Salt Stress Responses and Tolerance in Soybean

Abstract

Soybean is one of the major oil crops with multiple uses which is gaining popularity worldwide. Apart from the edible oil, this crop provides various food materials for humans as well as feeds and fodder for animals. Although soybean is suitable for a wide range of soils and climates, it is sensitive to different abiotic stress such as salinity, drought, metal/metalloid toxicity, and extreme temperatures. Among them, soil salinity is one of the major threats to soybean production and the higher yield of soybean is often limited by salt stress. Salt stress negatively affects soybean seedling establishment, growth, physiology, metabolism, and the ultimate yield and quality of crops. At cellular level, salt stress results in the excess generation of reactive oxygen species and creates oxidative stress. However, these responses are greatly varied among the genotypes. Therefore, finding the precise plant responses and appropriate adaptive features is very important to develop salt tolerant soybean varieties. In this connection, researchers have reported many physiological, molecular, and agronomic approaches in enhancing salt tolerance in soybean. However, these endeavors are still in the primary stage and need to be fine-tuned. In this chapter, we summarized the recent reports on the soybean responses to salt stress and the different mechanisms to confer stress tolerance.

1. Introduction

Soybean (Glycine max L.) is considered to be one of the major oilseed crops as well as an economically important leguminous crop as it supplies more than 25% of the global protein requirement for food and feed [1, 2]. Not only the food and feed values but also the root nodules of soybean enhance soil fertility through nitrogen (N) fixation. It also has other uses like forage crops, emergency crops, and even as a source of bioenergy [3]. A crop with such superior qualities needs to gain more attention than it has currently. Soybean production has been increasing over years, but the rate is not as much as other major cultivated crops like maize, wheat, and rice. The possible reasons are lack of suitable varieties, different environmental constraints, etc.

Environmental stresses including salinity, drought, waterlogging, toxic metals, extreme temperatures, etc. are nowadays the center of concern by the environmentalist and plant scientists worldwide as these are badly affecting food production. Salt stress is undoubtedly one of the worst conditions for plant growth as it creates both osmotic and ionic stress. Soybean is known to be partially sensitive to salt which may result in up to 40% yield loss depending upon salinity level. The presence of excess salt in the growing medium of soybean negatively affects the quality and quantity of seed, growth, and nodulation process [4]. Synthesis of protein, uptake and transportation of water and nutrient, translocation of assimilates, cytosolic and mitochondrial reactions, and several other metabolic pathways are adversely affected by salt stress [5]. Dehydration of cell and toxic ion accumulation occur when the rhizobia-legume symbiosis process is hampered in particular [4]. Higher magnitude of salt stress may even cause plant death [6]. In addition to these, salt stress impairs the photosynthesis process that ultimately results in oxidative stress by the excess production of reactive oxygen species (ROS). Ionic imbalances disrupt the normal metabolic processes and subsequently aggravate ROS generation making the situation worse for plants [5, 7, 8]. The antioxidant defense system is a vital strategy for the alleviation of oxidative stress in plants. It consists of different enzymatic and non-enzymatic components which act as saviors at their optimum levels. Besides, the use of salt-tolerant varieties, seed priming, use of exogenous protectants, beneficial microbes, or proper modulation of agronomic managements are other potential strategies to enhance salt stress tolerance in different crops including soybean.

In the present situation of the world where hunger and malnutrition exist, people are badly in need of proper nutrition to ensure a healthy future. In that case, soybean being a crop of versatile nutritional values and wide adaptability should be focused on the agriculture sector worldwide. But, due to some existing challenges the per unit yield of soybean is not increasing, although its global production has been increased remarkably over years. The underlying reason is the lack of proper knowledge about the identification of yield traits that could facilitate the per unit yield of soybean. Unlike cereal crops, soybean architecture is more complex, which makes the task by the breeders difficult to optimize the plant accessions. For example, simply increasing plant height will not give a higher yield due to lodging, and dwarf varieties will also reduce yield due to fewer internodes. Hence, Liu et al. [2] suggested that lodging resistant long plants with shorter internodes might be helpful in this regard. They also suggested that minimization of pod abscission is also required to ensure a better yield. So, it is clarified that high-yielding soybean variety development is not a piece of cake. But it can be made possible by an in-depth understanding of soybean plant architecture and yield characteristics. Moreover, environmental adversities like salinity are making these tasks more challenging as it hampers soybean growth and productivity by inhibiting germination and other vital physiological processes [3]. However, soybean is known to be a moderately salt-sensitive crop, and therefore, to ensure the potential production of soybean we need to minimize the salt stress-induced damages in soybean. To execute that, extensive studies on soybean responses to the different magnitude of salt stress and possible adaptive mechanisms are necessary. In this chapter, we have summarized the up-to-date findings focused on the responses of soybean to salt stress and exploring the tolerance mechanisms.

2. Soybean: a crop of multiple uses

Soybean is a crop of versatile uses with wide adaptability. Apart from the soybean oil, the whole soybean including pod and seed have multifarious uses like human food, animal feed, and in improving soil fertility through N-fixation (Figure 1). Soybean oil and protein contents are 21 and 40%, respectively including carbohydrate and ash contents of at 34 and 5%, respectively [9]. As a vegetable, early maturing soybean variety with green and immature pods is more feasible for fresh or frozen consumption [10]. Food products made from soybean e.g., soya milk, soya flour, tofu, boiled soybean, soya meat, etc. are gaining popularity day by day for their nutritional values. Soybean germ oil, produced as a byproduct at the time of protein preparation is able to reduce plasma cholesterol and so can be used in the treatment of hypercholesterolemia [11]. Having high concentrations of β-carotene, vitamin C, high calories and essential amino acid soybean also can be efficiently used in anti-hypertensive, antimicrobial, antioxidant, anti-diabetic, and anticancer activities that are very important for human health [12, 13]. Moreover, soybean is now being used in the preparation of meat analog, which has similarities with meat but is more beneficial in lowering lipid and blood pressure while increasing low-density lipoprotein (LDL), cholesterol oxidation, and digestibility [14].

Figure 1.

Versatile uses of soybean products [adapted from Hasanuzzaman et al. [3] with permission from Elsevier].

A rich amount of protein content with optimum amino acid profile, low amount of crude fiber, high phosphorus (P) content, and high level of digestibility is noticeable at soybean meal that makes it a desirable feed for livestock [15]. Whole plants, raw seeds, or processed products of soybean can be used efficiently in cattle feeds if proper ration components can be chosen considering the rate of N and energy release during rumen fermentation processes [16]. Additionally, soybean oil, due to its rich fatty acid profile, can be a convenient and affordable aquafeed in the aquaculture industry around the world [17, 18].

Soybean is a well-fitted crop in a wide range of climates and soils. It is easily cultivated as an intercrop, facilitates in proper utilization of available resources, and soil fertility improvement through biological N-fixation. When crop rotation was done comprising corn and soybean, minimum nutrient and water loss with higher yield and net income was recorded compared to conventional farming [19, 20].

Nowadays, this soybean solid waste is being widely used for different purposes, especially in bioenergy. If soybean waste is managed through proper hydrothermal treatment, can be efficiently used in industries as an eco-friendly way for its good nutritional, thermal, and fuel properties [21]. Soybean meal when used with bark, can be a raw material to prepare an affordable, formaldehyde-free, and environmentally sound wood adhesive for plywood production, which has a better performance in giving high water resistance, mechanical strength, and thermal stability [22, 23]. However, soybean could be a potential renewable energy source for different industrial purposes.

