Adaptive Mechanisms and Regulatory Strategies of Plants Under Saline Stress and Prospects for the Development and Utilization of Chinese Herbal Medicines in Saline Land

Abstract

Soil salinization has seriously restricted the growth of crops and the sustainable use of land resources. The exploitation and utilization of saline land has become an urgent problem of agricultural development and environmental management. Medicinal plants have “stress effect”, and some adversity stresses often become positive regulators of their quality, which provides new ideas for the development and utilization of saline land. Based on it, this review summarizes the adaptive mechanism of plants under saline stress, including the construction of plant phenotypic characteristics, osmotic regulation, ion homeostasis, and hormone regulation. We also outline management strategies for saline land, primarily encompassing physical, chemical, biological, and comprehensive improvements. We further discuss the prospects for the development and utilization of Chinese herbal medicines in saline land based on the resources of salt-tolerant medicinal plants and the effects of saline stress on the quality of Chinese herbal medicines, with a view to providing references for the improvement and utilization of saline land, as well as the solution of the dilemma of medicinal plants competing for land with grains.

1. Introduction

Saline soil is a land type that is widely distributed globally and poses a serious threat to the normal growth of plants due to its high concentration of salt and alkaline components [1]. According to incomplete statistics from FAO, the global saline soil area amounts to 1 billion hm², accounting for about 11% of the total land area [2]. China is a country with a wide distribution of saline soils, including North China, Northeast China, Northwest China inland and some coastal plains, with a variety of saline soil types. According to the results of the Third National Land Survey, the total area of saline soils in China is about 115 million hm², ranking third in the world. As a special type of soil, saline soil has a significant inhibitory effect on plant growth due to its high concentration of salts. High salinity leads to a reduction in soil osmotic potential, difficulty in water absorption by plants, reduction in photosynthesis, accumulation of reactive oxygen species (ROS), and physiological drought [3]. At the same time, excessive salinity also causes ionic toxicity, damages cell membranes, disrupts ionic balance as well as hormonal balance in plant cells, causes shortage of trace and massive elements, leads to nucleic acids and proteins as well as many metabolic disorders within the plant, and ultimately affects the normal growth and development of the plant [4,5]. Meanwhile, salinity and alkalinity in saline soils can damage the soil structure, making the soil compacted and poorly aerated, and affecting root growth and microbial activities. This not only restricts the absorption and utilization of nutrients by plants, but also reduces the decomposition and transformation of soil organic matter, further reducing soil fertility [2]. The hazards of saline soils are multifaceted, not only affecting plant growth and soil fertility, but also causing serious impacts on the ecological environment and agricultural production.

The adaptive responses of plants are primarily associated with persistent changes in morphology and physiological chemistry. In saline soils, plant adaptation mechanisms represent a complex and significant research domain, encompassing multiple aspects including plant physiology (phenotypic trait development, osmotic regulation) and molecular perspectives (ion homeostasis and hormone regulation). During the long-term evolutionary adaptation of plants in saline soils, they counteract these adverse conditions by regulating their physiological metabolism and the accumulation of secondary metabolites. Elucidating the adaptive mechanisms to saline stress is particularly important for the breeding of saline plant varieties and the development of ecological cultivation techniques.Saline land is a strategic reserve of national land resources. Improving and utilizing this land is an important way to alleviate pressure on arable land and is key to achieving agricultural ecological development and efficiency [6]. China has abundant medicinal plant resources, and some species have a high tolerance to salinity and alkalinity. Moderate stress from these conditions can improve the quality of medicinal herbs. Therefore, clarifying the adaptive mechanisms of plants in saline environments, screening medicinal plants that are suitable for growth in such environments and supplementing them with corresponding improvement measures can effectively alleviate the shortage of land for medicinal plants and promote the development of the traditional Chinese medicine industry, while preventing the cultivation of arable land for non-food purposes. Given the urgency of saline land improvement and the importance of traditional Chinese medicinal materials in the development and utilization of saline land, this review aims to comprehensively summarize the adaptive mechanisms and regulatory strategies of plants under saline stress. It also aims to explore the prospects for the development and utilization of traditional Chinese medicinal materials in saline land, reveal the physiological and molecular mechanisms of plant adaptation to saline environments, and analyze potential mechanisms. Concurrently, regulatory strategies have been proposed to provide a reference for the sustainable development and utilization of traditional Chinese medicinal materials in saline land. These strategies are based on current saline land improvement technologies.

