Current Status and Prospects of Salt Tolerance Mechanisms in Pepper (Capsicum annuum L.)

1. Introduction

Soil salinization is a process of continuous accumulation of soluble salts on the soil surface, resulting in deterioration of its chemical and physical properties. Unsustainable agricultural practices such as inappropriate cultivation and fertilization accelerate soil salinization, thus restraining crop yield and quality [1,2]. Plants suffer from physiological drought under saline conditions due to the high soil osmotic pressure, which impairs water uptake and disrupts normal physiological processes [3]. This sometimes causes tissue senescence and plant death. Moreover, previous studies have reported that excessive soil salinity not only changes microbial community structure and enzyme activities but also decreases SOC content and crop productivity. In serious cases, it can hasten soil desertification [4]. The FAO estimated that about 1.38 billion hectares of land — 10.7% of the world's total area — are salinized. Of the 1.73 billion hectares of arable land worldwide, nearly 25% has been suffering from salt stress. Simultaneously, unsustainable human activities, poor irrigation measures, and irrational fertilizer application are exacerbating the soil salinization further [5].

Pepper (Capsicum annuum L) is a highly valued vegetable crop belonging to the annual or semi-perennial species of the Capsicum genus under the Solanaceae family. It is rich in bioactive compounds, including vitamin C, carotenoids, and capsaicin, responsible for a sharp pungency that gives a special flavor and has several health benefits, such as anti-inflammatory and anti-cancer activity [6]. Due to these properties, pepper is extensively used in pharmaceuticals, condiments, and cosmetics [7]. It is considered one of the most economically and culturally important vegetable crops in the world [8].

However, pepper cultivation is confronting serious challenges due to soil salinization. In many areas, small natural precipitation, a lack of leaching from cultivated soils, and strengthened secondary salinization have resulted in the expansion of saline-alkali land every year. Similar to most vegetable crops, such as beans, carrots, eggplants, and tomatoes, pepper has a relatively low threshold for soil salinity, mostly with an electrical conductivity of the saturated paste extract (ECt) ranging between 1 and 2.5 dS m⁻¹ [9]. Salt stress seriously inhibits its growth, yield, and quality [10]. Therefore, It is very important to breed salt-tolerant pepper varieties to improve productivity in saline-affected areas and ensure the sustainable use of marginal lands.

This review systematically analyzes the current knowledge on salt tolerance mechanisms in pepper and strategies for improving its resistance. It will provide a scientific basis for the advancement of cultivation techniques, screening, and breeding salt-tolerant genotypes, thus promoting the effective use of saline-alkali soils.

2. The Impact of Salt Stress on the Growth and Development of Pepper

Currently, research on salt stress in peppers has primarily focused on the germination and seedling stages, during which phenotypic characteristics and physiological indicators of different pepper varieties undergo varying degrees of change. [11]. Under salt stress, germination rate, germination potential, and germination index all decreased with increasing duration of salt treatment and concentration, while root length and lateral root number correspondingly diminished [12]. Pepper seedlings exhibit heightened sensitivity to salt stress during their early growth stage [13]. On the one hand, this damage manifests as morphological changes, such as reduced leaf size, intensified leaf colouration, yellowing, and wilting of foliage [14]. On the other hand, physiological indicators such as leaf water content and relative electrical conductivity, alongside biochemical indicators like enzyme activity, were significantly reduced. This severely impaired the growth of the peppers, subsequently affecting flowering and fruiting, ultimately leading to a decline in both yield and quality [14]. Research indicates that when seedlings are exposed to 100 mM and 200 mM sodium chloride, their growth and development are significantly inhibited. As salt concentration increases, this inhibitory effect becomes more pronounced [15]. Additionally, under a 200 mM NaCl treatment, the number of flower buds and branches in pepper plants displayed a notable increase, indicating a reduced growth period for these plants [15].

