Innovative and Sustainable Management Practices and Tools for Enhanced Salinity Tolerance of Vegetable Crops

Abstract

The increasing threat of salinity, exacerbated by climate change and unsustainable agricultural practices, necessitates innovative and sustainable crop management strategies to safeguard vegetable crop production and global food security. This review highlights a comprehensive framework that combines physiological insights with practical interventions aimed at enhancing salinity tolerance in vegetable crops. Key strategies include grafting, precision irrigation and fertilization, biofortification, and biostimulant application. These practices are applicable to both soil-based and soilless cultivation systems, offering broad relevance across diverse production environments. Combining and adapting these strategies to specific crops and environments is essential for developing sustainable, productive vegetable farming systems that can survive rising salinity and secure future food supplies. Future research focus on optimizing these integrated methods and elucidating their underlying mechanisms to enable wider and more effective adoption.

1. Introduction

Climate change, characterized by rising global temperatures and increased frequency of extreme weather events (erratic rainfall, heat waves, droughts, floods), poses a significant threat to global agricultural productivity and food security [1,2]. The projected increase in global population to over 10 billion within 50 years [3] will exacerbate these challenges, demanding increased agricultural output under increasingly adverse environmental conditions. Salinity is one of the most relevant environmental stresses in horticulture, affecting over 800 million hectares of irrigated land and exacerbated by current practices and climate change [4]. Elevated soil and water salinity, exacerbated by natural processes and human activities such as saline irrigation water [5,6], negatively impacts soil physical properties. High sodium (Na) concentrations in saline soils disrupt soil–water and soil–air relationships, which directly impedes plant growth and productivity. In soilless culture, salinity issues can result from nutrient management imbalances leading to macronutrient oversupply or from high Na levels in the nutrient solution (NS) source water [7]. Salinity diminishes agricultural productivity through a sequential stress mechanism. Initially, osmotic stress, resulting from reduced root water absorption and increased leaf water loss from high salt accumulation in soil and plants, triggers various physiological changes [8,9,10]. These include membrane disruption, nutrient imbalances, impaired reactive oxygen species (ROS) detoxification, altered antioxidant enzyme activity, reduced photosynthesis, and decreased stomatal aperture [11]. Subsequently, ion toxicity develops due to excessive Na and chloride (Cl) accumulation, causing severe ion imbalance and inhibiting essential potassium (K) uptake [12,13], ultimately impacting plant productivity and survival, causing economic and societal losses [14]. Plants employ diverse physiological and biochemical mechanisms to survive high salt concentrations, including ion homeostasis and compartmentalization, ion transport and uptake, osmoprotectant biosynthesis, antioxidant system activation, polyamine synthesis, nitric oxide generation, and hormone modulation [15]. Developing effective cultivation strategies to improve the growth, yield, and quality of crops under salinity stress, particularly within the context of climate change, represents a significant challenge. To address the negative impacts of these environmental changes, the scientific community has explored various approaches.

In soil-grown vegetable crops, maintaining a balanced supply of essential nutrients plays a crucial role in helping plants cope with soil salinity [15]. Adequate levels of nutrients like K and calcium (Ca) can counteract the negative effects of excessive Na uptake, strengthening cell walls and regulating osmotic balance [16,17]. The incorporation of organic amendments like compost, manure, and biochar further enhances soil health [18,19]. Soilless culture (SC), and especially closed-loop systems (CLSs), has emerged as highly effective in enhancing resource efficiency in greenhouse vegetable production by reducing water and fertilizer consumption compared to open-field crops, due to precise control of root-zone conditions in terms of electrical conductivity (EC) and nutrient application [20]. However, Na, primarily introduced via irrigation water, poses a threat to crop growth and productivity in those systems [21], as its accumulation cannot be directly controlled through nutrient management [22]. Climate projections suggest rising salinity threats, with soil EC potentially reaching 6.5–9 dS m⁻¹ by 2050 if adequate leaching is not implemented [23]. This indicates the necessity of increasing irrigation volumes by 15–20% to manage salt levels under protected cultivation. Irrigation techniques and management practices play a crucial role in soil salt accumulation, affecting both crop productivity and soil health. Various irrigation techniques, including also deficit or precise irrigation (DI) strategies, have been studied for their efficacy in salt leaching and their impact on yield and water use efficiency/water productivity (WUE/WP) [24,25]. Complementary strategies such as optimized irrigation frequency [26] and delivery rates significantly influence salinity dynamics and crop response, highlighting the need for optimized irrigation management to mitigate salt stress [27,28].

Herbaceous grafting represents a relatively new propagation procedure that has gained popularity in the last few decades [29]. Moreover, herbaceous grafting is a suitable and friendly agronomic tool to counteract abiotic stresses, such as salinity, and to increase crop performance [30,31,32,33,34,35,36,37]. Beyond established techniques like grafting, the agricultural sector is continuously exploring novel solutions to address salinity stress. In this context, biofortification and products containing bioactive molecules such as nanoparticles and non-microbial biostimulants, as well as Plant Growth-Promoting Rhizobacteria (PGPRs), have notably gained significant interest in recent years for their potential to enhance plant resilience. Agronomic biofortification is achieved through the application of micronutrient fertilizers to the soil and/or the direct application of microelements to the leaves of the crop [38]. This method is considered a short-term intervention with immediate and effective results in improving plant nutrition, while numerous scientific studies have identified it as a complementary strategy to genetic biofortification [39,40]. Biofortification has emerged as a dual-purpose strategy to enhance crop resilience under saline conditions while improving nutritional quality. In parallel, novel non-microbial biostimulants, such as seaweed extracts, protein hydrolysates, humic substances, and amino acid-based formulations, containing bioactive molecules—including phytohormones, amino acids, and polysaccharides—have emerged as effective tools for modulating plant metabolism under salinity stress conditions [41]. Lastly, PGPR, a group of soil-inhabiting microorganisms that establish symbiotic relationships with plants, plays a significant role in alleviating salt stress by enhancing plant physiological and biochemical responses. In recent decades, PGPR application represents a sustainable and efficient agronomic strategy to control pests and plant diseases [42,43], to increase plant growth [44], to optimize plant nutrient uptake and assimilation [45], and to relieve the damage caused by abiotic stress like salinity [46].

This review presents a comprehensive synthesis of conventional and emerging strategies to mitigate salinity stress in vegetable production across both soil-based and soilless systems. By combining physiological insights with practical applications such as grafting (including the implementation of landraces and wild relatives), biofortification, microbial and non-microbial biostimulants, and precision irrigation and fertilization management, this work provides a multidimensional framework for enhancing crop resilience to salinity. Additionally, we highlight novel tools such as predictive modeling and closed-loop fertigation strategies, offering updated and actionable insights for sustainable vegetable production under salinity stress. Importantly, the novelty of this work lies in its integrative approach, bringing together diverse practices into a unified framework, which delivers unique and practical guidance for improving the resilience, productivity, and sustainability of high-value vegetable crops.

2. Fertilization Management in Soil Crops

Increased salinity in the root environment negatively affects vegetable crop performance by inducing both osmotic and ionic stress [16]. Specifically, excessive concentrations of NaCl in the root zone reduce water uptake due to the limited osmotic potential of soil solution, subsequently restricting nutrient absorption and causing further salt accumulation through either irrigation with saline water and/or fertilizer application, a salt build-up that gradually elevates EC in the root environment [17,18]. Moreover, elevated levels of Na and Cl ions in the root zone interfere with the uptake of essential nutrients. Na competes with critical micronutrients such as Ca and K, reducing their uptake and displacing them from soil colloids, which results in excess Ca and K leaching [19]. Similarly, opulent Cl levels disrupt NO₃-N uptake, hindering plant N utilization and soil N mobility [20]. Additionally, under osmotic stress, the uptake of N and phosphorus (P) is more severely disrupted than that of other nutrients and is strongly correlated with yield restrictions [21,22]. These interactions often result in severe nutritional imbalances and disorders in plants, ultimately limiting growth, yield, and overall crop health.

Overapplication of essential macronutrients has been explored as a strategy to increase their uptake under salinity stress. This strategy ameliorates the adverse effects of salinity and benefits growth, yield, and/or quality in most high-value vegetable crops, such as tomato [47,48,49,50,51,52,53], pepper [23,24,25,54,55,56], melon [57], and lettuce [26,27]. However, while effective in improving productivity, this strategy should not be considered sustainable. Excessive fertilization diminishes nutrient use efficiency and increases nutrient leaching, leading to environmental degradation and long-term salt build-up in the soil, which exacerbates salinity stress [28,29,30]. Foliar applications are considered as an alternative to overdose fertilization via root application. Particularly, foliar application of nutrients bypasses these soil constraints, supplying essential nutrients directly to plant tissues. According to Naz et al. [31], foliar application of K enhanced the K:Na ratio in plant tissues, a ratio that is closely related to plant salinity resilience [32,33], thus mitigating the adverse effects of salinity on spinach. The beneficial effects of K supplementation via foliar application in ameliorating saline pressure have also been reported in tomato [34,35], pepper [36], lettuce [37], and cabbage [58]. Except for K, Ca foliar application can benefit crop performance of fruit vegetables by suppressing the incidence of blossom-end rot due to the implications of high NaCl levels in the root environment on Ca uptake [59,60].

