Selenium Nanoparticles (Se NPs) as Agents for Agriculture Crops with Multiple Activity: A Review

Abstract

This review article is devoted to the use of selenium nanoparticles (Se NPs) in plant production. The review analyzes relevant literature data for the last 10 years, considering the effect of Se NPs application on morphometric and biochemical parameters of plants. A number of actual works demonstrating the efficiency of Se NPs use in the composition of nanocomposites based on synthetic and natural polymers are considered separately. Possible mechanisms of Se NPs absorption and transport and their further activity in plant cells of agricultural crops in the context of biostimulating, biofortification, nutraceutical, and antioxidant activities of Se NPs, as well as the efficiency of Se NPs application under stress factors are discussed. The review provides data demonstrating the antibacterial and antifungal activities of Se NPs in the context of their activity against a wide range of phytopathogens. Also, we conduct a detailed comparative analysis of the relative efficiency of Se NP application with mineral Se-containing compounds (SeO₃²⁻ and SeO₄²⁻), as well as organic forms of Se (SeCys and SeMet).

1. Introduction

Scaling up modern agriculture and increasing the volume of manufactured products require searching for new technological solutions, including the development of new fertilizers with high bioavailability, prolonged effect, and low toxicity [1]. Recently, much attention has been paid to selenium fertilizers due to the complex action of this element: participation in the biochemical processes of plant cells, stimulating effect on the growth of agricultural crops [2], participation in the formation of protective reactions to stress factors, as well as a positive effect on the organoleptic characteristics (aroma, taste, color) of plant agricultural products [3].

Selenium (Se) is a metalloid, occupying the 59th place in abundance in the Earth’s crust. Se is a trace element involved in various metabolic and physiological processes in both plants and animals. The necessity of selenium as an essential element for plant growth is still debatable. Se is a conditionally essential (or useful) trace element for plants, although it is not strictly considered essential for all species. Se is known to be a constituent of a number of plant selenoproteins, including a group of glutathione peroxidase enzymes that play an important role in protecting plant cells from oxidative stress [4]. In higher plants, Se is also involved in the regulation of photosynthesis [5,6]. In mammals, Se is a component of about 25 selenoproteins (protein molecules containing the amino acid selenocysteine) [7,8]. In plants, selenium is a component of 15 selenoproteins [9].

Despite the very low Se content in the human body, this element plays a unique role among other microelements, being the only one whose inclusion in proteins is genetically determined, as a component of the 21st amino acid, selenocysteine. In the form of selenocysteine, it is present in the active centers of Se-dependent enzymes involved in antioxidant protection and tissue development (glutathione peroxidase, thioredoxin reductase, and iodothyronine deiodinase) [10].

The Se content in soil is typically 0.01–2 mg/kg, with concentrations below <0.125 mg/kg considered deficient and concentrations above 3 mg/kg considered toxic to both plants and animals [11]. Most agricultural soils are Se-deficient, resulting in Se content in food products often being insufficient [12]. A number of studies have shown that the application of organic and mineral Se-containing fertilizers in low concentrations can stimulate plant growth [13,14,15]. Low concentrations of Se are capable of exerting both antioxidant, antimicrobial, and stress-modulating effects [16,17,18]. In general, soil Se fertilization is an effective method to improve overall plant productivity [19]. At the same time, high concentrations of Se often show phytotoxicity due to both the formation of malformed selenoproteins and the development of oxidative stress [5,20]. As a consequence, there is a need for dosage control when using Se-containing fertilizers.

The effects of exogenous Se depend on the chemical form in which it is supplied. Currently, Se fertilizers are used in the form of inorganic compounds: selenate (Se⁶⁺), selenite (Se⁴⁺), and selenide (Se²⁻), as well as in the form of organic compounds with amino acids (selenocysteine, selenomethionine) [21]. Selenate predominates in aerobic soils with neutral pH, while selenite predominates at lower pH and redox potential [22]. As a rule, organic selenium compounds are absorbed by plants more efficiently than inorganic ones [23]. At the same time, numerous studies have shown that the use of Se nanoparticles (Se NPs), compared to inorganic and organic Se compounds, allows an increase in bioavailability and biological activity, and at the same time reduces the toxic effect on plants [24,25,26]. Analysis of publications on the topic of “SeNPs in agricultural technologies” revealed an increase in the number of publications by ~22.5 times over the past 10 years (Figure A1a). The stimulating effect of Se NPs on the growth of agricultural crops was demonstrated for a large number of different groups of agricultural plants (Figure A1b). The growing interest in selenium-based fertilizers, particularly in the form of nanoparticles, highlights their promising potential to enhance crop productivity, improve food quality, and address selenium deficiency in agricultural systems, while minimizing environmental and phytotoxic risks. In this review, we summarize both the theoretical information on the effects of Se NPs on plant systems and review the results of current research demonstrating the efficacy of Se NPs on a wide range of crops.

2. Approaches for the Synthesis of Se NPs-Based Nano-Fertilizers and Their Modifications

2.1. Methods for the Synthesis of Se NPs

The expansion of the application areas of selenium nanoparticles is due to the discovery of new synthesis methods that allow for the active scaling of their production [27]. Existing methods for the synthesis of Se NPs can be divided into chemical, biological, and physical methods. These methods can be divided into “bottom-up” and “top-down” approaches (Figure 1).

Among the numerous methods for producing Se nanoparticles, chemical methods for reducing selenium ions predominate. Ascorbic acid, thiourea, glucose, L-cysteine, glutathione, and other compounds are used as reducing agents [28,29,30]. Nanoparticles obtained using chemical synthesis methods often have a rather wide size distribution, and the chemical reagents used are often unsafe for the environment. Biological synthesis of Se NPs is carried out using microorganisms, enzymes and fungi, and plant extracts. A number of microorganisms (mainly soil microorganisms) are able to reduce ionic forms of selenium to neutral nanoselenium due to the presence of specific reductases [31]. Plant extracts are rich in proteins, vitamins, and amino acids, which can act as biodegradable reducing agents and stabilizing agents to maintain the biological activity of Se-containing nanostructures. Thus, biological synthesis methods, being a special case of chemical synthesis method (cellular components of bacteria, fungi and plants act as reducing agents), allow obtaining Se NPs without using aggressive chemical compounds, preventing environmental pollution. It is also worth noting that the use of plant extracts can contribute to additional functionalization of nanoparticles, providing them with increased stability and biocompatibility. For large-scale production of Se nanoparticles, physical methods are the most suitable, among which ultrasonic synthesis, microwave synthesis, and laser ablation are the most commonly used. Ultrasonic liquid-phase synthesis is the most common method to produce fluorescent Se NPs less than 10 nm in size [32,33]. In general, the use of physical synthesis methods makes it possible to obtain Se nanoparticles with a narrow size distribution, possessing a high degree of purity, with a high degree of crystallinity and the possibility of controlling physical and chemical properties. At the same time, many types of physical synthesis are characterized by a low rate of nanoparticle production.

2.2. Modification of Se NPs

To enhance nanoparticle stability, improve the delivery control, release, and leachability of Se NPs, and reduce the environmental burden, the use of Se NPs in nanocomposite systems with both polymers and other NPs is often considered. Among the most popular synthetic polymer matrices, polyvinyl alcohol (PVA), poliacrylic acid (PAA) [34], polyethylene glycol (PEG) [35], polyethyleneimine (PEI), polylactide (PLA), and polyvinylpyrrolidone (PVP) are noted. Water-soluble natural polysaccharides differ from synthetic polymers in their high degree of biocompatibility with living systems and lower toxicity. The most common natural polymers used to create Se NPs-containing nanocomposites are polysaccharides and their derivatives. The interaction of natural polysaccharides with the surface of Se nanoparticles occurs mainly due to the formation of hydrogen bonds [36]. The nature of polysaccharides is a determining factor in the formation of Se-containing nanostructures and optimization of their parameters [37]. In the following, we will consider in more detail the use of both synthetic and natural polymeric materials for the creation of composite materials based on Se NPs. The biological effects of Se NPs-based nanocomposites and polymers reported in the literature with respect to agricultural crops are presented in

Table 1. Effect of nanocomposites based on Se NPs and polymers on the growth and functional state of plants, according to literature data.

Plant	Composition	Synthesis Method	Size (nm)	Treatment Method	Concentrations	Maximal Effective conc.	Effects	Ref.
Strawberry (Fragaria × ananassa)	Se NPs + alginate	γ-irradiation with sodium alginate	94	Fruit covering	75, 150, 300 mg/L	150 mg/L	↓ growth of Neopestalotiopsis rosae and Fusarium oxysporum, ↑ Se content ~2 times, ↑ fruit hardness by 26%, ↓ fruit weight loss by 3 times, ↑ preservation of appearance during storage, ↑ preservation of vitamins C and B9 during storage	[38]
Rice (Oryza sativa L.)	Se NPs + lignin containing cellulose nanofibers	Chemical reduction using ascorbic acid followed by microwave heating	32	Seed priming	10, 20 mg/L	20 mg/L	↑ germination rates, ↓ salt damage rates, Under salt stress (150 mM NaCl): ↑ chlorophyll content (~8 times), ↓ Na+/K+ ratio, ↓ OSLOL5, NHX1, SOS1 and HAK20 gene expression levels	[39]
Potato (Solanum tuberosum L.)	Se NPs + arabinogalactan + Stach	Chemical synthesis	40	Addition to Growth Medium	0.000625% 1 (Se)		Bactericidal and antibiofilm activity against Clavibacter sepedonicus, ↑ Root weight (25%), ↑ Peroxidase activity in tissues of potato leaves (85%), ↑ ROS production in plant tissues (520%), ↑ Vegetative part weigh (20%), ↓ Internode length, × effects against soil Acinetobacter quilouiace and Rhodococcus erythropolis × Se accumulation in tubers	[40,41]
Tomato plants (Solánum lycopérsicum L.)	Se NPs + PAA and thiophanate-methyl	Chemical synthesis	110	Foliar treatment	50, 100, 200 mg/L		Antifungal effect against Botrytis cinerea, ↑% of germination (34.28%), ↑ Root length (72.41%), ↑ Stem length (18.18%), ↑ Chlorophyll a (28.20%) and b (89.28%) content, ↑ Carotenoids content (61.76%), ↑ Total chlorophyll content (35–41%), ↑ Lycopene content in fruit (55–95%), ↑ Anthocyanin content in fruit (22.5%), ↑ Se content in fruit (~2600–3000%), × Fruit weight	[34]
Melon (Momordica charantia L., Cucumis melo L.)	Se NPs + CS	Chemical synthesis using chitosan	110	Addition to Growth Medium for Seedlings	10, 20 mg/L	20 mg/L	↑ Root length (32.39–16.22%), ↑ Shoot and stem length (30.93–22.61%), ↑ Fresh root mass (26.73–21.93%), ↑ Fresh shoot and stem mass (24.10–16.02%), ↑ Dry root mass (25.73–21.52%), ↑ Dry shoot mass (24.50–15.88%), ↑ Photosynthetic activity (SPAD 9.50–10.17%), ↑ Proline content (9.26–19.78%), ↑ Water content in leaves (RWC 7.85–5.69%)	[42]
	Se NPs + CS	Chemical synthesis with chitosan	50	Foliar treatment	10, 20 mg/L	20 mg/L	Under salt stress 50, 100 mM NaCl: ↑ Root fresh mass (10.29–13.59%), ↑ Root dry mass (7.14–13.35%), ↑ Shoot fresh mass (16.97–24.20%), ↑ Shoot dry mass (10.41–13.59%), ↑ Plant height (7.03–13.47%), ↑ RWC (13.46–27.53%), ↑ Yield (25.00–25.11%), ↑ Fruit mass (17.30–22.85%), ↑ Fruit count (10.00–13.95%), ↑ Photosynthetic activity (7.80–11.13%), ↓ MDA content (18.32–29.47%), ↓ H2O2 content (14.77–15.35%), ↑ CAT activity (14.97–16.50%), POD (36.66–63.33%) and APX (23.25–24.29%)	[43]
Rice (Oryza sativa L.)	Se NPs + CS	Chemical reduction	115.6	–	10 mg/L		↓ SOD activity in shoots (~1.3%) and roots (~2.6%), ↓ CAT activity in shoots (~3.2%) and roots (~7.2%), ↓ APX activity in shoots (~10.6%) and ↑ in roots (~5%), × Changes in GPX activity in shoots and roots, Under As-induced stress: ↑ SOD activity in shoots (~27%) and roots (~72.7%), ↑ CAT activity in shoots (~154.8%) and roots (~48.1%), ↑ APX activity in shoots (~181.8%) and roots (~193.3%), ↑ GPX activity in shoots (~45.4%) and roots (~40.8%)	[44]
	Se NPs + algal polysaccharides	Chemical reduction using ascorbic acid	80	Addition NPs to hydroponic medium and soil	0.08, 0.4, 0.8, 1.19, 1.6 mg/L	1.6 mg/L	× Root weight (%), ↑ Se concentration in roots (~25%), ↑ Total Se in roots (~20%), ↑ OsSULTR1;2, OsPT2 and OsNIP2 expression levels, ↑ Organic Se concentration in roots by a factor of 39.6 and 2.53 compared to SeO4 and SeO3, ↑ Inorganic Se concentration in roots by a factor of 2.2 and 2 compared to SeO4 and SeO3, ↑ Organic Se concentration in shoots by a factor of 10.8 and 1.91 compared to SeO4 and SeO2	[45]
	Se NPs+ polysaccharides from spent mushroom substrate Agrocybe aegerita	Chemical reduction using ascorbic acid	–	Root and Foliar Application	2, 20 mg/L	20 mg/L	↑ Root length (50–65%), ↑ Stem length (15–20%), ↑ Leaf length (30–35%), ↑ Antioxidant enzyme activities, Free radical scavenging	[46]
Red bean (Phaseolus vulgaris L.)	Se NPs coated with gum arabic	Chemical reduction	46.9	Foliar treatment	0.04, 0.2, 0.4 mg/L	0.4 mg/L	↑ Stem length (~2.5–22.5%), ↑ Root length (~5–70%), ↑ Dry shoot weight (~up to 40%), ↑ Dry root weight (~up to 27.3%), ↑ Fresh shoot weight (~2.7–70.3%), ↑ Fresh root weight (~4.3–78.6%), ↑ Leaf number (~up to 75%), ↑ Chlorophyll “a” (~4.5–40.9%), “b” (~4.3–47.8%), carotenoid (~20.9–58.1%) content	[47]
Barley (Hordéum vulgare L.)	Se NPs stabilized by PVP	Chemical reduction	70	Seed priming	0.465, 4.65, 46.5, 465, 4650 mg/L	4.65 mg/L	At 0.465–46.5 mg/L: ↑ Root length (54.9–120%), ↑ Root number (35–51.2%), ↑ Shoot length (27.8–87.2%); At >46.5 mg/L: ↓ Root length (45.8–100%), ↓ Root number (up to 100%), ↓ Shoot length (84.6–100%), ↑ Germination rate (45%) at 4.65 mg/L Se NPs and decrease (4.5–100%) at all others	[48]
Chili Pepper (Capsicum annuum L.)	Se NPs + ZnO + Arbuscular mycorrhiza fungus (AMF) inoculation	Chemical reduction	8.37–12.8 (ZnO NPs), 5.27–6.51 (Se NPs)	Foliar treatment	10 mg/L	–	Under combined application of ZnO NPs, Se NPs + AMF: ↑ Average fruit weight by 98.7%, ↑ Fruit number by 54.8%, ↑ Thylakoid grana thickness, ↑ Chloroplast number, ↑ Total ascorbic acid content, ↑ Total capsaicin content	[49]
Orange (Citrus × sinensis (L.) Osbeck)	Se NPs + CS	–	50–150	Foliar treatment	10, 20 mg/L	20 mg/L	↑ Chlorophyll a (40–80%) and b (25–42%) content, carotenoids (67–92%), ↑ Chlorophyll a/b ratio and SPAD value by 33% and 21% respectively at 20 mg/mL Se NPs with chitosan, ↑ Fv/Fm value, SPAD coefficient (20%), chlorophyll (25–42%) and carotenoid (67–92%) content under salt stress simulation	[50]
Apple (M. pumila Jinfu)	Compound consisting of Rosa roxburghii procyanidin, chitosan, and Se NPs	Biosynthesis	84, 56	Apple and red bayberry fruit treatment	0.03 mg/kg 1 (apple), 0.12 mg/kg 1 (red bayberry)	–	↓ Spread of P. piricola and S. aureus on apples, × Cytotoxicity	[51]
Moldavian dragonhead (Dracocephalum moldavica L.)	Se NPs + CS	Synthesis by ion gelation	70	Foliar treatment	5, 10 mg/L	5 mg/L	↓ Negative effects under Cd stress, ↓ MDA content (2.3 times), ↓ H2O2 content (1.3 times), ↑ Chlorophyll fluorescence parameters, ↑ Morphological characteristics, ↑ Photosynthetic pigment content, ↑ Proline content, ↑ Phenol content	[52]
Potato (Solanum tuberosum L.)	Se NPs in a carrageenan matrix	Chemical precipitation	70	Addition of NPs to the nutrient medium	–	–	↑ Number of leaves (13.2%), ↑ Root mass (37.9%)	[53]

