Understanding Salt Stress in Watermelon: Impacts on Plant Performance, Adaptive Solutions, and Future Prospects

Abstract

Soil salinity stress, intensified by extreme weather patterns, significantly threatens global watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai] production. Watermelon, a moderately salt-sensitive crop, exhibits reduced germination, stunted growth, and impaired fruit yield and quality under saline conditions. As freshwater resources decline and agriculture’s dependency on irrigation leads to soil salinization, we need sustainable mitigation strategies for food security. Recent advances highlight the potential of using salt-tolerant rootstocks and breeding salt-resistant watermelon varieties as long-term genetic solutions for salinity. Conversely, agronomic interventions such as drip irrigation and soil amendments provide practical, short-term strategies to mitigate the impact of salt stress. Biostimulants represent another tool that imparts salinity tolerance in watermelon. Plant growth-promoting microbes (PGPMs) have emerged as promising biological tools to enhance watermelon tolerance to salt stress. PGPMs are an emerging tool for mitigating salinity stress; however, their potential in watermelon has not been fully explored. Nanobiochar and nanoparticles are another unexplored tool for addressing salinity stress. This review highlights the intricate relationship between soil salinity and watermelon production in a unique manner. It explores the various mitigation strategies, emphasizing the potential of PGPM as eco-friendly bio-inoculants for sustainable watermelon management in salt-affected soils.

1. Introduction

Environmental stresses associated with a changing climate pose major challenges in the 21st century, directly impacting ecosystems, agriculture, and global food and energy security [1]. Rising temperatures, altered precipitation patterns, and increased soil salinization exacerbate osmotic and ionic imbalances in plants, leading to oxidative stress, reduced photosynthetic efficiency, and impaired metabolic functions [2,3]. Extreme weather events lead to alterations in precipitation patterns, soil degradation, and pathogen outbreaks [4,5]. Climate-driven events also increase the spread of pests and diseases, further threatening plant health and productivity [6]. For instance, rising temperatures and carbon dioxide (CO₂) levels can exacerbate drought stress while influencing pest populations and disease dynamics [3]. Changes in precipitation patterns contribute to water stress through prolonged droughts or excessive rainfall, which impair plant physiological processes, including photosynthesis and nutrient uptake [7]. An increase in temperatures disrupts plant physiological processes, such as photosynthesis, respiration, and reproductive development, and reduces yields and compromises the quality of crops [8].

Plants, being sessile organisms, are particularly vulnerable to a range of biotic and abiotic stresses. Biotic stresses, caused by pathogens, insects, and herbivores, further challenge plant survival and yield by triggering complex defence responses [9]. Abiotic stresses, including drought, salinity, extreme temperatures, heavy metals (HMs), and oxidative stress, directly impact plant growth and productivity by disrupting physiological and biochemical processes [10,11]. Among these constraints, salinity stress has emerged as a critical concern, particularly in arid and semi-arid regions where soil salinization is intensifying due to irrigation practices and sea-level rise [12]. Soil salinity is a major abiotic stress that significantly impedes plant growth and development worldwide. An excessive accumulation of soluble salts, mainly sodium chloride (NaCl), in the soil results in soil salinity, deteriorates soil quality and reduces crop productivity. The excess salt in plant tissues disrupts various physiological and biochemical processes within plants, leading to detrimental effects [13,14]. Soil salinization is a major environmental and agricultural issue affecting approximately 831 million hectares of land globally [15]. Excessive salt, especially sodium (Na), deteriorates soil quality and reduces crop productivity [16]. Soil salinity stress is exacerbated by factors such as industrial pollution, poor irrigation, and a growing human population, placing pressure on the agricultural sector to produce more food from saline soils [17]. The severity varies depending on salt concentration, crop species, growth stage, and duration [18]. Saline–alkali land has high Na⁺ levels that cause hypertonic conditions, limiting water and nutrient uptake by plants.

To avoid damage to vital processes, plants accumulate excess Na⁺ in their vacuoles, but high Na⁺ levels are toxic. Initially, osmotic stress affects water balance, followed by ionic stress, where Na⁺ uptake impairs K⁺ uptake, causing nutrient imbalances. Soil salinity stress also leads to oxidative stress due to reactive oxygen species (ROS), including singlet oxygen (O₂), the hydroxyl radical (^⋅OH), hydrogen peroxide (H₂O₂), and superoxide ions (O₂^⋅–) [19]. ROS disrupt subcellular structures, such as chloroplasts, mitochondria, and membranes, damaging macromolecules (proteins, lipids, DNA, and carbohydrates) and triggering cell death [20]. Plants activate endogenous antioxidant systems to combat oxidative stress, but excessive ROS can damage photosystem I (PSI) and II (PSII) in thylakoid membranes. Lipid peroxidation caused by ROS formation leads to membrane damage, organelle dysfunction, and increased markers of oxidative stress, such as protein oxidation and malondialdehyde (MDA) accumulation [21,22]. Plants under soil salinity stress accumulate various soluble sugars, such as sucrose, trehalose, and raffinose, as well as sugar alcohols, including sorbitol and mannitol, to regulate osmotic stress levels, maintain cell turgor pressure, and help stabilize cell membranes [23]. Amino acids are another essential metabolite in plants, not only for protein synthesis and other key cellular functions, but they also act as essential osmolytes to balance the cellular osmotic potential and control ion transport, as well as function as scavengers of reactive oxygen species (ROS) generated in plants under soil salinity stress [24]. For example, proline is widely recognized as an osmolyte that accumulates and protects plant cells from damage caused by salinity. Secondary metabolites are generally not required for the normal functioning of plant cells, but they play a crucial role in protecting plants against abiotic and biotic stresses. The naturally occurring alkaloids, which contain a nitrogen atom in a heterocyclic ring, have antioxidant activities and play important roles as ROS scavengers under soil salinity stress [25].

