Interactions of Fe and Zn Nanoparticles at Physiochemical, Biochemical, and Molecular Level in Horticultural Crops Under Salt Stress: A Review

Abstract

Salinity is a major abiotic stress that affects the growth and yield of horticultural crops. By raising the levels of sodium and chlorine ions in plant cells, salinity disrupts various morphological, physiological, epigenetic, and genetic traits, leading to excessive oxidative stress production. Through a variety of redox methods, the plants can partially alleviate this disorder and restore the cell to its initial state. At cell level, cellular redox adaptation plays a potential role coping with salinity stress in all plants; however, if the salt dose is excessive, the plants might not be able to respond appropriately and may even perish from salt stress. Scientists have proposed many solutions to this issue in recent years. One of the newest and most effective technologies to enter this field is nanotechnology, which has produced some extremely impressive outcomes. However, the molecular mechanism and interaction between nanoparticles in horticultural crops remains unclear. In order to take a step toward resolving the current doubts for researchers in this field, we have attempted to conclude the most recent articles regarding how iron oxide nanoparticles (FeO-NPs) and zinc oxide nanoparticles (ZnO–NPs) could aid salt-stressed plants in restoring cellular function under saline conditions in horticulture crops. Further, different inoculation modes of NPs mediated changes in physiological attributes; biochemical and genetic expressions of plants under salt stress have been discussed. This article also discussed the limitations, risk, and challenges of NPs in the food chain.

1. Introduction

Soil salinity is a critical environmental challenge that significantly threatens agricultural productivity, alters soil properties, and hampers plant development. The progressive accumulation of soluble salts in the soil is a growing concern, affecting over 100 countries worldwide, necessitating urgent and sustainable mitigation strategies. Sustainable agricultural production in the 21st century faces pressing global challenges, including climate change, environmental degradation, rapid population growth, water scarcity, and the increasing threat of soil salinization. Addressing these interconnected issues is crucial for ensuring long-term food security and ecosystem stability [1,2]. Sodicity and salinity accounted for over 6% of the world’s total surface (4434 million hectares) by 397 million hectares. The Food and Agriculture Organization (FAQ) reports that 0.25–0.50 million hectares of agricultural land are not suitable for agriculture due to salt stress [3]. Several exhaustive studies have focused on the adverse effects of salinity on crop production and their mitigation management [4,5]. The main effects of excessive salts in soils are osmotic and specific ionic, which impair a variety of crop plant metabolic functions. Thus, every stage of plant development is negatively impacted by salt stress, which eventually reduces crop productivity. It is now evident that plants’ reactions to salinity stress are highly complicated and dependent on a number of variables, including the different concentration of a solute, the plant’s growth stage and genetic potential, and also the kindand intensity of the stress [6].

High salinity induces a cascade of adverse effects on plant physiology, including osmotic stress, oxidative damage, nutrient imbalances, and ion toxicity. It disrupts membrane integrity, impairs cell expansion, destabilizes essential metabolic processes, and induces genetic toxicity, collectively leading to inhibited plant growth and development [6]. The excessive accumulation of ions, particularly sodium (Na⁺) and chloride (Cl⁻), along with nutrient deficiencies, disrupts plant growth and development by inducing water imbalance and cytotoxicity. Furthermore, salinity stress leads to the generation of reactive oxygen species (ROS), which contribute to oxidative damage, further exacerbating cellular stress and impairing physiological functions [7,8]. To cope with these issues, plants typically exhibit a two-phase response. The first phase involves the reduction in growth through osmotic adjustment, which inhibits cell expansion and leads to stomatal closure. The second phase is the activation of antioxidative enzymetic system, which help mitigate oxidative damage and enhance the plant’s ability to tolerate and endure salt stress [7]. Potassium (K⁺) homeostasis is thus important for the growth and adaptation of plants in K-deficient plants even under stressful conditions, and photosynthesis is also hindered [9,10]. It is attributed to the K⁺ prominent function in the conductance of mesophyll carbon dioxide (CO₂), organization of chloroplast, enhancement of rubisco enzyme development, and translocation of photosynthetic products into the phloem. Salinity also results in a reduction in K⁺ levels in both roots and shoots, where salt tolerance is closely linked to the plant’s ability to retain K⁺ and regulate the cytosolic K⁺/Na⁺ ratio. Potassium transport systems are central to signaling pathways that facilitate responses to environmental stress, influencing processes such as K⁺ uptake, translocation, and stomatal movement. Numerous channels and transporters involved in K⁺ transport have been identified in plants, playing crucial roles in maintaining cellular homeostasis under salinity stress (Figure 1) [11,12,13]. One of the most effective tactics for ensuring sustainable crop yields is the development of soil salinity-resistant crop cultivars. Documentation on signal transduction pathways and growing adjustments mainly comes from studies on the Arabidopsis thaliana model organism but has not yet been applied to crops. In order to increase salt tolerance in major crops, we need to understand which changes to salt productivity are effective for different species. Plant salt tolerance is a dynamic feature that can be graded in various ways, including the accumulation of ions, tissue-specific rates of growth, development of biomass, viability, and the development of seeds. It will be beneficial to optimize various salt response types of crops in order to boost yield. In this review, we will concentrate on potassium in and out of the melon rectifier channel transmitted genetically to Arabidopsis in order to address salt stress.

For modern agriculture to advance sustainably and meet the twin challenges of global food security, finding new technologies to combat global salt stress issues is crucial to achieving these goals because it will increase agricultural systems’ resilience, sustainability, and efficiency.

In recent years, the use of nanoparticles (NPs) has gained significant traction in agriculture, emerging as a powerful and innovative approach to promote plant growth and improve stress tolerance under saline conditions [14]. Due to their minute size and vast surface coverage, NPs can easily be absorbed and alter the properties’ macro materials. The substantial surface-to-volume ratio of NPs amplifies their responsiveness, making them highly capable of driving biochemical processes [15].