In soil-grown crops, one of the most promising and sustainable practices to alleviate plant tolerance to salinity is the application of organic amendments. These include manure, compost, and vermicompost that improve soil structure, increase moisture retention, and enhance nutrient availability, factors that contribute to improved plant resilience against salinity [38]. According to

Table 1. Impact of organic amendments on tomato resilience to salinity.

Organic Amendment	Salinity	Impact	References
Green compost 25 t ha−1	Saline water: 40–80 mM NaCl	Enhance plant growth up to 50%	[39]
Compost 55 g kg−1 soil	Saline water: 50–100 mM NaCl	Compost nullified the adverse effects of salinity at 50 mM NaCl levels	[61]
Municipal solid waste compost: 50 t ha−1	Saline water: 6.0 dS m−1	Enhanced yield by 10%	[41]
Straw compost: 40 t ha−1	Saline water: 40 mM NaCl	Enhanced growth and yield up to 100% and resulted in less BER by 50%	[43]
Straw compost: 45.0 t ha−1	Saline water: 3.0 mS cm−1	Almost nullified the adverse effects of salinity on yield	[42]
Urban waste biochar 5–10% v/v soil	Saline water: 100 mM NaCl	Enhanced growth by 75% (DB) and physiology parameters	[74]
Compost + biochar 2% w/w soil	Soil EC: 6 dS m−1	Enhanced biomass characteristics and quality performance	[46]
Manure biochar compost 3 t ha−1	Soil EC: 6.6 dS m−1	Enhanced growth and yield and NPK fruit content up to 100%	[75]
Wheat-straw biochar 2–8%	Saline water 1–3 dS m−1	8% biochar enhanced growth and yield by 40% and 50% under high saline conditions	[76]
Biochar 1–2 t ha−1	Saline soil EC 2.4 μS cm−1	Enhanced yield up to 30%	[77]

, compost application from various sources at a rate of 40–50 t ha⁻¹ can mitigate salinity effects to the extent of maintaining tomato crop productivity. The efficacy of compost in alleviating salinity stress is largely attributed to its influence on plant metabolic processes, including the accumulation of metabolites involved in osmotic adjustment and stress modulation [39,40], as well as the enhancement of antioxidant defenses and key C, N, and S assimilatory enzymes [61]. Beyond physiological benefits, compost application also enhances soil physicochemical properties, leading to increased microbial activity and availability of Ca and K, while concurrently reducing Na uptake by plants [41,42,43]. Among various organic amendments, biochar is emerging as a particularly promising soil conditioner for enhancing crop salinity resilience due to its higher stability, which contributes to sustained soil physical properties, and long-term strong Na absorbing capacity [44]. While biochar can extend the residence time of compost in the soil, compost is superior in its ability to boost soil fertility [42]. In this context, Qian et al. [45] suggested that combined applications of biochar and compost may offer synergistic benefits surpassing the effects of single compost or biochar application. This evidence corroborates that obtained by Ud Din et al. [46], where tomato productivity and quality performance under saline conditions were optimized under combined compost and biochar application. Finally, except for tomato, the salinity-alleviating effects of biochar have also been reported in pepper [62,63,64], eggplant [65,66], potato [67,68], cucumber [69], melon [70,71], and lettuce [72,73].

3. Nutrient Solution Adjustments in Soilless Culture Systems for Managing Salinity Stress

Soilless culture (SC), through the precise control of root zone conditions, can be considered a cultivation system highly effective in managing salinity issues in greenhouse vegetable production. This system enables optimal regulation of EC through precise nutrient application, ensuring optimal nutrient availability in the root solution (RS) while mitigating the effects of excessive salt accumulation [78]. Na is a key element whose accumulation may lead to salinity problems in SC. Originating primarily from irrigation water, its addition to the system cannot be directly controlled through nutrient management. As a result, Na accumulates in the RS, leading to EC levels that significantly impair crop growth and productivity [79].

Cultivation in CLS offers significant resource savings, reducing water consumption by 30–45% (depending on the drainage fraction) and fertilizer use by 30–60% [80,81]. In addition, the recycling of the drainage solution (DS) minimizes the environmental footprint as it eliminates the discharge and emission of nutrients into the environment [82]. Na accumulation is the main challenge in CLS, where the DS is recycled to prepare fresh NS. Salinity problems are exacerbated in CLS because Na removal from the RS/DS relies only on plant uptake. However, the Na uptake concentration (UC) of the plants is relatively low compared to its concentration in the root zone [83,84,85,86].

A simple and effective solution to avoid salinity stress, though a less resource-efficient alternative compared to CLS, is the use of a semi-CLS, where controlled discharge of the DS removes Na from the system. Voogt and Van Os [87] proposed a “smart” discharge strategy, where the discharge rate is adjusted according to daily Na⁺ accumulation. The necessary leaching fraction (LF) is calculated using Equation (1) based on Na⁺ concentration in irrigation water (I_Na), the Na⁺ plant uptake (U_Na), and the maximum acceptable concentration in the root zone (R_Na-max).

$$LF={\displaystyle \frac{I_{Na}-U_{Na}}{R_{Na-max}-U_{Na}}}$$

(1)

An alternative cultivation system that minimizes nutrient emissions and increases water and nutrient use efficiency is cascade hydroponics. By reusing the discharged DS as an NS for a more salinity-resilient crop, this system manages to enhance resource utilization while reducing environmental impact [88]. In cascade systems, the DS can be used directly as NS for the subsequent crop [89] or after suitable adjustment of the nutrient concentrations, as in the case of the CLS-based second crop requirements [7,78]. The implementation of such a system can increase the water productivity and the agronomic efficiency of nutrients by 20–50% compared to free-drainage systems [88,89].

Numerous models have been developed to predict Na accumulation in RS and Na uptake. Massa et al. [90] proposed a composite model to simulate water and mineral dynamics in greenhouse tomato crops grown in SCS. Firstly, the model calculates the plant water uptake based on the leaf area index and the solar radiation intensity. Subsequently, it calculates the proportion of Na⁺ that is taken up by the plants based on the Na⁺ uptake concentration (UC_Na) using the [Na⁺] in the root environment (C_NS) and Equation (2) suggested by Carmassi et al. [91] for tomato. Finally, through mass-balance calculations, the model calculates the ion concentration in the root zone and drainage solution volume that should be discharged based on the discharging strategy [81].

$${UC}_{Na}=0.18 C_{NS}$$

(2)

Varlagas et al. [86] proposed an exponential model for Na and Cl uptake in CLS tomato crops, which was subsequently validated in a semi-CLS by Katsoulas et al. [92], where the model simulation reduced the DS discharge. Similar models were suggested for cucumber [85], zucchini, and melon [84,93]. These models can be used for proactive nutrient management by predicting Na⁺ accumulation in the root zone. Recent technological advances include machine learning applications for estimation of pore water EC in substrate-based SCS by combining sensor data with predictive algorithms to achieve more accurate predictions [94]. These tools can be integrated into decision support systems (DSS) to optimize both nutrient application and salinity management.

EC management in the root zone in SCS aims to maximize yield by ensuring optimum plant nutrition while avoiding salinity stress. Sonneveld and Van Der Burg [95] suggested that the minimum sufficient nutrient concentrations correspond to lower EC values than the maximum EC threshold for yield loss. They also noticed that under mild salinity stress, yield losses are due to the increased EC rather than to Na or Cl toxicity. Furthermore, for tomato production, Sonneveld and Voogt [96] recommended an EC target of 4 dS m⁻¹ in the RS, slightly above the 3 dS m⁻¹ safety threshold to avoid yield losses, aiming to enhance fruit quality attributes.

The nutrient adjustment strategy, involving deliberate reduction in nutrient concentrations to compensate for Na⁺ accumulation and thus maintain EC at optimum levels, has emerged as a particularly effective management approach. Voogt et al. [97] tested this strategy on a pepper crop, and Voogt et al. [98] on a tomato crop, both suggesting that the yield remained unaffected despite lower nutrient and the higher Na⁺ concentration in the root zone. Additionally, Massa et al. [81], after testing four different strategies for salinity management in semi-CLS, suggested the aforementioned strategy as the most efficient in maintaining yield while avoiding nutrient losses. According to Voogt and Van Os [87], this strategy maintains acceptable EC levels by stimulating Na uptake, as it allows higher Na concentrations in the root zone while reducing the nutrient losses when DS is discharged.

However, Voogt et al. [97,98] and Kempkes and Stanghellini [99] pointed out that the concentrations of NO₃⁻ and K in the RS tend to deplete when this strategy is applied. To prevent such depletions, Giannothanasis et al. [100] proposed an algorithm that adjusts the target nutrient concentrations in the RS to compensate for Na⁺ accumulation while maintaining constant mutual ratios among nutrients (Figure 1). Firstly, the sum of the ion concentrations (ΣC_d) that correspond to the maximum accepted EC through an empirical equation [101,102], and then the sum of the nutrient concentration (ΣC_n) in the EC by subtracting the [Na⁺] or [Cl⁻], for cation or anion, respectively, from the total ion concentrations (Equation (3)). Subsequently, the ΣC_n is divided into K⁺, Ca²⁺, and Mg²⁺ in the case of cations and into NO₃⁻ and SO₄²⁻ in the case of anions based on their suggested mutual ratios to calculate the adjusted target values for the root zone.

$$\Sigma C_n=\Sigma C_d-C_{Na or Cl}$$

(3)

To achieve these adjusted targets in the RS, the nutrient addition in the solution was calculated based on the actual plant uptake concentrations [7]. Therefore, Giannothanasis et al. [100] suggested 21 mM of Na as the maximum acceptable concentration in RS that can be compensated by the reduction in the other cation concentrations in tomato (Figure 1), in contrast with Varlagas et al. [86] and Voogt et al. [98], who suggested 19 mM and 25 mM Na, respectively. For pepper, the maximum threshold is about 10 mM of Na⁺ [97], and for cucumber, 12 mM of Na⁺ [85,95] (Figure 1). In commercial practice, growers typically implement this strategy by adjusting the EC of the supplied NS to maintain the EC in the DS within optimal levels [99].