Watermelon belongs to the Cucurbits, native to tropical areas of Africa, particularly near the Kalahari Desert [26]. Watermelon is a popular fruit crop celebrated for its high water content, nutritional benefits, and economic value [27]. It thrives in tropical and subtropical climates, flourishing in well-drained soils with ample moisture and sunlight [28]. China is the world’s largest producer of watermelon, with an annual output of 60.4 million tons. At the same time, the United States is the eighth-largest global producer, with an annual production of 1.5 million tons in 2025 [29]. However, the cultivation of watermelon is increasingly threatened by abiotic stresses, especially salinity. Soil Salt stress has detrimental effects on watermelon, negatively impacting seed germination, vegetative growth, and physiological processes, reducing fruit yield and quality [30]. Watermelon is moderately sensitive to salinity, with a maximum salinity tolerance threshold of approximately 2.2 dS/m [31,32]. When the electrical conductivity (EC) of the irrigation water exceeds this threshold, watermelon plants experience reductions in photosynthesis, transpiration, and stomatal conductance. Additionally, salinity adversely affects nutrient absorption, transport, assimilation, and distribution, leading to reductions in plant growth and yield [33]. This review is the first attempt to focus on the salt tolerance studies in watermelon and its mitigation strategies.

2. Search Methodology

An iterative search approach was employed, utilizing Google Scholar, the World Wide Web, and citation searching to identify relevant papers on watermelon under salinity stress. The research articles were searched from 2020 to 2025 regarding studies on salinity stress in watermelon. The search words were watermelon, growth, salinity stress, adaptation to salinity, agriculture, and salinity. For specific information, such as salinity and agriculture, soil salinity, and crop production, we searched research articles older than 2020.

3. Soil Salinity and Plant Development

Crops encounter a variety of environmental challenges, including strong winds, extreme temperatures, drought, flooding, and soil salinity, all of which can significantly impact their growth and productivity. Among these issues, soil salinity is particularly damaging, leading to significant losses in farmland, crop yields, and produce quality [34]. Soil salinity negatively affects different stages of growth, from seed germination to the vegetative, reproductive, and fruit stages, and drastically affects yield [14,35]. When the Sodium Absorption Ratio (SAR) is 15 or higher, Na⁺ alters soil properties and hinders water uptake by plants. Salinity significantly impacts plant growth and quality, causing issues such as leaf burn, flower deformities, and stunted stems, which ultimately lead to lower yields [36]. In the section below, we discuss the impact of soil salinity on various growth stages of watermelon.

4. Effects of Soil Salinity on Watermelon Growth and Development

4.1. Seed Germination, Early Growth, and Plant Establishment

High salinity levels significantly hinder the germination of watermelon seeds and the vigor of the early seedlings. The osmotic imbalance caused by salt stress affects seeds’ ability to absorb water, thereby delaying or even preventing germination [37]. Early seedling growth is compromised, leading to stunted root and shoot development. Seedling production is the most critical stage in the fruit production [38]. A study carried out by Tehri et al. evaluating the effect of salinity NaCl (0, 2, 4, 6 dS/m) and salicylic acid (0, 0.5, 1 mM) on the germination of cucumber (Cucumis sativus cv. Super Dominus) and watermelon (C. lanatus cv. Crimson Sweet) seeds indicated that salinity significantly inhibited seed germination and seedling growth. At the highest salinity level (6 dS/m), cucumber and watermelon seeds showed the lowest germination rates (18.79 and 10.33, respectively), germination percentages (86.65% for cucumber and 69.63% for watermelon), and seed vigor indices (17.93 for cucumber and 9.59 for watermelon) [39].

A study evaluated the effect of different levels of irrigation water salinity on the emergence and initial development of ‘Crimson Sweet’ watermelon. The results indicated impacts on both emergence and initial growth, although the reductions during emergence were less pronounced. As the salinity levels of the irrigation water increased in the substrate from 0.17 to 5.5 dS/m, the emergence percentage decreased significantly. These findings suggest that high salinity levels significantly impair seed germination and early seedling development in watermelon. Salt stress creates osmotic imbalances that hinder water uptake, consequently delaying or inhibiting germination [40].

4.2. Vegetative Growth

Salt stress reduces leaf area, stem elongation, and overall biomass accumulation, directly affecting plant vegetative growth. The accumulation of sodium (Na⁺) and chloride (Cl⁻) ions within plant tissues disrupts cellular functions, leading to nutrient imbalances in watermelon [41]. This disruption results in stunted growth and diminished photosynthetic efficiency. A 2022 study examined the effects of salt stress on the germination, development, and physiology of various plants, including musk melon (Kalash, Durga), bottle gourd (Crystal long, Nuefield), and squash (Green round, Squash malika). The seeds were exposed to saline solutions ranging from 0 to 6.0 dS/m, with germination monitored for seven days and growth parameters assessed after thirty days. The results indicated that bottle gourd varieties exhibited the highest germination rates, with 93.42% for Nuefield and 85.56% for Crystal Long.

In contrast, the musk melon varieties showed the lowest rates, at 58.36% for Kalash and 54.54% for Durga. Increased salinity significantly reduced both shoot and root lengths across all cucurbits. In growth metrics such as leaf count, leaf area, and biomass, bottle gourd varieties outperformed the other plants. Moreover, chlorophyll content declined with salinity, although Nuefield retained the highest levels. Among the varieties tested, musk melons accumulated the most Na⁺ and Cl⁻ compared to the bottle gourds. Overall, bottle gourd demonstrated the highest salt tolerance, while musk melon proved to be the most sensitive. The best-performing varieties in response to salt stress were Nuefield (bottle gourd), Squash malika (squash), and Kalash (musk melon). In contrast, Crystal long, Green round, and Durga were the most adversely affected [42].

A study assessed the salt stress responses of the watermelon cultivar Crimson Tide and seven gourd genotypes under salinity levels (0–16 dS/m) over 30 days. Key measurements included stem length, dry weight, and leaf ion concentrations. All genotypes showed reduced growth due to salinity, with gourds generally performing better than watermelon, except for Luffa cylindrica and Benincasa hispida. Sodium (Na⁺) accumulation varied, with L. cylindrica having the highest and Birecik the lowest. C. maxima, B. hispida, and L. cylindrica accumulated more Na⁺ than watermelon. Higher Ca²⁺/Na⁺ and K⁺/Na⁺ ratios were linked to greater dry weight, whereas increased Na⁺ negatively affected biomass. Cucurbita and Lagenaria genera showed better salt tolerance than L. cylindrica, B. hispida, and watermelon [43].