2. Adaptive Mechanisms of Plants to Saline Stress

2.1. The Construction of Plant Phenotypic Characteristics

When subjected to saline stress, plants exhibit a series of adaptive mechanisms, including increased root-to-shoot ratio, reduced transpiration rates, and the formation of specific organs. The root system is the part of the plant that comes into direct contact with saline soil. In response to saline stress, plants have evolved to develop deeper root systems, enabling them to access water and nutrients. This adaptation helps them to withstand the water stress and ion toxicity caused by saline soil. However, different plant species exhibit varying levels of salt tolerance and employ distinct salt-tolerance mechanisms. At concentrations of NaCl below 300 mmol/L, a growth-promoting effect on Glycyrrhiza roots has been observed [7]. Conversely, 2 g/kg and 4 g/kg NaCl have been shown to inhibit Taraxacum officinale growth [8]. In response to salt stress, plants undergo changes in their aboveground parts, evolving specific morphologies that serve to reduce water evaporation and enhance water use efficiency within the plant. These changes include the development of smaller leaves and increased leaf thickness. In response to saline stress, plants undergo changes in their morphological structure, evolving specific organs for the storage or excretion of salts, thereby reducing saline toxicity. For instance, certain plants may develop waxy or hairy leaf structures, which provide additional space for water and store excess Na⁺, thereby reducing water loss and enhancing tolerance to saline stress. This is imperative for sustaining ionic equilibrium within cells [9]. Furthermore, salt-tolerant medicinal plants have evolved unique structures, such as salt glands, salt vesicles, and salt pores, to expel excess soluble ions from their bodies, thereby reducing the accumulation of Na⁺ and Cl⁻ ions. Ericaceous plants, such as Limonium bicolor and Tamarix, are archetypal examples of such vegetation [10]. In summary, plants frequently modify their phenotypic characteristics in saline soils. This adaptive mechanism enables plants to survive and reproduce under adverse conditions, ensuring the continuation of their populations.

2.2. Osmotic Regulation

Saline soils typically contain high concentrations of sodium salts, such as NaCl, Na₂SO₄, and Na₂CO₃. When these salts enter plant tissues, they increase osmotic pressure, and cause the plants to lose water and experience osmotic stress [11]. Osmotic stress disrupts photosynthetic pigments and disturbs the balance of membrane pH and potential, severely impairing photosynthesis. Additionally, oxygen within the plant is converted into ROS. Excessive ROS can cause oxidative damage to macromolecules, including lipids, nucleic acids, proteins, and carbohydrates. Ultimately, this leads to the gradual death of plant cells [12]. Plants have currently evolved unique strategies to cope with osmotic stress.

Osmotic regulation is the primary mechanism by which plants defend themselves against osmotic stress. To counteract excessive water loss, plants often close their stomata, which reduces carbon dioxide fixation [13]. Additionally, plants synthesize small molecules, such as mannitol and proline, to increase intracellular osmotic pressure and maintain cellular water balance. Under salt stress, the content of soluble sugars and proteins in the leaves of Salvia miltiorrhiza seedlings was significantly higher in the 100 mmol/L NaCl treatment group than in the control group, indicating that Salvia miltiorrhiza seedlings can alleviate salt stress by increasing the synthesis of osmoregulatory substances [14]. The soluble sugar and proline content in Limonium sinense and Medicago sativa were higher than in the control group under salt stress conditions [15,16]. P5CS is a key rate-limiting enzyme in the proline synthesis process in plants. When plants are subjected to salt stress, the transcription level of the P5CS gene is significantly increased, leading to higher P5CS enzyme activity and promoting proline synthesis. Under salt stress, the P5CS gene in various plants such as Lycium barbarum exhibits significant induced expression [17,18,19,20,21]. Other osmotic regulation genes, such as CMO and BADH, are key genes in betaine biosynthesis, while M1PDH and PhMT1D are key genes controlling mannitol synthesis. Studies have shown that the overexpression of these genes can enhance the salt tolerance of transgenic plants [22,23]. ROS are also important signaling molecules in response to osmotic stress, and maintaining ROS levels within a specific range is crucial for normal plant growth [24]. Plants utilize an effective antioxidant enzyme system to eliminate excess ROS. Superoxide dismutase (SOD) catalyzes the conversion of superoxide radicals into H₂O₂, which enzymes such as catalase (CAT), ascorbate peroxidase (APX), and peroxidase (POD) then decompose into harmless water and oxygen [25]. Exogenous application of salicylic acid attenuated the depressive effects caused by salt stress by reinforcing the antioxidant system and the synthesis of osmoprotectants such as glycine betaine, total soluble sugars, proline, and steviol glycosides (stevioside and rebaudioside A). Moreover, salicylic acid countered the decline in K, P, and Ca content induced by salt stress [26]. In Glechoma longituba and Nitraria tangutorum, salt stress increased the activity of SOD, CAT, APX, and POD; these enzymes exhibited a synergistic relationship during ROS scavenging [12,27].

2.3. Ion Homeostasis

In healthy soil, the ions near plant roots exist in a state of dynamic equilibrium. This equilibrium facilitates the exchange of ions and water between the roots and the surrounding soil, ensuring normal growth and development. However, in saline soil, the excessive presence of exchangeable Na⁺ ions disrupts this equilibrium, imposing ion stress on plants [28]. High Na⁺ concentrations reduce the water potential of the surrounding environment, leading to plant dehydration. This water loss causes stomata to close, reducing CO₂ fixation and directly inhibiting photosynthesis. Additionally, Na⁺ competes with essential nutrients, such as K⁺, Ca²⁺, and Mg²⁺, for absorption, which leads to reduced chlorophyll content. Elevated Na⁺ levels decrease the activity of various ion-dependent enzymes. These Na⁺-induced ion stresses are important causes of reduced plant biomass [29,30,31].