4. Salinity Management Through Irrigation Strategies

Deficit irrigation (DI) has emerged as a cultivation strategy to mitigate the negative impacts of saline water on vegetable crops. By applying water at levels below the full crop evapotranspiration demand (typically 60–80% of ETc), this method helps manage plant water balance, limits salt buildup in the root zone, and promotes physiological adaptations to stress. This controlled water application not only conserves resources but also reduces salt delivery to the roots over time, aiming to enhance WUE by carefully balancing both the quantity and quality of irrigation water and their combined influence on vegetable crop productivity [41,103,104]. This hypothesis was later validated by Alshami et al. [105], who demonstrated that applying moderate water deficits, via reduced ETc, can enhance crop adaptability and WP while maintaining efficient productivity in greenhouse tomato crops. However, the severity of water limitations should be carefully tailored to irrigation water salinity, as higher EC levels require milder deficits to prevent excessive salt buildup in the root zone. Similar evidence was also reported in a long-term potato crop, where mild DI (70–80% ETc) slightly restricted crop performance, which resulted in a relatively small increase in soil salinity at levels that did not substantially hinder productivity [106]. Contrary to Sabah et al. [107] reported that yield responses of red cabbage benefited fron full irrigation rather than deficit irrigation, even when irrigation water salinity increased. In parallel, Nagaz et al. [108] reported that a DI of 70% ETc, compared to 100% ETc, hindered rather than benefited the productivity of potato, carrot, faba bean, and pepper under saline conditions. However, the field trial was conducted in soil of poor water-holding capacity, minimizing the risk of salt accumulation. This background strongly indicates that crop responses to deficit irrigation practices are species-specific and the efficiency of the system relies on environmental conditions and applied farming practices, ultimately resulting in diverse case-specific outcomes. Although the deficit irrigation strategy evidently enhances WP in vegetable production, its long-term application poses potential risks in salt accumulation in soil [109]. Within this context, regulated deficit irrigation (RDI) serves as an alternative approach, where water deficits are applied only during specific growth stages that are less sensitive to stress. Indeed, according to Alghamdi et al. [110] and Zhang et al. [111], tomato crop performance is further benefited when DI is interrupted the during plant flowering stage, while both studies identified the fruit ripening stage as the most tolerant to combined salinity and drought stress, allowing for higher WP without compromising yield.

In addition to the water regime, the irrigation interval plays a critical role in determining WP and the overall performance of vegetable crops under saline conditions. Regardless of saline conditions, increased irrigation frequency maintains hydraulic conductivity and water availability, thereby supporting proper crop performance [112]. Frequent irrigation enhances root zone moisture levels, which facilitates the dilution and leaching of soluble salts away from the root environment, thus mitigating salt accumulation [113]. These beneficial effects of short-cycle irrigation systems on salt leaching proved to be even more critical in both soil [114,115] and soilless [116] cultivation of high-value vegetables when using irrigation water of poor quality, where high irrigation frequency mitigates the adverse effects of salinity on crop performance. Provided that various growing conditions strongly influence the evapotranspiration of crops, Mahmoodi-Eshkaftaki and Rafiee [117] and Rafiee and Mahmoodi-Eshkaftaki [118] developed and validated predictive models that determine the appropriate irrigation intervals to achieve efficient irrigation strategies that enhance eggplant tolerance to saline conditions.

The method of water delivery significantly influences salinity dynamics. Visconti et al. [119] compared flood, surface, and subsurface drip irrigation in melon cultivation. While flood irrigation exhibited greater salt leaching, subsurface drip irrigation achieved the highest yield and more uniform soil moisture distribution, enhancing WUE. Subsurface drip irrigation systems have demonstrated superior salinity control capabilities. Alomran et al. [120] showed that subsurface drip outperformed surface drip in yield enhancement, mitigating the salinity stress effect. In terms of water supply rate, a low rate is suggested when fresh water is applied to increase WUE, while a higher rate is suggested when saline water is applied to achieve the necessary salt leaching. Hanson and May [121] compared subsurface drip with sprinkle irrigation, highlighting that leaching of salts near the drip line in subsurface drip irrigation contributes to salinity stress reduction. In addition, it was found that regular sprinkler irrigation was required to remove salt accumulation above the buried drip lines, particularly in areas with insufficient rainfall. Except for subsurface drip irrigation, pulse drip irrigation serves as an alternative for improved WP. Pulse drip irrigation enhanced the WP and/or crop performance of tomato [122], eggplant [123,124], pepper [125], and green beans [126]. The enhanced efficiency of this system, compared to the daily single-dose drip irrigation, relies mainly on the beneficial effects of the frequent irrigation doses on soil moisture and crop WP.

Alternative furrow irrigation is a technique where the plants are irrigated only on one side, and thus only half of the cultivated area is irrigated [127]. Okasha et al. [128] compared drip, furrow and alternative furrow, irrigation in a cauliflower crop. Furrow irrigation resulted in lower soil salinity levels, followed by alternative furrow irrigation, while drip irrigation resulted in higher yield and WP. Using alternative furrow irrigation, the water savings and WP were improved compared to standard furrow irrigation. Meanwhile, drip irrigation consistently provided better soil moisture profiles compared to furrow irrigation when the same water volume is applied, contributing to salinity stress mitigation [129,130]. Karimzadeh et al. [131] highlighted that drip irrigation combined with additional leaching volumes was effective in salinity reduction, unlike sprinkle systems, which can increase soil salinity without high-frequency applications due to the higher evaporation.

Deficit irrigation strategies and alternative irrigation systems have been primarily developed to limit salt input by reducing the overall use of saline water. This is achieved either by precisely meeting crop water requirements or by improving WUE and WP. However, micro-irrigation systems, despite their efficiency, may lead to long-term salt accumulation in the root zone due to the limited leaching capacity associated with small irrigation volumes. Bonachela et al. [132] demonstrated that implementing micro-irrigation during crop growth, followed by a post-harvest over-irrigation event (e.g., 60–70 mm), serves as an effective management strategy to prevent tomato crops from experiencing severe salinity stress and to mitigate salt buildup for subsequent cropping cycles. Within the broader context of strategic salt leaching, Libutti and Monteleone [133] emphasize minimizing leaching during the spring and summer to reduce nitrate losses, while recommending leaching irrigation at the end of the growing season, when salt concentrations in the soil are highest and nitrate levels are relatively low. During the autumn–winter growing season, rainfall becomes the primary agent for leaching. Under these conditions, nitrogen should not be applied as a basal dressing; instead, a progressive top-dressing approach should be adopted, aligned with crop nutrient demand [133].

Innovative irrigation control methods also play a role. Contreras et al. [134] used electronic tensiometers to automate irrigation based on soil matric potential, achieving high water and nutrient efficiency while avoiding salt accumulation. Regardless of the irrigation threshold, increased soil salinity was evident by the end of the growing season, making leaching irrigation at that time an efficient way to prevent salt accumulation, minimize the environmental impact of nutrient leaching, and maintain long-term soil health.

Salinity control in soilless culture systems is crucial due to limited root zone volume and the propensity for rapid Na⁺ accumulation. According to Sonneveld and Voogt [96], effective leaching in substrate-based soilless crops during crop growth is necessary. The leaching fraction (LF) needed to avoid Na⁺ accumulation can be calculated using Equation (4):

$$LF={\displaystyle \frac{C_w- C_f-C_u}{C_d- C_u}}$$

(4)

In which

LF = leaching fraction;

C_w = concentration of a certain ion in the irrigation water, mmol L⁻¹;

C_f = the concentration increase in that ion from the fertilizer, addition mmol L⁻¹;

C_u = the apparent uptake concentration of the crop grown, mmol L⁻¹;

C_d = the accepted concentration of that ion in the drainage water, mmol L⁻¹.

Irrigation should be calculated according to the needs of the crop, considering evapotranspiration and the required LF, with additional water applied for salt leaching.