The plants’ exposure to NPs at various developmental stages might have both positive and negative impacts. NPs have been shown in numerous studies to have positive impacts on different plant species under salt stress [16]. These tiny particles exert notable effects on the structural, functional, and chemical attributes of various plants. Previous research indicates that NPs alter hormone levels, antioxidant enzyme activity, ion homeostasis, gene expression, and defense mechanisms to control salt tolerance in a variety of plants. Depending on the shape morphology and dose-dependent concentrations of NPs, these impacts may differ among various plant species or under various environmental circumstances [17]. For instance, depending on the quantities and characteristics of the NPs, different publications have shown the detrimental and adverse effects of high NP concentrations in plants, which differed among plant organs, growth stages, and species [18,19]. Thus, NPs’ interaction with different metabolic processes that result in harmful or advantageous effects may depend on their concentration, size, treatment application method, plant absorption, characteristics, responsiveness, and translocation into different tissues. Various NPs, including silver (Ag), nickel (Ni), selenium (Se), silver (Ag), copper (Cu), manganese (Mn), zinc (Zn), iron (Fe), and silicon (Si) have been utilized to enhance crop performance under both biotic and abiotic stress conditions [20]. Based on their chemical makeup, nanoparticles are grouped into four primary categories such as metallic (Fe, Zn, Cu, Ag, etc.), polymeric, carbon-based (graphene and car-bon nanotubes), and ceramic.

In general, there is ongoing discussion on the exposure of significant amounts of NPs in plant science and agriculture. The interaction and intracellular mechanism of Fe and ZnO-NPs in all horticultural crops under salt stress conditions are not widely known, despite several studies confirming the beneficial and detrimental effects of NPs on various crops in response to salinity stress. In the review paper, we aimed to examine the physiological, biochemical, and molecular levels of FeO-NPs, ZnO-NPs, and salt stress in plants grown under salinity stress. Deep understanding and interaction between NPs under salt stress provides novel insights on how to enhance plant growth in salt-affected soils in a sustainable manner.

2. Impact of Soil Salinity on Horticultural Crops

Salinity adversely affects plant growth through a combination of increased soil osmotic pressure and interference with nutrient uptake. High salt concentrations in the soil solution limit the plant’s ability to absorb water, leading to osmotic stress or water-deficit effects, which are akin to the metabolic changes caused by water stress, such as wilting. This often results in reduced crop growth, particularly when salt levels reach toxic concentrations in the plant. Additionally, salinity disrupts plant metabolism by causing specific-ion toxicities and nutritional imbalances. The impact of salinity on plant growth is a time-dependent process, with a two-phase model proposed by Munns et al. [21]. The first phase is rapid, driven by the onset of water deficit, while the second phase is slower, caused by the accumulation of toxic salt levels in the shoots. The overlapping nature of these two phases makes it challenging to assess their individual contributions to yield reduction. Salinity also alters soil properties, such as electrical conductivity, deflocculating soil clays, decreasing hydraulic conductivity, and disrupting soil activities like organic matter decomposition. Additionally, soil salinity can affect soil microflora, which is essential for maintaining soil structure. The osmotic stress caused by salinity reduces water availability, further impacting photosynthesis, leaf surface area, and CO₂ availability [22,23]. Visual symptoms of salt stress include early signs such as stunted growth, yellowing, and wilting, followed by more severe manifestations like leaf tip scorching, necrosis, curling, and leaf abscission [24].

3. Different Methods to Mitigate Salt Stress

Traditionally, various approaches were used to improve saline soils, involving physical, mechanical, biological, and engineering activities such as water leaching, chemical soil modification, organic modifications (compost and manure in the farmyard), microbial enhancement, and phytoremediation [25]. The breeding of plants is divided into two parts: One of them is the traditional breeding, which is used to classify the germplasm lines for salt tolerance [26]. The other one is conventional breeding like quantitative trait loci (QTL), marker-assisted recurrent selection (MARS), in vitro selection, physiological feature pooling, inter- and intra-specific hybridization, and marker-assisted selection (MAS), and the use of halophytes as substitute crops comprise the second traditional selection [27]. Traditional plant breeding methods have significantly enhanced crop yields in saline soils, leading to the development of salt-tolerant varieties [26,27,28,29]. However, these conventional approaches are labor-intensive and depend on access to diverse germplasm with desirable genetic traits [2,6,30,31]. Moreover, the identification of salt-tolerant genotypes, which play a crucial role in mitigating the risks associated with soil salinity and restoring contaminated lands, often stems from the inherent genetic variability within species [32].

