1. Introduction
Field pea (
Pisum sativum L.) is a grain legume of great importance for food and feed and one of the most important legumes next to soybean, groundnut, and beans. This pulse crop is a rich source of protein (21–25%), with a wealth of amino acids such as tryptophan, lysine, arginine, aspartic acid, and glutamic acid, as well as carbohydrate, fibers, minerals, vitamins C, B, B3, and E, and significant quantities of beta-carotene, lutein, and zeaxanthin, confirming the outstanding nutritional value of this crop [
1,
2,
3]. Pea has the lowest trypsin inhibitory activity of any legume, making it an essential food source for humans and livestock [
3]. Moreover, field pea has proven to be an excellent crop species for the intercropping system due to its particularly favorable effect on soil structure and quality as well as nitrogen fixation [
4]. In 2021, the global production of peas has reached over 7 million hectares with an average yield of 1.76 t ha
−1 [
5]. However, various biotic and abiotic factors have been identified to have an impact on pea productivity. Peas have proven to be sensitive to unfavorable conditions such as drought, salinity and heat stress in the initial stages of plant development, which leads to a significant yield loss.
Seed germination and initial plant growth are the most important stages in plant development. Because sowing on a large scale demands quick and uniform seed germination, fine-tuning seed germination at an adequate time is essential for crop yield. Exposure to various stresses decrease seed germination and early seedling growth traits, damage morpho-physiological parameters—which are commonly associated with biochemical, physiological, and molecular changes—and subsequently reduce crop yield [
6,
7]. Modern agriculture management is one weapon of choice for combating the harmful effects caused by environmental stresses. Precision agriculture technologies require that every single seed must be healthy and germinate quickly to ensure a high yield. These technologies in seed production have developed new study approaches to increase seed vigor, and recently, seed enhancement techniques have evolved. Among these techniques, seed priming stands out as the most important method of physiological seed improvement, which enables controlled hydration of seeds and induces pre-germination (phases I and II) but prevents radicle protrusion [
8,
9].
Recently, much attention has been paid to the development of nanomaterials for use in agriculture due to their multiple functions as a promising alternative to overcome stress and improve sustainable agriculture. Previous research has shown that seed priming improves seed germination and the initial growth of many crops under optimal conditions but also under stress [
10,
11,
12,
13,
14]. Many priming techniques have been used in recent decades, such as hydropriming, halopriming, osmopriming, hormopriming, and biopriming. Moreover, seed priming with nanoparticles has attracted a lot of attention due to the attributes and physical characteristics of nanoparticles such as high surface area-to-volume ratios and high reactivity, which make them suitable for agriculture applications [
12]. Nanoparticles (NPs) are being employed as priming agents to improve seed quality, such as greater seed germination, seedling growth, stress resistance or tolerance, and ultimately higher yields and food nutritional value [
9,
15,
16,
17]. In addition, NPs alter significant, beneficial changes in seed metabolism as well as physiological and biochemical changes, gene expression, antioxidant enzyme activity, and signaling pathways [
9,
15,
18,
19]. The beneficial effect of NPs is attributed to their small size ranging between 1–100 nm and unique physio-chemical properties, which make them suitable for seed priming [
20]. Nanomaterials have a wide range of physical-chemical properties depending on their shape, size, surface, surface area/volume ratio, chemical behavior, particle charge, production method, and coating [
8]. Moreover, their unique properties, such as a high surface area to mass ratio, allow them to improve catalysis and deliver materials of interest, as well as adsorb substances of interest. Several studies have been concerned with the application of metal-based, carbon-based, and polymeric nanoparticles as seed priming agents for the improvement of seed germination and plant growth under various conditions [
18,
21,
22,
23,
24,
25].
However, to our knowledge, there is a lack of information on the effect of comprehensive metal-based NPs seed priming on germination and initial growth of field pea. Therefore, the present study was carried out to examine the impact of comprehensive seed priming with metal-based NPs (Co, Mn, Cu, Fe, Zn, Mo, and Se) on seed germination and initial plant growth and development of three field pea cultivars under optimal conditions.