In order to cope with ion stress, plants have developed various mechanisms in response to salt stress (Figure 1) [32]. Research indicates that plants have evolved complex and effective methods to maintain appropriate levels of Na⁺, a process involving multiple carriers, channel proteins, cotransporters, and antiporters. The root epidermis primarily uses glutamate receptor-like (GLR) channels, cyclic nucleotide-gated (CNGC) non-selective cation channels, HKT2 high-affinity K⁺ transporters, and non-selective cation channels (NSCCs) to transport Na⁺ ions across the plasma membrane into cells [33,34]. Additionally, Na⁺ and K⁺ competitively enter the cell via the HKT2 and HAK5 cotransporters [35]. After Na⁺ enters root epidermal cells, a cascade of Ca²⁺, ROS, and hormone signals is activated. Salt stress initially causes a transient increase in Ca²⁺ concentration, which triggers salt tolerance responses and detoxification mechanisms [36]. Two main pathways are involved in reducing Na⁺ in the cytoplasm. The first pathway involves the activation of the SOS3-SOS2 protein complex by elevated intracellular Ca²⁺ concentrations [37]. The activated SOS2 then phosphorylates SOS1, thereby enhancing its transport activity. SOS1 is an Na⁺/H⁺ antiporter that facilitates Na⁺ efflux across the plasma membrane. This accelerates Na⁺ transport from the cytoplasm to extracellular vesicles, thereby maintaining the intracellular Na⁺ balance [38]. The second pathway involves activated SOS2 regulating the activity of ATPase, NHX, and V-H⁺-ATPase, which guide Na⁺ into the vacuole in a chelated form, thereby reducing the cytoplasmic Na⁺ concentration [39]. Slow (SV) and fast (FV) activated ion channels on the vacuole membrane influence Na⁺ chelation and may cause Na⁺ to leak back into the cytoplasm [40]. NSCCs promote the passive entry of Na⁺ into the xylem, while the symporters SOS1, CCC, and HKT2 actively load Na⁺ into the xylem. Additionally, Na⁺ can also be returned to the xylem parenchyma via HKT1. Under stress conditions, Ca²⁺ can activate the CBL-CIPK and CDPK signaling networks in response to salt stress [41,42]. In summary, plants activate cellular detoxification mechanisms through transport pathways, such as HKT, NHX, and SOS-Na⁺, and coordinate responses to salt stress through protein-mediated signaling networks and osmotic regulation.

2.4. Hormone Regulation

Plant hormone signal transduction is an important pathway for plants to respond to salt stress. The main plant hormones involved in plant stress responses under salt stress include abscisic acid (ABA), auxin (IAA), gibberellin (GA), brassinosteroids (BRs), etc. There are also complex interactions between different plant hormones, which regulate plants’ adaptation to salt stress in a positive or negative manner [44].

2.4.1. ABA

ABA, one of the most important stress response hormones, plays an irreplaceable role in salt stress defense [45,46,47]. Salt stress induces osmotic stress and water deficiency, stimulating plants to synthesize more ABA, activating SnRK2 kinases, and subsequently upregulating the expression of related genes to promote stomatal closure [48,49,50]. SnRK2s in the ABA signaling pathway also regulate osmotic balance by modulating the degradation of starch induced by BAM1 and AMY3, which converts starch into sugars and sugar precursors [51]. Additionally, ABA promotes the accumulation of K⁺, Ca²⁺, proline, and sugars in root vacuoles, ultimately inhibiting the uptake of Na⁺ and Cl⁻ by plants [52]. As the primary mediator of plant stress responses, ABA also participates in regulating Na⁺ and K⁺ absorption and transport by binding to the second messenger Ca²⁺, promoting K⁺ influx, and reducing the Na⁺/K⁺ ratio, thereby counteracting Na⁺-induced toxic effects [53].

2.4.2. IAA

Plant root systems exhibit growth plasticity in response to saline stress, and auxin plays a crucial role in this process [54]. Under salt stress, auxin accumulation in roots decreases, inhibiting root growth. Further studies have shown that salt stress alters the localization of auxin transporters AUX1 and PIN1/2, indicating that salt stress modifies auxin polar transport and the expression of auxin signaling genes. The reduced auxin accumulation in roots may be related to auxin polar transport [55]. Knocking out the SlMTC gene in tomatoes under salt stress affects the expression of auxin-responsive genes such as ARF3, IAA3, and LAX3, resulting in a significant reduction in the number of lateral roots in seedlings. Additionally, the mutants exhibit lower sensitivity to IAA and reduced resistance to IAA inhibitors compared to wild-type plants, indicating that the SlMTC gene is essential for the auxin response pathway [56]. Salt-tolerant, auxin-producing bacteria isolated from halophytes have also been shown to enhance root growth and nutrient absorption capacity in plants under salt stress [5].

2.4.3. GA

GA levels decrease under saline stress, thereby affecting growth processes such as root elongation and leaf expansion. However, exogenous application of GA can alleviate growth inhibition by promoting cell proliferation and elongation [57]. GA increases water use efficiency and the accumulation of osmotic regulatory substances such as proline and glycine in plants under salt stress, thereby helping to protect cell structure and macromolecules from oxidative damage caused by salt stress [58]. GA enhances antioxidant defense by influencing the expression of salt tolerance-related genes, thereby mitigating the harmful effects of ROS generated under salt stress. Under salt stress, GA regulates the expression of DELLA proteins, increasing their activity. DELLA proteins play a role in stress tolerance by controlling growth and enhancing ROS scavenging capacity, thereby improving plant survival rates [59].