4. Genetic Engineering in Coping Salt Stress in Horticultural Crops

Several studies on salinity tolerance focus on enhancing crop growth and development to ensure resilience in saline soils [33,34]. Understanding the detrimental effects of salt exposure on plants and developing salt-tolerant crops are crucial in addressing the increasing challenge of soil salinization in agricultural lands. Additionally, plants demonstrated a number of complex physiological and biochemical mechanisms for coping with salt stress, including osmotic adjustment, osmo-protection of harmful ions compatible with the buildup of osmolytes, and lowering ROS levels and membrane damage. In fact, one of the most crucial physiological markers for plants’ ability to withstand salt is the buildup of proline and other soluble sugars [35]. One important indicator of the metabolic changes that take place when plants undergo salt stress is the quantity of ROS generation, including both free radicals (superoxide O₂ •-and hydroxyl radicals OH•) and non-radical forms (hydrogen peroxide H₂O₂ and singlet oxygen 1 O₂). These ROS are extremely reactive and can affect several biological processes by oxidizing proteins, causing lipid peroxidation, and destroying nucleic acids. However, prior research has shown that malondialdehyde (MDA) accumulation due to lipid peroxidation should be viewed as a sign of oxidative stress and membrane integrity, which may be utilized to distinguish between genotypes that are salt tolerant and those that are salt sensitive [36]. In the last two decades, the scientific community of genetic engineering has spent a great deal of effort in developing transgenic plants like genetically modified (GM) crops by optimizing their ability to withstand salts [29,37]. No genes encode upregulated and downregulated proteins when plants under stress (biotic and abiotic) play a vital role in tolerating stress [38,39]. In plant TF families ERF/AP2, it has been shown that WRKY, bZIP, MYC, MYB, NAC, and zinc-finger proteins have regulatory functions involved in plant stress responses [40]. Protein targeting and trafficking, long-distance Na⁺ transport from root to shoot, salt accumulation, and even stomata functions are all linked to vacuolar NHX transporters. On plasma membrane-located Na⁺/H⁺ exchangers like SOS1, PM-located H⁺ pumps also help to create a pH gradient, which in turn helps to sustain the membrane potential that drives vital ion (K⁺) uptake through voltage-driven channels and excludes Na⁺ in exchange for H⁺ [41]. Additional types of cation carriers are expected to participate in cation homeostasis, according to genome-wide analyses, and their future investigation is essential to comprehending the mechanisms behind plants’ salt tolerance. Furthermore, it is yet unclear how K1 transporters contribute to salt tolerance, even though they might be involved in maintaining the cytosolic Na1 to K1 ratio [42]. To cope with salt stress, potassium transport has received great attention in many crops due to its ion homeostasis. There are many single gene plasma membranes that contain high potassium (K⁺) transporters (HKT) from higher crops recently transferred genetically to improve salt tolerance by overexpression like AtHKT1 (Arabidopsis thaliana) [43], HvHKT2;1 (Hordeum vulgare) [44], TmHKT7 a.k.a. TmHKT1;4-A2, TmHKT1;5-A (Triticum turgidum ssp.) OsHKT1;1, OsHAK5 (Oryza sativa cv. Indica) [45,46,47], and GmHKT1;4 (Glycine max) [6,48]. It was found that K⁺ is an important nutrient for plant metabolism; thus, the harmful effects of salt stress are sometimes correlated with K⁺ uptake disruptions and, hence, intracellular K⁺ homeostasis or the K⁺/Na⁺ ratio. Shaker-like K⁺ channels act in stomatal opening and closing by K⁺ uptake and exchange in guard cells [49]. To date, Arabidopsis and rice have identified 9 and 11 genes that encode Shaker-like potassium channels, respectively. The overexpression of Puccinellia tenuis (PutAKT1) in Arabidopsis led to a reduction in shoot and root Na⁺ content. Thus, potassium channel transcripts, Mkt1, Mkt,2 and Kmt1 from the common halophyte ice plant, which are homologs of Arabidopsis IRC Akt1 and Akt2/3 showed higher expression under salt stress. For instance, MKT1 protein amounts in rice that include three Shaker-like K⁺ channel proteins rectifying the interior (OsKAT1, OsKT2, and OsKAT3) enhanced salt tolerance and preserved ion homeostasis within cultured rice cells during salt stress. The potassium inward rectifier channel KCT2 from Brassica rapa is highly upregulated under salt stress [50]. KC1 from Arabidopsis [51], KST1 from potato (Solanum tuberosum) ZMK1/ZMK2 from maize (Zea mays) [52], CmSOS1, CMSKOR, and the PM H^± ATPase increase ion homeostasis under salt stress [11].

FeO-NPs and ZnO-NPs influence molecular pathways by modulating gene expression, signaling cascades, and antioxidant defense networks. Exposure to these nanoparticles upregulates stress-responsive genes such as SOD, CAT, and GPX, enhancing oxidative stress alleviation [53]. They also impact metal transporters (ZIP, NRAMP) to regulate Fe and Zn homeostasis [54]. Additionally, FeO-NPs and ZnO-NPs activate key signaling pathways, including the SOS response for cellular stress adaptation and MAPK cascades, which govern stress signaling and defense mechanisms [55]. Furthermore, nanoparticle-induced ROS generation triggers antioxidant networks, activating enzymatic (e.g., glutathione peroxidase) and non-enzymatic (e.g., ascorbate–glutathione cycle) [56] pathways to maintain redox balance.

Plants evolved Na⁺ exclusion and intracellular compartmentalization mechanisms to preserve Na⁺/K⁺ ratios in cellular homeostasis [57,58,59,60]. The mitigation of salt stress in plant cells relies on the coordinated action of plasma and vacuolar membrane transport systems (Figure 1) [61,62]. Therefore, ion homeostasis, even under stress conditions, is a requirement for plant growth, and regulation of the Shaker plasma membrane channels is voltage-gated. Shaker K⁺ subfamily depends on the inwardly (K in) or outwardly rectifying channels (K out) activated by hyperpolarization or depolarization of membranes, respectively (Figure 1). In Arabidopsis, AKT1, AKT5, AKT6, KAT1, KAT2, and AtKC1 subunits assemble as Kin channels. In contrast, two guard cell channels, SKOR (K⁺ outward rectifier) and GORK (guard cell outward rectifying channel), have been reported in K⁺ transport in plant cells under stress conditions [11,63]. After that, various salt tolerance varieties in melon were conducted: they focused mainly on the physiological level, for example, the evaluation of salt-tolerant varieties, the influence of salt stress on fruit quality and yield, and the physiological responses in melon when irrigated with salt water at different stages of growth [64,65,66]. The molecular mechanisms that control the resistance to salinity in melon, however, remain unclear. In our previous study, 82 NAC genes were reported, and the gene CmNAC14 was selected for functional verification for transformation into Arabidopsis thaliana. That gene’s overexpression was sensitive to salt stress. This research provides a valuable guide for the cloning and functional analysis in melon, as well as in cucurbitaceous plants and other members of the other gene family. For instance, further research employing insertion mutagenesis or an RNA interference approach is required to identify the precise biochemical pathways or mechanisms by which the melon gene family contributes to the plant’s ability to withstand salt stress. Previous analyses demonstrated that two transgenic melon cultivars showed distinct characteristics under salt stress [67,68]. Thus, the K⁺/Na⁺ concentration ratio in plants, by regulating the expression of particular ion channels, plays a critical role in the resistance of salts [49].