3. Soybean response to salt stress

3.1 Seed germination and emergence

Uniform seed germination and rapid seedling growth are prerequisites for better crop establishment [24, 25]. Salinity hampers the germination process either by delaying or reducing the germination rate [26]. Simaei et al. [27] observed that at lower concentrations of salt stress (0.05 and 0.1%) soybean seed germination was delayed whereas, at high salt dose germination rate was reduced. Shu et al. [28] reported that germination of soybean seed was delayed under 150 mM NaCl stress. Additionally, post germinative growth of soybean seedlings was adversely affected due to salinity. Salt stress downregulates gibberellic acid (GA) and upregulates abscisic acid (ABA) bio-production that ultimately reduces the GA/ABA ratio and thus delays seed germination in soybean [28]. Soybean seedling growth completely stopped when the leaf sodium (Na⁺) reached 6.1 mg g⁻¹ fresh weight (FW). It was also observed that compared to the control, seedling growth reduced by 14% at 160 mM NaCl whereas, at 330 mM NaCl stress soybean seedling growth got completely stopped [29]. Delayed seed germination processes in three cultivars of soybean in terms of germination rate, radicle length, and FW of germinated seeds were observed by Shu et al. [28]. During the seed germination process, changes in cell oxidative status were reported under salt stress. Xu et al. [30] conducted an experiment with two cultivars of soybean viz., Lee68, and N2899 to evaluate the response of soybean seeds to salt stress during germination. Seeds were soaked in 100 mM NaCl until radical initiation and reported that without impacting final germination, salt only delayed mean germination time (MGT) by 0.3 and 1.0 d in Lee68 and N2899, respectively. Abulfaraj and Jalal [31] experimented with three cultivars (Crawford, G-111, and Clark) of soybean viz., under 200 and 400 mM NaCl stress and observed that the final germination percentage (FGP) was reduced in all three cultivars irrespective of stress levels. However, it was prominent that the higher the stress, the lower was the FGP. At 400 mM salt stress the highest reduction of FGP (60%) was observed in cultivar Clark, whereas Crawford and G-111 showed 29 and 22% reduction, respectively. However, the MGT was increased with increasing salinity level where Crawford exhibited the best performance by lowering MGT values than other cultivars under both salinity stress and normal condition [31]. Essa [32] observed a remarkable reduction of germination percentage under different doses of salt stress in three soybean cultivars. They found that, when salinity level increased from 0.5 to 8.5 dS m⁻¹, it reduced germination by 54 and 63% for Lee and Coiquitt cultivars, respectively. Hashem et al. [33] observed that in two soybean cultivars, seed germination was also reduced with increasing salt doses. At 200 and 300 mM NaCl, germination reduced by 4 and 11%, respectively for the salt-tolerant variety Clark, whereas 11 and 20% were documented in salt-sensitive variety Kint, respectively. Khan et al. [34] experimented with different doses of NaCl treatments ranges (50–300 mM) and revealed that above 200 mM salt concentration, the maximum seed did not germinate whereas, most of the seeds germinated under the 50 and 100 mM NaCl treatments.

3.2 Growth

One of the obvious effects of salt stress in soybean is the reduction of plant growth, which is reported in many plant studies [28, 35, 36]. As salt stress inhibits water and nutrient uptake and translocation, it is obvious that this hampers the normal cell growth and development and hence, growth is retarded. Otie et al. [37] found that plant height, number of leaves, leaf area index (LAI) and the specific leaf area (SLA) of soybean plants were declined in a dose-dependent manner when they were grown under salinity (32.40, 60.60 and 86.30 mM NaCl). Salt stress also resulted in a lower dry weight (DW) of the leaf and stem [37]. The higher salinity declined the cell elongation and division that may lead to the inhibition of growth-related metabolic and physiological processes and eventually restricted the allocation of biomass [38].

Salt stress also hampers the plant’s anatomy and impairs the normal growth of plants. As salt stress allows the entry of high Na⁺ into plants roots, this reduces the elongation rate and disturbs root architecture [35, 36, 39]. It is evident that plants sometimes thicken the epidermis and endodermis root cells as a preventive measure of Na⁺ influx. But under severe salt stress, this phenomenon may lead to cell expansion and cell wall integrity [40, 41]. Silva et al. [36] observed that Na⁺ stress differently affected root anatomy. Interestingly, lower (50 and 100 mM Na⁺) salt stress improved the root epidermis and endodermis thickness, cortex thickness, vascular cylinder diameter, and metaxylem diameter at different root depths, while higher (150 and 200 mM Na⁺) salt stress reduced these parameters. Root epidermis and endodermis thickness were decreased by 44 and 56% under 200 mM Na⁺, compared to the control treatment, while root cortex thickness and vascular cylinder diameter were decreased by 8–17%, respectively [36]. Salt-induced decline of plant growth exclusively depends on the salinity levels and the duration of exposure. At the initial stage, salt-induced decline in seedling growth is associated with the seedling vigor and seed germination rate which is due to lower synthesis of particular phytohormones such as GA.

Amirjani [42] recorded some reduction of plant height and FW of soybean plants under salinity stress. A reduction of plant height of 30, 47, and 76% and FW of 32, 54, and 76% were found by increasing salinity levels to 50, 100, and 200 mM, respectively. Weisany et al. [43] observed that salinity stress caused a number of morphological and physiological changes in soybean plants. Salt stress caused a decrease in shoot length, root FW and DW, and shoot FW and DW. Lee et al. [44] found a marked decline (27%) in shoot length when soybean plants were exposed to 80 mM NaCl. The soybean FW and DW were also significantly decreased with NaCl application which was 16 and 40%, respectively lower than control. In our experiment, we found that soybean plants exposed to 300 and 450 mM NaCl stress showed 35 and 55%, respectively reduction in plant height. Moreover, shoot FW and DW were declined by 43 and 41%, respectively at 150 mM NaCl stress [45].

3.3 Photosynthesis

It is well-established that yield reduction under salinity stress is due to the reduced production of photo-assimilate, slower transportation of photosynthetic components, and transformation in cytosolic metabolism [46]. Under salinity stress, the concentration of Na⁺ ion absorption gets increased in plant tissues. As a result, uptake of other essential nutrients required for different biosynthesis processes gets decreased [47, 48]. Excessive salt accumulation in the cell causes necrosis and reduces the photosynthesis rate that ultimately diminishes plant growth [49]. In soybean, excess uptake of Na⁺ under saline conditions decreases potassium (K⁺) uptake which is a vital regulator of stomatal opening and closing during photosynthesis. Furthermore, prolonged salt stress results in chlorosis and reduced content of different photosynthetic pigments in soybean [27, 50]. Various experiments conducted with different cultivars of soybean reported that plant photosynthetic parameters significantly varied with the duration and doses of salt stress (Table 1). The experiment conducted by Zaki et al. [58] found that total chlorophyll (chl), carotenoid contents, and chl fluorescence were decreased in 150 mM NaCl stress compared to control. Hashem et al. [33] experimented with two soybean cultivars (Clark and Kint) under 200 mM NaCl stress and observed considerable reductions in chl a, chl b, total chl, and carotenoids by 33, 35, 35, and 50% for Clark and 71, 76, 74, and 81% for Kint, respectively. Here it is evident that in tolerant cultivar the rate of photosynthetic reduction was lower compared to sensitive one. Significant reduction in all photosynthetic traits such as net photosynthetic rate, stomatal conductance, and internal CO₂ concentration was observed in soybean plants when exposed to 100 mM NaCl stress. Considering the duration, under 6 h of salt stress, net photosynthesis was reduced by 47% whereas, 81% reduction was observed at 48 h stress. Furthermore, at 24 h and 48 h stress durations, intercellular CO₂concentration was decreased by 2 and 11%, respectively at the same dose of salinity stress [59]. In a similar experiment, Soliman et al. [53] observed that after 25 d of salt (100 mM NaCl) condition, stomatal conductance, chl and carotenoids contents, photosynthetic rate, and intercellular CO₂ concentration were reduced by 22, 46, 40, 42, and 26%, respectively. Yan et al. [60] experimented with a halophytic soybean (Glycine soja) to explore the salt adaptability in terms of photosystem coordination under 300 mM NaCl for 9 days (d). The result showed that under stress conditions, photosystem II (PSII) electron transport rate, stomatal conductance, and photosynthetic rate were reduced in both G. soja and G. max; however, the highest reduction was reported in G. max. Oppositely, PSII excitation pressure was increased by 72 and 50% in G. max and G. soja, respectively on day 9. During photosynthesis, chl plays a vital role in photon harvest in the PSII and PSI that belongs to the chloroplast. Under stress conditions, the production of different ROS is subjected to the destruction of many cellular organelles along with chloroplast. As a result, other photosynthetic activities are significantly hampered under salt stress [61]. Ning et al. [62] observed that under the same level of salt stress, sensitive cultivars showed higher photosynthesis inhibition compared to tolerant ones. It is assumed that due to the higher accumulation of P, K⁺, and magnesium (Mg²⁺) in the leaf, tolerant cultivar restored higher photosynthetic rates. When the Giza 111 cultivar of soybean was exposed to different levels of salt stress for 14 d, it resulted in a significant reduction of the photosynthetic pigments. Compared to control chl a, chl b, chl (a + b), and carotenoids contents were decreased by 27, 23, 26, and 27%, respectively [51]. Leaf size and area are two vital factors that regulate the amount of light captured for photosynthesis. Under salt stress, leaf area is reduced significantly and directly affects the production of photosynthetic pigments by reducing the amount of calcium (Ca²⁺) and iron (Fe) ions in leaves responsible for the chl biosynthesis [63, 64]. All these results reported in soybean under salinity stress are the consequences of osmotic stress, which triggers the higher accumulation of Na⁺ in leaf restricting the supply of K⁺ and thus inhibits photosynthesis, which eventually reduced the stomatal conductance. In addition, this excessive ion toxicity reduces chl content, damages chloroplast structure, and leads to nonstomatal inhibition of photosynthesis [62]. Guo et al. [65] also suggested that high Na⁺ notably reduces the K⁺, Ca²⁺, Mg²⁺, and other cations in the leaf which are vital for the photosynthesis in the plant.

