**1. Introduction**

Nanoscience and nanotechnology are rapidly evolving in di fferent applications having the potential to revolutionize human life. Considerable headways have been made for applications of engineered nanomaterials (ENMs) and nano-enabled products in medicine, energy, electronics, innovative materials and many others [1].

The flip side of nanotechnology is the release in the environment of tons of ENMs [2]. According to the ENMs flow models, soils and waters are the endpoints of such materials [3,4]. However, we still have patchy knowledge regarding the impacts of these materials on biota [5]. Since plant Kingdom represent about 82% of living organisms mass on Earth [6], and their ecological role is of paramount importance to understand the relationships between plants and ENMs. In particular, studying the behavior and fate of ENMs within plants is of grea<sup>t</sup> significance for exploring (i) ENMs uptake, translocation and storage in plant tissues, (ii) mechanisms of plant toxicity, and (iii) life cycle risk assessment of ENMs and risks of transfer to the trophic chain.

The early experimental demonstration regarding the negative influence of ENMs in higher plants was carried out not many years ago [7]. Subsequent studies reported physiological and morphological anomalies of plants exposed to nanomaterials [8,9]. Conversely, several studies of positive e ffects of ENMs applications to crops were reported. This is why applications of nano-enabled products in crop nutrition and protection are under investigation [10–12]. The first investigations revealed that the relationships between plants and ENMs are very complex. Up to now, the research has been paid almost exclusively to food crops, while the spontaneous plant species have been almost neglected. Although this was largely justified by the potential risks for ENMs human exposure, the potential negative impact of ENMs on primary producers could have very serious consequences on food webs and ecosystem services [13], and therefore, it should not be deemed less significant.

Experiments carried out on crops demonstrated that the chemical and physical properties of ENMs (e.g., size, shape, structure, composition, concentration, and others), the environmental conditions, the plant species and age contribute to determining the e ffects on plants [14,15]. It is not advisable to generalize the results on crops to other plants living on natural ecosystems, neither fertilized nor irrigated, and potentially more exposed to ENMs fluxes having a longer life-cycle than crops.

Studies have been conducted to investigate the flow of nanomaterials into aquatic and terrestrial environments. As regards plants, more aquatic [16–20] and wetland species [21–23] have been studied than terrestrial ones so far. To the best of our knowledge, *Pinus sylvestris* (L.) and *Quercus robur* (L.) are the only terrestrial wild plant species that have been investigated for exposure to silver nanoparticles (*n*Ag) and cerium oxide nanoparticles (*n*CeO2) so far [24].

Investigations on the e ffects of metal nanoparticles (MeNPs) on plant physiology are based on the assumption that nanomaterials can be absorbed by plants and that the former can subsequently move within the plant tissues while maintaining the nanoform, or that they can release elements in ionic form. Hence, the experiments in this field must be designed in order to verify whether the nanomaterials are taken up by the plant roots or internalized through other pathways such as stomata, leaf cuticle/epidermis, and hydathodes [25].

Given the estimated global production of 100–1000 tons per year, *n*CeO2 is among the most widely utilized metal oxide nanoparticle in Europe [26]. For this reason, the Organization for Economic Cooperation and Development (OECD) included *n*CeO2 among the nanoparticles to be studied and analyzed for the risk assessment [27]. *n*CeO2 could cause several e ffects on the plant system depending on *n*CeO2 particle size, treatment concentration and plant species. Literature reports contradictory results. Positive e ffects in terms of germination, biomass yield, photosynthesis and nutritional status have been observed on several species [28]. Other papers report a reduction of germination rates, reduction or inhibition of root growth, restrictions of biomass growth, and crop yield [29–31].

In this study, we evaluated the influence of *n*CeO2 having different concentrations and two particle sizes on the germination and root elongation in seedlings of the spontaneous monocot *Holcus lanatus* (L.), and the dicots *Lychnis flos-cuculi* (L.) and *Diplotaxis tenuifolia* (L.) DC. The plant species have been chosen since they are common and widespread in natural systems, highly competitive and easily adaptable to di fferent ecological conditions. *Holcus lanatus* L. (common velvet grass) is a hairy, tufted, fibrous-rooted and meadow soft perennial grass, growing between 50 and 100 cm tall, belonging to the *Poaceae* family. It has a wide climatic range and occurs over a wide range of soil types and fertility conditions [32]. *Lychnis flos–cuculi* L. (ragged-robin) is an herbaceous perennial plant belonging to the *Caryophyllaceae* family, and it is native and distributed throughout Europe [33]. It is found in open habitats, along roads and in wet meadows and pastures. Finally, *Diplotaxis tenuifolia* L. DC. (perennial wall rocket) is a perennial flowering herbaceous Mediterranean species, but it is native to Europe and Western Asia [34]. It grows in temperate climates and could be found in di fferent habitats, but in particular in ruderal plant associations.