5. Application of Nanoparticles and Their Impacts Against Salt Stress

Nanotechnology is a cutting-edge scientific method with many uses in many industries, with the pharmaceutical and agricultural sectors currently leading the way [69]. Major obstacles to the agriculture sector’s ability to meet the zero-hunger challenge include the slow development of commercially available salt-tolerant cereal crops, the sluggish progress of stress-resilient crop varieties, and social concerns about genetically modified foods in some parts of the world [70]. Food security, disease control, new techniques for detecting insect pests, better nutrient status, and packaging tools are just a few of the reasons why the use of nanotechnology in agriculture is growing quickly. Numerous nanomaterials have been created in tandem with the advancement of nanotechnology, including nano-biofertilizers, nano-pesticides, and nano-insecticides [71]. For example, cerium oxide nanoparticles (CeO₂NPs) showed catalytic activity in scavenging free radicals in plant cells’ chloroplasts. Therefore, CeO₂NPs mitigate oxidative damage during abiotic stress, thereby mitigating the adverse effects of oxidative damage on plant photosynthetic areas [72,73].

ZnO-NPs have been shown to improve superoxide dismutase (SOD) and glutathione peroxidase (GPX)-associated genes in tomatoes in response to salinity [17]. FeO-NPs have the potential to induce oxidative damage in mitochondria, which would harm genetic material and result in reduced cell viability and programmed cell death. Due to the oxidative damage caused by applying Fe₃O₄-NPs at greater concentrations, cytotoxic effects have also been reported. Accordingly, various effects have been documented for different combinations of NPs applications, as supported by literature research. However, NPs at lesser doses have shown greater effectiveness (Table 1 and Table 2) [74,75].

6. Inoculation, Uptake Mechanism, and Salt Mitigation Effect

The physicochemical characteristics of NPs, the concentration employed, and the application technique all affect their biological activity. NPs are employed in a variety of techniques, including direct injection, foliar spray, hydroponic substrate, irrigation, and priming [76]. When NPs supplement with soil, they move into plant tissue through the root hair and wound areas, the root epidermis’ cell wall, and cell membrane (by a variety of mechanisms like endocytosis, carrier proteins, plasmodesmata, or pore formation), enter the plant’s vascular bundle (xylem) (for example, by one of the two symplast or apoplast pathways) and then translocate to the stele symplast pathway [77]. Additionally, NPs can enter the cytoplasm of cells by passing through the cuticles, stomatal pores, hydathodes, and trichrome of leaves. The NPs may attach to various cytoplasmic organelles within the cytoplasm and disrupt local metabolic processes. Through diffusion in the cotyledon and entry into the coat through parenchymatic intercellular gaps, NPs can also be directly absorbed in seeds. Generally speaking, little is known about the processes by which NPs build up in plant cell walls [78,79]. Additionally, the mobility of NPs can be impacted by root- and leaf-associated microorganisms when they are applied as foliar sprays or in soil. According to one study, mycorrhizae of Ag-NPs is something that the Trifolium repens plant absorbs through its roots [69]. The first line of defense against the entry of harmful particles, mucilages also improve the absorption of good nutrients and the proliferation of soil microorganisms (e.g., symbiosis) [80]. It was stated that the mucilages can acidify the rhizosphere’s environment, which can encourage the disintegration of some insoluble NPs and thereby impact plants’ ability to absorb [81].

The size, concentration, climate, and application methods all had an impact on the efficient adsorption of NPs after foliar application. The form and chemical composition of the leaf (trichrome and waxes) are significant factors affecting the trapping of NPs on the leaf surface. For instance, it has been seen that topically applied chemicals, including pesticides, affect NP absorption [82]. For example, the pore size in the cell wall has been shown in multiple studies to be the primary barrier preventing NPs from entering plant cells. Reports state that the microscopic NPs can enter plant roots directly through the root hair epidermal cells, via capillary forces, or through osmotic pressure. However, the large NPs are limited by the semipermeable, tiny-holed epidermal cells of the root cell wall. Additionally, the cuticle acts as the primary leaf barrier, keeping NPs out of the leaf unless they are smaller than 5 nm [83]. The assessment of nanoparticles’ fundamental structure generally complements the analysis of their impact on the absorption, translocation, and accumulation of NPs in plants. Therefore, in order to comprehend and clarify the processes of absorption and translocation, a thorough investigation into the nature of the NPs is required. It is also critical to monitor and track NPs with high purity and stability in order to ascertain their mobility and localization to different plant structures and cell organelles [84].

7. Iron Oxide Nanoparticles’ Role of Mitigating Salt Stress in Horticultural Crop

Traditional agricultural practices are changing as a result of the incorporation of NPs, which is improving sustainability and production. In line with previous evidence, many NP types are suitable for optimizing growth and development since they can pass through cellular barriers. The ameliorating effects of several NPs under stress situations have been widely documented [74,75]. Further, the current literature indicates that, through a variety of established processes, distinct NPs can stimulate the growth and development of plants under salinity stress at concentrations below specific thresholds (Figure 2). The majority of these investigations were conducted in artificial treatment environments such as hydrophobic or pot conditions and plate growth media. We go over the benefits of NPs for enhancing plant resistance to salinity stress in order to comprehend how they affect plant growth [85].

Iron is a key regulator of plant growth and development; being involved in several metabolic processes, including the formation of chlorophyll, photosynthesis, and dark respiration, it is a crucial micronutrient that regulates several metabolic systems [86]. Additionally, iron is essential for the manufacture of several important proteins involved in plant metabolism, cell respiration, DNA repair, oxygen transport and balance, and photosynthesis, all of which affect crop productivity as a whole [87]. In plants under stress, iron absorption is also significantly impacted. Foliar or other inoculation methods of FeO-NPs in various forms (Fe, FeO, Fe₂O₃, Fe₃O₄) under stressful conditions may, therefore, be an effective method of reducing iron shortage symptoms and enhancing plant tolerance under salinity stress. FeO-NPs are nontoxic due to their small size and large surface area, strong electrical and thermal conductivity, high magnetic properties, and great dimensional stability. When FeO-NPs come into contact with oxygen or water, they can oxidize instantly, producing free Fe ions [88]. As a result, both stomatal and non-stomatal constraints impair the plant’s capacity to absorb CO₂. Finally, due to the direct activation of many ion channels in cellular membranes, salt stress causes a sharp rise in ROS levels in plant tissues, which seriously damages important cellular components and disrupts plant ion homeostasis (Figure 2). Larger application rates of NPs themselves were found to encourage oxidative damage in plants, while dose-dependent concentrations of (Fe) were discovered to inhibit it. Furthermore, it has been demonstrated that bioengineered FeO-NPs promote wheat development by increasing the plant’s levels of beneficial nutrients (N, P, and K) and decreasing its levels of Na⁺ and Cl⁻, thereby decreasing the negative effects of ROS [15].