2.4.4. BRs

BRs are positive regulators of plant salt tolerance. BR treatment significantly improves the germination of alfalfa seeds and the growth of alfalfa seedlings under salt stress [60]. In camphor trees, BRs significantly increase the relative water content of leaves, slow down the rapid decrease in relative water content with increasing sodium chloride concentration, enhance photosynthetic activity under salt stress, promote photosynthesis, alleviate the inhibitory effects of high-salt solutions on the photosynthetic system, and reduce the inhibitory effects of high salt concentrations on camphor trees [61]. In tomatoes and cucumbers, exogenous BR treatment induces ethylene production and alternase (AOX) activity, with these two molecules positively regulating each other under salt stress to enhance antioxidant enzyme activity [62]. In maize, BRs induce Ca²⁺ accumulation and the expression of the ZmCCaMK gene to enhance cellular antioxidant defense [63]. BR-induced nitric oxide (NO) accumulation also alleviates oxidative damage caused by salt stress [64].

2.4.5. Other Hormones

The application of the exogenous slinolide (SL) analog GR24 promoted plant growth under saline stress conditions while enhancing photosynthetic characteristics and antioxidant enzyme activity [65]. Jasmonic acid (JA), as a stress-related hormone, has also been reported to participate in antioxidant defense. Under salt stress, JA levels increase, activating the JA signaling pathway. The key inhibitory factor JAZ8 in the JA signaling pathway is degraded via the COI1 receptor-mediated 26S proteasome pathway, releasing the NF-YA1-YB2-YC9 complex to activate the expression of salt stress response genes such as MYB75, thereby alleviating salt toxicity by maintaining ROS or ion homeostasis [66,67].

2.4.6. Hormone Synergistic Regulation

Plant hormones exhibit complex interactions in mediating plant resistance to salt stress. As core plant hormones mediating plant adaptation to stress, ABA and BR signaling interact to regulate plant salt responses. Studies have shown that under certain conditions, BRs antagonize ABA in plant salt responses [68]. GA, produced by Bacillus species, increases endogenous JA and SA levels while reducing ABA synthesis. This improves plant salt stress regulation [69]. JA treatment increases endogenous ABA content, offsetting the reduction in GA induced by NaCl in rice and anthracnose-infected plants, which indicates that other plant hormones are involved in JA-mediated salt responses [70]. Salt-induced SLs mediate the upregulation of the ABA biosynthesis gene LsNCED2, thereby increasing ABA content in the root systems of mycorrhizal lettuce [71]. Salt stress abnormally promotes SL synthesis in mycorrhizal plants, suggesting that arbuscular mycorrhizal (AM) fungi contribute to SL production. SA positively affects AM symbiosis and nitrogen fixation in saline environments, suggesting that SL and SA may play positive roles in plant resistance to salt stress [72]. Previous studies have shown that SA promotes growth in wheat by inhibiting the decline of IAA and CK, while maintaining high ABA levels to enhance plant salt tolerance [73]. Under salt stress, SA treatment was found to downregulate the expression of ABA biosynthesis genes and upregulate the expression of GA biosynthesis genes in Limonium bicolor seeds, indicating that SA promotes seed germination by maintaining a favorable GA/ABA balance under salt stress [74]. In Arabidopsis, treating transgenic plants that overexpress SlWRKY23 with TIBA (an auxin transport inhibitor) or AgNO₃ (an ethylene perception inhibitor) inhibited lateral root growth and resistance to salt stress. This suggests that SlWRKY23 confers stress resistance to Arabidopsis by interacting with the auxin and ethylene pathways [75].

In summary, different plant hormones play distinct roles in mediating responses to salt stress. The process of plant resistance to salt stress involves a complex network of genes, with different hormones interacting synergistically to jointly defend against salt stress.

3. Strategies for Regulating Saline Land

Saline soil is a special type of soil that severely restricts crop growth and the sustainable use of land resources due to its high salt and alkali content. To effectively utilize these land resources and improve plant production potential, it is often necessary to implement improvement measures, including physical, chemical, biological, and comprehensive improvements.

3.1. Physical Improvement

Physical improvement refers to the use of physical methods such as sand covering, land leveling and deep plowing, irrigation and drainage to improve saline land. Sand covering involves covering the surface layer of saline land with a layer of clean sand to reduce soil capillary action and inhibit the rise of salt to the surface with water evaporation [76]. This method can effectively reduce the salt content of the topsoil, improve soil structure, and increase soil permeability and water retention capacity [77]. Studies have shown that sand covering effectively reduces the pH and total salt content of saline land compared with no sand covering. It has a more prominent inhibitory effect on salt ions (Na⁺, K⁺, SO₄²⁻, and Cl⁻) and significantly increases soil moisture content during the Lycium barbarum growing season. Additionally, different sand-covering treatments significantly promote the growth and development of Lycium barbarum. The 5 cm coarse sand-covering treatment shows the most significant overall salt suppression and yield enhancement effects [78]. Land leveling and deep plowing can break up soil compaction, improve soil structure, enhance drainage, and facilitate the leaching and removal of salts [79]. Deep plowing can also promote root growth and enhance a plant’s ability to absorb soil nutrients, thereby improving stress tolerance [2]. An effective irrigation and drainage system is also essential for improving saline land. Using water-saving irrigation technologies, such as drip and sprinkler irrigation, reduces water evaporation and avoids salt accumulation caused by rising groundwater levels [80]. Establishing a drainage system to remove excess moisture and salt from the soil in a timely manner can also help reduce soil salinity and improve the soil environment [81].