↑—Increase in indicators, ↓—Decrease in indicators, NR—Nitrate reductase, GSH—Glutathione, GR—Glutathione reductase, GPX—Glutathione peroxidase, APX—Ascorbate peroxidase, CAT—Catalase, POD—Peroxidase, SOD—Superoxide dismutase, PAL—Phenylalanine ammonium lyase, CS—Chitosan, MDA—Malondialdehyde, RWC—Relative water content. ¹—here and below, quantitative data in dimensions different from µg/mL are taken from the original works.

below.

2.2.1. Nanocomposites Based on Se NPs and Synthetic Polymers

The use of nanocomposites based on selenium nanoparticles (Se NPs) and synthetic polymers is considered in a number of works. The use of various water-soluble polymer matrices is designed to provide controlled release of the element, improving digestibility and minimizing losses of nano-fertilizer [54]. One of the proposed polymer matrices for creating nanocomposites is PVA, which has good film-forming properties, and a high level of biocompatibility and biodegradability [55]. Studies show that Se NPs-PVA nanocomposites have improved mechanical and antimicrobial properties, which contributes to improved fertilizer efficiency and plant pathogen protection. In particular, the application of Se NPs encapsulated with PVA and chitosan to grafted and ungrafted cucumber shoots promoted growth stimulation and increased fruit weight [56].

Another modifying agent under consideration is PAA, known for its ability to form hydrogels and ion exchange complexes. In agrotechnology, PAA is often used to create matrices with controlled release of fertilizers. In the context of Se NPs containing nanocomposites, repeated application of Se NPs-PAA nanocomplexes has no significant toxic side effects on plants and is able to improve the photosynthetic activity of plants under isolated conditions [34]. In turn, PVP is also a well-established water-soluble polymer known for its ability to stabilize Na nanoparticles and improve their bioavailability. In particular, priming of barley (Hordéum vulgare L.) Se NPs seeds coated with PVP together with ascorbic acid demonstrated an increase in shoot length, root length, and germination percentage [48].

Modification with PEI is of particular interest from a functional point of view. This polymer has a high density of amino groups, which makes it effective for ionic binding and delivery of active substances. An important ability of PEI-coated nanomaterials is the improved ability to penetrate cell membranes. The combination of PEI with nano-fertilizers can increase plant uptake, improving plant growth and development [57,58]. Currently, there are no publications that investigate the activity of Se NPs-PEI nanocomposites against plant objects, but these studies may be of interest in terms of evaluating the effectiveness and safety of such a combination for agrotechnologies in the future.

2.2.2. Nanocomposites Based on Se NPs and Biopolymers

The use of natural polymer systems as functional agents and/or stabilizers of Se NPs is a more popular approach for modifying Se NPs and has been widely investigated recently. It is important to note that unlike synthetic analogs, biopolymers do not accumulate in the environment and do not negatively impact the soil microbiota. Natural polymers are of particular interest for the creation of nanocomposites due to their biodegradability, biocompatibility, and ability for controlled release of active substances [59]. Polysaccharides are promising stabilizing agents for the synthesis of Se NPs due to a number of structural and functional features. Unlike other macromolecules, they demonstrate increased stability under synthesis conditions due to the presence of numerous reactive hydroxyl groups in their molecular structure. These functional groups are able to interact effectively with the surface of the formed nanoparticles, providing their stability and preventing aggregation. An important advantage of polysaccharides is their high biocompatibility and low toxicity, which are of fundamental importance for biomedical applications of the obtained nanocomposites. In addition, pronounced synergistic effects between the polysaccharide matrix and stabilized selenium nanoparticles are observed, which can potentiate the biological activity of the system as a whole. There are a number of works demonstrating the successful use of polysaccharide matrices for incorporation of Se NPs. For example, Perfileva et al. proposed to use a matrix based on starch and arabinogalactan in a number of studies. The obtained nanocomposites prevented the development of root rot in potatoes and also stimulated plant growth [40,41,60]. In turn, Peng et al. also reported high efficacy of treatment of rice seedlings with Se NPs stabilized with polysaccharides from a spent fungal substrate. These nanocomposites had a significant stimulatory effect, especially in foliar treatments [46,47]. Also, a recent work by Yang et al. proposed to modify Se NPs using polysaccharides derived from algae to enrich rice plants with selenium to improve nutritional properties [45]. Tran et al. also proposed the use of Se NPs within an alginate matrix as an edible coating for strawberries to increase the storage time of berries and their nutritional value [38]. A recent work by Shaban et al. proposed the use of nanohybrids based on lignin-containing cellulose nanofibers and Se NPs for seed priming to improve the salinity tolerance of rice plants [39].

Chitosan occupies a special place among other biopolymers used to modify Se NPs. As a derivative of chitin, chitosan molecules have a significant content of amino groups (-NH₂) and can serve as a source of nitrogen for plants [61]. At the same time, the effect of chitosan is observed in both root and foliar treatments. Also, due to its own antibacterial activity, chitosan shows activity against a number of bacterial phytopathogens. It has been shown that the use of chitosan has a significant stimulating effect on the growth and development of various plants [62], contributes to an increase in chlorophyll content and nutrient uptake [63], and reduces the negative impact of external stress factors [64]. Notably, chitosan also has the intrinsic ability to chelate heavy metal ions [65]. Se NPs stabilized by high-molecular-weight chitosan are characterized by significant aggregation, probably due to the formation of cross-links between macromolecular chitosan chains, while Se NPs stabilized by low-molecular-weight chitosan have smaller sizes and effective dispersion [36]. In general, the use of natural polymers, especially polysaccharides and chitosan, as stabilizers of Se NPs opens wide prospects for the creation of environmentally safe, biologically active, and functionally efficient nanocomposite systems. These systems not only provide stability and controlled release of Se, but also have their own biological activity, which allows significant expansion of the scope of their application in agriculture.

3. Mechanisms of Action of Se NPs on Agricultural Crops

3.1. Se NPs Uptake, Transport, Accumulation, and Biotransformation by Plants

The biological effect of Se NPs applied to crops is regulated by a number of systems responsible for their uptake, transportation within plant tissue, and accumulation. Despite the considerable amount of literature data demonstrating the effectiveness of both root and foliar applications of Se NPs, there is little data on the precise mechanisms involved in the uptake and biotransformation of elemental Se by plants. Since selenium is not an essential element for most higher plants, there is no specific mechanism for its uptake and transport [66]. It is believed that selenium-amino acids enter plant cells through amino acid transporters [67]. Se absorption in plants occurs similar to sulfur (S) pathways, through sulfate transporters of organic forms of Se found in soil due to physicochemical similarities [68]. The uptake of Se NPs by plants is either through root absorption or through leaves. Root absorption is the main pathway of Se NPs uptake and is realized through passive (through the apoplast) or active (through the symplast) absorption [69]. Inorganic forms of Se in the form of dissolved selenite (SeO₃²⁻) and selenate (SeO₄²⁻) enter plants from soil and accumulate in its edible parts [70], while Se NPs are able to effectively penetrate cell walls and membranes due to their small size (1–100 nm). The mechanism of penetration of NPs is generally determined by the size of nanoparticles: nanoparticles larger than 50 nm penetrate leaves through the stomata, 8–20 nm through pores and intercellular spaces, and less than 5 nm through cells [71]. Se NPs of smaller size are more actively absorbed from the rhizosphere and have higher bioavailability [72]. It was also found that not only do the size of Se NPs and their concentration have an important influence on the Se transfer coefficient to the plant, but so does the pH of the soil solution [73]. A number of studies have demonstrated the contribution of aquaporins to the absorption of Se NPs from soil [69,74]. Aquaporins are transmembrane channels responsible for water uptake from soil, having high density in the membrane cells of root hairs forming the absorption zone. Further movement of nanoparticles occurs through apoplastic and symplastic pathways by upward or downward flow through phloem and xylem [75]. The spherical shape and small size of Se NPs are thought to favor their freer penetration through cell membranes [76]. The pathways of Se NPs penetration into plants through the leaf were also discussed in detail in Wang et al., 2025 [77]. It is noted that there is a problem of aggregation of Se NPs on the leaf surface, but this does not prevent their penetration through the stomata and cell walls [74]. In addition, Se NPs can partially dissolve, releasing Se ions, which can then penetrate into cells through the cuticle [78]. This is actively promoted by the formation of biomolecular bonds and the so-called “corona” [79]. Using transmission electron microscopy, it was found that nanoparticles penetrated through the cell wall and were detected both in the intercellular space and inside cells, which also suggests the presence of apoplastic and symplastic transport pathways, as for NPs of other elements [75,77]. As we moved deeper into the mesophyll, the size of the aggregates of NPs decreased, indicating their gradual dissolution and release of Se ions [77]. Further pathways of Se NPs transformation and distribution remained unknown for a long time. With the improvement of high-precision detection methods using liquid chromatography (HPLC) and mass spectrometry, there are great opportunities for the determination of selenium forms transformed in the plant and their localization in the plant. During Se assimilation by plants, selenite is reduced to selenide and then incorporated into selenocysteine (SeCys). SeCys synthesis probably occurs in chloroplasts, the cytosol, and mitochondria; in turn, SeCys can be converted to selenomethionine (SeMet) [80]. In particular, on rice culture, thanks to HPLC with an anion-exchange column, it was found that Se NPs of 86 nm in size were rapidly transformed into organic forms dominated by SeMet [74]. In order to study the conversion of Se NPs in wheat plants, inductively coupled plasma mass spectrometry (SP-ICP-MS) was also used, which showed that Se nanoparticles are efficiently transported into the grain, where they are already in a safe ionic form for humans [77,78]. It is also worth noting that due to the biochemical competition for the uptake and transportation of Se and S, there is a risk of excessive accumulation of SeCys and SeMet in plant proteins, causing physiological disorders. Plants have a defense system against selenium excess, the 26S-proteasome, that prevents toxic effects and removes selenoproteins, which was proven in mutant Arabidopsis plants [81]. To offset the current inclusion of excessive amounts of SeCys and SeMet within protein molecules, some plants accumulate non-protein seleno-amino acids or their γ-glutamyl derivatives, which include selenocystathionine, Se-methyl-selenomethionine, γ-glutamyl-selenocystathionine, selenopeptides, and selenohomocysteine [70]. It is also believed that Se accumulator plants do not develop pronounced symptoms of Se-mediated toxicity because they contain a special enzyme selenocysteine methyltransferase in chloroplasts that prevents excessive incorporation of Se into proteins [82]. The subsequent pathways of Se transformation in plants have also been described in a number of other studies [66,83,84]. Different plant species accumulate Se differently, which is presumably related to the expression level of sulfate transporters [85]. For example, pickleweed (Allenrolfia occidentalis (S.Watson) Kuntze), asparagus (Asparagus officinalis L.), rice (Oryza sativa L.), broccoli (Brassica oleracea var. italica), and cabbage (Brassica oleracea L.) accumulate higher amounts of selenium while lettuce (Lactuca sativa L.), onions (Allium sp.), sugar beets (Beta vulgaris L.), and beans (Phaseolus vulgaris L.) accumulate the least [84]. Forms of selenium in plants also have species specificity depending on the presence and activity of various enzymes involved in its metabolism. Thus, the bioavailability, transformation, and distribution of Se NPs in plants depend on a complex of factors—from physicochemical properties of nanoparticles to species specificity of plants and environmental conditions—which requires further research for the safe and effective application of Se NPs in agrotechnologies.