The salt mitigation physiological attributes of FeO-NPs on horticultural crops under salt stress are listed in Table 1.

Table 1. List of the research reports showing the salt mitigation effects of FeO-NPs on horticultural crops.

Crop	Inoculation Method	FeO-NPs Concentration	NaCl Concentration	Physiological Attributes	Reference
Trachyspermum ammi L.	Foliar spray	5, 10, and 15 μM/L	0, 25, 50, and 75 mM	Improved uptake of beneficial nutrients like Fe, N, P, K, and Zn plant growth parameters (e.g., fresh root and shoot weight, root shoot length, and leaf number of leaves under severe salt stress)	[89]
Pistacia vera L.	Hydroponic	2.9 mg/L	0, 100, and 200 mM	Reduced oxidative stress and enhanced chlorophyll content	[90]
Trachyspermum ammi L.	Foliar application	4, 8, and 12 dS/m	3 mM	Increased endogenous SA levels, Fe content, and K+ intake and maintained the K+/Na+ ratio and antioxidant enzyme activity	[88]
Dracocephalum moldavica L.	Foliar application	30, 60, and 90 mg/L	50–100 mM	Enhanced the antioxidant enzymatic activities and phenolic compounds flavonoid and anthocyanin	[86]
Lycopersicon esculentum L.	Priming	25, 50, and 100 mg/kg soil	200 mM	Antioxidants and osmoregulatory substances significantly improved	[91]
Aloe vera L.	Foliar application	2 mg/L	50–100 mM	Alleviated the damaging effects of salinity	[92]
Lycopersicon esculentum L.	Soil drench	100 mg/L	200 mM	Impact on growth, biosynthesis of chlorophyll content, and metabolic activity	[93]
Fragaria x ananassa Duch.	MS medium	0, 0.08, and 0.8 ppm	0, 50 and 100 mM	Enhanced the antioxidant enzymatic activities and soluble sugar contents	[94]
Phoenix dactylifera L.	MS medium	1 mg/L	1%	Increased antioxidant activity	[95]
Moring oleifera L.	Foliar treatment	0, 20, 40, and 60 ppm	Saline mix soil	Enhanced crude protein, fiber, and ash percentages, as well as the activity of antioxidant enzymes	[96]
Vitis vinifera L.	Irrigation	0, 0.08, and 0.8 ppm	100 mM	Improved ion homeostasis	[97]
Vitis trifolia L.	Irrigation	0.8 ppm	0, 50, and 100 mM	Improved micronutrients in stressful conditions	[98]
Zea mays L.	Foliar application	0, 15, and 30 ppm	0, 50, 100 and 150 mM	It reduced the adverse effects of salinity stress by effectively maintaining the regulation of Na+ concentration, reduction in MDA levels, and H2O2 concentration	[99]
Triticum aestivum L.	Seed priming	0 and 500 mg/L	75 mM	Enhanced antioxidant activities and gene expression	[100]
Gossypium hirsutum L.	Irrigation	0.3 mg and 100 mg/L	10 mM	Increasing leaf number and fresh leaf weight, even under stress	[101]
Helianthus annuus L.	Foliar spray	2 g/L	100 mM	Shoot dry weight and seed yield of sunflower	[102]
Phaseolus vulgaris L.	Foliar spray	0, 10, 20, and 30 µM	200 mM	Seed germination percentage, shoot length, carbohydrate in the shoot and root, and soluble proteins in the shoot and root	[103]
Luffa cylindrica L.	Foliar spray	0.01, 0.025, 0.05, and 0.1 ppm	60 mM	Photosynthetic activity, pigments, primary and secondary metabolites, and antioxidant enzymatic activity	[104]

Table 1. List of the research reports showing the salt mitigation effects of FeO-NPs on horticultural crops.