3.2. Chemical Modification

Chemical improvement involves adding agents to saline soil to improve the land. These agents include both inorganic and organic materials. Inorganic and organic improvement agents that have been extensively researched and proven to have great application potential include Ca²⁺ compounds (e.g., CaCl₂, gypsum) and biochar. Substances such as CaCl₂ and gypsum (e.g., desulfurization gypsum) contain high levels of Ca²⁺. Through ion exchange, Ca²⁺ can replace Na⁺ in the soil, enhancing Ca²⁺ signal transduction within plants. This reduces soil alkalinity and alleviates salt stress in plants [82]. Exogenous CaCl₂ maintains ROS homeostasis, enhancing the activity of antioxidant enzymes and the content of antioxidant substances in Nitraria sibirica leaves under salt stress [83]. Gypsum is composed of fine particles with low water content and contains mineral nutrients such as Ca and S that are beneficial to plant growth. When applied to saline land, it can not only reduce soil pH and sodium adsorption ratio, but also improve soil structure stability and water retention capacity, which is conducive to improving the germination rate and yield of crops on saline land [84,85]. However, since gypsum itself has limited nutrient content, enhancing the improvement and reclamation of saline soils requires increasing the soil’s nutrient content [4].

Organic materials such as crop straw, livestock manure, and biochar contain abundant organic matter and nutrients, which can improve soil structure, enhance soil fertility, and promote plant growth [86]. The organic acids produced during the decomposition of organic materials can also neutralize soil alkalinity, further lowering soil pH [87]. Additionally, the application of organic materials can increase soil microbial activity, promoting the cycling and utilization of soil nutrients [88]. In recent years, biochar has garnered increasing attention as a potential method for improving soil quality and maintaining crop productivity. Research indicates that the application of biochar improves the nutritional and reproductive growth as well as biochemical characteristics of Catharanthus roseus under saline stress, enhances soil fertility, and that a 2% biochar application is more effective than a 4% biochar application in alleviating salt stress [89]. Research by Lei et al. showed that applying biochar reduced soil pH by 6.43–13.17% and ECe by 55.87–77.55%. It also increased total nitrogen and organic carbon content by 1.15–2.77 and 2.16–4.40 times, respectively. Additionally, biochar significantly increased microbial species richness, particularly bacteria (e.g., RB41, Vicinamibacteraceae), and also played a role in regulating phosphorus cycling in saline ecosystems [90]. The porous structure of biochar provides a “refuge” for soil microorganisms, protecting them from environmental threats, altering microbial abundance and activity, and promoting nutrient cycling [91]. In summary, the improvement effect of biochar is mainly reflected in the improvement of soil properties and nutrient content, rather than the fundamental alleviation of salt stress. It can be seen that the combined application of gypsum and biochar is an effective means of improving saline land.

3.3. Biological Improvement

Although biological methods for improving saline land are not as effective as physical and chemical methods, they have a smaller ecological impact, are less costly, and can be implemented on a large scale. This makes them the most ecologically and economically beneficial measures [76]. One effective way to biologically improve saline land is by planting salt-tolerant plants. These plants, such as Chenopodium quinoa, Tamarix, and Lycium barbarum, can grow in saline environments to a certain degree. They absorb and accumulate salt in the soil through their root systems, thereby reducing the soil’s salt content. Additionally, they improve the utilization rate of saline land and increase economic income [92]. Microbial fertilizers play an important role in improving saline land [93]. These fertilizers form a complex network with plants, directly or indirectly affecting growth. They also improve survival and yield by triggering the induction system resistance of plants and changing K⁺ uptake and Na⁺ excretion. This achieves the goal of improving saline land and restoring soil [94]. For instance, inoculation with arbuscular mycorrhizal fungi (AMF) significantly increased the chlorophyll content, auxin level, nodule number, nodule weight, and nitrogenase activity of Sesbania, indicating that AMF promotes Sesbania growth on saline land [95]. Composite microbial inoculants significantly increase soil available nutrients such as alkali-hydrolyzable nitrogen, available phosphorus, available potassium, and organic matter content, while reducing soil water-soluble salt content and soil pH. They promote the growth of Carthamus tinctorius and increase the yield of Carthamus tinctorius stigmas and seeds, achieving dual benefits of restoring saline soil and enhancing plant productivity [96].