In 2024, a study evaluated 48 watermelon genotypes under salt stress in a hydroponic greenhouse system using Hoagland nutrient solution. Salt stress was gradually applied by increasing NaCl levels to an EC of 8 dS/mover six days, followed by a 21-day exposure. Control plants were maintained at 1.5 dS/m without addition of salt. Most genotypes exhibited a reduction in growth parameters, with the most significant decreases observed in plant height (19.7%) and fresh stem weight (50.3%). However, certain genotypes, such as W3, W4, W8, W9, and W19, exhibited increased values in traits like root length and leaf number, indicating potential tolerance mechanisms. Photosynthetic pigments (chlorophyll a, chlorophyll b, and carotenoids) decreased, while SPAD values and PAR efficiency slightly increased, suggesting some physiological adjustment. W36 was highly sensitive, whereas W4, W14, and W64 maintained or improved pigment levels [44].

These findings suggest that exposure to salinity at the germination and seedling stages negatively impacts watermelon crop growth and development.

4.3. Physiological, Biochemical, and Molecular Responses

4.3.1. Physiological Responses

Watermelon, as a crop, has developed various adaptive mechanisms in response to salt stress, including osmotic adjustment, ion exclusion, and antioxidant defence systems. However, prolonged exposure to high salinity levels can overwhelm these mechanisms, leading to oxidative stress and subsequent cellular damage [45]. A study aimed to assess the salt tolerance of six watermelon genotypes (Crimson, Charleston Gray, Anarkali, Chairman, Sugar Baby, and Champion) under varying salinity levels (1.5, 3, 4.5, and 6 dS/m NaCl). At the highest salinity (6 dS/m NaCl), complete mortality was observed in Chairman and Champion, while other cultivars showed varied mortality rates (e.g., Crimson at 57%). Genotype Charleston Gray performed best across most growth parameters, including root and shoot length, biomass, and leaf number. Champion showed the most pronounced reduction in these traits, indicating its salt sensitivity. Salt stress reduced shoot and root lengths, with the highest root length observed at 3 dS/m NaCl, especially in Charleston Gray [46].

Furthermore, plant fresh and dry weights were significantly reduced at higher salinity levels, with Charleston Gray maintaining relatively higher values than other cultivars. Chlorophyll content, nitrogen, and protein contents were also affected by salinity, with Charleston Gray showing better retention of these traits. The study concluded that salt stress severely affects watermelon growth, with salt-tolerant genotypes such as Charleston Gray maintaining better growth and biomass accumulation. These results agree with previous studies that indicated salt stress reduces growth, root size, and biomass in various crops. A study on 22 watermelon genotypes, including a salt-tolerant C. colocynthis accession, examined the effects of salt stress in an unheated greenhouse. Salt stress was induced with NaCl at 0, 25, 50, and 100 mmol kg⁻¹, with the highest level applied in stages to prevent acute effects. Results showed that salt stress significantly reduced growth in shoot length, fresh and dry weight, decreasing by 61.44%, 60.75%, and 75.48%, respectively, under 100 mmol kg⁻¹ NaCl. These findings align with previous studies in other crops, highlighting the detrimental impact of salinity on plant development [47].

4.3.2. Biochemical Responses

Under salt stress, the natural endogenous cytokinin generally decreases, and there is an increase in the synthesis and accumulation of abscisic acid (ABA) [48].

In a study by Santos et al. on mini watermelon hybrid ‘Quetzali’, the plants were evaluated for the influence of two growth regulators, forchlorfenuron or [N-(2-chloro-pyridyl)-N-phenylurea (CPPU)] and indoleacetic acid (IAA). The treatments were composed of five proportions of growth regulators (0/100; 25/75; 50/50; 75/25 and 100/0%) corresponding to concentrations of 1.0 and 10.0 mg L⁻¹ CPPU/IAA, respectively, and two levels of salinity, one composed of water without salt addition (normal supply water) at 0.3 dS/m and another with saline water at 2.0 dS/m electrical conductivity (EC) [49]. The study found that the optimal leaf area (LA) was achieved with a 75/25% ratio of CPPU to IAA at a low electrical conductivity (EC) of 0.3 dS/m, resulting in a 26% increase in LA compared to higher salinity (2.0 dS/m). Under saline conditions, a 25/75% ratio performed better overall. CPPU alone (100/0%) led to higher total dry mass (TDM) and yield, indicating better adaptation to salinity. The best biomass accumulation (14.32 g) occurred with the 25/75% ratio under salinity, while CPPU alone increased TDM by 14.89% under salt stress. Fresh fruit mass (FFM) was the highest at 0.3 dS/m, but the 75/25% ratio still yielded a significant 634 g even under salinity. Salinity negatively impacted pulp firmness (PF), with the highest value (7.5 N) found under non-saline conditions at a 0/100% ratio of CPPU/IAA. The study concluded that the 25/75% ratio minimized reductions in LA and TDM under salt stress, while maintaining or improving both PF and soluble solid (SS) content. The interaction between cytokinin and auxin plays a key role in enhancing plant tolerance to stress.

Like heat and drought, salt stress often leads to the overproduction of reactive oxygen species (ROS) in watermelon [50] and melon [51]. Excessive ROS—including superoxide (O₂⁻), hydrogen peroxide (H₂O₂), and hydroxyl radicals (OH⁻)—can cause severe cellular damage by initiating lipid peroxidation, fragmenting DNA strands, inactivating vital enzymes, and disrupting redox balance, thereby hindering normal watermelon growth and development [52]. To counteract ROS accumulation, Cucurbitaceae have evolved sophisticated antioxidant defense systems comprising enzymatic and non-enzymatic components.

4.3.3. Molecular Response

In watermelon and other Cucurbitaceae, salt tolerance is a complex quantitative trait regulated by multiple genes and pathways. Recombinant inbred population studies confirm that, like cucumber and melon, watermelon’s salt tolerance is controlled by diverse loci [53,54,55,56]. Transcription factors such as DREB, bZIP, and NAC bind to cis-regulatory elements (DRE, ABRE, NACR), activating downstream genes that regulate osmotic adjustment, ion homeostasis, and antioxidative defenses in melon [51,57]. The Salt Overly Sensitive (SOS) pathway, critical for Na⁺ homeostasis, involves SOS3 sensing calcium signals, which activate SOS2, which in turn phosphorylates SOS1 to promote Na⁺ efflux [58,59]. Although poorly studied in watermelon, evidence suggests that polyamines may upregulate SOS2 [53]. Genome-wide identification studies revealed that transcription factors from the HD-ZIP (ClHDZ20, ClHDZ36, ClHDZ1, ClHDZ18) family, Dof, ERF, WRKY, NAC, bHLH, MYB, and others are upregulated in response to salt stress in watermelon, thereby coordinating stress responses [60,61,62]. Hormonal signaling pathways involving IAA, cytokinin, ABA, BR, and melatonin biosynthesis (ClCOMT1) are modulated during salt stress, thereby enhancing tolerance in melon and watermelon [57,63,64,65].

