*2.2. Experimental Setup*

Seeds of *H. lanatus* and *L. flos–cuculi* were purchased from SemeNostrum (Udine, Italy), while seeds of *D. tenuifolia* were provided by Sementi Bruni (Corbetta, Milan, Italy). The experiment was carried out in controlled conditions. 30 seeds were placed into 15 mm Petri dishes containing filter paper soaked with 10 mL of deionized water (control) and 0.2, 2, 20, 200 and 2000 mg mL−<sup>1</sup> of *n*CeO2 25 nm and *n*CeO2 50 nm suspensions. The suspensions of nanoceria were prepared and sonicated for 10 min to avoid aggregation. The Petri dishes were covered with aluminum paper to avoid light and set at room temperature (25 ◦C). The duration of the experiment was of two weeks. Each treatment was replicated three times. Germination was calculated as the ratio of germinated seeds out of the total seeds in each Petri dish. Seedlings were photographed, and Image J software [35] was used to measure roots length, which was calculated as the average of measures of all roots that emerged from seeds for each treatment.

### *2.3. Ce concentration in Plant Seedlings*

To quantify the total content of Ce inside di fferent plant species, seedlings were washed by agitation with HNO3 0.01 M for 15 min and rinsed with deionized water. The washed seedlings were oven-dried at 60 ◦C for three days, and 0.3 g of tissues were digested on a microwave oven (MARS Xpress, CEM, Matthews, NC, USA), using 9 mL of HNO3 and 1 mL of hydrogen peroxide (H2O2) in Teflon cylinders at 180 ◦C. Plant extracts were diluted and filtered with Whatman 0.45 μm PTFE membrane filters. During the ICP–MS analysis, yttrium was the internal standard used for the analysis [36].

### *2.4. Internalization of nCeO2 in Plant Tissues*

At the end of the germination experiment, the uptake of *n*CeO2 by plant seedlings was verified by enzymatic digestion. The digesting enzyme used was Macerozyme R−10 enzyme–pectinase from *Rhizopus* sp. (Sigma-Aldrich Co., St. Louis, MO, USA). The extraction of *n*CeO2 from homogenized samples of these species was performed according to Jiménez-Lamana et al. (2016) [37]. In particular, 0.03 g of fresh plant samples were harvested, rinsed with deionized water and homogenized with 8 mL of 2 mM citrate bu ffer at pH 4.5, using an ultrasonic bath for 5 min. After the homogenization, 2 mL of the enzyme solution (0.05 g of enzyme powder for roots, shoots, leaves and seedlings, dissolved in 2 mL of MilliQ water) was added. The samples were shaken in a water bath at 37 ◦C for 24 h, and the obtained suspensions were filtered with a 0.45 μm cellulose filters to remove the solid parts of seedlings. The final supernatants were appropriately diluted and analyzed using the single particle inductively coupled plasma mass spectrometer (sp-ICP-MS) NexION 350 (PerkinElmer Waltham, MA, USA).