3.4. Comprehensive Improvement

In practice, a single improvement measure often fails to simultaneously improve saline soil and increase plant biomass. The comprehensive application of multiple improvement measures will become an important way to develop and utilize saline land. For instance, using gypsum alone has limited effects on improving soil fertility and cannot effectively address the adverse characteristics of alkaline soil, such as stickiness, hardness, and thinness. However, combining gypsum with different functional improvement agents significantly increases soil organic matter and nutrient content, enhances Lycium barbarum agronomic characteristics, and increases Lycium barbarum production and income while reducing alkalinity and salt content in saline land [97]. A recent study by Li et al. revealed that applying a mushroom residue organic fertilizer in conjunction with desulfurization gypsum significantly reduced soil salinity, improved soil fertility, and enhanced carbon sequestration efficiency [98]. Xu et al. used a composite material of titanium gypsum and biochar to lower soil salinity indicators, including pH, ECe, SAR, and soluble sodium (Na⁺), by 20.74%, 77.24%, 68.77%, and 44.70%, respectively. This demonstrated a positive ameliorative effect on saline soils [91]. Tian et al. found that applying biochar and desulfurized gypsum to the root zone significantly improved the quality of the soil and sunflower yield in saline land [99]. Xing et al. demonstrated that amending saline soils with a composite additive (comprising 1% biochar, 1% salt-tolerant bacteria, 1% alkaline soil conditioner, and 10% earthworm castings) significantly elevated the abundance of salt-resistant bacteria, as well as microorganisms associated with carbon cycling, nitrogen cycling, and plant aggregation. Consequently, this amendment not only improved the soil’s carbon fixation potential and nitrogen cycling efficiency but also mitigated greenhouse gas emissions and lowered the soil’s HCO₃⁻ concentration [100].

The different regulatory strategies mentioned above can address various soil issues. A summary and evaluation of these measures is presented in

Table 1. Summary and evaluation of regulatory strategies for saline land.

Regulatory Strategies	Specific Measures	Effects on Soil Properties	Evaluations
Physical Improvement	Sand covering; Land leveling and deep plowing; Irrigation and drainage	Reduce soil capillary action; Break up soil compaction, improve soil structure, enhance drainage, and facilitate the leaching and removal of salts; Reduce water evaporation and remove excess moisture and salt from the soil	Advantages: The foundation for saline land reclamation; Rapidly alleviate salt stress; Effectively improve soil physical structure; Combined with other improvement methods yields better results; Disadvantages: Require significant labor and material resources; Short-term effect, lack long-term soil fertility improvement
Chemical Modification	Inorganic amendments (e.g., CaCl2, gypsum/desulfurization gypsum); Organic amendments (e.g., crop straw, livestock manure, biochar)	Ca2+ replaces soil Na+ via ion exchange, reduces soil alkalinity and salt stress; Lower soil pH, Ece and SAR, improve soil structure and fertility; Maintain ROS homeostasis and enhances antioxidant enzyme activity in plants; Increase total nitrogen and organic carbon; Provide microbial refuge, promote nutrient cycling and microbial richness	Advantages: Rapidly adjust soil chemical properties; Enhance long-term soil fertility and microbial activity; Cost-effective for large-scale use; Disadvantages: Have limited nutrient content, requiring additional fertilization; Potential environmental risks if chemical agents are overused.
Biological Improvement	Salt-tolerant plant cultivation (e.g., Chenopodium quinoa, Tamarix, Lycium barbarum); Microbial fertilizers (e.g., AMF, composite microbial inoculants)	Absorb and accumulate soil salt via roots, reducing salt content; Improve land utilization rate and economic benefits; Increase crop chlorophyll content, auxin level, nodule number/weight, and nitrogenase activity; Composite inoculants increase soil available nutrients; Reduce water-soluble salt and pH, and promote crop growth; Form plant-microbe networks to enhance stress resistance	Advantages: Low ecological impact; Low cost and suitable for large-scale application; Improve soil ecological function sustainably; Disadvantages: Slower effect compared to physical/chemical methods; Salt-tolerant plants have limited salt-removal capacity; Microbial activity is sensitive to environmental conditions
Comprehensive Improvement	Gypsum + organic amendments (e.g., mushroom residue fertilizer, biochar); Titanium gypsum + biochar; Biochar + salt-tolerant bacteria + alkaline soil conditioner + earthworm castings	Reduce soil salinity and alkalinity; Increase organic matter and nutrients; Enhance agronomic traits and yield; Improve carbon sequestration efficiency; Lower soil pH, ECe, SAR, and soluble Na+; Elevate salt-resistant bacteria and C/N cycling microbial abundance; Improve carbon fixation potential and nitrogen cycling efficiency; Reduce HCO3− concentration	Advantages: Synergistic effect of multiple measures, addressing both soil physical/chemical properties and ecological function; Long-term stability in salinity reduction and fertility improvement; Balance yield increase and environmental sustainability; Disadvantages: Complex preparation and application processes; Require precise matching of amendment ratios; Higher initial research and development costs.

. Therefore, more research should be conducted in the future on the effects of different mixtures of improvers on saline soil and plant growth. It is also important to combine different improvement methods. For instance, chemical improvers could be supplemented with physical improvement measures, such as land leveling and irrigation. Alternatively, microbial agents could be combined with chemical improvers and physical methods to improve saline land and increase plant production.