Joint metabolomic and transcriptomic analyses in melon reveal the central role of amino acid and carbohydrate metabolism under salt stress. Amino acid metabolites such as L-Glutamate, GABA, Proline, L-Asparagine, L-Aspartate, L-Alanine, and Citrulline accumulate [66]. Hormone signaling pathways respond dynamically; auxin is generally downregulated, while ethylene, BR, and JA pathways are variably activated in watermelon and melon [60,61,62,67]. In melon, osmotic balance is maintained by restricting Na⁺ uptake, extruding Na⁺ from the roots, or compartmentalizing it into vacuoles [68,69]. The role of

CmHKT1;1 in melon is well characterized, and MIRK, an inward-rectifying K⁺ channel, is important in salt-tolerant melon varieties [70,71]. Figure 1 summarizes the molecular response of watermelon to salinity stress.

4.4. Yield and Quality

Salt stress significantly reduces watermelon yield and fruit quality. The accumulation of salts in the soil limits fruit size, lowers sugar content, and decreases the overall nutritional value of the fruit [37]. These negative effects are more pronounced under prolonged or severe salinity conditions. The addition of NaCl to the nutrient solution resulted in a reduction in all production variables compared to the control treatment in watermelon, regardless of K and Ca concentrations. These reductions were approximately 46.39%, 20.01%, 19.25%, and 17.35% for production, longitudinal diameter of fruit, transverse diameter of fruit, and pulp thickness, respectively [72].

In an experiment, mini-watermelon plants were irrigated with five water mixtures composed of varying proportions of tap water (TW; EC = 0.54 d/m) and reject brine (RB; EC = 9.50 dS/m), resulting in salinity levels ranging from 0.54 to 6.90 dS/m (M1–M5). These mixtures were applied in an open hydroponic system using four substrates: coconut fibre (S1), washed sand (S2), and two sand–rice straw blends (S3 and S4). As salinity increased, fruit weight declined, with the sharpest drop (48.86%) seen in coconut fibre under the highest salinity (M5). In contrast, sand and mixed substrates showed smaller reductions (21–27%), with no significant differences between the control (M1) and low-salinity treatment (M2) in these substrates. Fruit size (longitudinal and transverse diameters) decreased with increasing salinity, although no significant interaction was found between substrates and water mixtures. Coconut fiber consistently produced the largest fruits, with diameters 11–14% larger than those of washed sand. Pulp pH showed a significant interaction (p < 0.01), being the highest in S1 and declining with increasing RB, especially in S1 and S3 [73]. Zong et al. studied the impact of saline water on the yield and quality of two Chinese Cucurbit species, melon (C. melo cv. Huanghe) and watermelon (C. lanatus convar megulaspemus). The melon yields decreased with an increase in water salinity. However, the concentrations of glutamic acid increased, while the concentrations of most other amino acids remained unchanged. The watermelon yields significantly decreased with an increase in water salinity. However, fruit number, firmness, crude protein content, and essential amino acid levels significantly increased with water salinity. Salt stress increased total soluble solids and Na⁺ concentrations, while Ca²⁺ and Cl⁻ concentrations were unaffected significantly in both Cucurbit species [74]. Figure 2 and

Table 1. Effect of salinity on watermelon growth and development.

Growth stages	References
Seed germination and early growth	[39,40]
Vegetative growth	[37,44]
Physiological, biochemical, and molecular responses	Physiological responses [46,47]
	Biochemical responses [49,50]
	Molecular responses [53,57,60,61,62,63,64,65,66,67,68,69,70,71]
Yield and Quality	[72,73,74]

describe the effect of salinity on watermelon growth and development.

5. Strategies to Mitigate Salt Stress in Watermelon

Several strategies are employed to enhance salinity tolerance in watermelon, including the use of salt-tolerant rootstocks, breeding salt-resistant varieties, and implementing agronomic practices such as drip irrigation and soil amendments [75]. Additionally, applying biostimulants and plant growth regulators has shown promise in improving watermelon’s resilience to salt stress [76].

5.1. Use of Salt-Tolerant Rootstocks

Currently, a fast and efficient way for horticultural crops to cope with biotic and abiotic stresses, under the prism of sustainable crop management, is through vegetable grafting. The grafting technique combines the desirable fruit traits of a scion (top plant) with the stress tolerance of a rootstock (bottom plant) [77,78]. Root characteristics are significant in determining salt tolerance in melon plants and salt-sensitive and salt-tolerant potato genotypes.

Goreta et al. observed that when watermelon (‘Fantasy’) was grafted onto ‘Strongtosa’ rootstock (C. maxima Duch. × C. moschata Duch.), it showed lower reductions in shoot weight and leaf area on salinity than in ungrafted plants [79]. Moreover, other experiments demonstrated that grafted ‘Crimson Tide’ watermelon [C. lanatus (Thunb.) Matsumet Nakai] onto C. maxima and two Lagenaria siceraria rootstocks resulted in higher growth performance than ungrafted plants under saline conditions (8.0 dS/m). A reduction in shoot dry weight was observed in ungrafted plants, ranging 41% to 0.8% in grafted plants under the same saline conditions [80]. Grafting these genotypes onto rootstocks capable of inducing salt tolerance to the scion can be used as one possible way to reduce the detrimental effects of salt stress on high-yielding cultivars [81]. Another study showed that Cucurbita (C. maxima and C. moschata) and Lagenaria (Lagenaria siceraria) rootstocks perform better than watermelon, Luffa cylindrica, and Benincasa hispida under salinity stress by avoiding physiological damage by accumulation of Na⁺ ions in leaves [43]. Watermelon salt tolerance can be improved by grafting watermelon onto salt-tolerant gourd (Lagenaria spp. and Cucurbita spp.) rootstock. Nongrafted watermelon plants show fewer adverse effects in plant growth parameters such as leaf surface area, leaf numbers, and total dry matter, which were negatively affected by salt stress, than control plants grown under normal conditions. Grafted plants outperformed nongrafted plants in terms of plant growth parameters under saline conditions.