3. Evaluation and Identification of Salt Tolerance Traits of Pepper

The seed germination period is one of the phases that is most susceptible to salt stress throughout the entire growth and development of crops [16]. A great deal of research shows that the salt tolerance of seeds at the germination stage can reflect the salt tolerance that will be displayed at other stages [17]. During the seed germination period, indicators such as the germination rate, germination potential, germination index, and relative salt injury rate are typically measured to evaluate the seed germination status [18,19]. Relevant studies treated pepper seeds with a concentration gradient of 30 mM NaCl, under 30 mM NaCl treatment the germination rate, relative germination rate, relative germination potential, and germination index of 10 of the 23 pepper varieties were increased, while there were no significant decreases in the indexes of 2 pepper seeds compared to the control [20]. With the increase in NaCl concentration, the germination rate, relative germination rate, relative germination potential, and germination index of 16 of the 23 seeds were all decreased under the 180 mM NaCl treatment, while the salt concentrations tolerated by different pepper varieties at the germination stage varied [20]. Moreover, the relative salt injury rate serves as a crucial indicator for assessing germplasm salt tolerance. This study categorised pepper salt tolerance into five grades—1, 3, 5, 7, and 9—based on the relative salt injury rates of different varieties, providing a significant basis for evaluating salt tolerance in pepper germplasm [21].

Research into salt tolerance during the seedling stage holds considerable significance for enhancing both yield and quality in pepper cultivation. The impairments caused by salt stress to pepper seedlings are visually manifested in certain phenotypes, such as leaf shape, leaf color, and the petiole, among others [22]. Certain research has employed nutrient solution hydroponics to investigate the morphological characteristics of pepper varieties subjected to salt stress. After salt stress treatment, the alterations in pepper plants primarily manifested in leaf color (leaf yellowing), leaf morphology (leaf drooping, leaf edge curling, wilting), petiole morphology (petiole browning), leaf abscission, stem morphology, and so forth. Furthermore, with the elongation of the salt treatment time, the phenotypic changes became more pronounced [22].

Additionally, the current certification method that is domestically used for salt tolerance in pepper mainly comprises the detection of physiological and biochemical indicators, such as chlorophyll, malondialdehyde, free proline, soluble sugar, soluble protein content, and related enzyme activities (including peroxidase, superoxide dismutase, etc.), in plant leaves [23,24]. Furthermore, it also involves the detection of the sodium and potassium ion contents in pepper plants. After salt treatment, the sodium ion contents in different parts of pepper plants increase, and the more pronounced the increase, the greater the damage to the plants [25]. Although the aforementioned physiological and biochemical indicators can precisely reflect the effects of salt stress on pepper seedlings, the determination of these indicators also involves certain issues, such as high determination costs and significant destructiveness to pepper seedlings. The Innovation Team of Fujian Agriculture and Forestry University designed and constructed a high-throughput platform capable of simultaneously collecting multi-fluorescence (multicolor-fluorescence, MF) and multi-spectral reflectance (multispectral reflectance, MF) images of plants. Through optical sensors, this platform can effectively provide plant morphological information, as well as biochemical and physiological information [26]. Compared with the traditional index measurement method, this method can solve the problem of the damage caused to pepper seedlings in measurement.

4. Research Advances Concerning the Salt Tolerance Mechanism of Pepper

In research on the salt tolerance of Solanaceae crops, it was discovered that pepper exhibits a lower salt tolerance in comparison to Solanum melongena L. and Solanum lycopersicum L. [34]. Therefore, investigating the mechanisms underlying pepper's response to salt stress holds significant practical importance for developing new pepper germplasm resources and enhancing both yield and quality. Presently, research into pepper's salt tolerance mechanisms by scholars both domestically and internationally has primarily focused on the following aspects.

4.1. Osmotic Regulation Mechanisms

Under salt stress conditions, the concentration of sodium ions (Na⁺) in the soil increases significantly, forming a steep concentration gradient at the soil-root interface [35]. Consequently, chilli plants activate self-regulating osmotic adjustment mechanisms to mitigate this gradient and maintain physiological functions. Changes in cell membrane permeability—typically assessed through indicators such as relative leaf conductance and malondialdehyde content—constitute a pivotal component of this adaptive process [36].