3.2. Multi-Activity of Se NPs

In agricultural technologies, Se NPs are widely used due to the wide variety of effects they have on plants. It is worth noting that the use of Se NPs, compared to selenium-containing organic and inorganic compounds, is often more effective due to their lower phytotoxicity, high bioavailability, and controlled action. In terms of crop yield, and biochemical processes in soil and plants, the use of traditional Se-containing fertilizers is considered less effective than the use of fertilizers with Se NPs [86]. It should also be noted that significant plant growth is observed even at high concentrations of Se NPs [87], which is not observed in the presence of inorganic selenium compounds. It was shown that Se nanoparticles at a concentration of 265–530 μM significantly increased organogenesis and root system growth (>40%) in tobacco (Nicotinia tabacum L.), while selenate at these and lower concentrations inhibited callus growth and root regeneration [88]. Application of selenite at a concentration of 20 mg/L inhibited the growth of rapeseed (Brassica napus L.); however, Se NPs at similar concentrations and higher dose-dependently increased the chlorophyll content and stimulated plant growth [89]. A comparative analysis of the effect of selenite and selenium nanoparticles synthesized by chemical and biological methods on the germination of pea seeds (Pisum sativum L.) showed that selenite inhibits germination (germination index 0.3) of seeds already at a concentration of 1 ppm, while Se NPs stimulate growth (germination index 120) at concentrations of 10 ppm and higher [90]. We also conducted a more in-depth comparison of the reported efficiency of Se NPs with the most commonly used mineral forms of selenium: selenite and selenate. Detailed information is provided in Section 4.

The effect of Se NPs on the growth and development of plant crops may depend not only on the plant species, but also on the morphological features of the nanoparticles, determined by the method of their synthesis and modification. In general, a number of beneficial effects of Se NPs on agricultural crops have been reported, which can be classified as biostimulatant, biofortification, nutraceutical, antioxidant, and antimicrobial (antifungal) activities [5]. The key biochemical and physiological mechanisms by which each of the listed effects is realized are discussed in more detail below in the relevant sections and are presented visually in Figure 2.

3.2.1. Biostimulant Activity

Among the possible areas of practical application of Se NPs in agrochemical technologies, the greatest attention is paid to their use as biostimulants. Active studies of Se-containing nanostructures as stimulants of plant growth and increasing their yields began only in recent years [91,92,93]. Nevertheless, by now, a significant amount of data has been accumulated illustrating the biostimulant effect of Se NPs (Figure 3).

The use of Se NPs promotes the enhancement of metabolic and photosynthesis processes in plants, activation of phytohormones such as auxins, abscisic acid (ABA), gibberellins (GA), and cytokinins (CK), responsible for cell proliferation and differentiation, and subsequent development and growth of plant organs [94]. In addition, the introduction of Se NPs promotes more efficient absorption and metabolism of key macronutrients involved in plant growth and development: nitrogen, phosphorus, and potassium [95,96], and also improves the physiological processes in the plant, stimulating the synthesis of photosynthetic pigments [97,98]. Recent studies have shown the stimulating effect of Se NPs on the growth of a wide range of crops, including wheat (Triticum aestivum L.), tomato (Solanum tomato L.), corn (Zea mays L.), soybean (Glycine max L.), bean (Phaseolus vulgaris L.), rice (Oryza sativa L.), etc. (

Table 2. Effect of selenium NPs on the growth and functional state of plants, according to literature data.