Citirex and Altinbas, two melon (C. melo L.) cultivars, were grafted onto commercial Cucurbita rootstocks (Kardosa and Nun9075) and grown at two electrical conductivity levels: 1.5 dS/m for control and 8.0 dS/m for salt stress. Under the hypertonic salt stress, statistically significant negative correlations existed between leaf proline and shoot dry biomass, leaf MDA, leaf and root ion leakages, and leaf Na⁺. Also, under salt stress, growth and biomass production of grafted melons improved by enhancing physiological (high leaf area and photosynthesis), biochemical (low leaf proline and MDA), and nutritional (low leaf Na⁺ and ion leakage and high K⁺ and Ca²⁺ contents) parameters. Citirex/Nun9075 and Citirex/Kardosa graft combinations exhibited the highest growth performance. Both Cucurbita cultivars have high rootstock potential for salt stress tolerance in melon [82].

A greenhouse experiment evaluated the growth, yield, fruit quality, gas exchange, and mineral composition of watermelon (‘Tex’), either ungrafted or grafted onto ‘Macis’ and ‘Ercole’ rootstocks, under two salinity levels (2.0 and 5.2 dS/m) using the Nutrient Film Technique (NFT). Salinity reduced the total yield due to a decrease in mean fruit mass rather than fruit number, while grafting increased total yield by 81% compared to ungrafted plants. Salinity improved fruit quality across all grafting combinations by increasing dry matter, total soluble solids, glucose, fructose, and sucrose, though grafting itself had no significant effect on sugar content. Grafted plants had higher juice electrical conductivity, but salinity increased the peel percentage and decreased the pulp percentage in grafted plants. Grafting significantly increased leaf area (149% larger than ungrafted), while salinity reduced it by 38%. Salinity also decreased stomatal conductance (gs) by 39% and lowered CO₂ assimilation (A), particularly in ungrafted ‘Tex’ plants, with A inversely correlated with leaf Na⁺ and Cl⁻ concentrations. Grafting enhanced potassium accumulation in stems and leaves, with ‘Tex/Ercole’ exhibiting the highest potassium levels, though salinity reduced potassium content, especially in ungrafted plants [41].

5.2. Breeding Salt-Resistant Varieties

Breeding involves modifying the genetic make-up of current types to improve their qualifications, efficacy, utility, and cost-effectiveness. A promising development in crop breeding is the creation of F1 hybrids that differ from cultivars in terms of superior output, plant uniformity, and fruit characteristics such as color, quality, size, ripening date, freshness, and resilience to abiotic and biotic stress [83]. There are no reports of the application of ZNF (Zinc Finger Nucleases) or TALENs (Transcription Activator-like Effector Nucleases) in cucurbits. Furthermore, a study from Zhu et al. (2018) [84] found that watermelon plants treated with a salt solution had higher levels of the transport protein HKT1;5, which is responsible for salt tolerance in plants.

Additionally, watermelon plants have also been found to contain high levels of antioxidant enzymes, which help protect the plant from the detrimental effects of stress factors. Therefore, watermelon plants exposed to salt stress exhibited higher levels of antioxidant enzymes, including peroxidase, superoxide dismutase, and catalase [32]. By using qRT-PCR, it was possible to ascertain the relative expression of genes linked to chlorophyll degradation, drought tolerance, and transcription factors (such as WRKY70- and MYB96), as well as ROS scavenging systems (including catalase Cu–Zn Superoxide dismutase, Glutathione reductase, and Ascorbate peroxide) [85].

Another study evaluated the salt tolerance of 121 watermelon germplasm resources at the seedling stage to identify salt-tolerant types for breeding purposes. Seedlings were cultured in Hoagland’s nutrient solution with or without 150 mmol/L NaCl, and traits such as shoot fresh weight, shoot dry weight, stem diameter, root length, root surface area, and SPAD value were measured after 8 days. Significant genetic variation was observed among accessions under salt stress, and correlation and principal component analyses highlighted shoot fresh weight and root length as key indicators of salt tolerance. Membership function analysis classified the germplasm into four groups: highly salt-sensitive, weakly salt-sensitive, moderately tolerant, and salt-tolerant. Three accessions (Zaohua, PI490377, and Zhongshihong) were identified as salt-tolerant, while three others (PI186489, PI494532, and Dahongzi) were highly sensitive. These findings provide valuable materials for breeding salt-tolerant watermelon varieties and for further research on salt tolerance mechanisms [86].

Six melon genotypes, including four winter and two summer melon landraces, were evaluated under increasing salinity levels (0, 30, 60, and 90 mM NaCl). Salt stress reduced relative water content (RWC) and membrane stability index (MSI), with salt-tolerant genotypes (Ghobadlu and Suski-e-Sabz) maintaining higher levels. Oxidative stress indicators (H₂O₂ and MDA) increased with salinity, particularly in salt-sensitive genotypes (Samsuri and Kashan), while Ghobadlu and Suski-e-Sabz exhibited stronger antioxidant enzyme activity, mitigating ROS damage. These tolerant genotypes also accumulated more osmolytes (proline and soluble carbohydrates), aiding stress resistance. Photosynthetic pigments declined under salinity, but Ghobadlu, Suski-e-Sabz, and Galia F1 retained higher levels, supporting better photosynthetic efficiency. Biomass production decreased by 16.8%, 28.2%, and 41.5% at 30 mM, 60 mM, and 90 mM NaCl, respectively, with Ghobadlu, Suski-e-Sabz, and Galia F1 showing superior growth. A principal component analysis and cluster analysis grouped these three as salt-tolerant, while Samsuri, Kashan, and Khatouni were classified as salt-sensitive. The findings highlight Δ¹³C as a reliable selection tool for salt tolerance, and Ghobadlu and Suski-e-Sabz as promising candidates for breeding salt-tolerant melon cultivars [51].