Crop	Inoculation Method	FeO-NPs Concentration	NaCl Concentration	Physiological Attributes	Reference
Trachyspermum ammi L.	Foliar spray	5, 10, and 15 μM/L	0, 25, 50, and 75 mM	Improved uptake of beneficial nutrients like Fe, N, P, K, and Zn plant growth parameters (e.g., fresh root and shoot weight, root shoot length, and leaf number of leaves under severe salt stress)	[89]
Pistacia vera L.	Hydroponic	2.9 mg/L	0, 100, and 200 mM	Reduced oxidative stress and enhanced chlorophyll content	[90]
Trachyspermum ammi L.	Foliar application	4, 8, and 12 dS/m	3 mM	Increased endogenous SA levels, Fe content, and K+ intake and maintained the K+/Na+ ratio and antioxidant enzyme activity	[88]
Dracocephalum moldavica L.	Foliar application	30, 60, and 90 mg/L	50–100 mM	Enhanced the antioxidant enzymatic activities and phenolic compounds flavonoid and anthocyanin	[86]
Lycopersicon esculentum L.	Priming	25, 50, and 100 mg/kg soil	200 mM	Antioxidants and osmoregulatory substances significantly improved	[91]
Aloe vera L.	Foliar application	2 mg/L	50–100 mM	Alleviated the damaging effects of salinity	[92]
Lycopersicon esculentum L.	Soil drench	100 mg/L	200 mM	Impact on growth, biosynthesis of chlorophyll content, and metabolic activity	[93]
Fragaria x ananassa Duch.	MS medium	0, 0.08, and 0.8 ppm	0, 50 and 100 mM	Enhanced the antioxidant enzymatic activities and soluble sugar contents	[94]
Phoenix dactylifera L.	MS medium	1 mg/L	1%	Increased antioxidant activity	[95]
Moring oleifera L.	Foliar treatment	0, 20, 40, and 60 ppm	Saline mix soil	Enhanced crude protein, fiber, and ash percentages, as well as the activity of antioxidant enzymes	[96]
Vitis vinifera L.	Irrigation	0, 0.08, and 0.8 ppm	100 mM	Improved ion homeostasis	[97]
Vitis trifolia L.	Irrigation	0.8 ppm	0, 50, and 100 mM	Improved micronutrients in stressful conditions	[98]
Zea mays L.	Foliar application	0, 15, and 30 ppm	0, 50, 100 and 150 mM	It reduced the adverse effects of salinity stress by effectively maintaining the regulation of Na+ concentration, reduction in MDA levels, and H2O2 concentration	[99]
Triticum aestivum L.	Seed priming	0 and 500 mg/L	75 mM	Enhanced antioxidant activities and gene expression	[100]
Gossypium hirsutum L.	Irrigation	0.3 mg and 100 mg/L	10 mM	Increasing leaf number and fresh leaf weight, even under stress	[101]
Helianthus annuus L.	Foliar spray	2 g/L	100 mM	Shoot dry weight and seed yield of sunflower	[102]
Phaseolus vulgaris L.	Foliar spray	0, 10, 20, and 30 µM	200 mM	Seed germination percentage, shoot length, carbohydrate in the shoot and root, and soluble proteins in the shoot and root	[103]
Luffa cylindrica L.	Foliar spray	0.01, 0.025, 0.05, and 0.1 ppm	60 mM	Photosynthetic activity, pigments, primary and secondary metabolites, and antioxidant enzymatic activity	[104]

8. Mitigation Impact of Zinc Oxide Nanoparticles in Horticultural Crop

Zinc is an essential micronutrient with critical functions in plant cells, including DNA transcription and intracellular and intercellular signaling. It plays a key role in various physiological processes, particularly in helping plants cope with stress conditions such as salinity [105]. Due to their widespread commercial use today, zinc oxide nanoparticles are anticipated to have positive effects on agriculture as well. Zinc nanoparticles are believed to be a good way to lessen severe environmental stressors [106]. ZnO-NPs are regarded as essential metal oxides that benefit plants in a variety of ways. It was shown that applying Zn-based NPs to plants enhanced their morphological and physiological traits in both normal and stressful situations, including salinity stress. (Figure 3) [107]. Recently, various reports have been published on the application of ZnO-NPs on horticultural crops grown under salinity stress, and positive impacts have been observed by regulating different salt tolerance defense mechanisms (Table 2).

Table 2. Previous and current research reports showing the salt mitigation effects of ZnO-NPs on horticultural crops.

Crop	Inoculation Method	ZnO-NPs Concentration	NaCl Concentration	Physiological Attributes	Reference
Solanum melongena L.	Seed priming	0.42, 0.49, 0.50, and 0.57 mg/g	50 and 100 mM	ZnO-NPs improved photosynthetic pigments, biosynthesis of chlorophyll contents, total soluble sugars, proteins, and proline contents	[108]
Capsicum annum L.	Foliar spray	0, 1000, and 2000 ppm	25, 50, and 75 mM	Improved antioxidant enzymatic system, water holding capacity, retention of K+ and proline content, reduction in electrolyte leakage (EL), Na+, and MDA level	[109]
Phaseolus vulgaris L.	Foliar spray	0, 10, 20, and 30 µM	200 mM	Enhanced the total chlorophyll contents, carbohydrates, and nitrogen in the shoot and root of the plant	[103]
Raphanus sativus L.	Supplemented with soil	1% w/w	100 mM	Promoted biomass and antioxidant activities	[110]
Vicia faba L.	Foliar application	50 and 100 mg/L	150 mM	Improved plant vegetative growth as well as the accumulation of antioxidants enzymes, osmolytes, and secondary metabolites and reduced reactive oxygen species level	[111]
Cucumis sativus L.	Foliar application	25 and 100 mg/L	200 mM	Improved proline contents, total sugar, glycine betaine, free amino acids, and antioxidant enzymes	[112]
Carthamus tinctorius L.	Foliar application	17 mg/L	250 mM	Reduction in the Na+ concentration in leaves and roots (74–60%)	[113]
Lagenaria siceraria L.	Soil drenching	250, 500, and 750 mg/L	500 mg/L	Increased total soluble sugars (23%) and chlorophyll (31%) while keeping the gas exchange parameters constant	[114]
Glycine max L.	Presoaking	25, 50, 100, and 200 mg/L	250 mM	Improved crop yield and enhanced antioxidant enzyme activities	[115]
Fragaria x ananassa Duch.	Ms media	0, 15, and 30 mg/L	0, 35, and 70 mM	Treatment resulted in increased proline content, peroxidase (POD), and catalase (CAT) levels	[116]
Coriandrum sativum L.	Ms media	100 mg/L	50 mM	Enhanced free radical scavenging activity reduced by up to 55%, antioxidant potential by up to 35%, and reduced power by 20% in leaves	[117]
Brassica napus L.	Priming	100 mg/L	150 mM/L	Enhanced plant growth development and gene expression	[118]
Solanum lycopersicum L.	Injection method	0, 20, and 40 mg/L	250 mM NaCl	Improved the endophytic bacteria and promoted plant growth under stress conditions	[119]
Camelina sativa L.	Foliar application	0, 20, 40, and 80 mg/L	0, 50, and 100 mM	Enhanced the macronutrients and plant growth	[120]
Olea europaea L.	Foliar application	200 mg/L	Saline soil	Enhanced the oil and total soluble solids (TSS) percentages of olive fruit as well as the N and P mineral content of the leaves	[121]
Pisum sativum L.	Seed priming	50 and 100 ppm	50 and 100 mM	Decreased levels of MDA, glycine betaine, and hydrogen peroxide	[122]
Oryza sativa L.	modified Hogland’s solution)	50 mg/L	0, 60, 80, and 100 mM	Improved antioxidant system that decreased MDA, proline, and H2O2 levels while lowering oxidative stress	[123]
Triticum aestivum L.	Supplemented with soil	0.12 g/pot	10 dS/m	Increased the total chlorophyll content by 24.6 and 10%, the height of the plant at the vegetative development stage and maturation stages by 34.6 and 37.4%, the length of the shoots and spikes by 30.7 to 27.6%, and the fresh and dry weights of the roots by 74.5 to 63.1%	[124]
Zea mays L.	Foliar application	0, 50, and 100 mg/L	60 and 120 mM	Significantly impacted features of plant development, physiological function, nutritional profiles, antioxidant activity, plant yield, and traits that contribute to yield	[125]
Hordeum vulgare L.	Seed priming	100 mg/kg	100 mM	Mitigation of salt adverse effect; improved fresh biomass, photosynthesis, and antioxidant enzymatic activities by reducing ROS level	[126]
Solanum tubersum L.	Soil drenching method	20, 12, and 15 ppm	Saline soil	Enhanced physiological characteristics, shoot dry weight, number of stems per plant, relative water content of leaves, photosynthetic rate, stomatal conductance, and chlorophyll content	[127]
Gossipyum barbadense L.	Foliar application	100 and 200 ppm	EC 52 dS/m (20%)	ZnO-NPs promoted the dry mass of root, stem and leaves; improved shoot/root ratio	[128]
Helianthus annuus L.	Foliar application	2 g/L	0 and 100 mM	Improved plant physiological attributes and antioxidant activities	[129]
Medicago sativa L.	Foliar application	25 and 50 mg/L	S100	Increased the levels of osmolytes (proline, total soluble sugar, and total soluble protein)	[130]
Phaseolus vulgaris L.	Foliar application	25, 50, 100, and 200 mg/L	200 mM	Increased total soluble sugar contents, proline contents, and antioxidant enzyme activity by promoting the physiological growth parameters	[107]
Vicia faba L.	Foliar application	50 and 100 mg/L	150 mM	Promoted fresh biomass and antioxidant activities	[111]
Abelmoschus esculentus L.	Foliar application	10 mg/L	0, 10, 25, 50, 75, and 100% SW	Significantly enhanced the antioxidant activities and total soluble sugar contents	[131]
Moringa peregrina L.	Foliar application	30, 60, and 90 mg/L	3000, 6000, and 9000 mg/L	Reduction in Na+ while increasing the uptake of beneficial nutrients (Fe, N, P, K, Zn, Mn, Mg)	[132]