Plant	Composition	Synthesis Method	Size (nm)	Treatment Method	Concentrations	Maximal Effective Conc.	Effects	Ref.
Grape (Vitis vinifera L.)	Se NPs	Biosynthesis using leaf extract of Vitis vinifera L.	28–50	Foliar treatment	1, 10, 100 mg/L	10 mg/L	↑ Shoot length (38–48%), ↑ Fresh (15.9–24.6%) and dry (21.6–27.5%) mass, ↑ Leaf number (20.7–29.3%), ↑ Leaf area (up to 25%), ↑ Photosynthetic activity (7.5–16.3%), ↓ H2O2 content (13.7–33.8%), ↓ MDA content (16.4–30.8%), ↓ Proline content (~18.2–31.8%), ↓ SOD activity (~18.5–40%), CAT activity (11–35.6%), and APX activity (29.7–67.7%), ↑ Proline content without drought (15–80%)	[103]
Pomegranate (Punica granatum L.)	Se NPs	Commercial Se NPs (Iranian Nanomaterials Pioneers Co., Mashhad, Iran)	10–45	Foliar treatment	0.08, 0.16 mg/L	0.16 mg/L	↑ Yield (4.2–16.5%), ↑ Leaf area (2.3–24.9%), ↑ Chlorophyll content (2.1–24.3%), ↑ Fruit count (7.4–28.9%), ↑ Mineral elements: N (2.6–22.6%), P (25–43.8%), K (6.6–12.3%), Ca (8.2–17.3%), Mn (1.6–6.6%), ↑ Total soluble sugars (4.2–19.8%), ↑ Phenolic compounds (3.5–8.1%), ↑ Antioxidant activity (6.1–25.6%), ↑ Anthocyanins (4.8–27.7%), ↑ Peel thickness (5–27.2%), ↑ Fruit mass (2.4–5.2%)	[3]
Melon (Momordica charantia L., Cucumis melo L.)	Se NPs	Commercial Se NPs (NanoSany Corporation, Iranian Nanomaterials Pioneers Company)	10–45	Addition to Growth Medium for Seedlings	1, 4, 10, 30, 50 mg/L	4 mg/L	↑ Total fresh leaf mass (50%), ↑ Expression of transcription factors WRKY1 (7.9 times), PAL (4.5–11.9 times) and 4CL (9 times), ↑ NR activity (52%), ↑ Proline content (80%), ↑ POD (35%), CAT (10%) and active phenols (50%)	[104]
	Se NPs	Chemical synthesis with ascorbic acid	72.4	Foliar treatment	5 mg/L	–	↓ Powdery mildew disease index (Podosphaera xanthii) (21–45%), ↑ PAL activity (22–38%), 4-hydroxylase (24–126%), 4-coumarate: Co A ligase (19–64%), CAT (42–65%), APX (24–94%), ↓ H2O2 content (9–25%) and MDA content (30–48%)	[105]
	Se NPs	Commercially available Se NPs (Guilin Jiqi Group Co. Ltd., Guilin, China)	–	Foliar treatment	2.5, 5, 10 mg/L	5 mg/L	↑ Fructose, glucose, galactitol, stachyose, lactic, tartaric, fumaric, malic, and succinic acid content in treated plants, ↑ Cucurbitacin B synthesis, ↑ Plant resistance to pathogens, ↑ Antioxidant enzyme activity	[106]
Field cabbage (Brassica campestris L.)	Se NPs	Biosynthesis using Allamanda cathartica L. extract	60.31	Seed priming in Se NPs solution	12.5, 25, 50 mg/L	25 mg/L	Under salt stress 200 mM: ↑ Germination rate (31%), ↑ Shoot length (92%), ↑ Root length (78%), ↑ Total chlorophyll content (49%) under salt stress	[107]
Strawberry (Fragaria × ananassa)	Se NPs	Commercially available Se NPs (NANOSANY Corporation)	10–45	Foliar treatment	10, 20 mg/L	20 mg/L	↑ Sucrose content (9.80–10.13%), ↑ Fructose content (10.21–19.80%), ↑ Glucose content (19.69–24.19%), ↑ Malic acid content (14.02–24.24%), citric acid content (21.33–21.78%), and succinic acid content (25.51–38.70%)	[108]
	Se NPs	Commercially available Se NPs (Iranian Nanomaterials Pioneers Co., Mashhad, Iran)	10–45	Foliar treatment	0.79, 7.90 mg/L	7.90 mg/L	Under salt stress: ↓ SOD activity (5.9–11.8%), ↓ MDA (~17.8–18.9%) and H2O2 (~20–60%), ↑ PAL activity (~5.1–43.6%)	[109]
	Se, Cu NPs	Chemical reduction	5–18 (Se), 35–42 (Cu)	Foliar treatment	100 mg/L	–	↑ Antioxidant enzyme activity (CAT, POD, SOD), ↓ Malondialdehyde content, ↑ Photosynthetic pigment content (chlorophyll a, chlorophyll b, and total chlorophyll), ↑ Drought resistance	[97]
Corn (Zea mays L.)	Se NPs	Chemical reduction using surfactants	–	Foliar treatment	100, 150, 200, 250 µg/pot 1	200 µg/pot	Se NPs were less effective than Na2SeO4 application	[110]
	Se NPs	Chemical reduction	–	Foliar treatment	20 mg/L	–	Cd-induced stress: ↑ chlorophyll content (40.2%), ↑ carotenoid content (50%), no growth changes, ↑ photosynthesis rate (84%), ↑ mineral element concentrations: Ca (26%), Fe (55.4%), Mg (27%), Na (28.6%) and Zn (10.1%), ↓ MDA content (15.7%) and H2O2 content (34.9%), ↑ GSH content (20.9% and 51.8%), APX (92% and 35.7%), CAT (23.1% and 10.4%), SOD (21.1%) in leaves and roots respectively, ↓ ZmHMA2 gene expression (58.8%) in roots, ↑ ZmHMA3 gene expression (~58.1%) in roots	[111]
	Se NPs	Chemical reduction	55–65	Foliar treatment	10 mg/L	–	↑ Flavor compound content, regulation of abscisic acid and sucrose accumulation, ↑ Expression of maturation-related transcription factors: FaMYB1, FaMYB10, FaRIF, FaSnRK1, FaSnRK2.6 and FaABI1 when combined with Se NPs, ↑ Antioxidant capacity of fruits, ↑ Selenium content	[112]
Sesame (Sesamum indicum L.)	Se NPs	Biosynthesis using Allium sativum extract	100	Pre-treatment of seeds, foliar spray	10, 20, 30, 40, 50 mg/L	40 mg/L	↑ Root length (55.7%), ↑ Leaf number (48%), ↑ Stem diameter (38%), ↑ SOD (147%), POD (140%) and CAT (76%) activity	[113]
Lemon balm (Melissa officinalis L.)	Se, ZnO NPs	Commercially available Se NPs (NanoSany Corporation, Iranian Nanomaterials Pioneers Company)	10–45	Watering with nutrient solution containing NPs	10, 50 mg/L	10 mg/L	↑ Leaf fresh mass (19.6%) with 10 mg/L Se NPs, and ↓ Leaf fresh mass (43.5%) with 50 mg/L Se NPs, ↓ NR activity (15%), ↑ POD activity (44.9–63.5%), ↑ Phenolic content in leaves (59.4–79.5%) and roots (61.7–100%)	[114]
Cucumber (Cucumis sativus L.)	Se NPs	Biosynthesis using Lactobacillus casei	50–200	Foliar treatment	25 mg/L	–	↑ Average fruit mass (6.1–16.1%), ↑ Plant height (11.7–12.3%), ↑ Leaf area (9.7–22.9%), ↑ Chlorophyll content (2.2–3.4%), ↑ CAT activity (10.8–11.4%), POD (40–48.4%), proline (15.8–21.1%), ↑ Relative water content (10.7%)	[115]
Jalapeno pepper (Capsicum annuum L.)	Se NPs	Chemical reduction	20	Watering at the base of the stem	1, 15,30, 45 mg/L	45 mg/L	↑ Plant height (up to 12.74%), yield (up to 52.75%), ↑ Root fresh mass (14.4–24.7%) ↑ Root dry mass (4.8–30.6%), ↑ Fruit density (5.5–59.3%), ↑ Chlorophyll content (up to 9.97%), ↑ Antioxidant capacity (14.6–35.7%), ↑ Total phenols (18.9–75.2%), ↑ CAT activity (6–40%), PAL (144.25–250.03%), ↓ APX activity (more than 60%) at 45 mg/L, ↓ GPX activity (20.9–48.49%)	[116]
	Se NPs	Commercially available Se NPs (NanoSany Corporation, Iranian Nanomaterials Pioneers Company)	10–45	Growing in nutrient medium with Se NPs	0.5, 1, 10, 30 mg/L	1 mg/L	At 0.5–1 mg/L: ↑ Leaf fresh mass (65.5%), ↑ bZIP1 gene expression (2.2–3.0 times) and WRKY1 (3.8 times), ↑ NR activity (48%), ↑ POD activity (21%), ↑ Catalase activity (56%), ↑ PAL activity (~41.6–58.3%), ↑ soluble phenols (~23.5–41.2%). At 10 mg/L: ↑ bZIP1 gene expression (5.4 times), ↓ WRKY1 gene expression (15.8 times), ↓ NR activity (–26.2%), ↓ POD activity (–24.7%), ↑ CAT activity (2.4 times), PAL (~100%), ↑ Soluble phenols (~88%)	[102]
Sunflower (Helianthus annuus L.)	Se NPs	Biosynthesis using Penicillium chrysogenum	3–15	Foliar treatment	10, 15, 20, 25 mg/L	20 mg/L	↑ Growth, ↑ Carotenoid concentrations, ↑ Chlorophyll content, carbohydrates, proteins, phenolic compounds, and proline	[117]
Wheat (Triticum aestivum L.)	Se NPs	Biosynthesis using Lactobacillus acidophilus	46	Seed priming	50–150 mg/L	100 mg/L	↑ Growth, ↑ Grain quantity and quality, ↑ Carotenoid and chlorophyll concentrations	[118]
	Se NPs	Commercially available Se NPs (Sigma–Aldrich)	5–70	Foliar treatment	10 mg/L	–	↑ Growth, ↑ Photosynthesis and gas exchange rates, ↑ Drought and heat resistance, ↑ CAT, APX, and SOD activity	[119]
	Se NPs	Synthesis using Melia azedarach L. extract	61	Foliar treatment	10, 20, 30, 40 mg/L	30 mg/L	↑ Growth, ↑ Chlorophyll, proline, phenol, and flavonoid concentrations, ↑ SOD activity	[120]
	Se NPs	Biosynthesis using Allium sativum L. extract	50–150	Foliar treatment	10, 20, 30, 40 mg/L	30 mg/L	↑ Plant height, ↑ Shoot length, ↑ Fresh and dry shoot mass, ↑ Root length, ↑ Fresh and dry root mass, ↑ Leaf area, ↑ Leaf number and length	[121]
	Se NPs	Biosynthesis using Bacillus cereus	41–102	Seed priming	50, 75, 100 mg/L	100 mg/L	↑ Germination parameters, ↑ Growth under salt stress	[122]
	Se NPs	Biosynthesis using lemon (Citrus limon L.) leaf extract	37	Foliar treatment	5, 10, 100 mg/L	100 mg/L	↑ Yield, ↑ 1000-grain weight, ↑ Chlorophyll a and b content, ↓ H2O2 and MDA concentration under salt stress	[123]
	Se NPs	Commercially available Se NPs (Xianfeng Nano Company, Nanjing, China)	25.2	Foliar treatment	0.24, 0.48, 0.96 mg/plant 1	0.96 mg/plant	↓ Cd content by 35.0%, ↑ Grain yield by 33.9%, ↓ Expression of Cd transport protein genes (TaNramp5 and TaLCT1), ↑ Expression of Cd vacuolar sequestration protein genes (TaHMA3 and TaTM20), ↑ Leaf antioxidant metabolite levels, ↓ Rhizosphere organic acid content, ↓ Cd bioavailability in rhizosphere soil, ↑ Development of carbon- and nitrogen-related soil microorganisms (Solirubrobacter and Pedomicrobium)	[124]
	Se NPs	–	–	Watering with a solution containing nanoparticles	2, 5 mg/L	2 mg/L	↓ Plant length (20.3–25.2%), ↓ Root length (24.7–34.1%), ↓ Shoot length (18.1–20.1%), ↓ Root dry weight (151.9–33.3%), ↓ Shoot dry weight (51.7–34.7%), ↓ Plant dry weight (52.1–28.8%), ↓ SOD content (37.5%) at 5 mg/L, POX (54.3–56.5%), ↑ SOD content (11.1%) at 2 mg/L, CAT (52.5–57.5), ↓ PAL activity (2.2–59%), ↑ Total chlorophyll content (199.6–95.7%) and carotenoids (101.6–154.3%)	[125]
Rapeseed (Brassica napus L.)	Se NPs	Biosynthesis using Comamonas testosteroni	167	Seed priming	3.95, 7.9, 11.84 mg/L	11.84 mg/L	↑ Germination rate (1.06–1.27%), ↑ Fresh shoot weight (1.59–4.49%), ↑ Dry shoot weight (29.56–26.94%), ↑ Root weight (22.8–25.5%), ↑ Shoot and root length (up to 8.47% and 24.74%), ↑ Total chlorophyll content (24.04%), ↓ Soluble protein content (8.48–5.86%), ↑ SOD (204–119%), POD (305.2–216%), and APX (191–106.1%) activity in shoots	[126]
	Se NPs	Biosynthesis using a bacterial isolate from the Caspian Sea	–	Foliar treatment	5, 10, 20 mg/L	20 mg/L	↑ Total dry weight (62–106%), ↑ Cd stress resistance, ↓ ROS formation, ↓ MDA by 73–81%, ↓ O2·– by 59–61%, ↓ H2O2 by 39–59%, ↑ Catalase activity (72%), ↑ Ascorbate peroxidase activity (75%) at 20 mg/L	[89]
	Se NPs, melatonin	Chemical reduction using melatonin	~50–60	Addition of NPs to nutrient medium	50 mg/L	–	↓ As-induced stress, ↑ Root dry weight (14.28%), ↑ Leaf dry weight (23.07%), ↑ Photosynthesis rate (50.73%), ↑ Transpiration rate (64.97%), ↓ MDA (17–29%) and H2O2 (14–24%), ↑ SOD (31%), POD (96%), CAT (50%), APX (20%), GR (50%), and GSH (60%) activity	[127]
Rice (Oryza sativa L.)	Se NPs	Chemical synthesis	50–100	Foliar treatment	0.08 mg/L	–	↑ Growth, ↑ Chlorophyll content, ↑ CAT and GPX activity, ↓ Malondialdehyde concentration, ↓ Cd impact on growth and chlorophyll content	[128]
	Se NPs	Biosynthesis using Vitis vinifera L. raisin extract	40	Seed priming	1.58, 1.97 mg/L	1.97 mg/L	↑ Germination, ↑ Seedling emergence and growth, ↑ Germination energy, ↑ Nutrient content, ↑ α-amylase activity (>60%), ↑ Antioxidant enzyme activity: SOD (>50%), CAT (>80%), APX (>80%)	[129]
	Se NPs	Chemical reduction	50	Foliar treatment	0.5, 5 mg/L	0.5 mg/L	↑ Leaf Se concentration (by 420%), ↓ Lead (Pb) accumulation in roots	[130]
	Se NPs	Chemical synthesis using ascorbic acid	50.1 ± 5.6	Seed priming	0.5, 2, 5, 20, 50, 200 mg/L	50 mg/L	↓ Root length (64–73%), ↓ Root number (48–79%) at Se NP concentrations > 50 mg/L, ↑ APX, CAT, and SOD activity, ↑ Antioxidant metabolism, ↓ ROS levels	[131]
	Se NPs in gel	Biosynthesis using Aloe vera extract	403	Cultivation in nutrient solution with NPs	15, 30 mg/L	15 mg/L	↑ Fresh root weight (100.7%), ↑ Chlorophyll content (>50%), ↓ Acid-soluble Cd fraction (4.01%)	[132]
Celery (Apium graveolens L.)	Se NPs	Commercially available Se NPs (Guilin JIQI Group Co. Ltd.)	50–78	Foliar treatment	5, 10 mg/L	5 mg/L	↑ Antioxidant capacity (46.7%), ↑ Apigenin biosynthesis (58.4%), ↑ Rutin biosynthesis (66.2%), ↑ p-Coumaric acid biosynthesis (80.4%), ↑ Ferulic acid biosynthesis (68.2%), ↑ Luteolin biosynthesis (87.0%), ↑ Kaempferol biosynthesis (105.7%), ↑ Leaf vitamin C content (26.7%)	[133]
Grain Sorghum (Sorghum bicolor L.)	Se NPs	Chemical reduction	10–40	Foliar treatment	10 mg/L	–	↓ O2·– content (29%), ↓ H2O2 content (38%), ↓ MDA content (39%), ↓ Membrane damage degree (25%) under temperature stress, ↑ SOD (22%), CAT (24%), POX (11%), and GPX (9%) activity, ↑ Yield under stress conditions (26%)	[134]
Soybean (Glycine max L.)	Se NPs	Biosynthesis using Penicillium chrysogenum	30–80	Seed priming	0.008, 0.04, 0.08, 0.16, 0.24 mg/L	0.08 mg/L	↑ Growth, ↑ Germination rate (93%), ↑ Germination energy (76.5%), ↑ Germination speed (19%), ↑ Average germination time (4.3 days)	[135]
	Se NPs	Commercial product (Sigma–Aldrich)	20	Foliar treatment	100, 150, 200 mg/L	150 mg/L	↑ Drought resistance and yield under drought conditions, ↓ Electrolyte leakage, ↓ ROS accumulation, ↓ MDA content	[136]
Tobacco (Nicotiana tabacum L,)	Se NPs	Biosynthesis using Lactobacillus acidophilus	–	Cultivation in nutrient medium with NPs	0.0412, 0.42, 4.2, 20.1, 42.0 mg/L	42.0 mg/L	↑ Shoot rooting rate (40% on day 8 and 10% on day 16), ↑ Root system growth (40%)	[88]
	Se NPs	Biosynthesis using Lactobacillus acidophilus	50–200	Addition to cultivation medium	100 mg/L	–	× Negative effect on seedling growth even at maximum Se NP concentration (100 mg/L), Minor effect on thylakoid membrane ultrastructure and photosynthetic apparatus	[137]
Tomato (Solanum lycopersicum L.)	Se NPs	Commercially available Se NPs (ID Nano, México)	50	Seed priming	1, 10, 50 mg/L	1 mg/L	↑ Seed germination, ↑ Total antioxidant capacity (TAC) (by 38.97%), ↑ Chlorophyll content (by 21.28%)	[138]
	Se NPs		50		1, 10, 50 mg/L	10 mg/L	↑ Growth, ↑ Germination rate (by 50%), ↑ Germination energy index (by 208%), ↑ Stem length (by 10.8%), ↑ Stem width (by 4.3%), ↑ Chlorophyll content (by 30.8%)	[139]
	Se NPs	Commercially available Se NPs (Sigma–Aldrich)	70–90	Seed priming	25, 50, 75, 100 mg/L	75 mg/L	↑ Yield under drought conditions, ↓ H2O2 content (by 39.3%) and MDA (by 28.9%), ↑ SOD activity (by 34.9) and CAT (by 25.4%), ↑ Carotenoid content (by 13.5%), ↑ α-Tocopherol content (by 22.8%), ↑ Flavonoid content (by 25.2%), ↑ Anthocyanin content (by 19.6%), ↑ Ascorbic acid content (by 26.4%), ↑ Reduced glutathione content (by 14.8%)	[140]
	Se NPs	Commercially available Se NPs (Iranian Nanomaterials Pioneers Co., Mashhad, Iran)	10–45	Foliar treatment	3, 10 mg/L	3 mg/L	↑ Shoot and root biomass, ↑ Antioxidant enzyme activity: CAT and POD, ↑ Ascorbate concentration, ↑ Non-protein thiol and soluble phenol concentrations, ↑ Phenylalanine ammonia-lyase activity, ↑ miR172 level (3.5 times), ↑ bZIP transcription factor expression (9.7 times), ↑ CRTISO gene expression in leaves (5.5 times)	[141]
		Biosynthesis from Trichoderma atroviride	–	Seed priming	100 mg/L	–	↑ Germination rate (22%), ↑ Plant height (51.2%), ↑ Fruit weight, ↑ (7–9 days), ↑ Resistance to Phytophthora infestans (72.9–49.5%)	[91]
	Se, Cu NPs	Chemical reduction	2–20	Addition to substrate	1, 10, 20 mg/L	10 mg/L	↑ Yield by 21%, ↑ Chlorophyll, ascorbic acid, glutathione, superoxide dismutase, glutathione peroxidase, and phenylalanine ammonia-lyase content in leaves, ↑ Ascorbic acid, glutathione, and flavonoid content in fruits	[142]
	Se NPs	Chemical reduction	2–20	Addition to substrate	1, 5, 10, 20 mg/L	20 mg/L	↑ Yield under salt stress, ↑ Photosynthetic pigment content in leaves, ↑ Antioxidant content in fruits: (lycopene, β–carotene, flavonoids, and phenols); Under salt stress: ↓ Plant height (up to 3.2%), ↓ Leaf number (2.5–4%), ↓ Cluster number (0.9–3.5%), ↓ Fresh (2.9–5.8%) and dry (3–6.2%) aboveground weight, ↑ Fruit number (4–11.1%), ↑ Average fruit weight (0.3–3.7%), ↑ Cluster number at 20 mg/L (1.1%), ↑ Fresh aboveground weight (12.3%), ↑ Dry aboveground weight (14.6%) at 10 mg/L, ↓ Fruit number at 5 mg/L (2.1%), ↓ Phenol content at 1 mg/L	[143]
	Se, Cu NPs	Chemical reduction	2–20	Foliar treatment	10, 20 mg/L	20 mg/L	Under A. solani infection: ↑ Plant height (up to 1.4%), ↑ Leaf number (up to 6.2%), ↑ Cluster number (up to 2.8%), ↑ Phenol content in leaves (up to 11.6%), ↑ Phenol content in fruits (1.7–27.1%), ↑ GPX activity in leaves (32.2–133.4), ↑ PAL activity in leaves (10.6–127.8%), ↑ APX activity in leaves (92.3–434.6%), ↑ APX activity in fruits (46.9%), ↓ GPX activity in fruits (16.3–42.2%), ↓ PAL activity in fruits (31.7–62.4%), ↓ SOD activity in leaves (up to 25.9%), ↓ SOD activity in fruits (15.3–40.5%)	[144]
	Se NPs	Biosynthesis using Nigella sativa extract	272 ± 72	Foliar treatment, soil addition	100, 300 mg/L	100 mg/L	Under Cd-induced stress: ↑ Shoot length by 45%, ↑ Root length by 51%, ↑ Branch number by 506%, ↑ Leaf number per plant by 208%, ↑ Ascorbic acid, protein, phenolic compounds, flavonoids, and proline content, ↑ Leaf area by 82%, ↑ Fruit yield (>100%), ↑ Fruit weight (>100%), ↑ Lycopene content (75%), ↓ Days to fruit set in tomato plants, ↓ Cd uptake	[145]
Potato (Solanum tuberosum L.)	Se NPs	–	4–8	Tuber treatment before planting	0.13 g/hectare ¹	–	↑ Protein content, ↑ Vitamin C content	[146]
Common bean (Phaseolus vulgaris L.)	Se NPs	Commercially available Se NPs (Sigma–Aldrich)	10–45	Foliar treatment	39.5, 79, 118.5 mg/L	79 mg/L	↑ Shoot length (32.5–98.8%), ↑ Number of leaves (18.5–54.4%), ↑ Leaf area (6.8–28.9%), ↑ Shoot dry weight (55.5–128.5%), ↑ Chlorophyll “a” (17.5–40.2%) and “b” (18.2–34.5%), ↑ Photosynthesis rate (68.6–121.8%), ↑ Transpiration rate (43.6–111%), ↑ Relative water content (2.3–16.9%), ↑ Membrane stability index (32.5–118.6%), ↑ Free proline content (7.1–22.8%), ↑ Soluble sugar content (14.1–39.3%), ↑ CAT content (~9.5–42.9%), POD (~28.6–107.1%), APX (~10.5–57.9%), SOD (~50–125%)	[147]
	Se NPs	Biosynthesis using Moringa oleifera leaf extract	~71.2	Foliar treatment	50, 100, 150, 200, 250 mg/L	100 mg/L	↓ Bacterial growth, biofilm formation, Antibacterial activity against Pseudomonas aeruginosa, ↑ Chlorophyll, tannin, flavonoid, and phenol content in plants	[148]
	Se NPs	Dissolution of Se powder in ethylenediamine (EDA) followed by dispersion in water	53	–	6, 10, 14, 20 mg/L	20 mg/L	↑ Seleno-amino acid content in bean sprouts up to 0.182 µg/g, ↓ DPPH (83.3%), ABTS (88.1%), ·OH (71.6%), × Significant cytotoxicity against HeLa and HepG2 cells	[149]
Barley (Hordéum vulgare L.)	Se NPs	Chemical reduction	50	Seed priming	1, 5, 10, 20 mg/L	5 mg/L	At 1–10 mg/L Se NPs: ↑ Root number (4.3–46.2%), ↑ Root length (8.6–139.3%), ↑ Shoot length (27.1–164.5%), ↑ Seed germination rate (~4–8%); At 20 mg/L Se NPs: ↓ Root number (15.1%), ↑ Shoot length (15.8%)	[150]
Radish (Raphanus sativus var. sativus), Arugula (Eruca vesicaria subsp. sativa (Mill.) Thell.), Eggplant (Solanum melongena L.), Cucumber (Cucumis sativus L.), Tomato (Solanum lycopersicum L.), Chili Pepper (Capsicum annuum L.)	Se NPs	Laser ablation in water followed by fragmentation	<100	Addition to substrate	1, 5, 10, 25 µg/kg 1	10 µg/kg 1	↑ Hyperthermia resistance, ↑ Leaf area at 1–10 µg/mL Se NPs, ↓ Leaf area at 25 µg/mL Se NPs, × Effect on Capsicum annuum under hyperthermia simulation, × Growth changes under non-stress conditions,	[92]
Chili Pepper (Capsicum annuum L.)	Se NPs + ZnO + Arbuscular mycorrhiza fungus (AMF) inoculation	Chemical reduction	8.37–12.8 (ZnO NPs), 5.27–6.51 (Se NPs)	Foliar treatment	10 mg/L	–	Under combined application of ZnO NPs, Se NPs + AMF: ↑ Average fruit weight by 98.7%, ↑ Fruit number by 54.8%, ↑ Thylakoid grana thickness, ↑ Chloroplast number, ↑ Total ascorbic acid content, ↑ Total capsaicin content	[49]
	Ag–Se bimetallic NPs	–	–	Foliar treatment	100 mg/L	–	↓ Disease index by 17.5%, ↑ Protection against Ralstonia solanacearum wilt by 78.1%, ↑ Vegetative growth, ↑ Photosynthetic pigment content, ↑ Soluble carbohydrate and protein content in infected plants	[50]
Pink Periwinkle (Catharanthus roseus (L.) G.Do), Marigold (Calendula officinalis L.)	Se NPs	Biosynthesis using Amphipterygium glaucum extract	40–60	Foliar treatment	0.8, 1.6, 3.95, 7.9 mg/L	1.6 mg/L	Catharanthus roseus: ↑ Fresh (37.7–72.1%) and dry (28.6–57.1%) flower weight, ↑ Fresh leaf weight (14.7–22%), ↑ Fresh (14.2–24.7%) and dry (5.3–13.2%) stem weight, ↑ Flower number (42.8–60.9%), ↑ Total carotenoid content (13.3–40%), ↓ Photosynthetic efficiency (up to 6.7%) Calendula officinalis: ↑ Dry leaf weight (20.7–34.5%), ↑ Fresh stem weight (13.2–27.2%), ↑ Fresh (45.1–53.4%) and dry (22.7–45.5%) root weight, ↑ Root length (up to 29.7%), ↑ Total chlorophyll (up to 78.6%) and carotenoid (up to 57.1%) content, ↓ Photosynthetic efficiency (up to 5.5%)	[100]
Spinach (Spinacia oleracea L. var. Stoik)	Se NPs	Laser ablation in water	~35	Foliar treatment	22.11 mg/L	–	↑ Average weight of male (~93.3%) and female (~90%) plants, ↑ Ascorbic acid content in male (16.9%) and female (15.3%) plants, ↓ Cd content in male (13.3%) and female (66.1%) plants, ↓ Lead content in male (−11.1%) and female (−19%) plants	[151]
Bok Choy (Brassica rapa subsp. chinensis)	Se NPs	Commercially available Se NPs (Chip Biology Co., Tianjin, China)	50	Foliar treatment/Soil application	1, 2 mg/kg 1 (Soil); 30, 60 mg/kg 1 (Foliar)	2 mg/kg 1 (Soil); 60 mg/kg 1 (Foliar)	↓ Cd concentration by 25.9–42.4% and Cd uptake rate by 33.4–37.8%, × Effect on available Cd, ↑ Se concentration by 3.1–6.3 times in bok choy and higher Se concentration in roots than shoots Under foliar application: × Effect on Cd uptake, ↑ Se accumulation by 2.4–33.0 times	[152]
				Foliar treatment	5, 10, 20, 40 mg/L	20 mg/L	↓ Cd toxicity, ↓ Cd accumulation, ↓ MDA formation, ↑ Plant growth, ↑ Antioxidant enzyme activity	[153]
	Se NPs	Laser ablation in liquid	60	Foliar treatment	5, 10 mg/L	10 mg/L	× Negative effect on growth at 10 mg/L Se–NP concentration compared to SeO32−, ↑ Uptake of macronutrients (Mg, P, K, and Ca) and micronutrients (Mn, Cu, Zn, and Fe), ↑ Se accumulation in leaves, ↓ Transport capacity compared to SeO32−	[95]
Shrubby Glasswort (Sarcocornia fruticose L.)	Se NPs	–	50–100	Addition to substrate	100 mg/L	–	Under salt stress (0.7–1 M NaCl): ↑ Fresh (32.4–10.8%) and dry (5.2–6.9%) shoot weight, ↑ Water content (4.2–2.2%), ↑ Chlorophyll a (9.4–5.8%), b (13.7–10.3%), and total (9.6–6.2%) content, ↑ Carotenoid content (37.6–7.1%), ↓ Proline content (46.6–33%), ↓ MDA content (66.7–44.4%), ↓ APX activity (−23.8%) at 1 M, SOD (−9.1–3.6%), ↑ APX activity (19%) at 0.7 M, GPX (130.4–30.4%), PAL (50–11.8%)	[154]
Garden bean (Vicia faba L.)	Se, ZnO NPs	Biosynthesis using grape seed aqueous extract	7.67–12.86	Seed priming	50, 100, 200 mg/L	100 mg/L	↑ Dry pod weight (35.4–104.4%), ↑ Seed number per pod (11.8–76.5%), ↑ 100–seed weight (2.3–26.3%), At 50–100 ppm: ↑ Shoot (18–26.1%) and root (52.6–73.8%) length, ↑ Fresh shoot (33.8–51.7%) and root (32.2–39.7%) weight, ↑ Pod number (18.8–42.5%), At 200 ppm: ↓ Shoot (−11.4%) and root (−10.6%) length, ↓ Fresh shoot (−8.3%) and root (−3.8%) weight, ↓ Pod number (−5%)	[154]
	Se NPs	Laser ablation in deionized water	90	Foliar treatment	100 mg/L	–	Beans of the “Belorusskaya” variety: ↑ Stem length (5.3%), ↑ Pod length (6.7%), × Pod width, ↓ Pod thickness (26.3%), ↑ Weight of 1000 seeds (94.7%), ↑ Number of seeds per plant (33.1%), Beans of the “Russian black” variety: ↑ Stem length (9.4%), ↑ Pod length (14.3%), ↑ Pod width (6.3%), ↓ Pod thickness (30.8%), ↑ Weight of 1000 seeds (37.4%), ↑ Number of seeds per plant (15.3%)	[155]
Sagebrush (Artemisia annua L.)	Se NPs	Laser ablation in deionized water	100	Foliar treatment	30.6 mg/L	–	↑ Plant height (5.4%), ↑ Plant diameter (14.6%), ↑ Leaf length (8.4%), ↑ Leaf width (5.4%), ↑ Plant weight (10.8%), ↑ Total chlorophyll (128.6%) and carotene (47.6%) content, ↑ Antioxidant activity in stems (8.8%), ↓ Antioxidant activity in leaves (1.3%) and roots (7%), ↓ Proline (17.4%) and MDA (17.4%) content	[156]