According to a study, the tolerance of different watermelon genotypes under saline conditions was found. Twenty-two watermelon genotypes and accessions were grown under control conditions (0 mmol kg⁻¹ NaCl) as well as at 25, 50, and 100 mmol kg⁻¹ NaCl for saline stress conditions. Stress indices were calculated based on plant dry weights at the 100 mmol kg⁻¹ salinity level to assess the salt tolerance of the genotypes. Stress intensity was calculated as 0.76, indicating the highest dose of extreme salt stress on the plants. The G04, G14, and G21 genotypes exhibited the highest K/Na and Ca/Na ratios in the plant tissue and were found to be salt tolerant. The decrease in dry mass at severe salt stress is 75.48%. The GMP (geometric mean productivity) and STI (stress tolerance index) indices indicated that G04, G14, and G21 could be prominent sources to develop salt tolerance varieties [47].

A study by Coskun et al. [44] reported that under salt stress (8 dS/m), the average root length of watermelon genotypes declined from 61.73 cm (control) to 55.85 cm, indicating an 8.16% reduction in root development. Chlorophyll a and b levels decreased by 17.1 and 13.6%, respectively, though specific genotypes (e.g., W7, W15, and W28) exhibited an increase in these parameters, suggesting potential tolerance mechanisms. Molecular marker analysis revealed that ISSR, SSR, and SRAP technologies effectively differentiate salt-tolerant and salt-sensitive genotypes. Notably, the ISSR-DBDACA7.540 band showed a strong association with photosynthetically active radiation (PAR) and MDA, achieving the highest regression coefficient (42.7%).

These findings highlight the varying salt stress responses among watermelon genotypes and underscore the critical role of molecular markers in evaluating and enhancing stress tolerance. The genetic resources identified from all these studies can be utilized to select the salt-tolerant genotypes and incorporate them into breeding programs to develop more resilient watermelon varieties.

5.3. Agronomic Practices Such as Drip Irrigation and Soil Amendment

Incorporating organic matter into the soil has been shown to enhance water-holding capacity and improve soil structure, thereby reducing salt accumulation [87]. Efficient irrigation practices, such as drip irrigation and sensor-based water application, have been found to minimize water loss through evaporation and leaching, thus preventing salt build-up [13]. Excess salts in the root zone can be leached by applying water in excess of crop requirements, but proper drainage is essential to avoid waterlogging. Applying specific soil amendments, such as gypsum or lime, can alleviate salinity stress by displacing sodium ions and balancing soil pH, respectively [88].

The effects of an apple–watermelon agroforestry system versus a watermelon sole-cropping system under three irrigation levels (105 mm, 210 mm, and 315 mm) were examined over three years in the arid region of central Ningxia, China. The research assessed how these systems influence resource availability and watermelon performance. Findings revealed that the agroforestry system extended the watermelon growth period, increased the leaf area index, and gradually enhanced shade intensity. However, it generally resulted in lower soil moisture, leaf photosynthetic rates, and yields than sole cropping. Notably, agroforestry slightly improved average fruit weight and total soluble solids under both low and high irrigation levels. Path analysis indicated that improved soil water content in the agroforestry system boosted yield under certain conditions [89].

A greenhouse experiment examined the effects of salt stress on the production of mini watermelons (Sugar Baby). Salinity was induced with NaCl (5.0 dS/m) and supplemented with potassium (50%) and calcium (100%). Plants were grown in a coconut fiber-sand substrate (1:1) with drip irrigation, manual pollination, and pruning. Data collected included fruit weight, dimensions, rind thickness, pulp firmness, soluble solids, vitamin C, titratable acidity, and colorimetric properties. Results showed that NaCl significantly reduced fruit production (−46.39%), longitudinal (−20.01%) and transverse (−19.25%) fruit diameters, and rind thickness (−17.35%) due to osmotic stress. Potassium and calcium supplementation improved yield (+46.12% and +17.38%) and fruit size. Pulp firmness increased under salinity, stabilizing cell walls, while vitamin C content declined with salinity (−39.13%). Salinity enhanced fruit color intensity and lycopene content, making the fruits darker red (Hue < 50), which is preferred by consumers. At the same time, salinity reduced yield, and vitamin C, potassium, and calcium improved fruit quality by enhancing firmness and color [90].

A study on Sugar Baby watermelon evaluated six salinity management strategies and two nitrogen doses (50% and 100%) using a randomized block design. Plants were grown in 20 L lysimeters with sandy loam soil and irrigated with water of low (0.8 dSm) or high (3.2 dS/m) salinity, applied at different growth stages. Physiological traits, including stomatal conductance, transpiration, CO₂ assimilation, and water-use efficiency, were measured at 75 days after sowing (DAS), while fruit yield parameters were recorded at 85 DAS. Results showed that salinity significantly reduced all physiological variables, with the greatest declines observed when stress was applied at the vegetative/flowering and fruit maturation stages. Stomatal conductance dropped by 35.02% at fruit maturation due to salt accumulation, while CO₂ assimilation was the lowest under high salinity, which was due to osmotic stress. Water-use efficiency improved with 50% nitrogen, which also increased fruit mass (1055.9 g), being18.27% higher than with 100% nitrogen. Salinity during early growth stages resulted in smaller fruit diameters, whereas 50% nitrogen application enhanced photosynthesis and fruit size. In conclusion, salinity at critical growth stages severely impacted physiology and yield, while moderate nitrogen application (50%) improved fruit quality and stress tolerance [91].

5.4. Nanobiochar and Nanoparticles

Biochar has a high capacity for adsorbing salt, which could lower plant Na⁺ absorption and mitigate the negative effects of soil salinity [92]. Additionally, it has been reported that biochar can reduce exchangeable acidity [93]. Rice straw biochar application positively influences the watermelon growth, yield, and soil nutrients [94]. However, we could not find any study on the use of biochar or nano-biochar on salinity tolerance in watermelon.