5. Grafting for Enhanced Salinity Tolerance

5.1. Rootstock-Mediated Responses to Salinity Stress

Grafting is an agamic plant propagation method involving the union of two plants, the rootstock, selected for its resistance to biotic and/or abiotic stresses, and the scion, selected for yield and quality traits. By combining a stress-resistant rootstock with a high-yielding scion, grafting merges desirable traits from two distinct plants, offering a unique solution to maintain productivity under saline conditions [135], while activating physiological and biochemical mechanisms that limit the accumulation of toxic salts in vegetal tissues. The effects of grafting on plant growth and yield traits under saline conditions may vary depending on scion and/or rootstock characteristics and the severity of salt stress [136]. Some rootstocks have the potential for enhancing plant water relations as well as growth and development during salt stress. Salt has an antagonistic effect on the plant nutrient absorption, which may disrupt the nutritional balance of vegetable crops [137]. In this respect, grafted plants exhibit several mechanisms to mitigate salt stress. One of the most important mechanisms concerns the restriction of Na and Cl uptake and translocation, accomplished by the rootstock [138]. The salt ion exclusion in grafted plants has been linked to morphological root system characteristics (such as root number, length, diameter, and total root surface) [139].

Roots play a crucial role in water and ion absorption and regulate several physiological processes [135]. The use of rootstock with a strong root system can efficiently modulate the negative effect of salt stress on scion cultivar performance [140]; in this scenario, recent research has focused on the selection of vigorous rootstocks capable of expressing their full potential to increase salt tolerance [141].

For example, according to Parthasarathi et al. [142], salinity reduces tomato plant performance. However, when plants were grafted onto potato rootstocks, they were able to avoid growth variations through a balanced distribution between vegetative and reproductive dry mass allocation. Nutrient uptake and translocation are often enhanced in salt-tolerant grafting combinations, resulting in a reduction in nutrient imbalances and deficiencies caused by salt stress. The efficiency in controlling Na accumulation in shoots has been related to the efficiency of the rootstock to modulate water uptake by the root system and water loss through transpiration. According to Behera et al. [143], the salt tolerance response of grafted plants is related to the modulation of water absorption and transpiration, ensuring a better water status in grafted plants compared to non-grafted ones. Similarly, Miao et al. [144] indicated that potato rootstock can alter Na transport to the shoots via exclusion and retention mechanisms.

A recent study investigating the interaction between grafting and exogenous salicylic acid in tomatoes, underlined a higher K content in grafted plants compared to ungrafted ones, suggesting the potential of this agronomic technique to restore the ionic balance and to improve salt tolerance. Additionally, the same authors reported an increase in Ca levels in grafted tomato plants compared to ungrafted ones. Ca plays a crucial role in increasing salt stress tolerance due to its involvement in salt overly sensitive (SOS) pathways, ROS regulation, ion absorption, and homeostasis [144]. Certain scion/graft combinations may mitigate abiotic stress, such as salinity, by regulating oxidative defense enzymatic and non-enzymatic mechanisms. Stressed plants enhance the activity of superoxide dismutase (SOD), ascorbate peroxidase (APX), peroxidase (POD), and catalase (CAT) [143], whereas non-enzymatic defense mechanisms include the biosynthesis of compounds with high antioxidant activity. Grafted plants subjected to abiotic stress enhance defense mechanisms by improving the biosynthesis of antioxidant enzymes (SOD, APX, CAT) [141]. Salinity stress also increases polyamine concentrations, which are associated with plant stress tolerance and play an important role in cell metabolism [145]. Grafting studies reported that certain scion/rootstock combinations provide a better ability to enhance salinity tolerance by regulating the antioxidative defense system in plants than other combinations.

Hormones affect nutrient uptake and transport of plants by modifying root architecture [146]. Grafting might affect hormone levels, influencing several morphological and physiological processes and abiotic stress tolerance. Recently, several research studies testified to the benefits of grafting in increasing salinity tolerance in cucurbitaceous crops [147]. In addition, Wang et al. [148] conducted a transcriptome analysis on Citrullus lanatus, either self-grafted or grafted onto Lagenaria siceraria, and evidenced that grafting significantly enhances salt tolerance. In particular, the transcriptome analysis showed that the transcription of most genes in the auxin signaling pathway of the Citrullus lanatus/Lagenaria siceraria combination was higher than that observed in self-grafted C. lanatus. These results indicate that L. siceraria rootstock may regulate auxin homeostasis to promote growth even under high saline conditions. Likewise, the expressions of ABA receptors were significantly higher during salt stress, but their levels in the self-grafted watermelon were lower than those in the Citrullus lanatus/Lagenaria siceraria combination. Thus, the ABA signaling pathway may also play a role in mediating L. siceraria rootstock-induced salt tolerance.

5.2. Grafting Limitations

Even though herbaceous grafting of vegetables appears to be a valuable approach for alleviating salt stress in plants, there are several constraints that affect its adoption on a large scale. The first one is the cost of grafted plants, which highly depends on labor and rootstock costs and the availability of specialized manpower [149]. Furthermore, compatibility issues between scion and rootstock can prevent successful grafting and hinder the feasibility of large-scale production [150]. Moreover, there are challenges related to the availability of rootstocks that meet the specific needs of farmers. To address these issues, scientific efforts should focus on developing economically viable grafting robots to reduce the cost of the grafted plants and on creating a comprehensive database providing all information on rootstocks (compatibility, cost, growth time, agronomic effects, etc.) to facilitate the exchange of information among farmers, nurseries, technicians, and scientists (

Table 2. Main effects of herbaceous grafting on vegetable species subjected to salinity stress.

Scion	Rootstock	Salinity	Main Effects on Plants	References
Cucumis sativus cv. Jinyan no. 4, Cs	Luffa cylindrica cv. Cuixiuhua, Yutu, Xiangbaiyu, Saijiali and Helanbiyu, Lc	50, 75, 100, 125 or 150 mM NaCl	Increase salt tolerance of cucumber, plant fresh weight, growth, photosynthetic rate, leaf number, vitamin C, soluble solids content, photochemical efficiency, K accumulation, and reduce the titratable acidity and Na uptake	[151]
Cucumis sativus cv. Jinchun No. 2	Cucumis sativus cv. Jinchun No. 2 or Cucurbita moschata cv. Chaojiquanwang	75 mM NaCl	Increase in transpiration rate and stomatal conductance, reduction in abscisic acid sensitivity	[136]
Citrullus lanatus ‘Jingxin No. 2	Cucurbita moschata ‘Quanneng Tiejia’, ‘Kaijia No.1’, and Lagenaria siceraria ‘Hanzhen No.3’	200 mM NaCl	Enhancement of photosynthetic capacity, chlorophyll concentration, photochemical efficiency of Photosystem II, reduction in electrolyte leakage, SOD, CAT, and APX	[152]
Solanum lycopersicum cv. Bark	Solanum lysopersicum cv. Bark, tomato accessions LA1995, LA2711, LA2485 and LA3845	100 or 200 mM	Increase in growth parameters, yield traits, vitamin C, firmness and total soluble solids, antioxidants, and proline	[153]
Cucumis sativus cv. 1010	Lagenaria siceraria, Cucurbita moschata, Citrullus lanatus var. Colocynthoide, Cucurbita maxima cv. Flexyl	50 or 100 mM NaCl	Increase in photosynthetic activity, auxin, gibberellin, cytokinin, salicylic acid, and antioxidant enzyme activity	[154]
Capsicum annuum × Capsicum annuum (Niber), Capsicum annuum (Adige)	Capsicum annuum × Capsicum annuum (Niber), Capsicum annuum (Adige)	70 mM NaCl	Increase in nitrate reductase activity, proline, and decrease in the Na/K ratio, abscisic acid content and POD	[155]
Solanum lycopersicum cv. Tom 174 and Tom 121	Solanum lycopersicum cv. Tom 174 and Tom 121	50 mM NaCl	Increase in yield, decrease in the Na concentration, fruit size, dry matter content, vitamin C, and transpiration	[156]
Cucumis melo cv. Citirex F1 and Kırkağaç Manisa Altinbas	Cucurbita maxima × C. moschata (Kardosa and Nun 9075)	1 dS/m and 8 dS/m	Enhancement of growth traits, SPAD indices, and shoot fresh weight	[157]
Cucumis melo SCP1 and SCP2	Cucumis melo TLR1, TLR2 and Albatros	200 mM NaCl	Enhancement of plant growth, leaf area, relative water content, chlorophyll, carotenoids, antioxidant enzymes, Ca and K uptake, and reduction in malondialdehyde, Na, and Cl uptake	[158]

).

5.3. Landraces and Crop Wild Relatives as Rootstocks for Salinity Tolerance

Landraces and crop wild relatives represent a valuable source of genetic variation, offering resilience to both abiotic and biotic stresses. Landraces, which are usually referred to as primitive, farmers’, traditional, local, or folk varieties, are genetically diverse and locally adapted populations with historical roots and distinct identities. Crop landraces contribute to improved nutritional and sensory qualities of products, along with consistent performance under challenging environments [159,160]. Utilizing the inherent salt tolerance of landraces as rootstocks represents a potentially valuable strategy for improving resilience to salinity in important vegetable crops such as tomatoes [161].

However, the use of landraces as rootstocks in vegetable crops under saline conditions remains poorly exploited. So far, only a few reports are available for certain major vegetable species, particularly in the case of tomato grafting. In an experiment conducted by Fullana-Pericàs et al. [162], researchers investigated the response of a commercial ‘de Ramellet’ tomato hybrid to water deficit conditions in a greenhouse when grafted onto either a traditional landrace rootstock, ‘Tomàtiga de Ramellet’ (TR), or the commercial rootstock Maxifort (Mx) [162]. The findings suggest that the ‘TR’ landrace could potentially serve as a suitable rootstock under salinity stress conditions, as it enhanced the hydraulic conductivity of the grafted scion and contributed to improved stress tolerance. Previous research on these rootstocks has shown that similar scion-rootstock interactions can contribute to tolerance against soil salinity and other environmental stressors [163,164], although additional studies are necessary to validate this hypothesis.