↑—Increase in indicators, ↓—Decrease in indicators, × — No differences in indicators, NR—Nitrate reductase, GSH—Glutathione, GR—Glutathione reductase, GPX—Glutathione peroxidase, APX—Ascorbate peroxidase, CAT—Catalase, POD—Peroxidase, SOD—Superoxide dismutase, PAL—Phenylalanine ammonium lyase, MDA—Malondialdehyde, RWC—Relative water content. ¹—here and below, quantitative data in dimensions different from µg/mL are taken from the original works.

). In particular, a recent study by Sariñana-Navarrete et al. [99] demonstrated the stimulating effect of Se NPs when treating Capsicum annuum L. jalapeno seeds with Se NPs (1 mg/L). There was an elongation of seedling roots by 12.5%, and an increase in the length of pinnates by 13.7% and 16.4% when using 20 and 25 mg/L Se NPs, respectively. The stimulating effect of Se NPs on development and seed germination was associated with an increase in the expression of genes responsible for the synthesis of ABA and GA. At the same time, with an increase in the concentration of nanoparticles to 45 mg/L, a decrease in the length of seedlings by 52.5–80.1%, and in the fresh weight of seedlings by 56.6% was observed. In another study [100], the stimulating effect of Se NPs was due to the activation of the enzyme nitrate reductase (NADH), a key enzyme in nitrogen metabolism [101]. In a number of cases, the effect of Se NPs induced the expression of genes responsible for the expression of nitrate reductase with subsequent initiation of the synthesis of amino acids and proteins and acceleration of plant growth [102]. For example, treatment of periwinkle Catharanthus roseus (L.) G.Don plants with Se NPs led to an increase in plant height by 6% at nanoparticle concentrations of 10 and 20 μM. This is explained by the activation of phytohormones CK and GA, which stimulate the growth of plant stems and seed germination. Se NPs at the concentration of 10–100 μM increased the number of leaves by 28%. A significant increase of 26% in the number of branches was observed at the concentration of 20 μM. The remaining parameters such as leaf fresh weight, stem fresh weight, stem dry weight, flower number, flower fresh weight, flower dry weight, root growth, and root fresh and dry weights also showed significant increase compared to the control [100].

The optimal concentration of Se NPs for biological activity depends on the method of plant treatment. Among the literature sources we analyzed, the most common method of applying Se NPs is leaf treatment or foliar treatment (Figure 4). Low concentrations of Se NPs are usually used to stimulate seed germination and are used by soaking them [107], while higher concentrations are used by spraying leaves with a solution of nanoparticles [89]. In particular, it was shown that soaking (priming) seeds with Se NPs contributed to an increase in the percentage of seed germination in rapeseed (Brassica napus L.) [126], wheat (Triticum aestivum L.) [122], soybean (Glycine max L.) [135], and tomato (Solanum tomato L.) [138,139]. It is reported that smaller Se NPs have higher bioavailability from soil: small Se NPs (30 nm) have strong adsorption capacity due to their large specific surface area, which can adsorb more soil organic matter and reduce the agglomeration effect [72].