Name of cultivars	Dose and duration of salt stress	Stress responses	Reference
Clark and Kint	200 mM NaCl, 60 d	Reduction of chl a, b, total chl and carotenoids contents by 33, 35, 35, and 50% for Clark and 71, 76, 74, and 81% for Kint cultivar, respectively	[33]
Giza 111	75 and 150 mM NaCl, 14 d	Reduction of chl a, b, a + b, and carotenoids by 13, 22, 16, and 7%, respectively at 75 mM salt stress and by 27, 23, 26, and 27%, respectively at 150 mM salt stress	[51]
C08	150 mM NaCl, 2d	Reduction of photosynthetic rate and stomatal conductance	[52]
Giza 22	100 mM NaCl, 25 d	Reduction of chl and carotenoid contents by 46 and 40%, respectively.; Reduction of photosynthetic rate, intercellular CO2 concentration, and stomatal conductance by 42, 26, and 22%, respectively	[53]
Giza 111	3 and 6 g L^−1, saline water (dissolving sea salt with tapwater), 30 d	Dose-dependent reduction of chl a, chl b, and carotenoid contents	[54]
M7	4, 7, and 10 dS m^−1, 60 d	Reduction of leaf chl, carotenoids, and anthocyanins contents; Reduction of Fv/Fm at above 4 dS m^−1 salinity	[55]
Crawford, Giza 111, Clark	200 and 400 mM NaCl, 69 d	Reduction of chl a content by 69, 79, and 85% in Crawford, Giza 111, and Clark, respectively, and chl b content by 61 and 71% in Crawford and Clark, respectively	[31]
Giza 22	7.46 dS m^−1 saline soil, 60 d	Reduction of total chl and carotenoid contents	[56]
M7	5 and 10 dS m^−1, 30 d	Reduction of chl a, chl b, chl a/b, and total chl contents; Reduction of RuBisCO activity with increasing salt concentration	[57]

Table 1.

Alteration of photosynthesis and associated parameters of soybean under salt stress.

3.4 Water relation

One of the major effects of soil salinity is the osmotic stress in plants. Under salt stress, the water uptake is hampered due to the lack of energy and also the imbalance between solute concentration in the soil solution and plant cells. As a result, plant cells lose their turgidity which results in osmotic stress. Katerji et al. [66] reported that water consumption declined gradually as the salinity increased. As a result, the water content of the leaves was reduced and with it the turgor. Stomatal closure and eventually transpiration and photosynthesis were reduced resulting in growth retardation. The leaf relative water content (RWC), xylem exudation, leaf water potential were evidently declined in soybean plants subjected to 75 mM NaCl [34]. This decline in RWC indicates a loss of turgor which is associated with impaired water availability required for cell growth and development [67]. Shoot water content and leaf water potential were decreased by salinity [68]. Ferdous et al. [69] conducted an experiment with soybean under salt stress and found that RWC was decreasing with increasing salinity levels. The control treatment showed 89% RWC and it reduced to 73% at 100 mM salinity level. Two soybean genotypes viz. Shohag and AGS 313 were tested against salinity (50 and 100 mM NaCl) for different durations (15, 30, 45, 60, and 80 d) and it was observed that salt stress caused a reduction in RWC, water retention capacity, leaf water potential, and exudation rate in a concentration and duration-dependent manner [70]. The decline in exudation rate indicated the lower flow of water into plants which is associated with lower water potential and eventually RWC. However, the moderately tolerant genotype i.e., AGS 313 showed relatively higher RWC, water retention capacity, leaf water potential, and ER compared to the cultivar Shohag. One of the obvious effects of salt stress in soybean is the reduction of osmotic potential. A dose-dependent decline of osmotic potential was observed in soybean plants upon 28-d exposure to salt (60 and 120 mM NaCl), with more negative values in the treatment with 120 mM NaCl [71].

3.5 Yield and quality

Exposure of plants to salt conditions causes morphological, physiological, and biochemical alterations in the plant, which ultimately pose negative impacts on plant reproductive attributes and subsequent yield [72]. The primary effect of salt stress is a shortage of water, which is a crucial element for soybean flowering and podding. Upon exposure to salt stress, soybean growth attributes (shoot length, stem diameter, number of branches, flower number, pod number, and seed weight) were recorded to decrease with a higher accumulation of Na⁺ in leaves (Table 2). These changes could be the reason behind the manifestation of reduced yield and oil content of soybean [55, 84]. Additionally, the osmotic effect of salt stress influences the augmentation of growth retardation, obliteration of growth promoter, imbalanced ions, water uptake, and inhibition of photosynthetic activities that eventually alter the growth traits responsible for yield, yield attributes, oil and protein content of seed [56, 85]. Soybean productivity depends on the root performance under different soil conditions. As salt-induced condition disturbs the nodule formation, impeding the activity of root, yield reduction is quite obvious here [23]. Salt stress minimizes the grain size along with the duration of protein and oil accumulation, in turn, decreasing oil and protein content [74, 82]. Decreased protein content also can be occurred due to the interruption of N metabolism and nitrate absorption within the plant, which is a common phenomenon under salt stress [86]. Under severe salt stress (80 mM NaCl) both the soluble sugar and soluble protein that are helpful in maintaining osmotic adjustment are recorded to decrease [80]. But the changes in sugar content under salt-induced conditions may vary according to soybean varieties. Tolerant variety may show increased sugar concentrations under salt stress to maintain turgor within plant species [87].

Name of cultivars	Stress level and duration	Effects on yield and quality	References
Peking	250 mM NaCl	The number of flowers and pods was decreased by 64 and 69%, respectively; Pod length and weight were reduced by 10 and 20%, respectively.; Reduction of seed yield by 55%	[73]
Giza 111	104.44 mM Sea salt, 30 d	Pod number and weight were decreased by 31 and 60%, respectively; Seeds number and weight were reduced by 41 and 72%, respectively	[54]
Williams	9 dS m^−1, 75 d	Reduction of grain, protein, and oil yield plant^−1 by 39, 38, and 39%, respectively	[74]
JS-335	100 mM NaCl, 45 d	The number of pods plant^−1 and harvest index were greatly reduced by about 51 and 18%, respectively	[75]
JS-335	50 mM NaCl, 45 d	The number of pods plant^−1, seeds, and seed weight were decreased by about 29, 18, and 17%, respectively	[76]
M7	10 dS m^−1, 70 d	Pods and seeds plant^−1 were reduced by 28 and 30%, respectively	[55]
M7	10 dS m^−1, 109 d	Seed yield reduction by 44%; Soluble sugar and protein contents were increased by 389 and 108%, respectively	[77]
Sohag	50 mM NaCl	Pods plant^−1 and seeds pod^−1 were reduced by 52 and 10%, respectively; Reduction of seeds and yield plant^−1 by 54 and 51%, respectively; 100-seed weight was decreased by 16%	[46]
Hwangkeumkong	140 mM NaCl, 40 d	Pods plant^−1 and pod DW were reduced by 40 and 78%, respectively; 100-seed weight and yield were decreased by 55 and 80%, respectively	[78]
Giza 22	7.46 dS m^−1, 70 d	The number of pods plant^−1, 100-seed weight, and seed yield were reduced by about 59, 57, and 55%, respectively; Reduction of protein and oil contents by 30 and 41%, respectively	[56]
Giza 111	150 mM NaCl, 39 d	Pod number plant^−1 and 100-seed weight were decreased by 40 and 45%, respectively; Seed protein and oil contents were decreased by 24 and 29%, respectively; Increased soluble sugar content by 17%	[58]
Pungsannamul	300 mM NaCl, 21 d	Increment of total protein content by about 57%	[79]
Giza 35	80 mM NaCl, 5 weeks	Soluble protein and sugar content were reduced by 26 and 24%, respectively	[80]
Giza 22	9.4 dS m^−1, 8 weeks	Pod number plant^−1 and 100-seed weight were decreased by 28 and 34%, respectively; Seed weight plant^−1 and yield were reduced by 27 and 27%, respectively; Reduction of protein and oil content at 19 and 11%, respectively	[81]
No. 62	12 dS m^−1	Pods plant^−1 was reduced by 54%; Protein and oil content were decreased by 24 and 10%, respectively	[82]
Galarsum, BD 2331, and BARI Soybean 6	50 and 75 mM NaCl, 35 d	The number of pods plant^−1 was reduced with the lowest in BD2331 (38% compared to control) at 75 mM NaCl; The number of filled pods plant^−1 was lowest in BARI Soybean 6 (18% compared to control)	[83]
Tachiyutaka8428h	86.3 mM, 2 weeks	Days to 50% podding required 39% more time than control plants; The number of pods plant^−1 reduced by 16.5% compared to the control plants	[37]

Table 2.

