm\) 0.03 & 1.02 \(\pm\) 0.21 & \textless{LOQ} \\ _Eruca vesicularia_ & 0.48 \(\pm\) 0.05 & 1.76 \(\pm\) 0.48 & 0.55 \(\pm\) 0.07 \\ _Sinapis alba_ & 0.23 \(\pm\) 0.01 & 0.21 \(\pm\) 0.03 & 0.16 \(\pm\) 0.02 \\ \hline
**Organic** & & & \\ \hline _Brassica rapa_ (turnip greens) & 0.18 \(\pm\) 0.05 & 1.02 \(\pm\) 0.46 & 0.33 \(\pm\) 0.23 \\ _Brassica rapa_ (turnip tops) & 0.12 \(\pm\) 0.05 & 0.85 \(\pm\) 0.22 & \textless{LOQ} \\ _Eruca vesicularia_ & 0.54 \(\pm\) 0.04 & 2.55 \(\pm\) 0.71 & 0.91 \(\pm\) 0.39 \\ _Sinapis alba_ & 0.07 \(\pm\) 0.01 & \textless{LOQ} & 0.45 \(\pm\) 0.31 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Total heavy metals concentration (dry matter) in _Brassicaaceae_ species analyzed (mean \(\pm\) standard deviation). Regarding Pb, there were statistically significant differences (\(p<0.05\)) between organic turnip greens (_Brassica rapa_) (0.33 \(\mathrm{\SIUnitSymbolMicro g}\)/g d.w.) and conventional ones (0.62 \(\mathrm{\SIUnitSymbolMicro g}\)/g d.w.) from Farm II (Table 4). Nevertheless, for the rest of the _Brassicaceae_ studied, the differences between conventional and organic cultivation were scant. The influence of the soil in which the plants are grown has also been demonstrated. Thus, we found statistically significant differences (\(p<0.01\)) for _Eruca vesiculara_ between conventional plants grown on Farm I (3.40 \(\mathrm{\SIUnitSymbolMicro g}\)/g d.w.) and plants grown on Farm II (0.55 \(\mathrm{\SIUnitSymbolMicro g}\)/g d.w.). Furthermore, while for organic and conventional turnip tops grown on Farm II, Pb concentrations were below the quantification limit (LOQ \(<0.160\)\(\mathrm{\SIUnitSymbolMicro g}\)/g), concentrations in conventional turnip tops grown on Farm I reached mean values of 0.90 \(\mathrm{\SIUnitSymbolMicro g}\)/g d.w. There are no conclusive results regarding a lower Pb content in vegetables grown through organic agriculture. Thus, [PERSON] et al. [18] found lower Pb concentrations in organic tomato, lettuce, onion and carrot but not in potato. [PERSON] et al.