## 3 Results and Discussion
However, considering that PGO had a sub-neutral pH (Table 1) and that the pH-neutral FLG and muscovite mica caused comparable results [19] to those of GO, the aforementioned effects might derive from a different interaction mechanism. Coating flowers of _Pistacia vera_ L. with dust before pollination [35] decreased the production of fruits. The authors hypothesized that the dust at the interface between pollen and stigma impaired the chemical signaling occurring between them, thus lowering the number of pollen grains able to germinate and to complete fertilization. This suggests that fewer pollen grains in contact with the stigma and a decreased pollen germination rate due to low pollen density over the stigmatic surface, i.e., a \"pollen population effect\" [36], are the cause of a reduced fertilization success. Accordingly, experiments on pollen loading and dilution of pollen [37] showed that lowering the density of pollen grains over the stigma decreased the percentage of germinated pollen grains and, consequently, the development of fruits and seeds [38]. Figure 1: SEM micrographs of stigmas of _Cucurbita pepo_ L. coated with 0 (CTRL) (**a,b**) or 1 mg of graphene oxide (GO) (**c**) and purified GO (PGO) (**d**), and pollinated after three hours (for more details, see Sections 2.2 and 2.3). Stigmatic papillae, pollen grains and GO flakes/nanoparticles are indicated with arrows, asterisks, and arrowheads, respectively. Bars = 50 \(\upmu\)m.  
The presence of reactive substances on the stigmatic surface can impair not only the quantity but also the quality of fruits. For _P. vera_, it was shown that the coating of flowers with dust reduces fruit size and increases particnocrapy (i.e., production of fruits without fertilization of ovules) [35]. In this study, CTRL fruits had a mass of 437.1 \(\pm\) 168.8 g, a length of 21.3 \(\pm\) 3.1 cm, a circumference of 26.1 \(\pm\) 5.7 cm, and bore 37 \(\pm\) 25 seeds per fruit. Differently, GO fruits developed defectively (Figure 4c,d), with significantly lower fresh mass, length, and maximum circumference than CTRL fruits (Table S2). Furthermore, ovules that did not develop seeds, dead tissues, and cavities in the upper part of the ovary were also observed (Figure 4c,d, asterisks). This could be caused by the highly reactive residues (such as H\({}_{2}\)SO\({}_{4}\), H\({}_{2}\)O\({}_{2}\) or KMnO\({}_{4}\)) of the GO production process. Once in contact with the stigmatic surface, they might dissolve in the intercellular spaces and injure the underneath pericarp tissues. A similar effect was observed as a consequence of the depositions of active molecules and materials, such as herbicides [35], fungicides [39], pesticides [26], or cement-skin dusts [40], which can dissolve and release active compounds causing necrosis and blocking seed and, subsequently, fruit development. In the case of PGO, fruits never reached maturation (Figure 4e,f), because ovule foundation did not take place. This is in accordance with the hypothesis that a physical interposition of planar NMs  
Figure 3: Fluorescence micrographs of stigmas of _Cucurhita pepo_ L. coated with 0 (CTRL) (**a**) or 1 mg of graphene oxide (GO) (**b**) and purified GO (PGO) (**c**), and pollinated after three hours (for more details, see Sections 2.2 and 2.4). Pollen tubes, pollen grains and GO flakes/nanoparticles are indicated with arrows, arrowheads, and asterisks, respectively. Bars = 100 \(\upmu\)m.  
Figure 2: Pollen adhesion (**a**) and pollen germination (**b**) observed on _Cucurbita pepo_ L. stigmas coated with 0 (CTRL) or 1 mg of graphene oxide (GO) and purified GO (PGO), and pollinated after three hours (for more details, see Sections 2.2 and 2.4). Values are means \(\pm\) s.d. (n = 6 stigmas). Statistically different groups are marked with different letters (GLM analysis, followed by Bonferroni \(t\)-test for more details, see Table S1).