To enhance the biostimulating effect of Se NPs, their combination with other nanoparticles and/or other bioactive compounds is often used. For example, the combined effect of adding Se NPs with melatonin on melon plants was studied [157]. As is known, the introduction of melatonin ensures the growth of lateral and adventitious roots of plants, stimulates root development, and is also able to stimulate photosynthetic activity. These effects are realized, first of all, due to the activation of calcium-dependent signaling pathways; for example, the signaling pathway of mitogen-activated protein kinase (MAPK) [158]. It is also worth noting that melatonin is responsible for the synthesis and transport of a number of phytohormones [159]. The combined use of melatonin and Se NPs led to an increase in the fresh weight of the stem of the variety by 6–10%, as well as an increase in auxin expression (up to 236%). Also, a number of studies have noted a significant stimulating effect on plants when using Se NPs together with Cu NPs [142,144] and ZnO NPs [114]. In particular, Hernández-Hernández et al. examined spraying tomato plants with a mixture of Se NPs and Cu NPs (10 mg/L). The combined application of Se NPs and Cu NPs also led to an increase in the content of chlorophyll, vitamin C, glutathione, 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid, SOD, glutathione peroxidase, and phenylalanine ammonia lyase [142]. Se NPs have a pronounced biostimulating effect on plants, promoting their growth and development. An increase in the intracellular concentration of selenium promotes the activation of a complex of biochemical processes responsible for the synthesis and accumulation of nutrients and secondary metabolites, the assimilation of macro- and microelements, the synthesis of biologically active compounds, which is accompanied by an increase in biomass, and, as a consequence, the nutritional value of plant crops.

3.2.2. Stress-Protective Activity

The use of Se NPs not only significantly stimulates plant growth, but also increases plant resistance to biotic and abiotic stress factors [14]. In the studies we reviewed, the stress-protective activity of Se NPs was studied when plants were exposed to abiotic stress factors such as salt stress, drought, and exposure to heavy metals.

Salt Stress

As is known, excessive soil salinity is one of the main negative factors affecting the yield of agricultural crops [160]. The content of high concentrations of salts in the soil prevents the absorption of water by root cells, which leads to a water deficit in plant tissues and disruption of the osmotic balance. One of the most important properties of the applied Se-containing fertilizers is an increase in resistance to salt stress [161]. In particular, it was shown that the addition of Se NPs (5.10 and 20 mg/L) to the substrate significantly increased the quantity and quality of the tomato (Solanum tomato L.) yield under conditions of salt stress induced by the addition of 50 mM NaCl to the nutrient solution [143]. Also, in another work, when simulating salt stress by adding 50 and 100 mM NaCl, spraying the leaves of bitter melon plants (Momordica charantia L.) with an Se NP solution contributed to an increase in tolerance to salinity due to an increase in the activity of antioxidant enzymes, an increase in the concentration of proline, the relative content of water and K+, and a decrease in the content of malondialdehyde and H₂O₂ in plant tissue [43]. Also, spraying the leaves with a solution of Se NPs (10 and 20 mg/L) contributed to an increase in the yield and the activity of antioxidant enzymes (superoxide dismutase and peroxidase) in strawberry plants (Fragaria × ananassa Duch.) under salt stress. In strawberry plants treated with Se NPs, an increase in the concentration of indole-3-acetic acid and abscisic acid, phytohormones involved in the regulation of stress responses of plants to salinity, was noted [108]. In the work of Ghazi et al., the effect of Se NPs on the germination of wheat seedlings under salt stress was also examined, and it was demonstrated that under salinity conditions (150 mM NaCl solution), parameters such as the final germination percentage, germination energy index, and germination rate index were significantly increased and were at their maximum when using 75 mg/L Se NPs [122]. Also, against the background of salt stress by adding 1% NaCl solution to the soil of 2-week-old wheat seedlings, the use of Se NPs at a concentration of 0.1% contributed to the stimulation of growth, which was due to a significant increase in both morphological and biochemical parameters [123]. Also, the effectiveness of the use of Se NPs, compared to the introduction of sodium selenite, was reported in modeling salt stress in rapeseed plants (Brassica napus L.) using 150 and 200 mM NaCl solutions. In addition to modulating Na⁺ and K⁺ uptake, application of 150 μM/L Se NPs significantly improved the phenotypic characteristics of rapeseed seedlings without signs of toxicity, significantly increased germination, growth, photosynthetic efficiency, and osmolyte accumulation, and also reduced the content of ROS and MDA due to increased expression of antioxidant enzymes [126]. When simulating both moderate (50 mM NaCl) and severe (100 mM NaCl) salt stress in orange trees (Citrus × sinensis (L.) Osbeck), foliar application of 10 and 20 mg/L chitosan-modified Se NPs significantly increased the content of photosynthetic pigments and photosynthetic activity (Fv/Fm value) [162]. Thus, when introducing Se NPs, a significant reduction in the negative impact of salt stress on plant growth is observed due to the induction of the synthesis of antioxidant enzymes, phytohormones, and the regulation of ion exchange.

Water Deficiency and Heat Stress

Another common stress factor is drought and heat stress. Selenium activity in plant adaptation to drought is associated with increased stability of the plant photosynthetic apparatus, protection of chloroplasts, and an increase in chlorophyll content under stress conditions caused by water deficit [163]. It is also well known that drought leads to the formation of ROS in plants under stress, causing oxidative damage to cellular components [164]. Selenium is able to provide antioxidant activity by regulating redox reactions under stress conditions [163], protecting plants from damage caused by oxidative stress [165], stimulating the activity of antioxidant enzymes, and initiating proline biosynthesis [166,167]. Spraying soybean leaves with a solution of Se NPs contributed to the accumulation of aboveground dry biomass, and an increase in the relative content of water and photosynthetic pigments under drought conditions [136]. Also, foliar application of Se NPs at a concentration of 10 mg/L contributed to increased resistance of wheat (Triticum aestivum L.) plants to drought and heat, and an increase in the activity of the antioxidant enzyme system: catalase, superoxide dismutase, and ascorbate peroxidase [119]. An increase in the resistance of wheat plants to drought was noted with foliar application of Se NPs at a concentration of 30 mg/L. It was also found that this concentration of Se NPs is optimal for stimulating wheat growth both under drought and normal conditions [121]. In turn, spraying the leaves of grain sorghum (Sorghum bicolor (L.) Moench) plants with Se NPs at a concentration of 10 mg/L against the background of heat stress contributed to an increase in the activity of antioxidant enzymes, a decrease in the content of oxidants, an increase in the level of unsaturated phospholipids, and a significant increase in seed yield [134]. In a recent study, Daler et al. [103] also showed an increase in the drought tolerance of grape varietal plants after foliar treatment with 10 ppm Se NPs. The increase in the drought tolerance of plants treated with the considered Se NPs was due to an increase in the relative water content, modulation of stomatal conductance, a decrease in H₂O₂ concentrations of malondialdehyde, and a decrease in electrolyte leakage. Another study also demonstrated the effectiveness of using Se NPs at a concentration of 75 ppm when priming tomato (Solanum lycopersicum L.) seeds with subsequent cultivation in field trials under dry conditions. The applied pre-treatment of seeds contributed to an increase in the resistance of plants to drought, caused by a significant increase in the activity of SOD and CAT (by 34.9 and 25.4%, respectively), an increase in the content of carotenoids, α-tocopherols, flavonoids, anthocyanins, and ascorbic acid, and reduced and oxidized glutathione (by 13.5, 22.8, 25.2, 19.6, 26.4, 14.8, and 13.12%, respectively) [140]. Thus, the use of Se NPs through foliar application and seed priming contributes to the formation of drought resistance in agricultural plants through the production of antioxidants, modulation of oyster conductivity, and electrolyte transport.

Exposure to Heavy Metals

Another negative factor is the impact of heavy metals on the growth of plant crops. Pollution of agricultural land with cadmium (Cd) is a widespread problem worldwide. Among the metal pollutants, Cd is the most harmful to plant growth and development [168]. Plant roots easily absorb Cd due to the higher mobility and hydrophilicity of this metal in the rhizosphere [169]. As a result of absorption by the root system and further transportation in plants, cadmium can accumulate in its edible parts, which significantly reduces the quality of the product and can cause significant harm to the consumer. Exposure to cadmium can lead to a decrease in chlorophyll content, inhibition of photosynthesis, deterioration in the activity of various functional enzymes, and changes in plant metabolism, which can subsequently affect the yield and quality of agricultural crops. A number of studies have shown that the introduction of Se NPs against the background of cadmium-induced stress can have a positive effect on the growth and development of agricultural crops by regulating the absorption, translocation, and sequestration of Cd in plant cells [170]. In experiments with spinach plants (Spinacia oleracea L.), it was shown that Se NPs, compared to ionic forms of introduced selenium, exhibit the maximum effect of reducing the cadmium level (by 66%) [151]. Also, foliar feeding of maize (Zea mays L.) plants with Se NPs (20 mg/L) reduced the toxic effect of cadmium by stimulating photosynthesis and antioxidant protection, as well as reducing cadmium transport into plants and enhancing intracellular packaging of cadmium in vacuoles by regulating the expression of the ZmHMA2 and ZmHMA3 genes [111]. The protective effect of Se NPs against Cd toxicity, as shown in rapeseed plants (Brassica napus L.), may also be due to a reduction in oxidative damage to membrane proteins and lipids by reducing the level of reactive oxygen species and inhibiting the expression of NADPH oxidases (BnaRBOHC, BnaRBOHD1 and BnaRBOHF1) and glycolate oxidase (BnaGLO) [89]. Wang et al. also demonstrated a decrease in the toxic effect of Cd upon foliar treatment of wheat (Triticum aestivum L.) plants with Se NPs. The authors associated this effect with a decrease in the expression of genes of the family of transport proteins responsible for Cd transport in cells (TaNRAMP), as well as an increase in the expression of genes of vacuolar proteins responsible for Cd binding (TaHMA3 and TaTM20) [124]. It was shown that the addition of Se NPs contributed to the leveling of the toxic effect of Cd on rice (Oryza sativa L.) shoots at a concentration of 15 mg/L [132] and 1 μM [128]. In turn, the application of Se NPs to maize (Zea mays L.) leaves at a concentration of 20 mg/L (0.25 mg/plant) significantly reduced cadmium toxicity, contributed to an increase in photosynthetic activity, antioxidant capacity, and cadmium fixation in the cell wall [111]. Treatment of tomato plants (Solanum lycopersicum L.) with 100 and 300 mg/L Se NPs, applied both to the leaves and to the substrate, contributed to a decrease in Cd absorption, as well as an improvement in a number of morphometric (length of shoots, roots, number of branches and leaves, fruit weight, yield) and biochemical (content of phenolic compounds, protein, ascorbic acid, flavonoids, proline, and lycopene) parameters [145]. Treatment of coriander seeds (Coriandrum sativum L.) with Se NPs at a concentration of 10 mg/L contributed to a decrease in Cd absorption, MDA, and H₂O₂ content and contributed to an increase in proline content. However, the level of proline content during Se NP treatment of control plants (without modeling Cd-induced stress) showed a 39% worse result, compared to plants affected by cadmium [168]. With soil application of Se NPs, a decrease in the Cd content in bok choy (Brassica rapa Pak Choi Group) by 25.9–42.4% was observed against the background of an increase in the Se content by 3.1–6.3 times [152]. Also, a number of studies have shown the effectiveness of Se NP application under the influence of other heavy metals, in particular, arsenic and antimony [127]. For example, the addition of Se NPs (0.05 g/L) to a hydroponic cultivation system led to a significant increase in the height of rice shoots (by 73.3%), fresh shoot weight (by 38.7%), and dry weight (by 28.8%) under antimony-induced stress [99]. Hydroponic application of Se NPs significantly reduced the accumulation of antimony in rice roots (by 77.1%) and shoots (by 35.1%), and also reduced its transport from the root to the shoot (by 55.3%). In addition, Se NPs supplementation was reported to regulate the expression of antimony detoxification-related genes in rice, such as OsCuZnSOD2, OsCATA, OsGSH1, OsABCC1, and OsWAK11 [171]. Foliar application of Se NPs (5 mg/L) to Chinese cabbage (Brassica chinensis L.) plants significantly reduced the cadmium, lead, and mercury contents in shoots and roots, and improved plant growth by enhancing the antioxidant defense system and stimulating chlorophyll synthesis [171]. Although the detailed mechanisms of Se NPs action on plant metabolism are still being investigated, the available data suggest that Se plays an important role in regulating the activity of antioxidant system components and is also able to modulate heavy metal transport in cells, providing plant protection from abiotic stress factors.

3.2.3. Biofortification and Nutraceutical Activity

The biofortification activity of Se NPs is associated with their ability to improve the nutritional value of agricultural products, as well as to improve the appearance of fruits [172]. This activity of selenium can potentially be useful in solving the problem of deterioration of the organoleptic properties of plant agricultural products due to active treatment with pesticides, contamination with phytopathogens, nutrient deficiency, and the influence of exogenous stress factors (ambient temperature, soil composition, excess/deficiency of water, etc.).