4. Prospects for the Development and Utilization of Chinese Herbal Medicines in Saline Land

4.1. Salt-Tolerant Medicinal Plants Resources

Currently, scholars both domestically and internationally have identified various medicinal plants that exhibit salt tolerance. For example, Glycyrrhiza can mitigate salt damage by compartmentalizing Na⁺ and secreting organic acids through its root system [7]. Nitraria tangutorum, as a typical halophytic medicinal plant, has a well-developed root system and strong salt tolerance. In recent years, it has been widely used in the ecological restoration of saline land. Research by Tang et al. shows that the roots of Nitraria tangutorum absorb Na⁺ and Cl⁻, transport them to the leaves, and mainly segregate them in the guard cell vacuoles [101]. Nitraria tangutorum can also resist salt stress by increasing the accumulation of osmotic regulatory substances, such as proline and soluble sugars, and by enhancing the activity of enzymes related to the antioxidant system, such as POD and SOD [27,102]. Yao et al. evaluated the salt tolerance of 61 Medicago sativa seed varieties during germination and found that “Zhongmu 3,” “WL440HQ,” and “WL712” have strong salt tolerance, making them excellent resources for the biological improvement of saline land [103]. After three years of growing Lycium barbarum on saline land, the desalination rates of the topsoil and deep soil were 78.7% and 64.0%, respectively, with significant improvements in soil structure and nutrients [104,105]. Zhang et al. found that under salt stress, Coicis Semen and Cichorium intybus increase nitrate nitrogen content in the soil, lower soil pH and electrical conductivity, and improve saline soil [106]. Cynomorium songaricum adapts to saline stress by shaping the rhizosphere microbial community (e.g., Sphingomonas and Saccharomyces) and regulating gene expression and metabolic processes [93]. In summary, salt-tolerant medicinal plants adapt to saline stress through ion compartmentalization, osmotic regulation, and antioxidant regulation. They also improve soil nutrients and the soil micro-ecological environment. Thus, developing salt-tolerant medicinal plant resources is important for utilizing saline land.

4.2. The Effect of Saline Stress on the Quality of Chinese Herbal Medicines

Medicinal plants have unique habitat requirements and, through natural selection, have developed the characteristic of “adversity breeds quality” [107]. Some medicinal plants exhibit adaptive morphological and physiological changes in response to saline stress and accumulate secondary metabolites (active components) as an important coping strategy (

Table 2. Effects of saline stress on the secondary metabolites of Chinese Medicinal Herbs.

Chinese Medicinal Herb Name	Stress Conditions	Enhanced Secondary Metabolites	References
Salvia miltiorrhiza	300 mmol/L NaCl	Tanshinone I; Tanshinone II; Cryptotanshinone	[108]
Isatis indigotica	200 mmol/L NaCl	Eugenol; Dihydrocinnamaldehyde; Dihydrocinnamaldehyde I	[109]
Catharanthus roseus	50 mmol/L NaCl	Vinblastine; Vinblastine base; Vincristine	[110]
Taraxacum officinale	NaCl ≤ 1 g/kg	Caffeoylquinic acid	[8]
Trigonella foenum-graecum	0.1% w/v NaCl	Phenolics; Flavonoids; Tannins;	[111]
Artemisia annua	4–6 g/L NaCl	Artemisinin	[112]
Rumex japonicas	200 mmol/L NaCl	Emodin	[113]
Glycyrrhiza	0.2–0.6% salt (NaCl:Na2SO4 = 1:2	Flavonoids; Glycyrrhizic acid	[7]
Saposhnikovia divaricata	15 g/kg alkaline salt (NaHCO3:Na2CO3 = 9:1)	Cimicifugin glycoside; Cimicifugin; Paeoniae root glycoside	[114]
Chenopodium quinoa	100 mmol/L alkaline salt (NaHCO3:Na2CO3 = 9:1)	Flavonoids;	[115]
Thymus vulgaris	90 mmol/L NaCl	Phenolics;	[116]

Note: The secondary metabolites listed in the table are the primary bioactive components of the corresponding Chinese medicinal herbs, directly influencing their quality.

).

