## 3 Results and Discussion  
The deposition of xenobiotics onto the stigmatic surface may have a negative effect on stigma-pollen interactions resulting in an impaired fruit and/or seed production [25; 26; 27]. For example, fungicides decrease pollen germination but do not alter fruit production in blueberry [25], apple [28] and grape, whereas they do affect seed production in grape [29]. Differently, pesticides lower pollen germination but not seed production in onion [27]. The GO tested in this study is a commercial product available in the market with dimensions in between those of PM 10 and of suspended powders with dimensions up to 30 \(\upmu\)m but with a bi-dimensional geometry and lightweight that might lead to a longer time of permanence in the air. This might increase the chances of being intercepted by the aboveground organs of seed plants. Hence, in this study, the effect of a commercial GO and its purified version (PGO) was tested on stigma, pollen\(-\)sigma interaction, and fruit and seed production of _C. pepo_. The stigmatic surface of _C. pepo_ is covered by clump-shaped structures made up of finger-like cells, i.e., the stigmatic papillae (Figure 1a,b). ESEM observations of CTRL samples revealed that the brushing of the stigmatic surface did not affect its integrity (Figure 1a). Papillae coated with GOs maintained their original shape even if they were slightly agglutinated and partially wilded at the apex. It is known that the direct interaction of GRMs with cells may cause physical damage, such as cuttings or piercings [30; 31]. However, signs of severe damage as cytoplasmic leachates were not detected (Figure 1c,d), as already observed on the stigmatic surface of _C. pepo_ coated with FLG and nanocrystals of the naturally occurring muscovitc mica [19]. The absence of severe physical damage is in good agreement with previous works suggesting that the cell wall of plant cells is very difficult to damage and pierce through [18; 32], even if the nanomaterials applied are very thin, have sharp edges, and their lateral dimensions are in the order of hundreds of nanometres. In fact, despite our GOs having lateral dimensions bigger than those of the NMs used in the previous study [19], it was as thin as FLG (6 vs. 3\(-\)4 layers, respectively) and thinner than muscovitc mica (16 \(\pm\) 13 layers). The presence of GO over the stigmatic surface might alter the interaction between pollen and stigma [19; 33; 34], affecting both pollen adhesion and germination. In our case, the presence of GO caused the slight modification at the apex of the stigmatic papillae described above. This might be caused by the reactivity of GO, conferred by the oxygen functional groups bound to the graphene lattice. However, a decrease in pollen adhesion and germination might depend also on a physical interposition of GO flakes between single pollen grains and the stigmatic surface. In CTRL stigmas, pollen adhesion was 70 \(\pm\) 11% (Figure 2a), whereas it decreased to 58 \(\pm\) 18% and 51 \(\pm\) 5%, in GO- and PGO-coated stigmas, respectively. This difference was statistically significant with respect to CTRL only in the case of PGO (\(p<0.05\), post hoc \(t\)-test for pairwise comparisons) (Table S1). PGO flakes in contact with the stigma appeared less folded than GO ones (Figure 1c,d), which might have decreased the surface of contact and the strength of the pollen adhesion to the stigma. Differently, pollen germination significantly decreased from 65 \(\pm\) 13% in CTRL stigmas (Figure 2b) to 17 \(\pm\) 9% and 18 \(\pm\) 19% in GO- and PGO-coated stigmas, respectively (Table S1). After three hours of germination, the pollen tubes generally penetrated deeper in the CTRL stigmatic tissues (Figure 3a) than in the GOs ones (Figure 3b,c). In vitro experiments reported that GO had an inhibitory effect on pollen tube emergence and elongation in _C. avellana_ and _N. tabacum_, which was mainly caused by GO acidic properties [17].