Notably, a Greek tomato landrace known as ‘Santorini’ demonstrated exceptional salt stress tolerance in the study by Kadoglidou et al. [165]. Among the genotypes tested under saline conditions, ‘Santorini’ showed minimal reductions in shoot length, leaf number, and shoot thickness, while also achieving the highest stress tolerance index. These results highlight the potential of ‘Santorini’ as a valuable genetic resource and a strong candidate for use as a salt-tolerant rootstock. Moreover, in the study of Sanwal et al. [166], salt tolerant eggplant landraces (IC-111056 and IC-354557) were utilized as rootstocks for cultivated tomato (cv. ‘Kashi Aman’). The results revealed that the activities of key antioxidant enzymes, including CAT, APX, SOD, and POX, were significantly higher in grafted plants than in those of non-grafted plants [166]. These findings suggest that the membrane damage in grafted plants exposed to high saline conditions is less severe than in non-grafted plants.

The study of Guo et al. [151], examined five different indigenous in the tropical zone luffa cultivars (Luffa cylindrica Roem.) as rootstocks for grafting cucumber plants. Their findings provide clear evidence that luffa confers salt tolerance in grafted cucumber plants by limiting Na⁺ transport to the shoot, thereby improving both yield and fruit quality. In another study on cucumber in Pakistan, Abbas et al. [167], conducted a pot experiment, using non-perforated containers to investigate the use of indigenous cucurbit landraces, bottle gourd (Lagenaria siceraria, cv. Faisalabad Round), pumpkin (Cucurbita pepo L., cv. Local Desi Special), sponge gourd (Luffa aegyptiaca, cv. Local), and ridge gourd (Luffa acutangula, cv. Desi Special), as rootstocks for enhancing salinity tolerance in cucumber (cv. Yahla F1) under four salinity levels. The results revealed that, while increasing salinity levels negatively impacted cucumber plants overall, grafted cucumbers demonstrated significantly greater salt tolerance compared to non-grafted plants. More specifically, among the grafting combinations tested, the researchers strongly recommended using the landrace ‘cv. Faisalabad Round’ as a rootstock for cucumber under mild saline conditions in pot experiments.

Usanmaz and Abak [168], evaluated two Cypriot landraces of Cucurbita moschata L. (Local-1 and Local-3) along with four commercial hybrids of Cucurbita maxima × Cucurbita moschata as rootstocks for the cucumber cultivar ‘Falconstar’ under three salinity levels and in three types of growing media. Among the tested rootstocks, Local-3 exhibited the highest root dry weight under the lowest EC condition (2.5 dS m⁻¹) and showed moderate tolerance at 5.0 dS m⁻¹ [168]. The results suggest that Local-3 is highly suitable as a rootstock not only for cucumber but also for other crops in the Cucurbitaceae family, particularly under salt stress conditions in both soil-based and soilless cultivation systems.

Studies by Uygur and Yetişir [169] and Yetişir and Uygur [170] showed that grafting watermelon cultivar ‘Crimson Tide’ onto local bottle-gourd (Lagenaria siceraria) and squash (Cucurbita maxima) landrace rootstocks markedly improved salt tolerance compared to non-grafted plants, while the grafted plants also maintained higher growth and biomass. Notably they also found that when L. siceraria rootstock was used, the shoot phosphorus content of watermelon plants almost doubled under irrigation with saline water, probably to offset the increased energy demands, while total N uptake declined above 8 dS m⁻¹. They concluded that Lagenaria rootstocks in particular conferred strong tolerance [169]. Likewise, these authors observed that grafted watermelons suffered much smaller reductions in shoot dry weight (≈0.8–22% loss) than non-grafted controls (~41% loss) under 8 dS m⁻¹ salinity [170]. Grafting also altered ionic balance. Both L. siceraria and C. maxima landrace rootstocks effectively enhanced watermelon salt tolerance by boosting nutrient (N, P) uptake and maintaining more favorable K/Na, Ca/Na and Mg/Na ratios, which contributed to better shoot biomass growth compared to non-grafted plants.

A similar study on pepper (Capsicum annuum), examined the effects of grafting on crop yield and fruit quality of the commercial pepper variety ‘Passion’ by utilizing three local Jordanian pepper landraces as rootstocks under salinity stress conditions. Physiological, biochemical, nutrient and yield-related parameters were evaluated. The results indicated that grafting pepper plants onto different landrace rootstocks improved salt tolerance by enhancing nutrient uptake and limiting toxic ion accumulation. Grafted plants showed higher levels of beneficial ions (K, Ca), increased antioxidant capacity, and reduced oxidative stress, resulting in improved productivity and fruit quality. Moreover, among the tested landraces, JO 207 emerged as a promising salt-tolerant rootstock for future grafting applications [171].

Wild relatives of vegetable crops also serve as invaluable genetic resources for enhancing salinity tolerance in cultivated varieties. These wild genotypes have evolved under diverse and often harsh environmental conditions, including saline soils, and thus possess adaptive traits that enable them to survive and thrive where domesticated varieties struggle [172,173]. Traits such as efficient ion exclusion, osmotic adjustment, and enhanced antioxidant defense mechanisms are commonly observed in salt-tolerant wild relatives [174]. Therefore, the exploration and utilization of wild species offer a promising path toward sustainable vegetable production under salt stress conditions. Grafting eggplant Solanum melongena L. cv. ‘Suqi Qie’ onto the rootstock of wild eggplant (Solanum torvum Schwarz cv. “Torvum vigor”) provided salinity tolerance due to the efficient removal of free radicals and the protection of antioxidant enzymes and polyamines [175]. Solanum tovrum similarly provided salt tolerance in grafted eggplant cv. ‘Madonna’ by more effectively distributing Na⁺ to the roots than the above ground parts [176]. Fullana-Pericàs et al. [177] demonstrated that grafting two ‘Tomàtiga de Ramellet’ (TR) genotypes onto commercial rootstocks (‘Beaufort’ and ‘Maxifort’) and a wild species rootstock (Solanumum pimpinellifolium) resulted in 100% graft compatibility and survival, confirming the feasibility of using landraces and crop wild relatives as rootstocks for commercial purposes. Variation in traits such as fruit yield, leaf morphology, and photosynthesis was influenced by the scion, the rootstock, and their interaction [178]. Additionally, Aydin [139] studied the effect of grafting tomato onto wild salt tolerant rootstocks (S. pimpinellifolium, S. habrochaites, S. lycopersicum × S. pimpinellifolium and S. lycopersicum × S. habrochaites) grown under salinity stress (8.0 dS m⁻¹). The author highlighted different responses of the scion/rootstock combination in terms of Na and Cl accumulation in shoots. Moreover, according to Aydin [139] the lowest Na concentration in tomato plants grown under saline conditions was recorded in ‘Galaxy’ tomato plants grafted onto S. pimpinellifolium LA1269 and S. pimpinellifolium LA2914 rootstocks, whereas the lowest Cl content was found in ‘Galaxy’ plants grafted onto S. habrochaites LAI1764 rootstock.

Landraces and crop wild relatives serve as important reservoirs of genetic diversity, providing valuable traits for resilience against both abiotic and biotic stresses. Leveraging the natural salt tolerance found in certain landraces and wild relatives as rootstocks presents a promising approach to increasing salinity resilience in vegetable crops. Harnessing these genetic resources contributes to developing more sustainable and climate-resilient agricultural systems.

6. Biofortification Strategies and Salinity Tolerance

Biofortification has proven effective in ameliorating plant stress in saline environments while enhancing their nutritional properties. In cucumber, for example, the combination of grafting onto pumpkin rootstock and selenium (Se) biofortification via sodium selenite application improved salinity tolerance, yield, and growth, while increasing potassium content and decreasing sodium accumulation in leaves and roots [179]. Similarly in tomato, Diao et al. [180] investigated the effect of exogenous Se (0.05 mM Na₂SeO₃) under NaCl^- induced stress (100 mM) under both salinity-resistant and salinity-sensitive cultivars. The observed effects were attributed to Se-mediated enhanced photosynthetic capacity and regulation of antioxidant enzymes and non-enzymatic systems in the chloroplast, and key enzymes in the ascorbate–glutathione cycle, alongside reduced H₂O₂ and MDA levels. Similarly, Wu et al. [181] obtained positive results of low level (25 μM) exogenous Se application of Se on photosynthesis, water use efficiency (WUE), nutrient content and salicylic acid biosynthesis, ultimately promoting overall growth of tomato plants subjected to salt stress.

