## 1 Introduction  
In 2004, the isolation of the first monolayer of graphene [1] kindled the research interest in graphene-related materials (GRMs) [2]. Their extraordinary chemical and physical properties boosted many fields of innovative technology, from opto-electronics and medicine to materials for automotive industry and constructions [3, 4]. To date, GRM-enriched products, such as tires, asphalts, and sports equipment, are in an advanced stage of development and have already reached markets and civil society (for a list see www.graphene5-Sinfo.com, accessed on 1 June 2021).  
Despite their advantages, GRM-enabled products are subjected to degradation and breakage and will be disposed of at the end of their life cycle. This could lead to an unintended and undesirable release of GRM nanoparticles into the environment. Moreover, applications involving a direct voluntary release of GRMs into the environment are under development, such as GRM-composites used as pesticides [5, 6], plant fertilizers [7, 8, 9], sand improvers for soil remediation [10], and drug enhancers [11]. GRM nanoparticles are extremely lightweight, thus, once dispersed in the atmosphere, they can be transported for very long distances, as described for carbon black in fine and ultrafine particulate matter (PM) [12]. These nanoparticles could settle on soil, water bodies, or vegetation with still unknown repercussions on organisms and functionality of ecosystems.  
So far, literature reported both positive and negative effects of GRMs on seed plants, possibly owing to different experimental conditions (materials, concentrations, exposure time, protocols, etc.), plant developmental stages (seed, seedling, adult plants, etc.), and/or species tested [13,14]. Notably, the aboveground organs of seed plants, such as leaves and flowers, act as natural traps for airborne PM [15,16], potentially including GRMs. Landing on flowers, airborne GRMs might affect one of the most important biological processes for life on Earth, i.e., the sexual reproduction of seed plants.  
Recent in vitro studies demonstrated that few-layer graphene (FLG) and graphene oxide (GO) impair pollen tube emergence and, in the case of GO, pollen tube growth in _Corylus avelman_ L. (common hazel) and _Nicotiana tabucum_ L. (tobacco plant) [17,18]. It was hypothesized that FLG might mechanically impair/damage the germination pore and affect the intracellular redox homeostasis of the pollen. However, in a following in vivo study on _Cucurbita peppo_ L. (summer squash) flowers [19], it was shown that FLG did not injure the structure of stigmas and pollen grains but affected pollen adhesion and germination over the stigmatic surface. Importantly, these effects were comparable to those of a naturally occurring 2D-nanomaterial (2D-NM), i.e., muscovite mica. Differently, the effects of GO on pollen were mainly related to the oxygen functional groups (e.g., carboxylic, epoxy oxydrilic) present on the GO lattice which makes GO acidic and able to bind/immobilize important cations, such as Ca\({}^{2+}\), for pollen tube germination and elongation [18].  
On the basis of these premises, in this study we tested the hypothesis that GO might be potentially more hazardous than FLG to the sexual reproduction of seed plants as it could alter the conditions of the stigmatic environment, leading to unsuccessful pollen germination and, consequently, hindering the fertilization process. To verify this, stigmas of _Cucurbita peppo_ L. ssp. _pepo \"Greyzini\"_, a crop that is a model species for pollination studies [20], were exposed to GO and GO purified from production residues (PGO). At first, the GO effect on stigmatic surface integrity and on pollen adhesion and pollen germination was tested. Subsequently, the effect on fruit and seed production was assessed.  
A better understanding of the possible negative effects of GO on the sexual reproduction of seed plants would help us to predict future scenarios and perhaps to adopt measures to engineer safely designed GRMs and derived applications.