*2.5. Ce Concentration in Plant Fractions*

Plant fractions were carefully washed with deionized water. The material was then oven-dried for three days at 60 ◦C. Subsequently, 0.3 g of dry plant fraction tissues were acid-digested on a CEM microwave oven (MARS Xpress, CEM, Matthews, NC, USA), using 9 mL of HNO3 (65%) and 1 mL of hydrogen peroxide (H2O2) in Teflon cylinders at 180 ◦C, according to the USEPA 3052 method [31]. After mineralization, plant extracts were filtered at room temperature under a fume hood with Whatman 0.45 μm PTFE membrane filters, and finally diluted and analyzed. Determinations of the total content of cerium were carried out using the NexION 350 ICP-MS. The accuracy of the analytical procedure adopted for ICP-MS analysis was checked by running standard solutions every 20 samples.

#### *2.6. Data Analysis*

Statistical analysis was carried out with one- and two-way ANOVA. A posteriori comparison of individual means was performed using Tukey's test (*p* < 0.05). Before ANOVA, arcsine and logarithmic transformations were used to determine seed germination percent-

age and Ce concentration data, respectively. spICP-MS data on *n*CeO2 size distribution were processed using Syngistix Nano Application Module software and interpolated with Gaussian curves.

## **3. Results**