Changes in yield and quality attributes of soybean under salt stress.

However, yield attributes e.g., branch number, pod number, number of seeds, etc. performed variably in different genotypes [46]. In addition, the duration of salt exposure also can influence the level of productivity loss. Pre-flowering stage of soybean is recorded to be more susceptive to salt stress as it reduces the capacity of soybean to uptake water and nutrient by accumulating a higher concentration of Na⁺ in the root [78].

3.6 Oxidative stress in soybean under salinity

Salt stress-induced stomatal closure leads to the reduction of the availability of CO₂ in leaves which reduces carbon fixation. Hence, the chloroplasts get exposed to higher excitation energy resulting in overproduction of ROS that includes superoxide (O₂^•–), hydrogen peroxide (H₂O₂), hydroxyl radical (OH^•), and singlet oxygen (¹O₂), etc. Besides, either osmotic or ionic stress resulting from salt exposure ultimately provokes the levels of ROS leading to cellular damages through the oxidation of lipids, proteins, and nucleic acids. The plant itself possesses some ROS scavenging mechanisms, such as an antioxidant defense system, and when there is an imbalance between the level of ROS production and the antioxidant defense system, the plant fails to cope with the oxidative stress (Figure 2) [72].

Figure 2.

Generalized outline of salt-induced oxidative stress and its consequences in soybean plants [adapted from Hasanuzzaman et al. [88].

Recent reports demonstrated the salt stress-induced oxidative damages in soybean plants (Table 3) and a notable number of those have also included possible mitigation strategies. For example, soybean seedlings grown on a pot containing 100 mM NaCl accumulated 38% higher H₂O₂ and caused 25% higher lipid peroxidation compared to the seedlings grown with no NaCl [91]. Exposure to salinity (6 dS m⁻¹) can also raise electrolyte leakage (EL) by 69% along with 75% and 56% increase in H₂O₂ and malondialdehyde (MDA) contents, respectively in soybean plants [92]. A similar increment of these oxidative stress markers in a dose-dependent manner of NaCl (75 and 100 mM) was also reported by Alharby et al. [5]. Reduction in membrane stability index (MSI) denotes the loss of cell membrane integrity and 21% lower MSI in soybean plants was recorded upon exposure to 100 mM NaCl stress [93]. They also observed 2.74 and 3.94-fold higher ROS (O₂^•− and H₂O₂, respectively) production and 2.28-fold higher lipid peroxidation in soybean. In another experiment with two cultivars of soybean differing in their salt tolerance levels (tolerant, Nannong 99-6 and sensitive, Lee 68), it was depicted that the oxidative or cellular damages are usually higher in the salt-sensitive one upon exposure to salinity [94]. Klein et al. [95] assessed the contents of H₂O₂ and MDA in root nodules of soybean exposed to 70 mM NaCl for 12 d and noted almost 98 and 75% enhancement of H₂O₂ and MDA, respectively.

Name of cultivars	Salinity levels	Level of oxidative stress	Reference
Taegwang	100 mM NaCl, 2 d	Levels of both lipid peroxidation and H2O2 content were higher	[89]
Giza 35	80 mM NaCl, 5 weeks	2.7-fold increase of MDA and 3.7-fold increase of H2O2 contents were recorded	[80]
Giza 111	150 mM NaCl, 40 d	EL was two times higher and MSI was 25% lower	[58]
Giza 22	7.46 dS m^−1, 70 d	A notable increase in MDA and H2O2 contents was recorded	[56]
FD92 and Z1303	150 mM NaCl, 20 d	MDA and H2O2 contents were 62 and 122% higher, respectively	[90]

Table 3.

Oxidative stress responses of soybean to salt stress.

Being one of the catastrophic abiotic stresses, salt stress induces oxidative stress in many possible ways including disrupted stomatal conductance, intruded photosynthesis, and altered activities of different enzymes [96]. The above-mentioned increase of lipid peroxidation, ROS production, EL, or reduction of MSI accounts for salt-induced oxidative damages in soybean plants. The bright side is that there are a number of mitigation strategies or adaptive mechanisms available for plants that facilitate the recovery or protection of plant cells from the cellular damages caused by excess salt.

4. Enhancing salt tolerance and adaptative mechanisms in soybean

4.1 Using salt tolerant genotypes

With the advancement of modern science and inventions in breeding technology; scientists, breeders, and agriculturists are working relentlessly to develop more tolerant varieties in different crops through genomic approaches and inbreeding technologies. Approaches towards genetic modification of soybean genotypes in order to increase their performances under salt-affected soil are of great importance. To identify salt-tolerant genotypes through a successful breeding program, the genetic diversity present in different soybean germplasm needs to be evaluated through a proper screening process that will help in understanding the mechanisms of salt tolerance in diverse genotypes [97]. United States Department of Agriculture (USDA) has compiled the salt tolerance level of more than 550 genotypes till today. Among these, 151 were declared as tolerant and 413 as susceptible genotypes [98]. The most effective way to uphold soybean yield under salinity stress is genetic improvement of the existing soybean varieties. To do so, in the beginning, requires the identification of genetic traits in soybean germplasm that are responsible for the successful improvement of crop tolerance. Later on, these genetic resources need to transfer to the subsequent cultivars. However, the CRISPR/Cas9 Genome-Editing System, phenotyping technology, genomic selection technology for molecular breeding are the few most advanced technologies to impart and enhance salt tolerance in soybean through varietal development [99, 100]. However, in different studies, soybean genotypes have been largely used to illuminate the morpho-physiological and biochemical responses under salt stress at different levels and durations (Table 4).

Accession name	Salt tolerance level	Tolerance traits	References
GmNcl1, GmNHX1	150 mM NaCl, 7 d	Photosynthetic rates, concentration of leaf K^+, Mg^2+, and P	[62]
Fengzitianandou, Baiqiu 1	100 mM NaCl, 7 d	Main root length, FW and DW of root, seedlings biomass, hypocotyls length	[101]
Java 7, Seputih Raman, Ringgit (JP 30217), Tambora, Sinyonya	100 mM NaCl, 35 d	Salt tolerance rate, leaf chl content (SPAD value), shoot DW	[102]
GmSALT3	200 mM NaCl, 18 d	Root and stem Na^+ content	[103]
PI 675847 A	200 mM NaCl	Leaf scorch scores, cell membrane stability, photosynthesis and biomass accumulation	[104]
SA 88, CX-415	150 mM NaCl, 23 d	Plant height, fresh and dry biomass of the shoot, relative leakage ratio, K^+/Na^+ ratio, proline (Pro) content	[105]
S04–05/150–114	90 mM NaCl, 8 h	Secondary structure contents of the protein isolates (α-helix, β-sheet, turn, and irregular conformations)	[106]
Clark	200 mM NaCl, 60 d	Nodule formation, leghemoglobin content, nitrogenase activity, auxin synthesis, MDA, and H2O2 content	[33]
En-b0–1, NILs72-T	150 mM NaCl, 21 d	Plant DW, nodulation, leaf greenness, and N uptake	[107]
HBK R5525	120 mM NaCl, 14 d	Leaf scorch score, leaf and root Na^+ and Cl^− concentrations	[108]
NILs72	120 mM NaCl, 28 d	Plant DW, photosynthetic rate, stomatal conductance, Na^+ and Cl^− content (leaf, stem, petiole, roots)	[109]
Pusa-9712, PS-1572	180 mM NaCl, 7 d	Seed germination, root and shoot length	[110]
GC840	160 mM NaCl, 15 d	Root and shoot length, FW and DW	[111]
F3	60 mM NaCl, 72 h	Level of gene and mRNA expression	[112]
Lee	150 mM NaCl, 45 d	Seed germination, shoot and root DW, and leaf mineral contents	[32]
BB52	300 mM NaCl, 7 d	Water potential, RWC, Pro and glycine betaine (GB) content, changes of ion content in young and mature seedings	[113]

Table 4.