Interestingly, recent studies have revealed some potential mechanisms underlying the changes in these secondary metabolites. For instance, Salvia miltiorrhiza is a traditional Chinese herbal medicine with tanshinone as one of the main bioactive components and has antitumor, antibacterial, anti-inflammatory properties, as well as other physiological functions. Research indicates that the accumulation of tanshinones (T-I, T-II, and CT) under salt stress is enhanced by AP2/ERF transcription factors (TFs)—specifically SmAP1, SmAP2, and SmERF2—through upregulating the expression of key genes (e.g., SmHMGR, SmDXS, SmGGPPS) and the abundance of corresponding proteins (e.g., HMGR, DXS) involved in the tanshinone biosynthetic pathway [108]. Moreover, the latest research have revealed that the overexpression of AtMYB2 can enhance the salt stress resistance of Salvia miltiorrhiza while upregulating the expression of key enzyme genes in the tanshinone biosynthesis pathway, thereby promoting tanshinone synthesis. This finding indicates that the increase in tanshinone content and the enhancement of salt tolerance can occur simultaneously [117]. In the medicinal plant quinoa (Chenopodium quinoa), saline stress downregulates the expression of CqbHLH216 and MYB4, thereby regulating flavonoid biosynthesis pathways such as the phenylpropanoid pathway to promote flavonoid biosynthesis, which contributes to the improvement of its quality. As potent natural antioxidants, flavonoids contain phenolic hydroxyl groups in their molecular structure that can act as electron donors to directly scavenge ROS induced by saline stress. In other words, the accumulation of flavonoids, in turn, enhances the ability of quinoa to resist saline stress. Notably, the accumulation of flavonoids is not an isolated stress response; instead, it acts synergistically with quinoa’s mechanisms such as osmotic regulation and pH homeostasis maintenance [115]. Since flavonoids are also bioactive components of Trigonella foenum-graecum and Glycyrrhiza (Table 2), similar mechanisms may exist in these plants, which warrant further investigation for verification. Similarly, salt stress induced by 90 mmol/L NaCl promotes the accumulation of phenolic compounds in the medicinal plant Thymus vulgaris. In turn, the accumulation of these phenolic compounds enhances the free radical-scavenging activity, thereby aiding Thymus vulgaris in resisting salt stress [116]. These findings demonstrate that a moderate saline environment affects the accumulation of secondary metabolites (e.g., flavonoids, phenolics) in medicinal plants. Through mechanisms such as synergistic osmotic regulation and ROS scavenging, this environmental condition enhances the ability of medicinal plants to resist salt stress and improves the quality of medicinal materials derived from them. Additionally, studies have reported that Chinese medicinal herbs including Lycium barbarum, Glycyrrhiza, and Astragalus membranaceus cultivated in saline soils of genuine producing areas (Daodi regions) in China exhibit satisfactory medicinal quality. Furthermore, these plants can reduce soil pH and salt content, thereby contributing to the remediation of saline soils [118,119,120]. Therefore, improving saline land through physical, chemical, and biological methods and developing and utilizing it for Chinese herbal medicines is of great practical significance. This approach can achieve the dual effects of improving the quality of medicinal materials and restoring the ecology of saline land.

5. Conclusions and Future Perspectives

As saline land continues to expand worldwide, developing and utilizing this land has become an urgent issue in agricultural development and environmental management. Plants, as sessile organisms on the soil surface, have evolved a highly complex and dynamically plastic response system over the course of their long-term growth. They regulate their physiological metabolism and the accumulation of secondary metabolites by constructing plant phenotypic characteristics, regulating osmosis, maintaining ion homeostasis, and regulating hormones to withstand damage caused by saline stress. A deeper understanding of these adaptive mechanisms can also provide technical pathways for screening key functional factors and breeding salt-tolerant plant varieties. Additionally, to promote the efficient development and utilization of saline land, an effective strategy is to select salt-tolerant medicinal plant varieties from the abundant resources and combine physical, chemical, biological, and comprehensive improvement measures to facilitate efficient development and utilization of saline land, as well as the sustainable development of the Chinese herbal medicine industry.

Due to global climate change and the scarcity of agricultural resources, the development and utilization of saline land has become a hot topic in agricultural scientific research. Against the backdrop of the growing health industry, the demand for Chinese herbal medicines continues to rise, creating more competition for land between food and medicine. In-depth research on the adaptation mechanisms of medicinal plants to saline land and their development and utilization is significant for effectively utilizing saline land resources and improving the yield and quality of Chinese herbal medicines on this land. In the future, utilizing saline land for developing and producing Chinese herbal medicines can focus on the following aspects.

(1) Strengthen the screening of medicinal plants that are suitable for saline land. Then, develop supporting soil improvement technologies based on the salt tolerance characteristics of these plants. Screening salt-tolerant medicinal plants is essential for utilizing saline land. Some medicinal plants, such as Lycium barbarum, salvia miltiorrhiza, and Limonium sinense, have developed a certain degree of tolerance to saline stress through long-term evolution. Further investigation of saline land resources is necessary to discover more salt-tolerant plants and expand the “reserve pool” of salt-tolerant medicinal plants. According to the physiological characteristics of the selected salt-tolerant medicinal plants and the degree of saline land, soil improvement should be carried out to accelerate the process of saline land development and utilization.

(2) Conduct in-depth research on the physiological response mechanisms of medicinal plants under saline stress. Using modern omics and molecular biology technologies, analyze the physiological and biochemical changes in medicinal plants under saline stress, as well as the underlying molecular mechanisms. The integrated analysis of multi-omics data reveals the regulatory mechanisms by which medicinal plants adapt to saline environments. This will provide a theoretical basis for selecting salt-tolerant varieties and developing salt-tolerant cultivation strategies.

(3) Actively promote the demonstration and promotion of medicinal plant cultivation techniques in saline land. While using genetic engineering to cultivate new salt-tolerant medicinal plant varieties, combine modern agricultural technology, intelligent agricultural equipment, and other means to comprehensively utilize different improvement methods to improve the efficiency and effectiveness of saline land improvement, promote the introduction and cultivation of medicinal plants, and explore the role and function of medicinal plants in saline land ecosystems. Study the interactive relationship between medicinal plants, soil microorganisms, and other plants to provide a scientific basis for constructing a model for the ecological restoration and sustainable use of saline land, and ultimately promote the rational use and sustainable development of saline land resources.

This study complied with ethical standards.