Table 2. Previous and current research reports showing the salt mitigation effects of ZnO-NPs on horticultural crops.

Crop	Inoculation Method	ZnO-NPs Concentration	NaCl Concentration	Physiological Attributes	Reference
Solanum melongena L.	Seed priming	0.42, 0.49, 0.50, and 0.57 mg/g	50 and 100 mM	ZnO-NPs improved photosynthetic pigments, biosynthesis of chlorophyll contents, total soluble sugars, proteins, and proline contents	[108]
Capsicum annum L.	Foliar spray	0, 1000, and 2000 ppm	25, 50, and 75 mM	Improved antioxidant enzymatic system, water holding capacity, retention of K+ and proline content, reduction in electrolyte leakage (EL), Na+, and MDA level	[109]
Phaseolus vulgaris L.	Foliar spray	0, 10, 20, and 30 µM	200 mM	Enhanced the total chlorophyll contents, carbohydrates, and nitrogen in the shoot and root of the plant	[103]
Raphanus sativus L.	Supplemented with soil	1% w/w	100 mM	Promoted biomass and antioxidant activities	[110]
Vicia faba L.	Foliar application	50 and 100 mg/L	150 mM	Improved plant vegetative growth as well as the accumulation of antioxidants enzymes, osmolytes, and secondary metabolites and reduced reactive oxygen species level	[111]
Cucumis sativus L.	Foliar application	25 and 100 mg/L	200 mM	Improved proline contents, total sugar, glycine betaine, free amino acids, and antioxidant enzymes	[112]
Carthamus tinctorius L.	Foliar application	17 mg/L	250 mM	Reduction in the Na+ concentration in leaves and roots (74–60%)	[113]
Lagenaria siceraria L.	Soil drenching	250, 500, and 750 mg/L	500 mg/L	Increased total soluble sugars (23%) and chlorophyll (31%) while keeping the gas exchange parameters constant	[114]
Glycine max L.	Presoaking	25, 50, 100, and 200 mg/L	250 mM	Improved crop yield and enhanced antioxidant enzyme activities	[115]
Fragaria x ananassa Duch.	Ms media	0, 15, and 30 mg/L	0, 35, and 70 mM	Treatment resulted in increased proline content, peroxidase (POD), and catalase (CAT) levels	[116]
Coriandrum sativum L.	Ms media	100 mg/L	50 mM	Enhanced free radical scavenging activity reduced by up to 55%, antioxidant potential by up to 35%, and reduced power by 20% in leaves	[117]
Brassica napus L.	Priming	100 mg/L	150 mM/L	Enhanced plant growth development and gene expression	[118]
Solanum lycopersicum L.	Injection method	0, 20, and 40 mg/L	250 mM NaCl	Improved the endophytic bacteria and promoted plant growth under stress conditions	[119]
Camelina sativa L.	Foliar application	0, 20, 40, and 80 mg/L	0, 50, and 100 mM	Enhanced the macronutrients and plant growth	[120]
Olea europaea L.	Foliar application	200 mg/L	Saline soil	Enhanced the oil and total soluble solids (TSS) percentages of olive fruit as well as the N and P mineral content of the leaves	[121]
Pisum sativum L.	Seed priming	50 and 100 ppm	50 and 100 mM	Decreased levels of MDA, glycine betaine, and hydrogen peroxide	[122]
Oryza sativa L.	modified Hogland’s solution)	50 mg/L	0, 60, 80, and 100 mM	Improved antioxidant system that decreased MDA, proline, and H2O2 levels while lowering oxidative stress	[123]
Triticum aestivum L.	Supplemented with soil	0.12 g/pot	10 dS/m	Increased the total chlorophyll content by 24.6 and 10%, the height of the plant at the vegetative development stage and maturation stages by 34.6 and 37.4%, the length of the shoots and spikes by 30.7 to 27.6%, and the fresh and dry weights of the roots by 74.5 to 63.1%	[124]
Zea mays L.	Foliar application	0, 50, and 100 mg/L	60 and 120 mM	Significantly impacted features of plant development, physiological function, nutritional profiles, antioxidant activity, plant yield, and traits that contribute to yield	[125]
Hordeum vulgare L.	Seed priming	100 mg/kg	100 mM	Mitigation of salt adverse effect; improved fresh biomass, photosynthesis, and antioxidant enzymatic activities by reducing ROS level	[126]
Solanum tubersum L.	Soil drenching method	20, 12, and 15 ppm	Saline soil	Enhanced physiological characteristics, shoot dry weight, number of stems per plant, relative water content of leaves, photosynthetic rate, stomatal conductance, and chlorophyll content	[127]
Gossipyum barbadense L.	Foliar application	100 and 200 ppm	EC 52 dS/m (20%)	ZnO-NPs promoted the dry mass of root, stem and leaves; improved shoot/root ratio	[128]
Helianthus annuus L.	Foliar application	2 g/L	0 and 100 mM	Improved plant physiological attributes and antioxidant activities	[129]
Medicago sativa L.	Foliar application	25 and 50 mg/L	S100	Increased the levels of osmolytes (proline, total soluble sugar, and total soluble protein)	[130]
Phaseolus vulgaris L.	Foliar application	25, 50, 100, and 200 mg/L	200 mM	Increased total soluble sugar contents, proline contents, and antioxidant enzyme activity by promoting the physiological growth parameters	[107]
Vicia faba L.	Foliar application	50 and 100 mg/L	150 mM	Promoted fresh biomass and antioxidant activities	[111]
Abelmoschus esculentus L.	Foliar application	10 mg/L	0, 10, 25, 50, 75, and 100% SW	Significantly enhanced the antioxidant activities and total soluble sugar contents	[131]
Moringa peregrina L.	Foliar application	30, 60, and 90 mg/L	3000, 6000, and 9000 mg/L	Reduction in Na+ while increasing the uptake of beneficial nutrients (Fe, N, P, K, Zn, Mn, Mg)	[132]