Additional studies have examined the effect of iodine (I) in salt-stressed vegetable crops. In a hydroponic experiment with lettuce conducted in southern Spain, plants were grown under salinity stress (100 mM NaCl) and received I in 3 different concentrations (20, 40, and 80 μM KIO₃) [182]. The results showed that 20–40 μM KIO₃ enhanced plant biomass and stimulated the phenolic compound synthesis pathway, suggesting a potential role of I in mitigating salinity-induced oxidative stress. Similarly, strawberry plants grown under 10 mM NaCl stress and treated with two iodine sources (commercial products and KIO₃) exhibited higher yields and improved fruit quality [183]. These plants also displayed increased enzymatic and non-enzymatic antioxidant activity in both leaves and fruits, along with enhanced nutrient status, under both normal and saline conditions. In contrast, Fuentes et al. [184] found that foliar KIO₃ application in tomato plants under severe salinity stress did not significantly alleviate the negative effects on fresh or dry biomass. However, it increased fruit production by 23%, indicating that I biofortification may still benefit yield even when overall plant growth remains constrained by salt stress.

Molybdenum (Mo), an essential micronutrient, has also been successfully employed in agronomic biofortification strategies to enhance crop resilience under salinity stress. In a study by Bouzid & Rahmoune [185], Phaseolus vulgaris L. seedlings were subjected to varying NaCl concentrations (0, 3000, 6000, 9000, and 12,000 ppm) alongside Mo treatments (0, 0.1, 0.2, and 0.4 ppm). The results revealed that under mild salinity stress (3000 ppm NaCl), Mo application at 0.1–0.2 ppm optimized total chlorophyll content and improved morphological parameters, including shoot length, root length, and biomass. Similarly, Zhang et al. [186] demonstrated that exogenous Mo application enhanced salinity tolerance (8000 ppm NaCl) in Chinese cabbage by increasing photosynthetic pigments (chlorophyll a, chlorophyll b, and carotenoids). This improvement was attributed to Mo’s role in ion homeostasis regulation and protection of plant membranes from salt-induced damage.

As a field of research, genetic biofortification aims to enhance the ability of plants to tolerate and accumulate beneficial microelements such as selenium (Se), zinc (Zn), I, Mo, etc. The ability to tolerate and accumulate such micronutrients is crucial for two major reasons. Firstly, from a phytoremediation perspective, it enables plants to perform effectively even under adverse conditions, such as high concentrations of trace elements, making them valuable for soil detoxification. Secondly, from a nutritional standpoint, it improves the nutrient density and overall quality of crops, addressing dietary deficiencies in human populations. For genetic modification to be considered agronomically viable, it must target key genes involved in micronutrient uptake, translocation, and storage while simultaneously enhancing metal tolerance. Such modifications should enhance crop yields without jeopardizing quality characteristics, safety, or commercial value, ensuring both farmer profitability and consumer acceptance. The primary objective of genetic biofortification is to identify and isolate the genes that are responsible for the accumulation and tolerance of heavy metals. In their study, Hung et al. [187] analyzed the transcript profile of Se hyperaccumulator A. racemosus treated with 20 μM selenate (K₂SeO₄) and selenite (K₂SeO₃). They identified 125 Se-responsive candidate genes, 6 of which responded to both Se forms. Among these, a novel Se-responsive gene (CEJ367) was induced by both selenate and selenite and was mainly expressed in shoots and roots, respectively. Significant outcomes were also demonstrated by LeDuc et al. [188], where double transgenic Indian mustard plants overexpressing the genes encoding ATP sulfurylase (APS) and selenocysteine methyltransferase (SMT) showed better performance in terms of uptake and conversion of selenate to MetSeCys, the non-toxic form that it is accumulated to high concentrations. These plants also showed higher biomass and shoot Se concentrations compared to wild types. Moreover, the role of plant metal tolerance proteins (MTP), in plants resilience under excessive/toxic concentrations of microelements, is being investigated extensively. For instance, MTP1 has been linked to Zn hypertolerance in Zn hyperaccumulator plants [189]. Several candidate genes involved in zinc (Zn) uptake, transport, and homeostasis have been identified in maize. These include ZmIRT1, ZmHMAs, ZmNRAMP6, and ZmVIT, which play a role in Zn acquisition and redistribution [190]. Further evidence comes from studies on A. thaliana and P. alba (cv. Villafranca) expressing the Saccharomyces cerevisiae ZRC1 gene which accumulated more Zn in vegetative tissues and were more Zn tolerant than compared to wild-type plants. The elevated zinc (Zn) levels observed in transgenic plants were associated with enhanced superoxide dismutase (SOD) activity. This suggests that the activation of defense mechanisms may have occurred to prevent cellular damage, supported by the fact that o toxicity symptoms were observed in the presence of cadmium [191]. These findings provide a compelling dataset, offering a substantial foundation for enhancing the effectiveness of genetic biofortification for abiotic stress tolerance, such as salinity.

7. Biostimulants for Enhanced Salinity Tolerance

7.1. Microbial Biostimulants

7.1.1. Plant Growth-Promoting Rhizobacteria (PGPRs)

The interconnection among microorganisms, soil and plant lasts throughout the entire plant life cycle, from germination to senescence, promoting plant growth and development and increasing the resistance of host plants to biotic and abiotic stresses [192,193,194]. Specifically, PGPR contribute to alleviating salinity stress in plants supported by withstanding adverse osmotic conditions, increasing plant salinity tolerance and ameliorating the overall soil quality [195]. The mechanisms behind salinity stress mitigation in plants are diverse and include the activation of plant stress response systems, the synthesis of anti-stress compounds for ROS detoxification, the production of exopolysaccharides which can bind Na ions, the synthesis of phytohormones and the decrease in ethylene presence via the enzyme 1-aminocyclopropane-1-carboxylate (ACC) [196,197,198,199,200]. Among these mechanisms, one of particular interest is the production of ACC deaminase enzyme, which reduces the excessive ethylene concentrations, thereby supporting plant growth [193]. Since N uptake is impaired under saline conditions, the utilization of ACC by plants as N source has positive outcome on their growth [201,202]. At this regard, there is evidence that many PGPR with ACC deaminase enzyme can degrade ACC to ammonia and a-ketobutyrate, contributing to a better N nutrition and, at the same time, to a lower ethylene production [203,204]. Moreover, under salt stress, PGPR can modulate K and Na homeostasis, promote the accumulation of osmolytes, stimulate exopolysaccharides biosynthesis, stabilize lipids of cell membranes and induce the transcription of anti-stress compounds [205]. Under abiotic stress conditions, PGPR can also activate plant antioxidant defenses by regulating key ROS-scavenging enzymes, such as CAT and SOD, which in turn protect plants from the salinity-induced oxidative damages [206]. Previous research indicates that two strains of Bacillus subtilis (NBRI 28B, NBRI 33N) and B. safensis NBRI 12 M enhance the production of ACC deaminase, biofilm, exopolysaccharides, and alginate depending on the concentration of NaCl [207]. In addition, according to Mukherjee et al. [208] the production of exopolysaccharides by Halomonas sp. Exo 1 is strictly related to NaCl concentration. Thus, the ability of PGPR to mitigate salinity stress largely depends on bacteria species and strain selection [193]. For example, several studies pointed out that PGPR species, such as Pseudomonas putida UW4 [201], Pseudomonas fluorescens 002 [209], Bacillus subtilis strains NBRI 28B and NBRI 33 N, Bacillus safensis (NBRI 12 M) [207], B. subtilis subsp. subtilis NRCB002, B. subtilis NRCB003 [210], and Kosakonia sacchari [211] produce ACC deaminase, indole-3-acetic acid, siderophore, exopolysaccharides, and proline.

7.1.2. Arbuscular Mycorrhizal Fungi (AMF)

The colonization of plant roots by arbuscular mycorrhizal fungi (AMF) is very common, as approximately 80% of terrestrial plants can develop mutualistic associations with these fungi [212]. In this respect, it was underlined that the application of AMF on plants could be considered as an eco-friendly and efficient strategy to increase plants yield and quality [213,214,215] and to relieve the negative effect of abiotic stresses [216], such as salinity. Indeed, there is evidence that the positive effects of AMF on plants subjected to salt stress make them suitable for the bio-amelioration of saline soils [217]. AMF inoculation boosts plant salinity tolerance through diverse mechanisms, including modulation of plant water and nutrient uptake, alteration of physiological and biochemical traits and protecting chlorophyll from degradation [218]. Salinity stress impairs plant growth and development, primarily due to reduced availability of water and nutrients. However, under such adverse conditions, AMF symbiosis can support plant function by facilitating the release of essential nutrients, such as N and P, from organic components [219]. Moreover, AMF inoculation promotes plant water and nutrient uptake even under saline conditions, partly due to its positive influence on root growth and physiology [219]. In saline environment, AMF also benefits the selective uptake of low-mobility nutrients, such as P [220].