Plant cell membranes contain multiple ion transporters capable of regulating the flux of sodium ions (Na⁺) and potassium ions (K⁺). For instance, components of the salt over-sensitivity (SOS) signaling pathway—including SOS1 and HKT-type transporters—facilitate the efflux of sodium ions from the cytoplasm [37]. Concurrently, NHX-type transporters promote sodium ion influx into vacuoles or other organelles. Under high sodium conditions, plants typically reduce sodium influx via non-selective cation channels (NSCCs) while enhancing sodium efflux and vacuolar compartmentalisation [8]. Salt stress also promotes sodium influx through NSCCs, which activates outwardly rectifying potassium channels and stimulates potassium efflux. The plasma membrane H⁺-ATPase generates a proton motive potential by expelling protons, driving sodium/proton countertransport systems such as SOS1 to achieve sodium efflux [38]. The resulting proton flux further promotes membrane potential repolarisation, inhibits potassium leakage through KOR channels, and facilitates potassium reabsorption (KIRC), collectively maintaining ionic homeostasis [39].

To maintain an optimal Na⁺/K⁺ ratio and support normal metabolism, plants selectively absorb and distribute inorganic ions such as Na⁺, K⁺, Cl^-, and NO^3- [40]. Research indicates that Na⁺ accumulates simultaneously in the roots of both salt-tolerant and sensitive pepper genotypes. For instance, after 14 days of exposure to 70 mM sodium chloride, the salt-tolerant cultivar “A25” exhibited more pronounced root Na⁺ accumulation than the sensitive line – a trait potentially linked to enhanced compartmentalisation within vesicles or vacuoles [41]. This ion accumulation not only aids osmotic regulation but also maintains turgor pressure, thereby enhancing salt tolerance [42,43]. Furthermore, calcium ions (Ca²⁺), as key signaling molecules in plant stress responses, participate in salt stress signal transduction and regulate the expression of stress-related genes [44].

The integrity of the cell membrane system is crucial for maintaining the metabolic activities, physiological functions, and osmotic balance of plant cells and organelles. Under salt stress conditions, membrane integrity is typically assessed through indicators such as leaf relative electrical conductivity (closely related to membrane permeability) and malondialdehyde (MDA) content, with MDA levels reflecting the degree of membrane lipid peroxidation [28]. Studies on salt-stressed pepper plants indicate that leaf relative electrical conductivity typically rises initially before declining, whereas MDA content continues to increase with prolonged stress duration. This pattern suggests that extended salt stress enhances membrane permeability and exacerbates oxidative damage [45].

When subjected to osmotic stress, plants accumulate osmotic regulatory substances such as soluble proteins, soluble sugars, and free proline, thereby mitigating the damage caused by salt stress by reducing intracellular osmotic pressure. The levels of these osmolytes generally exhibit a biphasic response, increasing initially under moderate or short-term stress but declining as stress intensity or duration escalates [46,47]. Moreover, salt tolerance in pepper has been linked to efficient chloride ion exclusion, which reduces Cl⁻ accumulation in leaves. This mechanism helps preserve photosynthetic function by minimizing ion-specific toxicity within photosynthetic tissues.

