**5. Conclusions**

Under our experimental conditions, the presence of *n*CeO2—even at high concentrations—did not cause negative e ffects on *H. lanatus*, *L. flos-cuculi* and *D. tenuifolia*. On the contrary, *n*CeO2 had a stimulating e ffect in the early stages of development of the plants. The plants' response with respect to the di fferent *n*CeO2 sizes has not been very evident. This aspect requires important insights that must take into account the aggregation/dissolution dynamics of *n*CeO2 and the forms of Ce taken-up by plants, as well as the fate of the *n*CeO2 assimilated by plants. Our results are quite in accordance with the literature, in which—it must be remembered—there are still rather conflicting results, obtained to a very large extent by observing crop species.

Finally, the knowledge of the e ffects of the exposure of plants to ENM is limited. It is acknowledged that the flux of ENM in the ecosystem involves the primary producers. That is the reason why it is important to focus the research on this field as well as to develop new methods of investigation suitable to non-food species.

As previously reported, the number of works dedicated to the study of the e ffects resulting from the exposure of spontaneous terrestrial species to ENMs is very low. This constitutes "per se" the major novelty element of this paper. Our observations were made during the early stages of vegetative development. It will be necessary to extend the study to evaluate the e ffects of the treatments over the whole plant life-cycle. It will be equally important to compare the responses of plants with respect to single and repeated treatments over time.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/10/12/2534/s1, Table S1: Theoretical ratio calculated at a hypothetical equal mass of nanoparticles uptaken by plants exposed to nCeO2 25 nm and 50 nm. The ratio is calculated by dividing the number of *n*CeO2 25 nm by the number of *n*CeO2 50 nm; both numbers are calculated dividing the mass by the estimated the mass of a single nanoparticle (g/g). Table S2: The observed ratio calculated at the measured mean Ce uptake by the plants exposed to the two *n*CeO2 25 nm and 50 nm for the treatments 200 and 2000 ppm and the two species *L. flos-cuculi* and *D. tenuifolia*. The ratio is calculated by dividing the number of *n*CeO2 25 nm by the number of *n*CeO2 50 nm; both numbers are calculated dividing the mass of Ce derived from the measured mean Ce by the estimated the mass of a single nanoparticle (g/g).

**Author Contributions:** Conceptualization, D.L. and L.M.; formal analysis, D.L. and L.M.; funding acquisition, L.M.; investigation, D.L.; methodology, A.M., B.P. and A.A.; supervision, L.M.; writing—original draft, L.M.; writing—review and editing, G.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Regione Friuli Venezia Giulia, General Directorate of Environment, gran<sup>t</sup> "Nanomateriali 2017–20".

**Conflicts of Interest:** The authors declare no conflict of interest.