Some of the salt-tolerant soybean genotypes as reported by researchers.

Plant tolerance to salt stress mostly depends on the capacity to regulate Na⁺ or chloride (Cl⁻) transportation to different parts of the plant from soil. Generally, tolerant soybean genotypes accumulate less amount of Cl⁻ in their leaf thus they can produce more photosynthates whereas the salt-sensitive lines accumulate more Cl⁻ in their leaves that reduces the net chl concentration in the plant [108, 114, 115]. Khan et al. [67] worked with 41 soybean genotypes under 100 and 150 mM NaCl stress where 7 genotypes performed as salt-tolerant under 150 mM NaCl, 21 genotypes performed as moderately tolerant under 100 mM NaCl, and rest was susceptible genotypes. Zhang et al. [101] worked on 257 genotypes with SSR markers to estimate epistatic association mapping for salt tolerance at the germination stage where 83 quantitative trait loci were detected. In addition, an experiment conducted by Luo et al. [116] with two wild soybean genotypes BB52 and N23232 collected from coastal and inland showed significant salt tolerance. In the greenhouse condition, an experiment was conducted hydroponically where 123 soybean accessions were found as salt-tolerant [117]. Salt stress highly restricts plant growth through imbalanced water status and disturbed ion uptake mechanisms. These phenomena were more focused in an experiment conducted by Shereen and Ansari [118] with four soybean cultivars viz. AGS-160, Loppa, Egyptian, ICAL-132 where ICAL-132 showed more tolerance in terms of the above-mentioned parameters. Cao et al. [102] experimented on 51 Indonesian accessions with 100 mM NaCl salt stress to speculate the salt tolerance rate, chl content (SPAD value). Among which 6 genotypes viz. Tambora, Sinyonya (late), Java 7, Sinyonya (early), Seputih Raman, and Ringgit (JP 30217) performed best as salt-tolerant genotypes. Hamayun et al. [119] conducted a screening of 69 cultivars of soybean under 100 mM salt stress for 2 weeks. The cultivars were placed in three groups (highly susceptible, susceptible, and tolerant) considering their performances under selected parameters. Salt stress remarkably affected root and shoot length and weight, photosynthesis, chl contents, transpiration rate, and nodule weight in most of the cultivars. Among 69 cultivars only 10 were finally considered as tolerant, 3 were susceptible, and the rest 56 were highly susceptible according to their overall performances. Four soybean cultivars were tested for salt tolerance under 80, 120, and 160 mM NaCl for 2 weeks. The tolerance level was estimated by leaf scorch score on a 1–9 scale (1 = no chlorosis; 9 = necrosis). The prominent differences between tolerant and sensitive cultivars were obtained at 120 mM NaCl in soil. Through this process, Williams and Clark were found as the most sensitive cultivars where the most tolerant were HBK R5525 and AG5905. In addition, leaf and root Na⁺ and Cl⁻ concentrations were analyzed where it was observed that in the leaf of sensitive genotypes accumulated higher Na⁺ than the tolerant one. At root, the opposite results were reported [108]. The presence of a higher amount of soluble protein in a plant is a good sign of plant physiological state. Under salt stress, it triggers the plant signaling to express tolerant gene by protein upregulation and enhanced enzymatic activities [120]. Saad-Allah [121] conducted an experiment with six varieties of soybean (Crawford, G21, G22, G35, G82, and G83) under different levels of sea salt (8, and 16 dS m⁻¹) and found that the least salt-affected cultivar was G82 in terms of the highest protein content. In addition to the genotype screening method, superior characteristic gene can be identified and isolated through exploring natural allelic variation. In this way, the isolated superior gene can be inserted into the targeted better yield performing cultivars to develop salt stress tolerance. This gene-based allele-specific multiple markers genotyping could be a prospective approach in developing more salt-tolerant varieties in the future [122].

4.2 Seed priming for salt tolerance in soybean

Priming, in the early stage of germination, can stimulate the metabolic processes that result in an enhanced germination rate with uniform emergence, which is very helpful for seeds to withstand different stresses particularly abiotic stress (Figure 3). Various types of priming (hydropriming, osmo-priming, nutrient priming, chemical priming, bio-priming, etc.) with different agents like water, inorganic compounds, hormones, and nutrients are mostly used to improve the performance of plant morphology, physiology, leaf gas exchange, transpiration, photosynthesis and antioxidant activities [123, 124].

Figure 3.

Seed responses raised from primed seed lead to salt stress tolerance.

During the priming process, seeds are treated to enhance enzyme activity, decrease the imbibition period, increase metabolic reparation and improve germination-promoting metabolites. Additionally, primed seeds showed better osmotic adjustment by activating a cellular defense system that increases tolerance levels against abiotic stressors [125].

The advantageous effect of priming is also prominent in soybean, like many other plants. For instance, when pretreated (100 mg L⁻¹ GA for 12 h) soybean seeds were sown, germination (88%), root and shoot length (16%), dry matter content of seedling (30%), vigor index (22%) and field emergence (8%) were increased compared to control. The number of pods plant⁻¹, seed pod⁻¹, and seed yield of pretreated seed also seen to perform better by giving a higher value at 47, 19, and 26%, respectively than untreated soybean seed [126]. A positive interaction between genotypes and priming was seen in the first pod length of soybean. Whereas, using proper priming procedure that specially adapted to cultivar, resulted in better first pod length of soybean which is desirable for the breeder [127]. Priming with polyethylene glycol (PEG) for 12 h with controlled low osmotic potential (−1.2) generated a higher final germination percentage while lowering MGT and electrical conductivity at soybean seedling compared to unprimed seed [128].

To mitigate the devastating effect of salinity in soybean, priming can be a promising measure. For example, soybean seed primed with benzyladenine resulted in higher leaf area, stomatal density, RWC at 11, 51, and 46%, respectively, and in turn improved plant growth and morphology even under high salt stress (250 mM NaCl) compared to a non-primed seed. Moreover, the number of flowers and pods, which are considered as principal attributes of soybean yield, were recorded to increase by 71 and 64%, respectively in primed seed under salt stress in comparison to non-primed salt-stressed soybean. The same trend of improvement was also recorded in biochemical and physiological parameters, especially increased antioxidant activity (92%) of priming seed was noticeable [73]. Besides, improving the early growth characteristics (germination percentage, seedling shoot, and root lengths) priming can enhance the tolerance of soybean seedlings through uplifting the activities of 𝛼-amylase, protease, and nitrate reductase along with the higher O₂^•−, H₂O₂, and nitric oxide (NO) contents [75]. When 5% saponin was used as a biopriming agent in soybean under salinity, shoot characteristics were seen to develop with increasing length, FW, and DW by about 15, 17, and 12%, respectively over the NaCl stressed plant (without saponin). Saponin minimized the salt-induced damages not only by ameliorating the declination of chl and carotenoid contents by about 31 and 14%, respectively, but also improving the MSI (6%) through decreasing H₂O₂ and lipid peroxidation by about 25 and 28%, respectively in comparison to stressed plant without saponin. They also made a remark that 5% saponin priming boost soybean seedling’s ability to withstand salt stress by improving the antioxidant system through enhanced enzyme activities and ascorbic acid (AsA) content and decreased glutathione (GSH) [53]. A similar trend of the beneficial effect of soybean was also recorded by Sheteiwy et al. [94] by using JA as a seed priming agent with a decreased Na⁺ and increased K⁺ concentrations under salt stress conditions compared to non-primed seed. In addition, a balanced regulation of endogenous hormones (e.g., ABA, GA, and JA) was recorded in the case of priming that in turn enhance protein production to induce a defense mechanism under salt stress. Interestingly, priming also can take part in anatomical changes to minimize the harmful effects of salinity. Generally, salt stress causes the destruction of mesophyll cells, as plasmolysis is a common phenomenon here and consequently water absorption deficiency occurs. On the contrary, priming can elude the plasmolysis of mesophyll cells of soybean under salt stress conditions and the number and size of plastoglobuli of leaf chloroplast were also lesser than normal salt-induced seedlings, which is the sign of less damage of thylakoid and photosynthetic capacity of mesophyll cells [129]. We also observed that soybean seeds pretreated with zinc sulfate (ZnSO₄·7H₂O) showed better growth performance under high salt stress (150 mM NaCl) compared to salt-stressed plants (without priming) (Figure 4; unpublished data). From this evidence, it can be concluded that priming not only improves the growth and developmental process of soybean seedlings but also enriches the metabolic performance and antioxidant capacity as well under salt conditions.