3. Results and Discussion
The germination test was used in the current study primarily to characterize seed quality in relation to seed priming treatment under optimal conditions and to predict the field emergence of primed pea seeds. The purpose of this test was to assess the percentage of viable seeds that germinate and produce normal seedlings with well-developed essential structures, as well as to identify abnormal seedlings. The results presented in
Table 2 clearly show that the examined parameters of different field pea cultivars were significantly altered by priming treatments, excluding abnormal seedlings and dry root weight. The significant effect of the cultivar on all examined parameters was observed, while cultivar × treatment interaction significantly influenced all parameters, excluding abnormal seedlings and fresh and dry root weight.
Generally, priming treatments led to an increase in the examined parameters of all pea cultivars (
Table 3,
Table 4 and
Table 5). The results indicated that the tested nanopriming treatment did not significantly affect the germination energy of Dukat and Partner pea cultivars in comparison to the control, while hydropriming led to a decrease in the germination energy of cv. Dukat (
Table 3). However, the energy of germination in cv. E-244 was significantly improved by nanopriming, followed by hydropriming. Furthermore, significantly increased seed germination was observed by nanopriming in cv. E-244 and nanopriming and hydropriming in cv. Dukat compared to the control seeds, while no significant differences were observed for pea cv. Partner. In this regard, nanopriming led to an increase of seed germination between 3.19% (cv. Dukat) and 4.51% (cv. E-244) in comparison to the control. Recently published studies also revealed the beneficial effect of seed priming metal-based NPs such as zinc oxide (ZnO) NPs, copper oxide (CuO) NPs, silver (Ag) NPs, and cerium oxide (CeO
2) NPs on seed germination of different crops [
8,
35,
36,
37,
38]. Positive effects of priming with metal-based nanoparticles such as TiO
2, ZnO, and FeO NPs on seed germination under various conditions were observed in wheat [
39,
40], rapeseed [
41], and rice [
42]. Nanoparticles of metals such as Zn, Ag, Cu, and Fe have been proven to be more effective in improving seed germination in many plant species than metal salts, without adverse effects on the appearance of abnormal seedlings [
22,
36]. Moreover, Chau et al. [
43] also reported the beneficial effects of Co and Mo NPs on the seed germination of soybean. These benefits of seed nanopriming have been attributed to nanoparticle exposure-induced changes in seed metabolism as well as several physiological, biochemical, and signaling pathways in the seed [
15].
Moreover, in cultivars E-244 and Dukat, no significant differences were observed regarding the appearance of abnormal seedlings, while in cv. Partner, an increased percentage of abnormal seedlings was observed when priming with the nanoparticle solution (3.67%) compared to the control. It can be assumed that it is the result of phytotoxicity, considering the primary roots were stunted, retarded, and/or deeply broken. As stated by previous research [
22,
36], nanoparticles are absorbed on the surface of the seed and are gradually released over a germination period, which can lead to stress in the germination process. Moreover, the accumulation of metals such as Zn from the treatment of nanoparticles is much higher compared to the value from an aqueous solution of the same concentration, which can cause phytotoxicity to occur, and it depends on the plant species, as well as the cultivar, together with nanoparticle size [
35].
Pea cultivars differed in their shoot length in control (
Table 3). The results indicated that both priming treatments significantly improved the shoot length of the tested pea cultivars compared to the control. Nanopriming markedly improved shoot length up to 51.38% (cv. Partner), while hydropriming increased shoot length up to 50.49% (cv. Dukat) compared to the control. Regarding early seedling growth, the root length of seedlings varied among pea cultivars in the control and ranged between 85.93 mm (cv. E-244) and 106.83 mm (cv. Partner). The obtained results indicated that root length generally increased due to both priming treatments compared to the control. However, the highest root length was discovered in the nanopriming treatment, which was significantly improved prior to the control and hydropriming treatments. Priming with nanoparticles led to an increase of root length up to 44.04% (cv. E-244) as compared to the control seeds, while hydropriming increased root length up to 32.40% (cv. E-244). As stated by [
15], nanopriming has also been proven to have an effect on plant growth, stability, and physiology, in addition to modulating seed germination. Similarly, previous studies reported augmentation in plant growth of maize primed with TiO
2 NPs [
44], rapeseed primed with ZnO NPs [
41], fenugreek plants primed with Ag NPs [
45], wheat, pea, and mustard primed with SiO
2 NPs [
46], and chickpea primed with Fe
2O
3 NPs [
47]. Nanoparticles, as the main actors in plant morphology, growth, and physiology, affect physiological characteristics through changes in the formation of reactive oxygen species (ROS), peroxidase, superoxide dismutase (SOD), catalase (CAT), and enzymatic activities and modify leaf protein, chlorophyll, and total phenolic content (TPC) [
48,
49,
50].