Nanoparticles (NPs) are tiny particles ranging from 1 to 100 nm in diameter and exhibit distinct physico-chemical properties compared to their larger counterparts [95]. On one side, the use of nanoparticles is promising, but there are side effects, including environmental pollution and potential harm to animal and human health [96]. The use of nanoparticles in agriculture has emerged as a promising approach to enhance crop productivity under both optimal [97] and adverse environmental conditions, including salt stress [98,99,100]. Calcium nanoparticles have been utilized in postharvest fresh-cut watermelon production [101]. Turmeric oil nanoemulsions (TNEs) and silver nanoparticles (AgNPs) enhance seed germination, growth, and yield when used as priming agents for diploid (Riverside) and triploid (Maxima) watermelon seeds [102]. Nanoparticles are used to mitigate salinity stress in other cucurbitae plants such as cucumber. Under salt stress, some nanomaterials, such as cerium oxide nanoparticles (CeO₂), have been found to elevate cucumber seedling salt tolerance by modulating the antioxidant system [103].

Regarding the reasons why silicon improves cucumber salt tolerance, on the one hand, silicon enhances the hydraulic conductivity of the root system, thereby improving seedling moisture content. On the other hand, silicon stimulates the accumulation of polyamines, which reduces the Na⁺ content and alleviates ion toxicity [104,105]. However, nanoparticles have not been explored in watermelon to mitigate salinity stress.

To date, no studies have explored the use of nanobiochar and nanoparticles in watermelon under salt stress, representing a promising and largely untapped area of research. Investigating how nanobiochar and nanoparticles could help watermelon plants cope with salt stress might open new doors for developing sustainable solutions to this major agricultural challenge.

5.5. Application of Biostimulants and Plant Growth Regulators

5.5.1. Biostimulants

Biostimulants in modern sustainable agricultural practices are emerging as a promising approach to enhance crop performance. Biostimulants are substances or combinations of naturally occurring organic compounds that promote plant growth, particularly under challenging environmental conditions [106]. Biostimulants come in various forms, including botanical extracts such as seaweed extract, protein hydrolysates, vitamins, amino acids, anti-transpirants, non-microbiological products, humic acid, fulvic acid, and their products. Unlike fertilizers or manures, they are applied in small quantities, which sets them apart from these inputs [107].

According to the Fertilizer Amendment Order of 2021, biostimulants are defined as materials, microbes, or combinations designed primarily to improve nutrient absorption, enhance growth, increase yield and quality, and help plants withstand stress. Importantly, these do not include plant growth enhancers or pesticides controlled by the 1968 Insecticide Act. In a study, two biostimulants, Ascophyllum nodosum (Asc) seaweed and a silicon-based (Si), were tested on watermelon. Three salinity treatments, i.e., 0 mM, 50 mM, and 100 mM NaCl, were applied to watermelon seedlings, and a foliar spray of biostimulants was performed. Asc increased the relative water content at the high salinity level. The plant area, shoot dry weight, and leaf number were decreased with an increase in salinity level. However, total root length and surface area were increased by 50 mM salt, as well as by Asc in some cases. The OJIP transient of the photosynthetic apparatus was also assessed. Following the application of Asc, specific OJIP parameters decreased under high salinity conditions. Overall, it is concluded that Asc induced a positive phenotypic response after salt stress, whereas silicon (Si) did not mitigate the effects of salinity stress in transplanted watermelon [108].

Another study reported that salt-stressed watermelon plants exhibited the most favourable morphological and biochemical responses when treated with a combination of silicon (4 mM), Glomus mosseae, and Gigaspora gigantea. This treatment also led to reduced osmotic activity, electrolyte leakage, and peroxide content. Treatments involving silicon (4 mM) with either G. mosseae or G. gigantea individually showed similarly significant improvements across most evaluated traits, outperforming treatments with either mycorrhizal species alone. Additionally, the antioxidant capacity of watermelon was notably enhanced under salinity stress when inoculated with the AMF–silicon combination. Overall, the joint application of arbuscular mycorrhizal fungi (AMF) and silicon appears to be an effective strategy for alleviating salinity stress in watermelon [109].

Melatonin (MT) is a widely studied biomolecule with dual functions, serving as both an antioxidant and a signalling molecule. Trichoderma Harzianum (TH) is widely recognized for its effectiveness as a biocontrol agent against many plant pathogens. However, the interplay between seed priming and MT (150 μm) in response to NaCl (100 mM) and its interaction with TH was investigated. The study aimed to evaluate the potential of MT and TH, alone and in combination, to mitigate salt stress in watermelon plants. The results demonstrated that treatments with MT and TH individually mitigated the adverse effects of salt stress. Notably, the combined application of MT and TH produced a pronounced positive impact by enhancing plant growth, photosynthetic activity, gas exchange parameters, chlorophyll fluorescence indices, and ion homeostasis (with decreased Na⁺ and increased K⁺ levels) [110].

MT and TH effectively reduced oxidative stress by suppressing hydrogen peroxide accumulation under both saline and non-saline conditions, as evidenced by decreased lipid peroxidation and electrolyte leakage. The combination also significantly alleviated salt-induced oxidative damage through the activation of antioxidant defence mechanisms, including the AsA-GSH (ascorbate–glutathione) cycle, glyoxalase pathway, accumulation of osmolytes, and the upregulation of stress-responsive genes. Furthermore, transmission electron microscopy [111] confirmed the preservation of chloroplast ultrastructure under salt stress [110].

5.5.2. Plant Growth-Promoting Microbes

The plant’s microbiome plays a significant role in its growth and development. The soil microbe–plant interactions are complex and are significant in plants’ growth and development [112]. Around 10 billion microbes are found in 1 g of rhizosphere soil and plant roots [113]. These microbes, which can be bacteria, fungi, and viruses, exhibit several plant growth-promoting traits such as phosphate solubilization, Indole-3-acetic acid (IAA), Nitrogen fixation, siderophore, catalase, ammonia production, ACC deaminase, and protease activity, and help plant’s growth and development [114,115]. The bacteria and fungi associated with the plant’s rhizosphere affect the host plant’s immunity, nutrient acquisition, stress tolerance, and pathogen abundance [116]. Plants recruit plant growth-promoting bacteria to mitigate the effects of soil salinity stress [117]. In a study [118], the isolation of PGPR from the rhizosphere of native plant Ceanothus velutinus exhibited several plant growth-promoting traits, such as the ability to fix nitrogen, siderophore production, and phosphate solubilization. While extensive research has explored the application of plant growth-promoting rhizobacteria (PGPR) across various cucurbit crops, investigations specifically targeting watermelon (C. lanatus) have been comparatively limited. Recent studies, however, have begun to illuminate the potential benefits of PGPR in watermelon cultivation.