Generally, the ameliorated growth of AMF-inoculated plants grown under salinity is attributed to the beneficial effects of AMF in facilitating nutrient uptake, particularly N and P. The increase in P uptake is promoted by the extensive hyphal network developed by AMF, allowing inoculated plants to explore greater volume of soil compared to non-mycorrhized ones [221]. Moreover, the increased P uptake in AMF-inoculated plants can mitigate the detrimental effects of Na and Cl ions by preserving the vacuole membrane, which plays a key role in reducing ion leakage and enhancing ion selectivity [218]. A recent study revealed that inoculation of sweet pepper plants subjected to salinity stress enhances the P uptake and leaf water content [62]. Similar findings were reported by Kakabouki et al. [222] who found that inoculated flax plants grown under salt stress exhibited higher yield, chlorophyll content, N and P concentrations compared to non-inoculated plants. These studies underlined the importance of AMF inoculation in enhancing P uptake under saline conditions, a response associated with improved plant growth and increased antioxidant production [218]. Salinity also restricts N uptake and utilization, with negative consequences on plant growth and development [223]. Under such conditions, inoculation with AMF can increase the plant N uptake via their influence on nitrogen fixation and/or on enzymes linked to nitrogen metabolism, such as pectinase, xyloglucanase and cellulase [218]. McFarland et al. [224] reported that up to half of plant N requirement may be met by AMF symbiosis and that inoculation can also increase the CAT and SOD activity in plant roots exposed to salinity stress. Giri and Mukerji [225], in a study on Sesbania grandiflora and S. aegyptiaca, indicated that inoculated plants exhibited higher N shoot concentrations compared to non-inoculated ones. Nevertheless, limited information is currently available on the precise mechanisms regulating N uptake and transfer from AMF to plant roots. Some authors hypothesize that ammonia transporters may contribute to N transport during AMF symbiosis [226]. Moreover, it was reported that AMF inoculation can influence the activity of nitrate reductase, an key enzyme in plant N metabolism. At this regard, Azcón et al. [227] supported that nitrate reductase activity, typically suppressed by salinity stress, increases in AMF-inoculated plants, even under saline conditions, indicating improved salinity stress tolerance.

Water stress is another consequence of high saline levels in soil and irrigation water. In fact, plants exposed to salinity experience a form of a physiological drought caused by Na+ and Cl⁻ ions [218]. Several studies highlighted that AMF inoculation help plants to maintain a higher cell turgor and water content compared to non-inoculated plants [228]. The enhanced water status in inoculated plants is associated with improved water uptake and transport mediated by AMF [229]. Moreover, AMF can influence the osmotic balance via solute accumulation, thereby improving water uptake under saline conditions [218]. The efficacy of AMF inoculation has also been demonstrated in studies on tomato and lettuce, two most widely cultivated vegetable crops globally. For example, Liu et al. [230] reported that Paraglomus occultum inoculation improved aquaporins gene expression in tomato under salinity, thereby enhancing plant tolerance. Similarly, Hajiboland et al. [231] reported that AMF inoculation alleviates salt stress by increasing P uptake, stomatal conductance and the production of antioxidant compounds in tomato plants. Moreover, Santander et al. [232] evidenced that the inoculation of lettuce with AMF promotes salt stress tolerance by altering ionic balance. Cela et al. [233], investigating AMF inoculation under P-deficient conditions, reported increased chlorophyll, carotenoids, phenols, gas exchange and N, P and Mg uptake in inoculated lettuce plants compared to controls. Collectively, these studies suggest that the application of AMF in vegetables represents an effective strategy for enhancing plant tolerance to salinity stress (

Table 3. Effect of plant growth promoting rhizobacteria (PGPRs) and arbuscular mycorrhizal fungi (AMF) on different vegetable species grown under salinity stress.

Microbial Biostimulant	Plant Species	Salt Stress	Effect on Plant Performance	Article
IG 2 (Acinetobacter bereziniae), IG 10 (Enterobacter ludwigii), and IG 27 (Alcaligenes faecalis)	Pisum sativum	75 or 150 mM of NaCl	Increase in growth parameters, chlorophyll content, total soluble sugars and decrease in electrolyte leakage	[234]
Pseudomonas aeruginosa HG28–5	Solanum lycopersicum	200 mL of 200 mM NaCl	Improvement in growth parameters, fresh and dry weight, photochemical efficiency, antioxidant enzymes activities and reduction in ROS accumulation, malondialdehyde and electrolyte leakage	[235]
Bacillus megaterium TV-6D, Paenibacillus polymyxa KIN37, and Pantoea agglomerans RK92	Solanum melongena	100 mM NaCl	Mitigation of the negative effects of salinity via the reduction in Na and Cl uptake, malondialdehyde and hydrogen peroxide concentrations, as well as reduction in electrolyte leakage and increase in leaf water content	[236]
Pseudarthrobacter oxydans SRT15 and Bacillus zhangzhouensis HPJ40	Beta vulgaris var. Bressane	85 mM NaCl	Increasing growth, leaf dry weight, chlorophyll and carotenoid concentrations, stomata conductance, and antioxidant capacity and reduction in electrolyte leakage and Na+ uptake	[237]
Bacillus velezensis JB0319	Lactuca sativa	50, 100, or 150 mM NaCl	Promotion of shoot height, root length, shoot biomass, SOD, POD, and reduction in malondialdehyde concentration	[238]
Bacillus subtilis	Lactuca sativa	150 mM NaCl	Enhancement of germination rate, polyphenols, flavonoids, tannin, POD, PPO, and PAL activity	[239]
Glomus spp.	Capsicum annuum	4, 8 and 12 dS m–1	Increase in marketable yield, proline, potassium, phosphorous, Brix and reduction in Na accumulation	[240]
Claroideoglomus etunicatum	Solanum lycopersicum	100 mM of NaCl	Upsurge of growth parameters, soluble sugars, soluble proteins, proline, SOD, POD, and CAT	[241]
Paraglomus occultum	Solanum lycopersicum	150 mM of NaCl	Improvement of plant growth, chlorophyll index, nitrogen balance index, modulate aquaporins expression	[242]
Glomus intraradices, Glomus aggregatum, Glomus mosseae, Glomus clarum, Glomus monosporus, Glomus deserticola, Glomus brasilianum, Glomus etunicatum, and Gigaspora margarita	Lactuca sativa	50 mM of NaCl	Alleviation of the salinity negative effect on plant weight, height, leaf number and leaf area. Increase in plant yield, stomatal conductance, chlorophyll content, nutrient uptake and plant water status. Decrease in malondialdehyde concentration	[243]
Glomus mosseae	Solanum lycopersicum	5.6 g of NaCl in 1 L of water	Enhancement of plant height, leaf area, stem diameter, number of leaves, number of flowers and highest fresh weight	[244]

).

7.2. Non-Microbial Biostimulants

7.2.1. Seaweed Extract Applications

Seaweed extracts (SEs) enhance the activity of key antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX), which play crucial roles in mitigating oxidative stress induced by salinity [245,246]. A study by Hernández-Herrera et al. [247] demonstrated that SEs derived from Padina gymnospora and Cystoseira tamariscifolia significantly improved the growth of tomato plants irrigated with 300 mM NaCl. Notable increases were observed in several growth parameters, including root length (20%), shoot length (19%), root surface area (45%), shoot surface area (35%), and fresh weight (49% for roots and 42% for shoots). These enhancements were accompanied by elevated antioxidant enzyme activity and enhanced carbon and N metabolism, contributing to increased salinity stress tolerance. Similarly, soil drench applications of a brown algae extract from Cystoseira tamariscifolia (CTE) [248] and a red algae extract from Jania rubens [249] promoted the growth of tomato seedlings under 50 mM NaCl stress. These treatments effectively reduced ROS while enhancing the activity of antioxidant defense enzymes, as well as enzymes involved in N and carbon metabolism. Additionally, non-enzymatic antioxidant mechanisms were also stimulated, resulting in notable improvements in shoot length and in both root and shoot fresh weight.

SEs have also been reported to improve nutrient uptake under saline conditions by enhancing the absorption of K, Ca, and Mg, while limiting the uptake of Na, thereby maintaining an optimal K/Na ratio, an essential factor for plant salinity tolerance [250,251,252]. For instance, Ntanasi et al. [253] observed that the application of Ascophyllum nodosum extract in the hydroponic cultivation of two Greek tomato cultivars under 30 mM root-zone salinity resulted in increased biomass, elevated leaf K and Ca levels, and yield improvements of 23% and 34% in cherry-type and mid-type varieties, respectively. In the same context, the A. nodosum extract Super Fifty was shown to promote K accumulation while limiting Na uptake under moderate salinity stress [254]. Supporting this evidence, Cruz and Ajit [255] demonstrated that A. nodosum effectively maintained Na/K homeostasis and improved tomato growth under NaCl concentrations up to 200 mM. Overall, these findings highlight the multifaceted role of SEs in supporting plant adaptive responses to salinity through improvements in physiological, biochemical, and nutritional traits. For instance, Kelpak^®, a commercial seaweed-based product, significantly enhanced growth, reduced oxidative damage, and improved the nutritional profile of spinach grown at 300 mM salinity [256]. A. nodosum extract has also been shown to enhance vegetative growth and dry matter content in Capsicum annuum L. cultivated under 100 mM salinity, further underscoring the broad efficacy of SEs in mitigating the detrimental effects of salinity stress [257].

7.2.2. Use of Microalgae-Based Products to Enhance Crop Growth and Salt Stress Mitigation

Microalgae are increasingly recognized as sustainable and effective source of biostimulants in agriculture for improving crop resilience under salinity stress. These unicellular, photosynthetic organisms exhibit remarkable metabolic plasticity and synthesize a broad spectrum of bioactive compounds with plant growth-promoting effects, including essential amino acids, vitamins, phytohormones (auxins, cytokinins, gibberellins), polysaccharides, polyphenols, carotenoids, betaines, and mineral nutrients, components known to enhance plant growth, productivity and stress resilience [258,259,260]. Under saline conditions, microalgal extracts have shown to mitigate stress-induced damage by improving ion homeostasis, enhancing antioxidant defense mechanisms, and maintaining osmotic imbalance [261,262]. Although the literature remains relatively limited, emerging studies underscore the diverse mechanisms through which microalgae-based biostimulants enhance salinity tolerance, particularly in vegetable crops.