Figure 4.

Effect of zinc (Zn) priming in soybean under salt stress (150 mM NaCl) (unpublished result).

4.3 Agronomic managements

Like many other crops, various agronomic management practices can enhance salt tolerance in soybean. These management practices are mainly but not limited to manipulation of sowing times, mulching, water management, tillage management, soil amendments, fertilizer management, and so on. Manipulation of agronomic practices may affect soybean tolerance to salinity because salt gradient differs among the environmental conditions such as temperature, precipitation, and humidity. For example, An et al. [130] found that elevated humidity provided soybean better tolerance to severe salinity (120 mM NaCl, 3 weeks). Elevated humidity inhibited Na⁺ influx and improved root activity as well as stomatal conductance which are vital traits of salt tolerance in plants [130].

Various nutrient management showed enhanced tolerance of soybean to salt stress. In the salinity stress management of soybean, K⁺ plays an active role in increasing plant growth, development, and productivity [56]. Exogenous foliar K⁺ application was more efficient than seed priming in modulating soybean salt tolerance through growth, physio-biochemical attributes, yield, and seed quality. In brief, K⁺ caused more than 50% improvement in shoot length, shoot DW, leaves number, leaf area in salt stressed-soybean with higher total chl and carotenoid contents. This was because of the K-induced reduction in Na⁺ (69%) and Cl⁻ (59%) accumulation as well as higher K⁺ accumulation (47%) and K⁺/Na⁺ ratio (361%). Higher content of total soluble sugar, Pro, and α-tocopherol with antioxidants activity by K application in stressed-plant also contributed to the suppression of ROS and membrane damage with better water content. In addition, K-mediated higher seed yield (92%) with seed protein (63%) and oil (59%) content in soybean showed its regulatory role in enhancing yield and quality attributes under salinity and proved its potentiality to be used as a sustainable approach for crop production under saline condition [56]. Calcium (Ca) is another essential plant nutrient which actively involved in plant signaling responses for increasing plant salt tolerance like soybean [131]. From the proteomic analysis, it was revealed that exogenous Ca-activated 80 and 71 proteins in cotyledon and embryo were involved in signal transduction, energy pathways, transportation, and protein biosynthesis. In addition, Ca supplementation caused the inhibition in proteolysis as well as increment in the gamma-aminobutyric acid (GABA) and polyamines, osmolytes, and secondary metabolites accumulation for attuning salt tolerance in germinating soybean [131]. Likely, as a micronutrient for instance boron (B) not only perform a significant role in plant physiology and development but also provide protection in soybean against salinity (up to 150 mM NaCl). Supplemental B regulates the different physiological processes and improves the cellular structural integrity in plants under salinity, as evidence it increased shoot FW, chl content, leaf RWC, and Pro accumulation with lower EL as growth enhancement, osmotic stress mitigation, and membrane stability indicator, respectively [92]. This elevated Pro indicated B-mediated osmoregulation to enhance osmotic status (Leaf RWC) under salinity in soybean. Thereafter, Rahman et al. [45] studied the response of soybean plants under varying degrees of saline condition with supplementation of B alone or in combination with selenium (Se). Although separate supplementation of B and Se caused better growth and water content in salt-stressed soybean, their combined foliar application was better to alleviate stress toxicity under different levels of severity. For instance, the combined application of Se and B improved leaf RWC, leaf area, shoot height, and FW [45]. Further comprehensive studies are required to find out how and which nutrient will give more salt tolerance mechanisms together besides their nutritional value. In the study of Alharby et al. [5], only Se was applied as a foliar spray on salt-stressed soybean and confirmed Se-induced restoration of Pro, leaf RWC, higher chl accumulation. Therefore, such better growth, water status, and chl contents by 50 μM Se contributed to improvement of yield contributing and yield of soybean upon salinity. When soybean plants at an early stage were supplemented with foliar application of K (KCl and K₂SO₄), they showed enhanced tolerance to salt stress (6 and 12 dS m⁻¹) [132]. Especially, K enhanced the levels of antioxidant activities and secondary metabolites such as total polyphenol, flavonoid, chl, and carotenoid contents. However, these authors suggested that the appropriate K concentration should be fine-tuned and they also found that K₂SO₄ showed a better positive effect than KCl [132]. Organic amendments help in providing plants in improving soil and providing plant defense systems. For instance, chitosan-modified biochar (CMB) increases soybean tolerance to salt stress by enhancing plant morpho-physiology and antioxidant defense as reported by Mehmood et al. [133]. The addition of CMB resulted in 55 and 29% reduction of root Na content compared to the plants treated with salt (40 and 80 mM NaCl) alone. Shoot Na content was also decreased in the same way which was as low as 65 and 51%, respectively. These CMB treated plants produced higher osmolytes (GB and Pro), decreased H₂O₂ and MDA levels in plants. This was due to the upregulation of the antioxidant enzymes viz. catalase (CAT), ascorbate peroxidase (APX), peroxidase (POD), and superoxide dismutase (SOD) [133]. Adding water hyacinth compost and rice husk biochar (5 and 10 t ha⁻¹) also provided soybean plant tolerance to salinity (5 and 10 dS m⁻¹) as reported by Ferdous et al. [134]. However, rice husk biochar at the rate of 5 t ha⁻¹ provided better results and it was prominent on the plants grown at 10 dS m⁻¹ salinity.

4.4 Application of different stress elicitors

There is a necessity to utilize external stimulation in parallel with plants’ inherent tolerance mechanism to tolerate salinity. Use of exogenous protectants such as phytohormones, antioxidants, amino acids, osmolytes, signaling molecules for regulating soybean salt tolerance mechanism to sustain better growth, development, and yield (Table 5).

Extent of salt stress	Stress elicitors	Tolerance responses to salinity	Reference
100 mM NaCl, 14 d	5 μM kinetin	Enhanced endogenous JA and SA contents with reduction of ABA; Improved growth and chl content; Improved isoflavones concentration	[135]
100 mM NaCl, 23 d	5 μM GA3	Promoted shoot length, plant FW and DW, chl content; Increased daidzein and genistein contents; Improved bioactive gibberellins (GA1 and GA4) and JA; Declined endogenous ABA and SA	[136]
10 dS m^−1 NaCl	1 mM SA	Increased N and sulfur (S) contents; Improved seed yield and quality with higher content of protein and amino acid	[86]
100 mM NaCl	200 mg L^−1 SA	Increased chl content; Augmented Pro accumulation; Increased sugar and starch contents	[137]
100 mmol L^−1 NaCl, 21 d	60 μmol L^−1; JA	Improved water content, water potential, osmotic potential, and WUE; Increased net photosynthetic rate, transpiration rate, stomatal conductance, intercellular CO2, and total chl content by 60, 51, 78, 40, and 42%, respectively; Improved ABA, GA, JA by 61, 63, and 52%, respectively; Decreased Na^+ concentration with higher K^+ in both leave and root tissue; Increased shoot and root length with both FW and DW	[94]
80 mM NaCl, 16 d	10 μM NO as 2,2′(hydroxynitrosohydrazono) bis-ethanimine	Improved plant growth as evidenced by higher shoot, root, and nodule weights and nodule numbers; Lowered H2O2 to basal levels; Reduced cell death	[138]
100 mM NaCl, 7 d	100 μM NO as SNP	Decreased Na^+ content with higher K^+ and Ca^2+ uptake; Increased germination percentage; Increased anthocyanins and flavonoid content	[27]
70 mM NaCl, 15 d	50 μM NO as SNP	Improved shoot and root length; Reduced browning, tissue drying, and increased plant survivability; Increased chl and Pro contents with better RWC; Stabilized Na^+ and K^+ ion ratio	[139]
100 mM NaCl, 28 d	10 μM NO as SNP	Increased root FW (38%), shoot FW (29%), whole plant DW (75%), root DW (58%), shoot DW (27%), and whole plant DW (29%); Increased chl a (38%), and chl b (44%) contents; Reduced Na^+ content in leaf (117%), and root (119%); Improved K^+ content in leaf and root	[140]
200 mM NaCl, 12 h	150 μM NO as SNP, 2 d	Increased ABA content and reduced stomatal conductance; Increased RWC and chl content	[141]
15 mM NaCl	Pro, 25 mM	Lowered Na^+ accumulation.; Elevated K^+ and N contents; Increased shoot DW and Pro content with reduction of EL	[142]
11 dS m^−1 NaCl	Foliar spray of GB at 10 kg ha^−1	Reduced Na^+ and Cl^− uptake; Increased number of lateral branches (33%), pods plant^−1 (49%), and grain yield (71%); Increased endogenous GB content	[143]
50 mM NaCl, 7 d	2 mM GSH	Increased plant height, branch number; Increased pod number about 12–60% with increment in seed number pod^−1; Improved 100-seed weight as well as yield plant^−1; Reduced H2O2 and MDA contents	[46]
150 mM NaCl, 39 d	1 mM GSH	Increased shoot length, leaf number, and leaf area; Enhanced pod number, 100-seed weight, seed protein, and oil percentage; Improved total chl content, chl florescence, and performance index.; Increased RWC and membrane stability with reduction of EL; Elevated AsA, GSH, and α-tocopherol content; Reduced Na^+ uptake with higher accumulation of N, P, K, and Ca	[58]
3 and 6 g L^−1 NaCl, 30 d	40 mg L^−1 cysteine	Augmented the content of chl a, b, and carotenoid; Elevated Pro accumulation and the content of N, P, and K; Lowered the generation of H2O2 and MDA; Increased yield with higher seed oil content	[54]
7.46 dS m^−1 NaCl, 70 d	6 mM K as K2SO4	Enhanced leaf number (74%), leaf area (52%), and shoot DW (56%); Increased chl and carotenoid contents by 185 and 20%, respectively; Enhanced K^+ content with reduction of Na^+ and Cl^− uptake.Improved Pro accumulation; Reduced the contents of H2O2 and MDA; Improved seed yield and quality	[56]
6 dS m^−1 NaCl, 45 d	50 μM Se, as Na2SeO4	Increased plant growth and chl content; Improved Pro content with better water status.; Reduced H2O2 generation and membrane damage; Improved seed yield	[5]
9 dS m^−1 NaCl, 8 weeks	AsA at 100 and 200 mg L^−1	Increased nutrients contents like N, P, K, Fe, manganese (Mn), and Zn; Reduced Pro content; Improved chl and carotenoid content; Improved growth, yield attributes, and quality.	[81]
Sandy coastal soil	Seaweed compost, 60 t ha^−1.	Increased plant growth rate, root DW, and pod number	[144]