It was reported that the use of Se NPs contributed to an increase in the content of sugars and acids, directly affecting the taste of strawberries treated with the fungicides boscalid and pydiflumetofen [112]. While boscalid and pydiflumetofen decreased the anthocyanin content, negatively affected the levels of glucose, fructose, sucrose, total dissolved solids, and soluble sugar, and negatively affected the volatile compounds, the application of Se NPs in the fungicide experimental groups contributed to the increase in the content of anthocyanins by 26.38%, and sugars (glucose by 45.79 and 28.43%, fructose by 40.24 and 54.52%, and sucrose by 40.29 and 33.72% compared to the application of boscalid and pydiflumetofen, respectively) [112]. Similarly, in another study, the root addition of 10 mg/L Se NPs to tomato plants contributed to the change in the metabolic profile of plants and the biochemical composition of fruits. Improvement of biochemical parameters affecting nutraceutical qualities of tomatoes was noted: an increase in naringenin and chlorogenic acid content, a decrease in coumaric acid content, and a change in volatile compounds such as hexanol and 1-penten-3-one responsible for the pleasant aroma of tomatoes [173]. Another study also noted an increase in the content of vitamin C, glutathione, phenols, and flavonoids in tomatoes with the introduction of Se NPs together with Cu NPs [144]. The use of a 1% nanocomposite consisting of Se NPs, chitosan, and rosmarinic acid contributed to the preservation of persimmon fruit firmness by 85.7%. At the same time, the efficiency in preventing the growth of black rot (Alternaria alternata) during fruit storage was higher than when using the fungicide imazalil [174]. Notably, Se NPs also contributed to improving the taste of Camellia sinensis (L.) Kuntze summer tea by reducing bitterness and astringency [175]. Thus, the growing interest in Se NPs in the context of their biofortification and nutraceutical activity highlights their potential for use in improving the nutritional and gastronomic value, as well as improving the appearance of fruits. The high bioavailability of selenium in the form of Se NPs makes their use a promising option for increasing the selenium content in food crops, thereby solving the problem of selenium deficiency in the population.

3.2.4. Antioxidant Activity

An integral property of agricultural crops is their ability to withstand various external stress factors: both abiotic (climatic, edaphic and topographic threats) and biotic [176,177,178,179]. An important component in the response to stress in plants is the activation of antioxidant enzymes [180]. Numerous studies analyzed during the literature review showed that the addition of Se NPs leads to an increase in the activity of the main enzymes involved in the antioxidant protection of cells, such as superoxide dismutase, catalase, and glutathione peroxidase [181]. Increased plant resistance to stress can also be due to a change in the redox state of cells caused by stimulation of the synthesis of non-enzymatic antioxidants, such as polyphenols and carotenoids. It was shown that foliar treatment of celery plants (Apium graveolens L.) with Se NPs significantly increased the total antioxidant capacity of leaves by 46.7%, the content of β-carotenes by 61.4%, and vitamin C by 26.7%. The synthesis of such flavonoids as apigenin (by 58.4%), rutin (by 66.2%), p-coumaric acid (by 80.4%), ferulic acid (by 68.2%), luteolin (by 87.0%), and kaempferol (by 105.7%) increased [133]. The observed increase in antioxidant capacity under the action of Se NPs may be due to the regulation of the α-linolenic acid biosynthesis pathway, which leads to greater biotransformation or accumulation of various nutrients in celery [133]. Also, in another recent work, it was demonstrated that soaking in an Se NP solution at a concentration of 0.5 mg/L stimulated the activity of antioxidant enzymes ascorbate peroxidase, catalase, and superoxide dismutase, and decreased the content of S-nitrosothiols, nitrites, and hydrogen peroxide in rice seeds without affecting the physiological characteristics of seedlings. It is noteworthy that an increase in the concentration of Se NPs had the opposite effect: the content of antioxidant enzymes decreased and seedling growth slowed down [131]. The improvement in the rate of photosynthesis and a decrease in the loss of photosynthetic pigments under stress conditions can also be partially explained by the fact that the action of Se NPs reduces lipid peroxidation in thylakoid membranes, providing them with stability and increasing the synthesis of photosynthetic pigments [182]. It was shown that foliar application of Se NPs increased antioxidant activity (decreased malondialdehyde concentration, increased glutathione peroxidase and catalase activity, reduced glutathione content) in wheat sprouts, and induced an increase in the level of melatonin (by 88.6%) and 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) (by 64.3%) [183]. A number of other studies also demonstrated that high concentrations of Se NPs (10 mg/L and higher) contributed to an increase in the concentration of proline, an amino acid actively accumulated by plants under stressful conditions [184], as well as the accumulation of hydrogen peroxide and lipid peroxidation products in plants, which indicates the development of oxidative stress and disruption of the integrity of cell membranes [141]. In general, the use of Se NPs improves the physiological and biochemical responses of plants under stress conditions by modulating both enzymatic and non-enzymatic antioxidant systems. However, questions remain regarding the optimal concentrations of Se NPs required for their maximum efficiency in using these nanoparticles.

3.2.5. Antibacterial and Antifungal Activity

Bacterial and fungal pests cause various diseases in agricultural crops, which leads to crop losses and reduced product quality. Recently, active research has been conducted to improve existing approaches to combat phytopathogens. Long-term use of pesticides has contributed to the development of resistance in most strains of phytopathogenic microorganisms. The use of nanomaterials to combat bacterial and fungal pathogens is intended to prevent the volumes of fungicides used. In addition to the properties of Se NPs discussed above, their activity against bacterial and fungal cells has also been reported.

The antimicrobial activity of Se NPs against Gram-negative and Gram-positive bacteria and fungi is due to several mechanisms. It is assumed that nanoparticles disrupt the integrity of the cell wall of the pathogenic microorganism due to electrostatic interaction between the negatively charged surface of bacterial cells and positively charged nanoparticles [100]. This interaction is accompanied by an increase in the permeability of the bacterial cell wall and membrane and, as a consequence, leakage of intracellular components and cell death [185]. Another frequently described mechanism of antibacterial and antifungal activity of Se NPs is the production of reactive oxygen species: hydrogen peroxide (H₂O₂), hydroxyl radicals (^∙OH), and superoxideanion radicals (O₂^−∙), chemical compounds that cause oxidative damage to biomolecules [25]. In particular, in the work of Garza-García et al., the antibacterial activity of Se NPs against Serratia marcescens, Enterobacter cloacae, and Alcaligenes faecalis was demonstrated [100]. Soaking tomato seeds in a suspension of Se NPs (100 ppm) not only increased germination (by 22%), but also increased resistance (up to 72.9%) and plant defense responses to the causative agent of late blight (Phytophthora infestans) [91]. In turn, soaking seeds with Se NPs at a concentration of 25 mg/L improved the seed germination and growth parameters of Brassica campestris, and also had a pronounced antibacterial effect against Pseudomonas marginalis, a causative agent of plant rot, and Pseudomonas aeruginosa at the concentrations of 2.5, 5, and 10 mg/mL. Also, the work of Ge et al. demonstrated the antibacterial activity of Se NPs when used in combination with chitosan and the plant extract Rosa roxburghii Tratt. procyanidin against Physalospora piricola, the causative agent of physalospora blight. Using electron microscopy, it was found that the antimicrobial effect was due to the adhesion of nanomaterials to cell walls and their subsequent destruction, damage to the cell membrane and nucleus [51]. In turn, spraying Momordica charantia L. melon plants with Se NPs contributed to the protection of plants from powdery mildew (Podosphaera xanthii) infection, and also significantly reduced the development of this disease: an increase in the levels of putrescine (PUT; 43–112%), spermine (SPM; 36–118%), indoleacetic acid (IAA; 43–172%), salicylic acid (SA; 24–73%), the activity of phenylalanine ammonia lyase (PAL), transcinnamate-4-hydroxylase (C4H), and 4-coumarate: Co A ligase (4CL) of the phenylpropanoid pathway (22–38%, 24–126%, and 19–64%, respectively) was observed [105]. Foliar application of Se NPs (10 and 20 mg/L) together with Cu NPs (10 and 50 mg/L) to tomato crops infected with Alternaria solani, the causal agent of late blight, contributed not only to a decrease in plant damage and disease severity, but also to the induction of superoxide dismutase, ascorbate peroxidase, glutathione peroxidase, and phenylalanine ammonia lyase enzyme activity in leaves, as well as the induction of glutathione peroxidase activity, vitamin C, glutathione, phenols, and flavonoid content in fruits [144]. Shahbaz et al. reported the activity of Se NPs synthesized using Melia azedarach L. leaf extract against Puccinia striformis, the causal agent of yellow rust of cereal crops. When using these Se NPs at a concentration of 30 mg/L, a decrease in the severity of the disease in wheat plants was observed, which was due to an improvement in the morphological, biochemical, and physiological parameters of the plants [120]. In turn, coating persimmon fruits with a composite based on Se NPs, chitosan, and rosmarinic acid made it possible to inhibit the growth of the Alternaria alternata fungus, which causes black rot, by 97.2%. Treatment with a 1% solution of the considered Se NP-containing composite contributed to the complete (100%) prevention of the disease in the coated fruits, which was more effective than the results after treatment with the fungicide imazalil [174]. Biosynthesized Se NPs in combination with chitosan exhibited antifungal activity and leveled the spread of green mold Penicillium digitatum in lemon fruits [186]. Also, Se NPs showed antifungal activity against Rhizoctonia solani (MIC 0.0625 mM); however, the maximum inhibition was achieved at 1 mM with an inhibition zone of 45 mm [187]. The use of 100 μg/mL bimetallic Ag-Se NPs contributed not only to an increase in vegetative growth, the content of photosynthetic pigments, and soluble carbohydrates and proteins, but also to a decrease in the disease index by 17.5% and an increase in protection by 78.1% against Ralstonia solanacearum wilt, the causative agent of brown rot of solanaceous plants [50]. In a recent study, Abdelsalam et al. demonstrated the 100% fungistatic activity of Se NPs applied at a concentration of 20 mg/mL against Rhizocotonia solani, Fusarium oxysporum, and Fusarium solani [135].

For the treatment of gray mold caused by Botrytis cinerea, pH-sensitive drug delivery vehicles based on mesoporous nanoselenium, antimicrobial agents, and polyacrylic acid have been proposed [34]. Mesoporous Se NPs were used to deliver the antifungal drug thiophanate-methyl, which increased the solubility of the drug in water and reduced its dosage, and polyacrylic acid was used for controlled release of the drug. As a result of using the combined drug (100 ppm) for the treatment of gray mold of tomatoes, it was found that it has a rapid and effective antifungal effect against Botrytis cinerea, does not have toxic side effects with repeated use, and does not cause drug resistance, while increasing the efficiency of plant photosynthesis and promoting plant growth by 11% [188]. These results indicate that the simultaneous use of Se NPs as carriers of pesticides and nano-fertilizers is a new approach to the creation of environmentally friendly and sustainable pesticides.

Thus, the data accumulated to date indicate the high efficiency of using Se NPs as an additive capable of exhibiting multiple biological activity, providing not only stimulation of growth and ripening of agricultural crops, but also realizing a powerful protective effect under biogenic and abiogenic external stress factors.

4. Comparison of the Efficiency of Se NPs with Inorganic (SeO₃²⁻, SeO₄²⁻) and Organic (SeMet, SeCys) Forms

In a number of the works we analyzed, the efficiency of using not only Se NPs, but also volumetric forms of selenium—sodium selenite and selenate—was assessed, and, therefore, it was of great interest to summarize these data. We also analyzed a number of modern works considering the effect of application of organic forms of Se: selemethionine and selnocysteine. Figure 5 illustrates graphs reflecting the efficiency of application of Se NP, inorganic selenium-containing fertilizers (Na₂SeO₃ and Na₂SeO₄) and organic (SeMet, SeCys) relative to control values (without application of selenium-containing fertilizers). Among the parameters most frequently considered in literary sources, we selected and categorized into groups morphometric, photosynthetic, and biochemical parameters, as well as indicators of crop yield and accumulation of heavy metals (Figure 5).

Data analysis showed that the use of Se NPs was more effective compared to Na₂SeO₃ and Na₂SeO₄. The relative efficiency of Se NPs was 20.6 ± 27.6%, 17.3 ± 20.0%, and 38.8 ± 53.5% for morphometric parameters, photosynthetic, and biochemical parameters, respectively. At the same time, for Na₂SeO₃ and Na₂SeO₄, these parameters were −6.9 ± 35.1% and 4.1 ± 26.8%, −5.4 ± 24.9% and 12.0 ± 17.4%, and 8.2 ± 36.2% and 25.5 ± 47.1%, respectively. It is noteworthy that Se NPs also contributed to a significant increase in the yield and leveling of heavy metal accumulation by plants compared to the use of Na₂SeO₄.

4.1. Organic Se-Containing Fertilizers

At the same time, a number of studies have noted that organic Se(II) complexes, including Se-amino acids (selenocysteine (SeCys) and selenomethionine (SeMet)), act as a highly available source of selenium [189]. Several studies have comparatively analyzed the effects of organic and inorganic forms of selenium as well as Se NPs in relation to crops. In particular, Kikkert and Berkelaar observed that selenocysteine (SeCys) and selenomethionine (SeMet) were assimilated by Brassica napus L. and Triticum turgidum L. plants 2–20 and 40–100 times faster, respectively, compared to SeO₄ or SeO₃ [190]. Thus, it is suggested that synthetic organic Se(II) compounds may also be of interest in terms of effective agents for efficient Se enrichment in plants [67]. In general, organic biofortification of Se is a very common approach in global agricultural practices, since Se-enriched wastes can be used as a resource to improve Se concentration in agricultural soils [191]. Bañuelos et al. proposed the use of Se-hyperaccumulator plants (Stanleya pinnata (Pursh) Britton) as an organic Se fertilizer to enrich agricultural soils [192]. However, the key limitations of organic selenium fertilizers are their low storage stability, sensitivity to temperature, humidity and microbial activity, high leaching or volatilization rates, and increased risks of excessive plant accumulation and subsequent phytotoxicity. We analyzed published data demonstrating the effect of organic Se-containing fertilizers and compared their effectiveness with inorganic and Se NPs on five key parameters. We found that in case of application of organic selenium compounds, morphometric indices (Figure 5a) and indices of photosynthetic activity of plants (Figure 5b) were comparable with the use of Se NPs and were 14.56 ± 3 3.73%, and 15.98 ± 32.31%, respectively. When the effects of organic Se-containing fertilizers on biochemical parameters (Figure 5c) and yield (Figure 5d) were analyzed, no differences in effects were found compared to the use of both Se NPs and inorganic forms of Se; the values were 54.61 ± 70.0% and 3.3 ± 22.8%, respectively. In the case of the parameters reflecting the effects of heavy metals (Figure 5e), there were significant differences between the application of organic Se-containing fertilizers (−26.43 ± 33.0%) and Se NPs, as well as the application of Na₂SeO₃.