Arbuscular mycorrhizal fungi (AMF) are the most extensively studied plant growth-promoting microbes, playing a significant role in sustainable agriculture and mitigating environmental stresses in plants. We found only one study on the role of AMF in salinity tolerance in watermelon [119]. Ye et al. studied watermelon seedlings combined with the beneficial fungus Funneliformis mosseae—a type of arbuscular mycorrhizal fungus (AMF)—and various physiological characteristics, including water absorption, reactive oxygen species levels, antioxidant enzyme activity, photosynthesis efficiency, chloroplast health, and the expression of stress-related genes, were measured. Under salinity–alkalinity conditions, untreated plants exhibited stunted growth, reduced water and nutrient uptake, damaged chloroplasts, disrupted gene expression, and impaired photosynthetic performance. However, when inoculated with AMF, the seedlings exhibited significantly better growth, regained water and nutrient absorption, and displayed enhanced antioxidant enzyme activity (including SOD, CAT, APX, and GR). Photosynthesis, which typically drooped under stress, was restored to much healthier levels, and chloroplast structure was preserved. Gene expression also rebounded: RBCL levels rose back up, PPH—a chlorophyll-degrading gene—was suppressed, and antioxidant-related genes were even more active [120].

These findings support using beneficial microbes to enhance crop productivity in saline soils as a sustainable strategy. A description of the mitigation strategies is given in

Table 2. Mitigation strategies for salt stress.

Strategies	References
Use of salt-tolerant rootstocks	[41,43,79,80,82]
Breeding salt-resistant varieties	[44,47,86]
Agronomic practices	[73,89,90,91]
Biostimulants	[108,109,111]
Plant growth-promoting microbes	[120]

and Figure 3 below.

6. Challenges and Future Perspectives

Salt stress is a crucial factor in breeding economically essential crops for resistance, as it severely inhibits plant growth and development, ultimately influencing yield and quality. To address the issue of salt stress in plants, future efforts should focus on two main areas. Firstly, we should actively use whole-genome analysis and transcriptome sequencing technologies to identify salt-tolerant genes from germplasm resources. It includes conducting practical research on gene editing, Agrobacterium infection, virus-induced gene silencing, and nanoparticle technologies should be used to validate salt tolerance genes in cucurbit crops functionally. Secondly, we should employ biotechnological methods, such as transgenic engineering, along with improving soil conditions and adjusting irrigation practices. While the increasingly refined reference genomes of cucurbit plants and the continuous development of modern molecular biology tools have made it possible to study the salt response mechanisms of these species, challenges such as time-consuming genetic transformation systems and low transformation rates mean that the molecular mechanisms of salt tolerance, such as in cucurbits, are continuously lacking and primarily understood at the physiological and biochemical levels. Therefore, promoting the development of an effective genetic transformation system is a crucial avenue for exploring the molecular mechanisms of salt tolerance in cucurbit crops. Additionally, grafting and applying exogenous nanomaterials and hormones can be employed to mitigate salt stress in Cucurbitaceae. Nevertheless, the donor’s application rate and the rootstock selection require further investigation tailored to specific plant species and growth stages.

In the broader agricultural landscape, numerous studies have successfully applied nanoparticles and nanobiochar—such as silicon, zinc, selenium, and calcium–silicon composites—to alleviate salt stress in various crops, including tomato. However, no similar research has explored the use of nanoparticles or nanobiochar specifically in watermelon under salt stress. It represents a compelling opportunity for future investigation. Given the positive outcomes observed in other crops, leveraging nanotechnology—whether via soil amendment or foliar application—could offer an innovative and impactful strategy to enhance watermelon salt tolerance.

Moreover, plant growth-promoting microbes remain an underexplored tool in mitigating salt stress. Particularly under abiotic stresses, despite their proven potential to alleviate plant stress, the application of these techniques in managing salinity stress in crops such as watermelon has received limited research attention. Future studies should focus on elucidating the underlying mechanisms by which PGPB confer salt tolerance in watermelon, alongside efforts to develop effective and crop-specific microbial formulations.

7. Conclusions

Climate-driven events worsen abiotic and biotic stresses by increasing extreme weather, soil degradation, and pathogen outbreaks. The watermelon crop is susceptible to drought, floods, extreme temperatures, heavy metals (HM) toxicity, and especially to salinity, which can disrupt its growth, productivity, and crop quality. Soil salinity poses a critical threat, as excessive sodium degrades soil quality and inhibits seed germination and growth, ultimately reducing fruit yield. Strategies to improve salt tolerance in watermelon include grafting with salt-resistant rootstocks, breeding salt-tolerant cultivars, and employing agronomic practices such as drip irrigation and soil amendments. Biostimulants and plant growth regulators can further enhance salt tolerance. Root traits are crucial for determining salt tolerance, and breeding programs aim to improve stress resilience and efficiency. Beneficial microbes, including AMF, also support plant health by enhancing nutrient acquisition and suppressing pathogens. Although the phenomenon of salt stress has been extensively studied across various crop species, research specifically focused on watermelon is notably limited. This review highlights the physiological impacts of salinity and potential mitigation strategies, while also emphasizing the need for targeted research on watermelon. To date, no published study has evaluated the use of nanoparticles and nano biochar as an agent to alleviate salt stress in watermelon. Similarly, the application of beneficial plant growth-promoting rhizobacteria (PGPR), which is well-documented in other crops, has been largely absent in the context of enhancing salt tolerance in watermelon. The existing literature contains only a few studies examining stress responses and adaptive treatments for watermelon, revealing a significant gap in research related to nanoparticles, nano biochar, and microbial inoculants aimed at improving salinity resilience.

In conclusion, despite substantial research conducted on other cucurbits and horticultural crops, watermelon represents an underutilized field of inquiry regarding adaptation to salt stress. This review not only highlights the known physiological effects of salinity but also makes a compelling case for urgent research initiatives targeting nanoparticles, nano biochar, and PGPR interventions for watermelon cultivation in saline environments. Such efforts could pave the way for more resilient watermelon varieties, ultimately benefiting agricultural productivity and sustainability.