9. Global Risk, Limitations, and Challenges of NPs Application in Future Agriculture

Due to its impressive characteristics and metabolic activities, NPs are particularly helpful in eliminating the harmful impacts of ROS at the molecular, enzymatic, non-enzymatic, and biochemical levels when salt stress is present [133]. However, NPs supplemented with enough quantities can impair plant life and have hazardous effects on the environment [114]. NPs that have been applied to plant leaves or soil can have an impact on the deep lung tissue during inhaling. Since NPs are so tiny, they can enter the circulation and build up in organs, including the liver, kidneys, and lungs. There, they can cause oxidative stress, lipid peroxidation, genotoxicity, protein fibrillation, and inflammatory reactions. Silver nanoparticles, zinc oxide nanoparticles, nano-zerovalent nanoparticles, and TiO₂ nanoparticles were the main targets of research because of their harmful environmental consequences. Implementing NPs in an agroecosystem presents a number of challenges, including toxicity, the ideal dosage, bioavailability, and a lackluster regulatory environment (Figure 4) [134]. Improper NP application can lead to environmental toxicity and negatively impact plant physiology and growth. Excessive NP exposure generates reactive oxygen species (ROS), causing oxidative stress that harms plants [135]. Plants respond to high NP concentrations through physiological, morphological, and molecular changes. The uptake and ecotoxicity of NPs are largely influenced by biological processes in the rhizosphere [136]. Researchers have identified several factors affecting NP toxicity and activity in soil, including plant species, NP concentration, morphology, size, surface charge, and type (carbon-based, magnetic, non-metallic, or metallic). The concentration of NPs especially impacts the vegetative growth and development of horticultural crops. It is commonly known that higher NP application rates pose risks to vegetation, animals, and people. The extensive application of NPs for the alleviation of plant salt and/or environmental stressors may be limited as a result of the previously noted safety concerns. Therefore, intervention techniques must be used to reduce the environmental dangers related to nanotechnology. Technology advancement, omics technology, and interdisciplinary approaches could be convenient for sustainable development and policy implementation for the agriculture and health sectors. The combination of all these approaches will give us a more comprehensive idea about how to tackle such lethal effects of NPs.

10. Conclusions

To achieve sustainable agriculture, it is crucial to address crop yield losses caused by salinity stress through environmentally friendly solutions. Nanotechnology offers a novel and highly effective approach to modernize farming practices, enhance agricultural productivity and crop quality, and meet the food demands of a growing global population without compromising food security. Various NPs are being explored for their potential to enhance crop productivity and promote plant growth while mitigating the detrimental effects of abiotic stress. Salinity disrupts the cellular redox balance by inducing excessive ROS generation, which subsequently leads to biomolecule degradation and impairs essential cellular functions in plants. To alleviate the detrimental effects of salinity stress, FeO-NPs and ZnO-NPs stimulate plant defense mechanisms, enhancing stress tolerance and resilience. NPs are small enough to efficiently penetrate plant tissues, where they enhance physiological, biochemical, and morphological processes, promote plant growth and development, and improve crop yields, even under salinity stress conditions. Additionally, the large surface area of NPs makes it easier for a variety of particular nutrients to be absorbed and distributed. However, the application of NPs for crop enhancement and sustainable agriculture remains in its early stages, with limited high-quality research in the field. Further studies are needed to minimize the potential adverse effects of NPs on ecosystems and horticultural crops, particularly in understanding their interactions with plants at the molecular and cellular levels, their influence on metabolic processes, and their impact on gene expression. Additionally, thorough risk assessments are needed to ensure the safe and sustainable use of NPs in agriculture and human health.