4.2. Comparison of Effective Doses of Se NPs, SeO₃²⁻, and SeO₄²⁻

Since the stimulating effect of Se on plant crops is dose-dependent and when threshold concentrations are exceeded, the opposite, inhibitory effect is observed, we also compared the generalized data on the reported values of the maximum effective concentration for the applied Se NPs and Se-containing mineral analogues. As can be seen from the graph (Figure 6), Se NPs are characterized by a wider range of effective concentrations, while the use of selenium compounds is limited to ~25 mg/L. At the same time, the average value of the effective concentrations of Se NPs and selenium-containing “traditional” fertilizers are about 20 mg/L and 6 mg/L, respectively. Thus, according to the analyzed literature data, the introduction of Se NPs contributes to a more pronounced stimulating biological effect in relation to agricultural crops, compared to Na₂SeO₃ and Na₂SeO₄.

5. Negative Aspects, Limitations, and Risks of Using Se NPs

At present, it is believed that for the widespread use of Se NPs in the open ground, there is a lack of clear recommendations that take into account the content of the element in the soil, the specifics of the growing crop, the method of nanoparticle production, and the method of plant treatment. At excessive doses of selenium application, its content in plants may decrease, causing growth suppression and yield reduction, while small doses may stimulate growth and increase yield [193]. In terms of approaches, both physical (ultrasound, laser ablation) and chemical methods (selenic acid reduction, sodium selenite salts, selenourea reactions, etc.) are often used to produce Se NPs, which have high production costs and, in some cases, lead to the formation of hazardous chemical compounds within the nanoparticles, limiting their large-scale application [194]. Consequently, one of the strategies to reduce the negative environmental impact is to scale up biological “green” pathways for the synthesis of Se NPs, which allow leveling toxicity [195]. In this case, the methods of “green” synthesis are optimal from the economic point of view, ensuring the cost-effectiveness of their application [196]. However, it is difficult to produce plant-based nanoparticles on a production scale due to standardization problems—their size and shape are unstable, and clear production regulations have not yet been developed [194]. When using nanoparticles in field conditions, it is necessary to take into account various environmental factors to choose the method of their application. It was found that foliar application increases the efficiency of Se assimilation, but the possibility of its application depends on weather conditions and is inadmissible in heat, rain, and high humidity. In addition, a high concentration of Se NPs in the air causes the need to use personal protective equipment both directly in the treatment area and in the surrounding areas. This problem can be partially solved by surfactant additives that reduce the surface tension of the suspension and improve Se transportation along plant organs, including roots, which is especially important for root crops [197]. In addition, given the antibacterial and fungicidal properties of Se NPs, it is also extremely important to consider their negative impact on soil beneficial microflora, so it is necessary to choose the right form of nanomaterials application. The most promising in this sense is the use of Se nanocomposites based on biopolymer matrices [198]. The application of selenium-containing fertilizers in the soil can also contribute to their leaching and getting into wastewater, causing their pollution [199]. Excess selenium ingested by animals and humans, including with water, can cause various serious diseases. The toxicity of Se NPs obtained by biological and chemical methods was evaluated on the example of Zebrafish. Chemically synthesized Se NPs were five times smaller in size than those obtained by biosynthesis, and their toxicity to the liver and gills of fish was higher than that of biosynthesized Se NPs [200]. The toxicity of Se NPs was also established for crustaceans of Daphnia magna species and natural bacteria Aliivibrio fischeri [201].

Thus, a comprehensive understanding of the dangers of uncontrolled use of Se NPs is necessary to develop rational strategies for their application for biofortification, growth enhancement, phytopathogen control in agriculture, and ensuring the environmental safety of agricultural products and reducing the burden on the environment. Due to stability, high bioavailability and biocompatibility, low toxicity, and greater environmental friendliness [194], Se NPs obtained by green synthesis are of greatest interest. In addition, they have great stress-protective and antibacterial properties, and their production and application are considered economically feasible [202]. Scaling up and widespread implementation of Se NPs obtained using the green synthesis method requires the development of production regulations and purity control of the plant extracts used, which can be difficult. Therefore, this issue remains unresolved for the time being. The other methods of synthesis (chemical and physical) are still considered unprofitable for mass production. Taking this fact into account, it is reasonable to use them in agricultural biotechnology, where with a small and more efficient consumption of applied components, it is possible to recoup the costs of their production. In addition, Se NPs can be used particularly effectively under controlled growing conditions (for vertical farms and growing rooms), where losses of nanoparticles to the environment will be minimized. The use of Se NPs can also be justified for rare, endangered, and particularly valuable plant species and varieties whose price is so high that the use of Se NPs would be economically feasible.

Analysis of the Phytotoxicity of Se NPs

Toxic effects induced by Se NPs are discussed in detail in a number of publications [101]. In the case of bulk forms of Se-containing fertilizers, phytotoxicity can be determined by their chemical form. For most plant species, SeO₃ is usually more toxic than SeO₄ [203]. Thus, with the exception of Se hyperaccumulator plants, a threshold of ≥ 2 μg/g dry weight was considered toxic for most crop species [204]. In the context of nanoforms of Se-containing fertilizers, their toxicity is also due to exceeding the required dose (as for bulk analogues) and the subsequent induction of oxidative stress (intrinsic to nanoparticles), provoking damage to cellular structures. As a result, there is an inhibition of root growth, green biomass, a reduction in the number of flowers and ovaries, and, as a result, a decrease in yield and stress indicators (SOD, POD, CAT, GR, APX, etc.). As well as for volumetric forms of application, due to the replacement of excessive amounts of sulfur in the protein cycle with selenium, the formation of amorphous protein aggregates occurs, which affects the activity of chloroplasts and mitochondria due to the replacement of Fe-S cluster centers with Fe-Se in the electron transfer chain [82].

To evaluate the effect of Se NPs on different crops and to determine the minimal stimulating and maximum permissible concentrations of their application, it is advisable to consider only those studies in which there are no additional stress-inducing factors (drought, heavy metals, etc.), because they can shift the stress response curves. In

Table 3. Negative effects of foliar treatments with chemically obtained Se NPs when the maximum beneficial concentrations for various plant species are exceeded.

Size, nm	Crop	Botanical Family	Optimal Conc., mg L−1	Depressing Conc., mg L−1	Negative Reaction	Ref.
10–45	Cichorium intybus L.	Compositae	4	40	↓ 26% root and shoot FW, ↓ 43% number of flowers	[206]
50–150	Cyamopsis tetragonoloba (L.) Taub.	Fabaceae	100–300	400–500	↓ 21.6 times yield, ↓ 37.5% number of grains, ↓ 36% FW, ↓ 2.6 times DW	[207]
33.4	Vigna unguiculata (L.) Walp.	Fabaceae	100–150	300	↓ 22% plumule length, ↓ 16.7% radicle length	[208]
22	Melissa officinalis L.	Lamiaceae	10	50	↓ 30% FW	[114]
19–45	Mentha x piperita L.	Lamiaceae	2	20	↓ 31% leaf length, ↓ 40% leaf surface area, ↓ 33% FW, ↓ 20% total chlorophyll content, ↓ 32% activity of nitrate reductase,	[205]
10–45	Capsicum annuum L.	Solanaceae	0.5–1	10–30	↓ 20 times FW, ↓ 33% activity of nitrate reductase, ↓ differentiation of xylem tissues, ↑ proline concentration, ↑ 1.5 times CAT, ↑ 2 times total soluble phenols, ↑ abnormalities in the structure of the stem apical meristem	[102]
50–78	Capsicum annuum L.	Solanaceae	20	>50	↓ 17% total chlorophyll content, ↓ 29% flavonoids, ↓ 30% capsacin, ↓ 20% soluble sugars (compared to 20 mg/L), ↑ 17% SOD activity, ↑ 50% total phenols	[209]
2–20	Solanum lycopersicum L.	Solanaceae	10	>20	↓ 5% yield, ↓ 12% flavonoids, ↑ 7% total soluble phenols	[142]
81–152	Fagopyrum dibotrys—(D.Don.) Hara.—PFAF.org	Polygonaceae	5	>20	↓ 14% leaf FW, ↓ 47% stem FW, ↓ 33% tuber FW, ↓ 3 times leaf transfer factor, ↓ 1.5 times stalk transfer factor, ↓ 3 times tuber transfer factor (compared to 5 mg/L), ↑ 2 times SOD activity, ↓ 20% POD	[210]
10–50	Coffea arabica L.	Coffeeae	40	>80	↓ 33% total chlorophyll content, ↓ 41% carotenoids, ↓ 20% total pheophytin, ↓ 10% total sugars, ↓ 40% APX ascorbate peroxidase activity, ↑ 12.5% SOD activity, ↑ 2 times CAT activity, ↑ 2 times glutathione reductase activity,	[211]

↑—Increase in indicators, ↓—Decrease in indicators, FW—Fresh weight, DW—Dry weight, APX—Ascorbate peroxidase, CAT—Catalase, POD—Peroxidase, SOD—Superoxide dismutase.

, we have selected “clean” experiments from which we can judge the toxicity of chemically produced Se NPs. More or less similarly sized nanoparticles have different susceptibilities and, consequently, minimal limiting concentrations that are toxic to plants. For example, within the family Lamiaceae, the very close species Mentha x piperita L. and Melissa officinalis L. have different minimum threshold toxicity concentrations of 20 and 50 mg/L, respectively [114,205], and for strawberry (Fragaria × ananassa (Duchesne ex Weston) Duchesne ex Rozier) of the family Rosaceae, this concentration will stimulate growth and yield [108].

For plants of the Solanaceae family—tomato and pepper [102,142]—the minimum toxic concentration of Se NPs (particle size 2–20 nm and 10–45 nm, respectively), was in the range of 10 to 30 g/L, while larger nanoparticles (50–78 nm) in high concentrations (50 mg/L) did not cause negative effects in pepper cultivation in another experiment [209]. At the same time, for pepper culture, the range of effective concentrations shifted to a larger side when Se NPs of larger size, 50–78 nm, were treated, and the positive effect of the treatments disappeared at a concentration of 50 mg/L [209].

In part, the difference in the responses of different crops can be explained by their origin. Evolutionarily, in areas with excessive selenium, plants have developed special defense mechanisms against damage. According to Se accumulation and, consequently, resistance to its excessive presence, plants are divided into three groups: non-accumulators (accumulate less than 100 µg/g dry weight), which include most forage plants; secondary accumulators (accumulate from 100 to 1000 µg/g dry weight), which include some representatives of the Compositae and Brassicaceae families; and primary accumulators (accumulate more than 1000 µg/g dry weight) [19,212]. As can be seen from Table 3, within a single plant species, the beneficial and suppressive concentrations of Se NPs may depend on other stress factors present, cultivation/growing conditions, and plant age (plants are more vulnerable at early stages of development), while within families, they may differ by tens of times, as, for example, for the Fabaceae family. It is difficult to assume whether such a difference in the toxicity of Se NPs is due to peculiarities of metabolism or differences in cultivation conditions. Thus, the establishment of toxicity limits for different cultures and their analysis is a promising area of research, which should be carried out under controlled conditions to ensure the representativeness of the results obtained. At present, however, such extensive experiments have not yet been conducted.

6. Conclusions

It is assumed that the use of nanoparticles in the agricultural sector as an alternative to “traditional” additives such as mineral fertilizers and pesticides is intended to increase the yield of crop plants [213]. Se NPs are a widely discussed candidate, considered in studies in recent years [214]. The accumulated significant volume of literature data indicates a wide spectrum of action of Se NPs in relation to both food and industrial agricultural crops, including stress-protective, stimulating, biofortification, antibacterial, and antifungal activity. It is important to emphasize the frequently reported combined activity of Se NPs. The addition of Se NPs contributes to a significant increase in the morphometric parameters of plants, including an increase in the biomass of green parts, roots, and fruits, as well as an increase in the content of nutrients, vitamins, and microelements. A significant increase in resistance to infection by a wide variety of bacterial and fungal phytopathogens, as well as an improvement in the physiological state of plants when using Se NPs against the background of diagnosed infection, has been demonstrated. The antifungal effect of Se NPs, comparable to the use of systemic fungicides, reported in a number of recent studies, deserves special attention.

Against the background of mineral forms of selenium-containing compounds—selenites and selenates—that have proven themselves in agricultural practice, the use of Se NPs in most cases showed more encouraging results. However, the unsolved problem remains the control of the concentration of Se NPs introduced. Despite a wider range of effective concentrations of Se NPs, high concentrations are capable of inhibiting plant growth. Also, the possibility of wide use of Se NPs is limited by the fact that the range of effective concentrations of Se NPs can vary depending on a large number of factors, including the type of plant, the method of introducing NPs, the irrigation regime, the characteristics of the growth substrate, etc. Also, unresolved issues remain the safety of Se NPs for animal organisms, especially representatives of the soil fauna, as well as the possible negative impact of the introduced nanoparticles on the soil microflora during long-term use.