4.2. Regulatory Mechanisms of the Antioxidant System

Certain substances with potent oxidizing properties are found in plants, including ROS. When plants are exposed to adverse circumstances, the equilibrium of the metabolic system regulating the ROS content within the cell will be disrupted, and the antioxidant system will come into operation [48]. This boosts the actions of different antioxidant enzymes that operate to get rid of the ROS to ensure that the plants are growing and developing normally [49]. Under the influence of xanthine oxidase, ROS will generate superoxide anion radicals, which can oxidize and degrade lipids and proteins. SOD is capable of catalyzing the disproportionation reaction of superoxide anion radicals so as to maintain normal physiological functions in plants [50]. Research indicates that when plants are subjected to salt stress, the SOD activity of peppers with poor salt stress resistance will decline, while the SOD activity of plants with strong resistance to saline–alkali environments will increase to eliminate the superoxide anion radicals generated within the body, inhibit the occurrence of membrane lipid peroxidation, and maintain the stability of the intracellular environment; however, as the intensity of the stress deepens, this activity diminishes [51]. Peroxidase is an antioxidant enzyme that cooperates with SOD to decompose the H₂O₂ produced by the disproportionation reaction of SOD [52]. Studies have demonstrated that the elongation of salt treatment time leads to a gradual increase in POD activity in the leaves of some pepper varieties, somewhat reducing the damage caused by salt to the membrane system of pepper seedlings [53]. In addition, after researchers inoculated the rhizosphere of pepper plants with Magnaporthe oryzae, the activities of peroxidase and ascorbate peroxidase, along with the transcriptional levels of related genes (such as ascorbate peroxidase, peroxidase, etc.), were all significantly upregulated, and this decreased the contents of malondialdehyde and reactive oxygen species in the plants [54]. These findings suggest that when pepper plants are subjected to damage by a salt environment, ion homeostasis is enhanced and the antioxidant defense system is upregulated to mitigate damage [54]. Ethylene response factor (ERF) transcription factors play a critical role in plant responses to salt stress. In chili pepper, for instance, the AP2/ERF-type transcription factor CaERF2 has been shown to enhance salt tolerance by modulating reactive oxygen species (ROS) homeostasis [55]. Under adverse environmental conditions, heat shock transcription factors (HSFs) activate the expression of heat shock proteins (HSPs). These proteins help maintain protein folding homeostasis by binding to misfolded or aggregated proteins, facilitating their restoration to native conformation and thereby minimizing cell damage caused by aggregation. Research indicates that the small heat shock protein CaHsp25.9 significantly enhances plant tolerance to high temperatures, salt stress, and drought stress by reducing reactive oxygen species (ROS) accumulation, enhancing antioxidant enzyme activity, and upregulating the expression of key stress-related genes [56].

4.3. Regulatory Mechanism of Photoreceptors

Light is an indispensable key element in the plant environment, playing a vital role in regulating plant growth and responding to abiotic stresses. Research indicates that factors interacting with phytochromes (PIFs) suppress plant salt tolerance. Under light conditions, phytochrome A (phyA) and phytochrome B (phyB) within plants become activated, thereby enhancing the activity of SOS2 kinase [53]. SOS2 interacts with the photosensitive pigment-interacting factors PIF1 and PIF3, degrading them, thereby eliminating their inhibitory effect on plant salt tolerance. This enhances the plant's stress resistance and adaptation to salt stress [53]. Research has revealed that under salt stress conditions, CaPIF8 exerts a positive effect on enhancing salt tolerance by promoting the expression of key abscisic acid (ABA) biosynthesis genes [57]. In a related study, pepper seedlings at the stage six pieces of true leaves were treated with a 50 mM NaCl concentration gradient (maximum concentration 200 mM), and their photosynthetic rate, fluorescence parameters, and chlorophyll content were measured after treatment. The results show that, with the increase in NaCl concentration and the extension of NaCl treatment time, the net photosynthetic rate and chlorophyll content of pepper leaves decreased, the initial fluorescence Fo increased, and the maximum light energy conversion efficiency Fv/Fm of PSII decreased [58].

Moreover, relevant studies have demonstrated that short-term salt treatment can facilitate the rise in chlorophyll content in pepper leaves, enhance plant photosynthesis, and improve plants’ abilities to resist external salt stress within a short period [8]. Nevertheless, as the treatment time and concentration increase, the chlorophyll content in pepper leaves gradually declines; the degree of decline in varieties with strong salt tolerance is smaller than that in varieties with weak salt tolerance [49].