Regarding biomass accumulation, fresh and dry shoot and root weights of the tested pea cultivars differed from the control (
Table 4). According to the obtained results, fresh and dry shoot weight of cv. Dukat were considerably higher in both tested treatments compared to the non-primed treatment, while the same parameters of cv. Partner were significantly improved only in the priming treatment with comprehensive nanoparticles. However, no statistical difference regarding the fresh root weight in all treatments was observed. Dry root weight of cv. E-244 seedlings was significantly higher when primed with NPs solution as compared to non-primed seedlings, while for other pea cultivars, no difference was observed. Previous studies have reported similar results to our findings on the seedling biomass accumulation of SeO NPs-treated wheat seeds [
51]. Likewise, beneficial effects of ZnO NPs, CuO NPs, and FeO NPs on plant biomass accumulation was also reported by [
52,
53,
54], respectively. Moreover, Chau et al. [
43] also reported a significant increase in dry biomass of soybean due to seed priming with Co and MoO
3 NPs. Our findings are consistent with the findings of [
55], who regarded metal NPs as an agent that promotes microelements to infiltrate plant cells and participate in enzymatic activities, hence increasing the rate of plant growth and development.
In addition, the root/shoot ratio was also assessed, and the results revealed that no clear pattern was observed with respect to this parameter (
Table 5). The root/shoot ratio varied among pea cultivars as well as among treatments within the same pea cultivar. The highest value of root/shoot ratio in cv. E-244 was observed in nanopriming (2.44), while for cv. Dukat, the highest root/shoot ratio was observed in the control (1.90). In cv. Partner, both priming treatments led to a significant decrease of root/shoot ratio in comparison to the control. Also, shoot elongation rate (SER) varied among pea cultivars and ranged between 6.31 (cv. E-244) and 10.79 (cv. Partner) in the control (
Table 5). The results indicated that all pea cultivars responded differently to priming treatments. For cv. E-244, no statistical difference was observed, while for two other cultivars, priming treatment significantly increased SER compared to the control. However, in cv. Dukat, the highest value of SER was observed in hydropriming, while nanopriming had the best effect in cv. Partner. Moreover, root elongation rate (RER) in the control treatment also differed between pea cultivars (
Table 5). Nanopriming led to a significant increase of RER in cv. E-244 and cv. Partner compared to the control treatment and hydropriming. Contrary to this, it was observed that nanopriming significantly reduced RER in cv. Dukat compared to other treatments. In regard to these results, numerous studies report a beneficial effect of seed priming on the seedling growth, especially under stressful conditions. For instance, SiO
2 NPs were noticed to improve photosynthetic parameters, maintain biochemical balance, and amplify biomass production in wheat seedlings under drought [
37]; ZnO NPs caused an increase in the shoot height and root-shoot biomass in wheat seedlings facing salinity stress [
56]; and application of TiO
2 NPs induced an increment in the root-shoot length of seedlings and their fresh and dry biomass of maize in both optimal and salinity stress conditions [
44]. Priming with ZnO NPs has been reported to have numerous advantages over other nanoparticles such as Fe
2O
3 NPs, CuO NPs, Ag NPs, CeO NPs, etc. in terms of plant growth, especially root growth, considering the fact that it is an important transitional metal and is an essential micronutrient that plays a vital role in the growth and yield of plants by maintaining cell membrane integrity, cell elongation, and protein synthesis [
35,
57]. However, the results of the RER reduction in cv. Dukat can be justified by the fact that this genotype is more sensitive to the metal-based nanoparticles that were tested, and the concentration of nanoparticles might affect the agronomic effectiveness of nanoparticles. In this regard, Li et al. [
58] reported that Fe
2O
3 NPs at concentration of 20 mg L
−1 significantly promoted root elongation by 11.5%, while concentrations of 50 and 100 mg L
−1 remarkably decreased root length. Moreover, it has been reported that zero-valent Zn and Fe NPs at higher concentration could lead to reduced water flow and limit root hydraulic conductivity, thereby inhibiting the root elongation of maize and mung bean, respectively [
59,
60].