Table 5.

Exogenous stress elicitors-induced salt tolerance in soybean.

Plant hormones cause plant growth and development and are widely used for that, but they also play significant roles in mitigating the adverse effects of salinity to crop production. Exogenous application of kinetin (a synthetic cytokinin) significantly improved soybean growth attributes including plant height, plant FW and DW, chl contents under saline conditions [135]. Kinetin directly enhances isoflavones biosynthesis as well as modulates phytohormone crosstalk which are involved in soybean growth regulation and stress resistance. Very recently, exogenous salicylic acid (SA) recovered significant soybean growth and yield along with elevated RWC, osmolytes accumulation, the content of chl, sugar, and starch in soybean as well as the reduction in the accumulation of toxic Na⁺ under salinity [145]. Therefore, SA is able to alleviate the salt-induced both ionic and osmotic stress in soybean through regulating ionic and osmotic balance, respectively. Similarly, JA foliar spray showed better water content and water use efficiency (WUE) along with improved chl content, intercellular carbon dioxide concentration, stomatal conductance, and transpiration rate which ultimately resulted in a higher net photosynthetic rate in salt treated-soybean [94]. Therefore, JA-mediated lower Na⁺ and higher K⁺ accumulation, regulation of osmolyte content, and improvement in plant hormone syntheses like ABA, GA, and JA resulted in better salt tolerance mechanisms in soybean [94].

Signaling molecules have important roles in improving various physiological attributes and adaptive mechanisms to recover plant growth against salinity. Nitric oxide (as sodium nitroprusside, SNP) supplementation significantly increased K⁺ and Ca²⁺ uptake with inhibition of toxic Na⁺ uptake and thus promoted ion homeostasis followed by stimulated activities of polyphenol oxidase and phenylalanine ammonia lyase of soybean in salt exposure [27]. Thus, NO-mediated higher content of flavonoids and anthocyanins suggested the NO might be affected the biosynthesis pathways of pigments to confer salinity tolerance. But the combined application of NO and SA showed more effective functional roles in salt mitigation by decreasing NaCl-induced damaging effects than individual use. The exogenous NO is also effective in alleviating the long salt toxicity of soybean as evidenced by higher growth parameters like not only shoot and root growth but also nodule weights and nodule number [138]. In addition, NO-mediated stimulation in antioxidants activities was reported which contributed to attaining higher salt tolerance and growth. From the recent study, it has been disclosed about NO-mediated higher chl content, better growth attributes, and maintenance in ion homeostasis [140]. The role of osmoprotectants including Pro and GB had been proved to increase salt tolerance in soybean with the indication of suppression in osmotic, ionic, and oxidative stress markers significantly [146].

As an important bioregulator, the amino acid has effective regulatory roles in plant growth, development, and productivity with active roles in scavenging excess ROS, thus increasing researchers’ thirst to use in the regulation of plant stress management. In accordance, as a precursor of GSH containing non-protein thiol, cysteine actively improves plant stress responses upon adverse environmental conditions [54]. Foliar application of cysteine significantly improved nutrient accumulation by plants with higher osmoregulation reflected as augmented Pro content in salt-treated soybean. Not only that, cysteine-treated plants showed lower ROS generation and membrane damage as recovered from salt-induced oxidative stress. Therefore, salinity-mediated growth inhibition with suffering from lower photosynthetic pigments contents alleviated by cysteine application and thus soybean showed higher tolerance attributs under stress condition.

Glutathione is a vital component of plant antioxidant defense mechanism and also plays a key role in regulating ROS management, thus GSH had been used as a protectant to increase salt tolerance of soybean as well as to explore the mechanism of GSH [46]. Such exogenous GSH application improved soybean salt tolerance behavior through minimizing stress-induced oxidative stress and thus caused improvement in yield attributing characters leading to higher yield.

4.5 Use of beneficial microbes

Besides plant growth improvement, microbial inoculants including bacteria, fungi, and microbial symbiosis have been already enlisted for increasing plant stress tolerance including salinity [72]. Plant growth-promoting rhizobacteria (PGPR) are the potential to improve plant growth by mitigating salt toxicity, where plant and soil health are benefitted from the interaction of roots with these microorganisms and plant roots [80]. Bacillus firmus SW5 showed the protective role on salt stressed-soybean by increasing growth and biomass production [80]. Bacterial inoculation also caused higher nutrient accumulation, the content of chl, osmolytes (GB and Pro), soluble sugar, phenolic compound and also gas exchange parameters in stressed-soybean. Both Bradyrhizobium japonicum USDA 110 and Pseudomonas putida TSAU1 coordinately enhanced soybean growth and root architectural traits under saline conditions due to higher auxin production [147]. This better root growth in length, number, and surface area later contributed to higher nutrient absorption from the soil. Five different strains of PGPR including Arthrobacter woluwensis AK1, Microbacterium oxydans, A. aurescens, B. megaterium, and B. aryabhattai significantly improved soybean tolerance to salinity [4]. These salt-tolerant stains not only improved nutrient accumulation and chl biosynthesis, but also caused the improvement in hormonal regulation like higher indole-3-acetic acid (IAA), GA production with decreasing ABA content in soybean under 200 mM NaCl. In addition, halotolerant PGPR bacteria could be used as a biological safe tool for increasing plant growth by alleviating salt toxicity [4]. Arbuscular mycorrhizal fungi (AMF) were used for increasing the salinity tolerance of both salt-tolerant and sensitive cultivars of soybean [33]. Salt-stressed soybean showed better plant growth with higher nodule formation, leghemoglobin content, and nitrogenase activity under salinity with AMF inoculation irrespective of tolerance level, whereas higher AMF inoculation was detected in tolerant genotype. Plant hormones have a stimulating role in plant growth maintenance as an essential member of metabolites and in such fungal symbiosis with soybean, auxin exhibited a prime signaling role in between AMF and host plants. Hashem et al. [33] observed such increment in IAA and indole-3-butyric acid (IBA) level in AMF inoculated salt-stressed soybean regardless of cultivars. Therefore, the salt tolerance ability of soybean through microbial association is enhanced due to the stimulation in endogenous growth hormone followed by better root growth, nutrient acquisition [33, 147]. These AMF also contributes to mitigate salt-induced oxidative stress in soybean by suppressing membrane damage and ROS generation [33]. Beneficial microbes-induced modulation of different mechanisms for attaining better salt tolerance in soybean has been summarized in Table 6.