4.4. Regulatory Mechanism of Signal Transduction

Signal transduction-regulating mechanisms employ many different signals, including ion signaling and hormone signaling. Under salt stress conditions, in addition to sodium and potassium ions, calcium ions act as one of the important salt signals [59]. In plant cells, calcium ions can function as secondary messengers in the transmission of signals in response to a range of abiotic stimuli known to cause adversity [60]. They can be classified into two major types based on whether Ca²⁺ participates in the signal transduction of plant salt stress. First, there is the Ca²⁺-dependent signal transduction pathway, which is further classified into two categories: Type I and Type II. The Type I Ca²⁺-dependent signal transduction pathway mainly comprises the Salt Overly Sensitive (SOS) signal transduction pathway and the Calcium-dependent Protein Kinase (CDPK) cascade reaction pathway [61]. The Type II Ca²⁺-dependent signal transduction pathway mainly encompasses the abscisic acid (ABA) signal pathway and the phospholipid signal pathway. The second type of signal involves the MAPK cascade reaction pathway, namely, the Ca²⁺-independent signal transduction pathway [62].

4.4.1. Ca²⁺-Dependent Signaling Pathways

The SOS signal pathway is a representative mechanism that plants have evolved in adaptation to salt stress. SOS1 controls the Na+/H+ exchange activity by encompassing a broad cytoplasmic domain (CPD), which contrasts it with conventional sodium/proton exchangers [63]. The research provided a molecular basis to explain the mechanism of SOS1-regulated transporter activity as well as the cultivation of salt resistance in plants by studying the structural and functional activities of SOS1 in plants [63]. One of the six main phytohormones, abscisic acid (ABA), is essential to how plants react to harmful external stressors. In response to external adverse stress, regulatory elements that promote ABA synthesis express themselves at higher levels in plants, whereas those that promote ABA breakdown express themselves at lower levels[8]. Meanwhile, the stress activation of ABA biosynthesis genes and the expression genes that feedback and stimulate ABA biosynthesis are all mediated by Ca²⁺-dependent phosphoprotein cascade reactions [64]. Studies have demonstrated that in under NaCl stress, potassium ions efflux, leading to the closure of the stomata of plant leaves and a decrease in the photosynthetic rate, and at the same time, the ABA content is reduced and the IAA content is increased. The downstream transmission of IAA signals enables guard cells to absorb potassium ions [65]. Thus, under conditions of salt stress, potassium ions interact with calcium signals, IAA, and ABA hormone signals and promote stomata opening. Additionally, potassium ions can control antioxidant enzymes to get rid of the excess ROS introduced by salt stress, which controls the salt tolerance of plants [66].

4.4.2. Mitogen-Activated Protein Kinase (MAPK) Cascade Pathway

The MAPK signaling pathway is essential to pepper growth, and its effects are mostly seen in the contexts of growth and development, as well as in the plant's response to stress [67]. Recent studies have identified specific components of the MAPK signaling cascade (e.g., CaAIMK1) in pepper that can interact with other proteins to synergistically regulate osmotic stress responses [68,69]. In addition, the MAPK kinase CaDIMK1 has been shown to play a role in the signaling pathway in pepper [70]. While the specific downstream targets of MAPKs in pepper remain unknown, the control of the MAPK cascade signaling network is mediated by specific transcription factors, such as WRKYa [71]. Relevant genome-wide studies have found that raf-like proteins involved in the MAPK cascade play important roles in a variety of signaling pathways [72], and the specificity of the MAPK cascade signaling pathway in peppers also plays an important role in regulating the defense responses of peppers to pathogens [73,74]. In addition, the activation of MAPK cascade responses, such as CaADIK1, is also required to facilitate the ethylene signaling pathway in fruit development [75]. The MAPK cascade pathway of pepper is a complex signaling network, and its defense-related genes and pathways have been revealed by certain analytical methods and techniques (such as transcriptomic analysis) to play a crucial role in the growth, development and stress response of pepper.

5. Research Advances in Salt Tolerance Genes Associated with Pepper

Currently, research concerning the molecular mechanisms of salt tolerance associated with pepper predominantly centers on the identification and functional investigation of salt tolerance genes in pepper. Researchers have identified certain genes and proteins associated with the salt tolerance of pepper via techniques such as transcriptomics, proteomics, and genomics.