For example, Mutale-Joan et al. [263], found that a combined aqueous extracts from Dunaliella salina, Chlorella ellipsoidea, Aphanothece sp., and Arthrospira maxima significantly improved growth, photosynthetic activity and Na/K homeostasis in salt-stressed tomato plants. Similarly, El Arroussi et al. [264] reported that exopolysaccharide (EPS) extracts from Dunaliella salina alleviated salinity stress in tomato by promoting shoot and root development, while maintaining K content thereby restoring the K/Na ratio, and also reducing the stress-induced accumulation of proline. Further evidence comes from studies on leafy greens. Chaudhuri and Balasubramanian [265] found that EPS derived from Spirulina sp. enhanced salinity tolerance in hydroponically grown basil and spinach by increasing biomass by 30% and 21%, respectively, under 80 mM NaCl. EPS also increased the accumulation of photosynthetic pigments, phenolic compounds, and osmoprotectants such as proline and soluble sugars while improving K/Na ratio and the activity of antioxidant enzymes, including CAT and SOD. These findings suggest microalgal EPS modulate physiological and biochemical defense pathways, including jasmonate signaling. In lettuce, seed priming with extracts from two microalgae species, Chlorella vulgaris and Arthrospira platensis, combined with macroalgal extracts improved germination rates, seedling vigor, and antioxidant enzyme activity under saline conditions. These treatments also promoted osmolyte accumulation, enhanced photosynthetic pigment levels, and increased stress resilience in mature lettuce plants, highlighting the potential of microalgae-based seed priming as a practical and sustainable approach to improving early-stage stress adaptation [266]. Additionally, Francioso et al. [267] demonstrated that a commercial microalgae-based formulation enhanced biomass production, total protein content, chlorophyll concentration, antioxidant enzyme activities and K uptake in hydroponically grown salt-stressed Lactuca sativa L. var. Gentile Rossa, while reducing Na accumulation and oxidative damage.

In summary, microalgae-based biostimulants offer a promising and eco-friendly strategy for enhancing crop tolerance to salinity by modulating ion transport, activating antioxidant systems, and promoting osmotic adjustment. Their integration into sustainable agricultural practices can contribute to maintaining productivity on saline soils while reducing reliance on synthetic agrochemicals. However, future research should focus on optimizing extraction methods, application protocols, and species-specific formulations to maximize their efficacy.

7.2.3. Protein Hydrolysates and Other Organic Compounds

Protein hydrolysates (PHs) are a class of plant biostimulants composed of amino acids, oligopeptides, and polypeptides obtained through the partial hydrolysis of various protein sources [268]. Their capacity to enhance plant tolerance to salt stress has been demonstrated to a considerable extent [269], with primary effects attributed to enhanced nutrient uptake and boosted antioxidant defense mechanisms. The utilization of PHs has been demonstrated to enhance nutrient use efficiency, particularly under conditions of nutrient deficiency. For instance, Colla et al. [270] reported that foliar application of PH significantly improved the growth of baby lettuce, even when the nutrient solution was reduced to one-tenth of its standard concentration. PHs treatment resulted in a 50% increase in yield, accompanied by an 11% rise in both leaf chlorophyll content and N uptake. Similarly, the application of PHs to the roots and leaves of salinity-stressed lettuce can enhance the marketable yield by improving N metabolism and facilitating the uptake of key nutrients, including P, K, Ca, and Mg. These nutrients, often less available under saline conditions, are critical for sustaining photosynthesis and overall plant health [271]. In a comparable study, PHs derived from pumpkin seeds mitigated the detrimental effects of salinity in Phaseolus vulgaris by restoring the balance of all essential macronutrients, leading to improved growth and yield [272]. According to Sorrentino et al. [273], PH-based biostimulants enhanced growth and salt stress tolerance in lettuce and tomato grown under 40 mM NaCl. In lettuce, this response was associated with shifts in phytohormone levels such as auxin and ethylene. In tomatoes, chlorophyll fluorescence parameters were identified as significant indicators of plant performance under salinity stress and were strongly related to the accumulation of stress-related metabolites from the shikimate pathway. Furthermore, PH application under saline conditions has been associated with increased activity of key antioxidant enzymes, including CAT, POD, and SOD, as well as elevated levels of non-enzymatic antioxidants such as glutathione (GSH) and phenolic compounds. In a study by Zhou et al. [274], the beneficial effects of PH treatment in reducing the harmful effects of salinity on Solanum lycopersicum were attributed to the enhancement of both enzymatic and non-enzymatic antioxidant responses.

In addition to PHs, other organic compounds, such as humic substances and amino acid-based formulations, have shown potential in enhancing salt stress tolerance in vegetable crops. Humic substances, in particular, have been associated with the stimulation of both primary and secondary metabolic processes, contributing to improved plant performance under salinity [275]. Their application has been linked to several beneficial effects, including improved root development, increased water and nutrient uptake, enhanced photosynthesis, accumulation of osmoprotectants, and activation of both enzymatic and non-enzymatic antioxidant defense systems [276]. Research by Çimrin et al. [277] demonstrated that humic acid (HA) application in pepper seedlings alleviated salt stress by stimulating root growth, optimizing mineral uptake, and reducing membrane damage, ultimately enhancing the plant’s tolerance to salinity. Similarly, a study by Amerian et al. [278] on cucumber plants found that applying HA at 200 mg L⁻¹ reduced Na and Cl accumulation while increasing K and Ca levels in both roots and leaves. These effects helped maintain physiological processes and promoted growth in saline conditions, highlighting HA’s role in improving root function and nutrient availability. Fulvic acids have likewise been shown to possess the capacity to enhance plant growth, boost photosynthetic performance, and mitigate stress-related effects [279]. In a pot experiment, Gabr et al. [280] investigated spinach (Spinacia oleracea L. cv. Balady) treated with fulvic acid at two concentrations (1.5 and 3.0 g L⁻¹) under four salinity levels (tap water, 1500, 3000, and 4500 ppm). At the highest salinity, the higher fulvic acid dose increased leaf N, P, and K contents and improved growth traits, including fresh and dry biomass, leaf number, and leaf area. Amino acid-based formulations also play a key role in mitigating salinity stress. Acting as osmoprotectants, amino acids help stabilize proteins and cellular membranes and serve as precursors for the synthesis of stress-related metabolites. For instance, Saddique et al. [281] found that applying a mix of amino acids to spinach boosted the plant’s antioxidant defense, promoted the accumulation as osmolytes, such as proline, and helped maintain nutrient balance, thereby enhancing stress resilience. Likewise, Abdelkader et al. [282] reported that amino acid treatments in lettuce exposed to 50 mM NaCl improved growth, increased chlorophyll content, and stabilized ion levels, reducing cellular damage caused by salinity.

8. Conclusions and Future Perspectives

This review synthesizes current knowledge on innovative strategies to enhance salinity tolerance, with a focus on advanced agronomic and crop management strategies. Key findings underscore the effectiveness of soilless and closed-loop cultivation systems in optimizing nutrient and salinity management, though sodium accumulation remains a critical constraint requiring predictive modeling and adaptive discharge strategies. Deficit irrigation techniques, particularly regulated deficit irrigation (RDI) and subsurface drip systems, have demonstrated significant potential in improving water use efficiency while mitigating salt stress, though their efficacy is highly dependent on crop-specific responses and environmental conditions. Grafting with salt-tolerant rootstocks contributes to reducing toxic ion uptake, enhancing antioxidant activity, and improving nutrient homeostasis. However, broader adoption is hindered by economic and logistical barriers, including high production costs and limited rootstock availability. Landraces and crop wild relatives possess valuable traits such as stress tolerance and adaptability. Biofortification—both agronomic and genetic—offers a dual benefit by enhancing stress resilience while improving nutritional quality, with selenium, iodine, and molybdenum showing promise in alleviating oxidative damage. Furthermore, biostimulants, including PGPR, AMF, seaweed extracts, and protein hydrolysates, support plant adaptation to salinity by enhancing nutrient uptake, osmotic regulation, and antioxidant defense mechanisms. Looking ahead, future research should prioritize integrated management systems that combine grafting, biostimulants, and precision irrigation for synergistic effects. The development of salt-tolerant rootstocks through the exploitation of landraces and crop wild relatives should be accelerated, supported by genetic markers for improved compatibility. Standardization of biostimulant formulations and application protocols is essential to maximize their efficacy and revolutionize precision stress mitigation. Additionally, refining deficit irrigation strategies based on crop-specific salt sensitivity and growth stage dynamics will be crucial for sustainable water use in saline environments. Economic and policy interventions are equally vital to facilitate farmer adoption, including cost-reduction strategies for grafted seedlings and incentives for biostimulant use. Long-term soil health must also be addressed through organic amendments and strategic leaching to prevent salt accumulation. By bridging the gap between innovation, agronomic practice, and socio-economic feasibility, the agricultural sector can develop resilient vegetable production systems capable of withstanding increasing salinity pressures while ensuring food and nutritional security in a changing climate.