Salinity levels	Microbial inoculation	Tolerance responses to salinity	Reference
200 mM NaCl	Mycorrhizal fungi Funneliformis mosseae, Rhizophagus intraradices, and Claroideoglomus etunicatum	Improved the nodule formation and plant root structure; Increased nutrient accumulation; Enhanced the content of auxin and chl; Lowered H2O2 production and membrane damage	[33]
100 mM NaCl, 22 d	Bradyrhizobium japonicum	Prompted root nodulation and seedling growth; Increased chl and carotenoids content; Improved maximum photochemical efficiency of PSII; Decreased EL; Maintained ultrastructure of thylakoid and chloroplast of mesophyll cells; Raised root isoflavone content	[148]
120 mM NaCl, 10 d	Bacillus amyloliquefaciens H-2-5	Enhanced plant growth with 10% higher shoot length and GA4 content; Lowered the concentration of ABA, SA, JA, and Pro	[149]
150 mM NaCl	Rhizobium sp. SL42 and Hydrogenophaga sp. SL48, co-inoculated with B. japonicum 532C	Increased shoot biomass, seed weight, and grain yield.Enhanced N assimilation and shoot K^+/Na^+	[150]
100 mM NaCl, 7 d	Pseudomonas sp. strain AK-; 1, and Bacillus sp. strain SJ-5	Increased plant FW with higher chl content; Enhanced water content with higher Pro accumulation and thus lowered osmotic injury; Elevated photosynthesis activity; Suppressed MDA production	[14]
70 and 140 mM NaCl, 7 d	Porostereum spadiceum AGH786	Increased seedlings’ growth and reduced transpiration rate; Enhanced GA content with reduction of ABA and JA production; Increased isoflavones content	[151]
120 mM NaCl, 10 d	Pseudomonas putida H-2-3	Enhanced the shoot length and FW; Increased chl content.Reduced the contents of ABA and SA with higher content of JA; Decreased Na^+ content; Lowered the total polyphenol with increment in total flavonoid content	[152]
75 mM NaCl, 42 d	B. japonicum BDYD1; Stenotrophomonas rhizophila ep-17	Increased plant growth attributes like root length, shoot length, shoot and root DW; Elevated nutrient uptake including N and P with a higher number of nodules	[147]
75, and 150 mM NaCl, 46 d	Arthrocnemum macrostachyum, 7 and 14 d	Increased shoot and root length, FW of shoot and root, and their DW with higher leaf number; Reinforced the chl a, b, and carotenoids, soluble sugars, and proteins contents; Decreased MDA and H2O2 contents; Increased Pro, total free amino acids, total phenols, and AsA content	[51]
70 and 140 mM NaCl, 7 d	Metarhizium anisopliae pretreatment, 21 d	Increased plant growth with higher leaf area and chl content; Improved transpiration and photosynthesis rate; Elevated Pro content with suppression of MDA generation; Reduced ABA with higher JA contents; Increased isoflavonoids content from 9 to 17%	[153]

Table 6.

Beneficial microbes-mediated mechanism in increasing salt tolerance of soybean.

4.6 Enhancing antioxidant defense

Enhancing the productivity of the antioxidant defense system and synthesis of antioxidant enzymes may provide a safeguard against salinity-induced oxidative stress [154]. In soybean, salt tolerance can be elevated by enhancing the activity of antioxidant enzymes viz. CAT, APX, glutathione peroxidase (GPX), glutathione reductase (GR), POD. Higher enzymatic activity helps to repair the ROS-induced membrane dysfunctions, which ultimately accelerates plant growth by maintaining chloroplasts and other cell organelles. Such regulatory effects of the antioxidant defense system were observed in different experiments (Table 7). Application of different types of phytoprotectants, trace elements, nutrient elements, or organic acids play vital roles to increase plant antioxidant defense capacity under salt stress [154]. In addition, different gene expression is triggered by salt stress that can enhance the activity of antioxidant enzymes to provide a defense under prolonged salt conditions. For example, Mehmood et al. [133] observed the higher activity of four antioxidant genes (CAT, APX, POD, and SOD) and two salt-tolerance conferring genes (GmSALT3 and CHS) under 40 and 80 mM salt stress in 13 soybean cultivars. The result suggested that when the CMB was used as a protectant under salt stress, the expression profile of salt-tolerant genes GmSALT3 and CHS get increased and their expressions are high during the vegetative stage of the crop which is significantly affected during salinity stress.

Dose and duration of salt stress	Changes in antioxidant defense level and enzyme activities	Reference
150 mM NaCl, 39 d	The activities of SOD, CAT, and GPX were increased; Enzymatic protein content in leaf was reduced	[58]
200 mM NaCl, 21 d	FeSOD, POD, CAT, and APX activities in root were improved	[94]
3 and 6 g L^−1 NaCl, 30 d	Increased CAT activity, but reduced SOD activity	[54]
100 mM NaCl, 25 d	Salt stress increased SOD, CAT, APX, and GR activities by 31, 16, 20, and 11%, respectively	[53]
7.46 dS m^−1 NaCl	The activities of SOD, CAT, APX, and GPX were increased	[56]
100 mM NaCl, 2 d	CAT and APX activities were increased by 3.6 and 1.4-folds, respectively	[59]
200 and 400 mM NaCl, 69 d	Activities of CAT, APX, and GR were increased with increasing salt concentrations; APX activity was higher in Clark under 200 mM salt treatment	[31]
100 mM NaCl, 30 d	SOD, APX, and GR activities were increased by 37, 40, and 33%, respectively; Reduced AsA by 16% while increasing GSH and tocopherol content by 27 and 15%, respectively	[91]
100, 200, 300 mM NaCl, 10 h	Increased the activities of polyphenol oxidase and POD; Total protein content was increased; Reduced GSH concentration was observed	[4]

Table 7.

Regulation of antioxidant defense system by enhancing enzyme activities in soybean.

Superoxide dismutase, as an antioxidant enzyme plays a vital role under different abiotic stresses, and their functions are well documented during oxidative stress. However, the role of SOD family genes under salt stress is little explored. Lu et al. [155] observed the SOD gene expression under 50 mm NaHCO₃ for 6 and 12 h. However, the soybean transcriptome data under alkaline stress revealed that six soybean SOD genes were differentially expressed under salt stress. Among them, only GmFSD3, GmFSD5, and GmCSD5 were all up-regulated under alkaline and salt stress, which denote that they might have a positive regulatory role under such stress condition. The differential expression of GmFSD3 and GmFSD5 in soybean leaves and roots suggests that these two genes may be involved in different signaling pathways under salt stress.

5. Conclusion and perspectives

Salinity has a destructive effect on plants by imparting osmotic stress as well as ionic imbalance and toxicity. Soybean is a moderately salt-sensitive crop and cannot withstand saline conditions for a long duration. Most of the cultivated soybean are glycophytic in nature and originated through the domestication or cross-breeding with the wild type of soybean. This is why the yield performance, growth, and quality of the cultivated soybean are lower under salt stress. Soybean is a crop of versatile uses as both human food and animal feed. Therefore, the yield or prior reproductive development should be emphasized while considering different attributes for studies. Avoidance or escape mechanisms should be introduced to ensure the expected yield from soybean whether it is cultivated in favorable or unfavorable environmental conditions. Many research works have been conducted related to the soybean responses and tolerance to salt stress. However, these results are largely inconsistent due to the genotypic and experimental variations. However, the precise mechanisms of salt stress tolerance and finding the biochemical, molecular, and genetic bases of such mechanisms should be investigated comprehensively. Tailoring salt-tolerant traits is, therefore, a vital task for future plant biologists. Screening large number of genotypes and finding the appropriate genotype for saline environment would overcome the hindering of soybean production in those areas. These interventions would provide breeders and agronomists with climate-resilient soybean cultivation packages in the changing world to ensure global food security.

Acknowledgments

We acknowledge the Ministry of Science and Technology (MoST), Government of the People’s Republic of Bangladesh, and Sher-e-Bangla Agricultural University Research System (SAURES), for providing funds in the research on salt stress in soybean. We thank Md. Rakib Hossain Raihan, Shamima Sultana, Ayesha Siddika, Maliha Rahman Falguni, and Khadeja Sultana Sathi for providing important literature on soybean and salt stress.

Conflict of interest

The authors declare no conflict of interest.