The development of salt tolerance in pepper is closely linked to the expression of specific functional genes. Through genome-wide identification and bioinformatics analysis of the SBP-box gene family in pepper, combined with virus-induced gene silencing (VIGS) experiments, researchers have preliminarily revealed that two differentially expressed genes, CaSBP11 and CaSBP12, function as positive regulators under salt stress [76].

In another study based on the whole-genome data of the pepper cultivar ‘Zunla-1’, the expression patterns of NAC family genes were analyzed in leaves of a salt-tolerant inbred line ‘H1023’ and a salt-sensitive line ‘XWHJ-M’ under varying durations of stress treatment. The results suggest that NAC genes may regulate multiple physiological processes in pepper, including hormone signaling and abiotic stress responses. Quantitative real-time PCR analysis demonstrated that CaNAC61 responds to all six abiotic stresses tested, indicating a potential role in pepper adaptation to salt stress [77].

Further characterisation of the pepper B-box (BBX) transcription factor family revealed that under salt stress conditions, the expression of CaBBX4, CaBBX5, CaBBX7, and CaBBX10 was significantly upregulated. Conversely, CaBBX1, CaBBX2, CaBBX6, and CaBBX9 exhibited fluctuating expression patterns throughout the time course of treatment [78].

The bHLH transcription factor family plays a crucial role in regulating plant responses to abiotic environmental stresses [79]. Studies indicate that transient expression of CabHLH035 enhances pepper tolerance to external salt stress, whereas reduced expression diminishes salt resistance. Furthermore, CabHLH035 binds to the promoter regions of CaSOS1 and CaP5CS genes, directly and significantly activating their expression. Collectively, CabHLH035 exerts a pivotal role in the pepper's response to external salt stress by regulating the intracellular sodium-potassium ion ratio and proline synthesis [79]. Photochromin interaction factors (PIFs) in the HLH family of transcription factors are negative regulators of photomorphogenesis. By silencing CaPIF4 in peppers, it was found that the dynamic balance between ROS formation and the clearance ability of the antioxidant defense system was disrupted, thereby enhancing salt tolerance. In addition, after the silencing of CaPIF8, the expressions of the CBF1 and ABA biosynthesis genes were inhibited, and the resistance of pepper plants to salt stress was reduced [57,80].

Research has shown that heterologous overexpression of the pepper transcription factor CaPF1 (Pathogen and Cold Tolerance-related Protein 1) significantly improves tolerance to drought, freezing, and salt stress in Pinus densiflora [81]. Another transcription factor, RAV1, originally identified from Xanthomonas-infected leaves, has been demonstrated to play an essential role in resistance to bacterial infection, drought, and salt stress [82]. Similarly, the pepper transcription factor CabZIP1 acts as a core regulatory element, enhancing plant resistance to pathogen invasion and environmental stress [83]. Subsequent studies revealed a physical interaction between the redox enzyme CaOXR1 and CaRAV1, which is crucial for pepper adaptation to salt and osmotic stress [84]. Furthermore, the capsicum RING-type E3 ubiquitin ligase CaFIRF1 (Capsicum annuum FAF1-interacting RING-finger protein 1) has been demonstrated to interact with CaFAF1 and mediate its ubiquitination, thereby promoting degradation of this protein. Under high salt stress conditions, plants with the CaFIRF1 gene silenced exhibited enhanced salt tolerance [85]. Parallel in-gel kinase assays revealed that CaSnRK2.6 phosphorylates CaSAP14 under ABA, dehydration, and high-salinity stress, confirming CaSAP14 as a direct substrate of CaSnRK2.6 and a positive regulator of osmotic stress adaptation [86]. Collectively, the identification and functional analysis of these genes deepen our understanding of the molecular basis underlying salt tolerance in pepper.

