## 2 Materials and Methods  
### Plant Material  
The cultivation of microgreens of three rare oilseed plants--nigella (_Nigella sativa_ L.), safflower (_Carthamus tinctorius_ L.) and amelina (_Camelina sativa_ L.)--was performed in the greenhouse of the Faculty of Biotechnology and Horticulture at the Agricultural University of Krakow. The seeds originated from the horticultural industry. Seeds were worn into seed boxes filled with TS 2 peat substrate, which consisted of PG Mix fertiliser and surfactant (pH = 6). The fertiliser (at a density of 2 g/L) was composed of 16% phosphorus (P\({}_{2}\)O\({}_{5}\)); 14% nitrogen (of which: 5.5% N-NO\({}_{3}\) and 8.5% N-NH\({}_{4}\)); 0.8% magnesium (MgO); 19% sulphur (SO\({}_{3}\)); 0.03% boron (B); 0.12% copper (Cu); 0.09% iron (Fe); 0.16% manganese (Mn); 0.20% molybdenum (Mo); 18% potassium (K\({}_{2}\)O); and 0.04% zinc (Zn). The temperature conditions in the greenhouse during their growth were controlled: the day temperature oscillated around 21 \({}^{\circ}\)C, and the night temperature was 18 \({}^{\circ}\)C. Plants were watered daily to keep the moisture content of the substrate at the level of household water. Tap water was used, with the following parameters (Table 1).  
\begin{table}
\begin{tabular}{c c c} \hline General hardness & mg/dm\({}^{3}\) & 301 \\ \hline Sodium & mg/dm\({}^{3}\) & 21 \\ \hline Ammonium ion & mg/dm\({}^{3}\) & 0.021 \\ \hline Magnesium & mg/dm\({}^{3}\) & 11.6 \\ \hline Calcium & mg/dm\({}^{3}\) & 101 \\ \hline Fluorides & mg/dm\({}^{3}\) & 0.08 \\ \hline Chlorides & mg/dm\({}^{3}\) & 44.3 \\ \hline Nitrites & mg/dm\({}^{3}\) & \textless{}0.01 \\ \hline Nitrates & mg/dm\({}^{3}\) & 16.3 \\ \hline Sulfphates (A) & mg/dm\({}^{3}\) & 48 \\ \hline General iron (A) & mg/dm\({}^{3}\) & \textless{}0.01 \\ \hline Manganese & mg/dm\({}^{3}\) & \textless{}0.005 \\ \hline Total chromium & mg/dm\({}^{3}\) & \textless{}0.001 \\ \hline Cadmium & mg/dm\({}^{3}\) & \textless{}0.001 \\ \hline Copper & mg/dm\({}^{3}\) & \textless{}0.01 \\ \hline Nickel & mg/dm\({}^{3}\) & \textless{}0.001 \\ \hline \end{tabular}
\end{table}
Table 1: Selected quality indicators of the tap water used.  
Plants were observed daily and, apart from watering, did not require any maintenance. Microgreens were harvested twice; at the first harvest, the plants had cotyledons and their first true leaf, and at the second harvest the plants had 1-2 true leaves. According to the BBCH (Biologicalesnatsalt, Bundessortenamt, und Chemische Industrie) scale [16], the first harvest was performed at BBCH 11, and the second one at BBCH 12. Camelina and sawflower developed similarly and reached the BBCH 11 stage at the same time, i.e., 11 days after sowing. Nigella grew more slowly, reaching its development stages about 12 days later, which means 24 days after sowing. The second harvest was performed at BBCH 12, it was 18 days after sowing for safflower and camelina, and 30 days for nigella. The Supplementary Materials include photographs of the microgreens.  
During harvest, the above-ground part of the microgreens was cut with sterile scissors. The raw material was rinsed, drained using filter paper and frozen at \(-22\)\({}^{\circ}\)C and finally freeze-dried (Christ Alpha 1-4, Osterode am Harz, Germany). The material obtained was then milled (Knifetec 1095 Sample Mill, Tecator, Hoganas, Sweden) using special titanium-nib-coated knives for grinding without heavy metal contamination. Finally, a homogeneous sample was obtained with small-in-diameter particles. Afterwards, the freeze-dried material was examined for selected heavy metals, as well as its nitrate and nitrite content. Analyses were conducted in triplicate.