Boreal forest, the world’s largest terrestrial biome, represents significant land areas in Canada, China, Finland, Japan, Norway, Russia, Sweden, and the United States (Larsen, 1980). Boreal forest ecosystems develop under several climate conditions, including a short growing season and freezing winter temperatures (Sykes and Prentice, 1995). These environment factors result in relatively low tree species diversity, with only a few genera being dominant across the biome, e.g., Larix, Pinus, Picea, Populus, Betula, and Alnus (Johnston, 2017). Boreal forests represent the largest pool of living biomass on the terrestrial surface. The aboveground net primary productivity of boreal forest in North America ranges from 25 to 675 g C m^–2 y^–1, with an annual mean of 296 g C m^–2 y^–1 (Gower et al., 2001).

Seed germination constitutes a fundamental part of plant growth and development. Some boreal forest species of the Northern hemisphere exhibit deep dormancy, which prevents seeds from germinating when environmental conditions are unfavorable, thus, allowing more seedlings to survive over time (Herranz et al., 2010; Baskin and Baskin, 2014; Hu et al., 2018). Seed dormancy is a mechanism that ensures seeds are able to germinate at times when environmental conditions are ideally suited for germination and seedling establishment (Baskin and Baskin, 2014). As such, germination is often synchronized with appropriate environmental cues, such as temperature, moisture, and photoperiod, to improve the establishment and survival of the germinant (Baskin and Baskin, 2004; Bewley et al., 2013b).

Seeds are known to exhibit different types of dormancy, including physical, morphological, or physiological (Bewley et al., 2013a; Baskin and Baskin, 2014). Physical dormancy is often associated with the inability of the seed to exchange gases and imbibe water due to the presence of militating physical barriers such as an impermeable seed coat. Morphological dormancy is observed in seeds with an immature embryo. Physiological dormancy is characterized by the release of inhibiting hormones, preventing the initiation of germination under different prevailing environmental conditions (Bewley et al., 2013a; Baskin and Baskin, 2014). The hormonal balance of endogenous abscisic acid and gibberellin compositions constitute vital factors inducing the germination of dormant seeds (Finch-Savage and Leubner-Metzger, 2006). A number of native boreal tree species that have been identified as important for land reclamation are challenging to propagate from seeds because of seed dormancy or seedling vigor issues (Smreciu et al., 2013; Ali et al., 2020a,b; Arnott et al., 2021).

For instance, green alder (Alnus viridis) is known to exhibit physiological dormancy with germination rates of less than 55% (Kaur et al., 2016). The embryos of these seeds are characterized by an inability of the radicles to emerge from the seed. This is considered to be a vital factor in this type of dormancy (physiological), which may be influenced by the seed coat or endosperm. Several strategies have been applied to successfully overcome the challenges associated with different seed dormancy of native forest species (e.g., Kaur et al., 2016; Mackenzie and Naeth, 2019; Deng et al., 2021). These include scarification, stratification, hormone treatments, and seed priming methods. Recently, nanotechnology has been demonstrated to be an effective approach to alleviate seed dormancy in several crop species (e.g., Villagarcia et al., 2012; Tiwari et al., 2014; Javed et al., 2021) and a few native boreal forest species (Ali et al., 2020a,b). This mini-review focuses on current studies that have examined the effect of carbon-based nanoparticles (CNPs) on seed germination of crops and native boreal forest species. Finally, the review provides an outlook on potential future applications of CNPs on native boreal species for use in forest reclamation or restoration.

Nanotechnology has the potential to revolutionize the way ecologists approach plant propagation to improve plant regeneration during forest reclamation, food, and crop production (Parisi et al., 2015; Servin et al., 2015). NPs used in this sector are atomic or molecular aggregates with dimensions between 1 and 100 nm (Ball, 2002; Roco, 2003). In fact, NPs are prepared directly from natural compounds or synthetic materials by grinding or milling, high-pressure homogenization, and sonication to reduce the particle to nanometer scale (Podsiadlo et al., 2007; Vasquez et al., 2008). The small size of the particles can drastically modify the physicochemical properties of seeds compared to the bulk material (Nel et al., 2006). NPs are also observed to be present in the ecosystem and can originate from a variety of sources. Origins of NPs include manufacturing processes of different industries, treatment and disposal of wastewater, and incorporation of biosolids into the soil (Morales-Díaz et al., 2017). Engineered NPs can be grouped into four primary groups based on the materials used: (1) carbon, (2) metal, (3) dendrimers, and (4) composites (Majhi and Yadav, 2021). However, the ideal NPs used for plant propagation should be economical, eco-friendly, biocompatible, and non-toxic (Liu and Lal, 2015; Mahakham et al., 2016). Plant-based materials are good candidates for synthesizing biocompatible NPs due to the biochemical diversity of plant extracts, non-toxic phytochemical constitutes, non-pathogenicity, low cost, and flexibility in reaction parameters compared to chemically synthesized nanoparticles (Mahakham et al., 2016; Singh et al., 2016).