Pepper plants display a considerable increase in the expressions of several genes involved in the SOS pathway and the scavenging of reactive oxygen species (ROS) under conditions of external salt stress. In addition, after salt treatment, the roots of varieties salt tolerance expressed more ROS clearance and SOS pathway genes than the roots of salt-sensitive varieties [87]. After silencing the CaAnn9 gene through VIGS and subjecting pepper seedlings to salt stress treatment, it was discovered that the silenced seedlings were more susceptible to salt, with the accumulation of ROS being one of the biochemical indicators of this [88]. Genes in the CUL family have been identified in pepper. After treating pepper seedlings with NaCl, it was found that CUL genes are essential to plant responses to external salt stress since it was discovered that most of them show altered expression levels in both roots and leaves, albeit to different degrees [89]. Studies have shown that CaCP15 functions as a negative regulator of salt and osmotic stress tolerance in pepper, likely through modulating reactive oxygen species (ROS) scavenging processes [90]. Meanwhile, MADS-box transcription factors, known for their crucial roles in plant growth and development, also contribute to stress adaptation. Among them, CaMADS has been identified as a positive regulator in response to cold, salt, and osmotic stress [91]. In addition, CaLEA1 enhances drought and salinity tolerance via an ABA-mediated cellular signaling pathway [92].

Besides the aforementioned, there are also transcription factors, such as MYB, BBX, AP2/ERF, that may play substantial roles in peppers' responses to external salt stress [93]. Furthermore, other genes have also been found, such as CaFtsH06, PepRSH, CaXTH3, CaBZ1, and related ones [94,95]. Some proteins, such as CaHsp25.9, act as vesicles for the formation-related protein CaSec16 [96,97]. Some studies have also found that certain microRNAs (miRNAs) play vital roles in the salt tolerance of peppers.

Overall, certain advancements have been achieved in the ongoing research regarding the molecular mechanisms of salt tolerance in peppers. Nevertheless, more in-depth investigations are still required to reveal the molecular mechanisms of salt tolerance in peppers and to furnish a theoretical foundation for cultivating pepper varieties with salt tolerance.

6. Research Advances in the development of Cultivation Techniques for Pepper Resistant to Saline–Alkali Soil

The research on techniques for cultivating peppers with salt tolerance has been progressing gradually, and has mainly focused on the selection and breeding of varieties with salt tolerance, the utilization of relevant substances with salt tolerance, and the implementation of these practices in cultivation management. Researchers, via large-scale screenings and genetic enhancement, have sought to develop pepper varieties with higher salt–alkali tolerances. These varieties possess a relatively elevated salt–alkali tolerance capacity, and are capable of growing and producing a relatively high yield in a high salt–alkali environment. A wide range of methods and approaches have been introduced to improve salt tolerance in crops, such as high-throughput phenotypic screening methods, molecular genetics, genetic engineering, and different types of “genomics” (genomics, transcriptomics, proteomics, microbiomics, etc.). However, transferring the desired traits via the cultivation of crops remains challenging [112].

6.1. The Application of Relevant Salt Tolerance-Regulatory Substances

Researchers have explored the physiological response mechanisms of pepper under salt stress in order to explore the methods employed in regulating the internal ion balance and water content of plants. For example, studies have shown that certain pepper varieties salt tolerance can accumulate higher amounts of organic solutes, such as proline and soluble sugars, under salt stress, allowing them to maintain the intracellular osmotic balance [36]. The research has sought to determine the tolerance of pepper to salt stress via the application of salty irrigation water. Application of gibberellin via foliar spray can alleviate the adverse effects of salt stress on pepper growth parameters and yield [113]. Soaking the pepper germplasm in gamma-aminobutyric acid (GABA) could improve the salt tolerance of peppers at the seedling stage [114]. Spraying the leaf with salicylic acid and 5-aminolevulinic acid (ALA) is conducive to reducing the harmful effects of salt stress on pepper germplasms or seedlings, thereby resolving the problems related to the interference of endogenous hormones and the lack of available nutrients under salt stress, and generally enhancing the adaptability of pepper to these conditions [115,116,117]. Exogenous application of chlorophyll lactones (SLs) mitigates salt-induced accumulation of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), whilst alleviating the detrimental effects of salinity on chlorophyll content and photosynthetic performance [118]. Beyond plant hormone regulation, photoregulation—particularly red light LED pretreatment—has been demonstrated to enhance salt tolerance by improving water and carbon dioxide transport capacity, maintaining stomatal function, and strengthening antioxidant systems [119]. These findings provide practical strategies for sustainable crop management in environments prone to salinisation.