In addition, the seedling vigor index (SVI) was markedly improved by nanopriming, followed by hydropriming in cv. E-244 and cv. Partner. In pea cv. Dukat, both priming treatments significantly improved SVI compared to the control, but in nanopriming to a lesser extent than hydropriming. Janmohammadi and Sabaghnia [
61] also reported beneficial effects of nanopriming with Si on SVI in sunflower. Moreover, Raja et al. [
62] revealed a significant increase in the SVI of black gram seeds primed with ZnO NPs and Cu NPs, which is in agreement with our findings. Prasad et al. [
63] also found that ZnO NPs seed priming had a beneficial effect on the SVI of peanut seeds. Additionally, Dehkourdi and Mosavi [
64] and Zheng et al. [
65] have demonstrated that the SVI of parsley and spinach seeds were positively influenced by priming with TiO
2 NPs.
The effect of priming treatments on chlorophyll content of the studied pea cultivars is shown in
Table 4. In cv. Dukat and cv. Partner, the highest values of chlorophyll content were observed in nanopriming followed by hydropriming. The relative increase in chlorophyll content due to nanopriming was 10.88% in cv. Dukat and 24.43% in cv. Partner. Contrary to this, no significant difference in priming treatments in terms of this parameter was observed in cv. E-244. In this regard, it has been shown that priming treatment with ZnO NPs [
56,
66], MnO NPs [
67], and Fe
2O
3 NPs [
68] positively affect the increase of chlorophyll content and photosynthetic pigments in plants. Besides, it is suggested that Mn, as an essential metal for plant growth, has an important role in the organization of the thylakoid membrane and photosynthetic electron transport [
69]. However, Kasote et al. [
67] showed that MnO NPs had no considerable effect on the chlorophyll content of watermelon seedlings. In addition, Faraz et al. [
53] indicated that
Brassica juncea had a significantly higher chlorophyll content when the seeds were primed with CuO NPs. No available research data was found on the effect of seed priming with CoO
3 NPs and Mo NPs on chlorophyll content. However, available data shows that the activity of photosystem I, and thus the Hill reaction, is inhibited by Co ions in peas, but the role of cobalt in photosystem I has not yet been elucidated [
70,
71,
72]. Cobalt has also been found to harm the chloroplast membrane [
73]. Regarding Mo NPs, the application of Mo NPs by root irrigation has been shown to have a beneficial effect on the chlorophyll content of tobacco [
74].
Correlation analysis verified the favorable effects of seed priming treatments in optimal conditions (
Table 6). Overall, a positive interrelationship was established between seed germination and other parameters, with an exception of abnormal seedlings, shoot and root elongation rate, and chlorophyll content, while the interrelationship between seed germination and shoot growth and biomass accumulation was not significant. Moreover, research revealed a strong correlation between fresh root weight, dry shoot and root weight, and root and shoot length. A positive interrelationship was also established between chlorophyll content and abnormal seedlings, and fresh and dry root weight, while for germination parameters, a negative relationship was established. In addition, the seed vigor index was significantly positively correlated with all examined parameters, except abnormal seedlings and shoot and root elongation rate. Therefore, seed nanopriming treatments could result in enhanced seed vigor and quality, early plant growth, and subsequent grain production at later developmental stages.
Moreover, the biplot of the principal component analysis (PCA) (
Figure 2) illustrated the relationship between priming treatments and field pea cultivars. Priming treatments were clearly separated from the control, except nanopriming in cv. E-244, due to its lower effects on the examined pea parameters. Besides, PCA showed that pea cultivars were separated from each other within the same group. Thus, the results indicated that the comprehensive nanopriming treatment had the greatest effect on cv. Partner, followed by hydropriming treatment, while hydropriming and nanopriming treatment effects were more alike/uniform in cv. E-244 and cv. Dukat. The obtained results clearly emphasized the role of seed nanopriming treatment in improving seed germination and enhancing the initial growth of field pea plants. However, the different response of pea cultivars to nanopriming treatment implies the possibility of improving seed germination and pea seedling growth by adjusting the appropriate nanopriming seed treatment, which includes the concentration of the priming solution and the duration of priming treatment, to the requirements of each cultivar in the laboratory and in the field.