In recent years, carbon-based NPs (CNPs) (fullerene and carbon nanotubes) have been applied as seed pre-treatment agents to promote seed germination, seedling vigor, and growth in crops (e.g., Villagarcia et al., 2012; Tiwari et al., 2014; Mahakham et al., 2017). CNPs were shown to enter the seed and alter various biological pathways, which modified their physicochemical properties and resulted in different effects on plant growth (Nadiminti et al., 2013). Rapid and uniform seed germination and seedling emergence are essential determinants of successful stand establishment (Rajjou et al., 2012; Chen and Arora, 2013). Seed germination begins with water uptake by the mature dry seed (imbibition) and terminates with the elongation of the embryonic axis, usually the radicle, through the seed envelope, ultimately resulting in the protrusion of root and shoot (Rajjou et al., 2012; Baskin and Baskin, 2014).

Plant responses to treatments with CNPs were observed to vary depending on the chemical composition of the CNPs, concentration, aggregation state (Liu et al., 2009), metabolic abilities of the treated plant species, plant growth environmental conditions (Schultz et al., 2015), and exposure time (Mrakovcic et al., 2013). However, a thorough understanding of the effects induced by CNPs on plant physiology at the molecular level is still lacking. In addition, the methods used for detecting such nanostructures in plant tissues are not well established, and most of them are time-consuming and labor-intensive. Existing detection methods for NPs usually lead to the destruction of samples in the process of detecting the presence of nanomaterials. As a result, the same plant samples cannot be assessed for genomic/proteomic analysis (Khodakovskaya et al., 2011).

The information presented indicates that selected CNPs treatments significantly improved seed germination in several plant species, including the native boreal forest species, green alder, which has challenging seed dormancy issues with low germination rates (Ali et al., 2020a; Sobze and Thomas, 2022). Anthropogenic disturbance and fragmentation of global forest ecosystems are recognized as major threats to biodiversity and essential ecosystem services. To mitigate any potential environmental and social impacts, it is important to achieve forest establishment within the natural range of ecotypes found in the forest seed zones, capable of supporting wildlife habitats similar to pre-disturbance conditions (Rowland et al., 2009). The challenges associated with native boreal species germination and establishment must be considered for effective reclamation purposes and biodiversity conservation. As such, we believe nanoparticles (NPs) could be an excellent source of improving the germination rate and propagation of many native boreal forest species. General benefits of this technology include a new approach to enhance plant propagation and efficient use of limited native boreal seed resources through an improvement in germination and seedling stock quality. Very low amounts (microgram quantities) of NPs have been shown to confer beneficial effects to the tested plants reported in our study (Ali et al., 2020a; Sobze and Thomas, 2022) and the literature (e.g., Mondal et al., 2011; Smirnova et al., 2012; Javed et al., 2021), indicating this is a very promising technology with low inputs and cost. The ultimate benefit is that select CNPs could be used to produce superior boreal planting materials (seeds and stock plants) ideal for boreal forest reclamation across a range of field sites following industrial disturbances. As a first step, NPs imbibed seeds (aspen and fireweed) should be tested under adverse conditions in greenhouse, growth chambers, and field sites to determine the effectiveness of the improved seed vigor on seed germination, growth, and establishment when exposed to adverse fields or environmental conditions. The size and concentrations of the NPs used to achieve the most effective responses would be an appropriate direction for future work. Moreover, further studies are required with other species not only in boreal regions, but also in other geographical regions, to confirm the effectiveness of NPs on both germination and seedling vigor. For instance, a recent study from the tropics found MWCNTs in the burned wood of native plants, Pinus oocarpa, and P. pseudostrobus, following forest wildfire (Lara-Romero et al., 2017). These MWCNTs produced by wildfire were applied to other native plants such as Eysenhardtia polystachya and found to be effective to speed up its germination time (Juárez-Cisneros et al., 2020). Many fire-adapted plant species are difficult to germinate due to their impermeable and hard-coated seeds (Moreira and Pausas, 2012; Baskin and Baskin, 2014). Applying NPs could possibly increase water uptake during seed imbibition by penetrating seed coat and cell wall, leading to seed germination improvement, which can assist reforestation process and biodiversity conservation.

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