6.2. Application of the Grafting Techniques

Traditional breeding methods typically involve selecting superior varieties through natural variation or artificial mutagenesis, followed by techniques such as hybridisation to cultivate disease-resistant cultivars. Conversely, grafting technology can also effectively enhance plant stress tolerance. Research indicates that grafting pepper seedlings onto rootstocks with strong salt tolerance and normal photosynthetic function leverages the rootstock's extensive root system to ensure normal growth of both scion and rootstock, thereby increasing commercial yields [111]. The application of salt-tolerant rootstocks significantly increases pepper yields in saline-alkali soils by maintaining photosynthetic efficiency and nutrient uptake capacity. This advantage stems from elevated proline levels, which effectively stabilise enzyme activity and counteract salt-induced damage [136].

6.3. Methods for Soil Improvement

Implementing appropriate cultivation management practices is crucial for developing salt-tolerant chilli varieties. Soil improvement measures, such as organic matter and gypsum, can enhance the structure and properties of saline-alkali soils, thereby mitigating the adverse effects of salt stress on chilli growth. Furthermore, scientifically planned irrigation and drainage management, coupled with judicious selection of crop rotation and intercropping systems, can both reduce the impact of salt stress on chillies and improve soil workability [137,138]. These research findings provide a robust theoretical and practical foundation for developing salt-tolerant pepper varieties and harnessing the potential of saline-alkali soils.

7. Concluding Overview

Pepper (Capsicum annuum L.), as a crop of significant economic value, is widely cultivated and consumed globally. However, soil salinisation, triggered by climate change and the expansion of human activities, poses an increasingly severe threat to their cultivation and yield. Consequently, investigating the salt tolerance mechanisms of chilli peppers and developing strategies to enhance their tolerance and productivity in saline-alkali environments has become a critical research focus. Current pepper salt tolerance research faces three major challenges: Firstly, the absence of a systematic research framework has left the molecular regulatory networks governing salt stress responses poorly defined. Existing studies predominantly focus on individual genes or proteins, failing to establish comprehensive models. Secondly, methods for assessing salt tolerance across different varieties are inefficient. Current screening primarily relies on physiological and biochemical indicators, which are not only time-consuming and labour-intensive but also susceptible to environmental interference, operationally complex, and potentially damaging to seedlings. Thirdly, there is a scarcity of salt-tolerant varieties adapted to diverse agricultural environments. Although multiple salt-tolerance-associated genes have been identified, their application in breeding practices remains inadequate. Moving forward, it is imperative to systematically screen pepper germplasm resources for genotypes exhibiting salt stress tolerance, thereby enhancing yield and quality while ensuring robust plant growth in saline-alkali environments.

8. Future Perspectives

To unravel the mechanisms underlying salt tolerance in peppers, future research must advance in three key directions. Firstly, multi-omics technologies—including genomics, transcriptomics, and metabolomics—should be employed to systematically analyse gene expression changes in peppers under salt stress. It will help identify key salt-tolerant genes and signaling pathways, providing a comprehensive understanding of the associated molecular networks and regulatory mechanisms. Secondly, an efficient evaluation system for salt-tolerant cultivars should be established, enhancing testing efficiency while minimising damage to plant material. Finally, genetic improvement research is paramount. Through techniques such as genetic engineering, marker-assisted selection, and gene editing, we can not only develop pepper varieties with enhanced salt tolerance but also improve the salt resistance of existing cultivars while achieving overall yield increases.

In conclusion, current research on salt tolerance in peppers has established a complex and significant framework. Future studies should adopt a multi-faceted approach, aiming both to elucidate the molecular mechanisms underlying salt tolerance and to lay the theoretical groundwork for developing salt-tolerant pepper cultivars.