Oilseed Brassica Responses and Tolerance to Salt Stress

Abstract

Salinity interrupts osmoregulation, hinders water uptake, reduces water content, causes ionic toxicity, reduces chlorophyll content, alters stomatal conductance/movement, decreases enzymatic activity, alters transpiration and photosynthesis, disrupts the antioxidant defense system, and results in the oxidative burst. In turn, seed and oil yield is greatly declined. To overcome salinity-induced osmotic and ionic stress, plants evolve avoidance or tolerance mechanisms in order to protect the cellular components from sustaining growth and development. Ion homeostasis, vacuolar compartmentalization, accumulation of secondary metabolites, hormonal regulation, osmolytes production and by activating defensive responses, plants endure the salinity-induced damages, and enhance the stress tolerance. However, these salt-tolerant traits are greatly varied with species and genotypes as well as the extent of salt stress. Therefore, precise studies in understanding the physiology and molecular biology of stress are important to understand Brassica oilseed crops’ responses and tolerance to salt stress. In this chapter, we summarize the recent findings on the Brassica plants’ response to salt stress and later discuss the possible ways to enhance salt stress tolerance.

1. Introduction

Soil salinity can commonly be caused by the excess amount of carbonate, bicarbonate, sulfate, and chloride salt of magnesium, calcium, potassium, and sodium. Having a high quantity of adsorbed sodium ions (Na⁺) in sodic soil, the soil structure is degraded, for which aeration and water movement are limited [1]. According to the report of FAO [1], more than 424 million hectares of topsoil (0–30 cm) and 833 million hectares of subsoil (30–100 cm) are affected by salinity. Among the salt-affected soil topsoils, 85% are saline, 10% are sodic, and 5% are saline-sodic. On the other hand, 62% are saline among subsoils, 24% are sodic, and 14% are saline-sodic. The data (data on 118 countries covering 73% of the global land area) also represent more than 4.4% of topsoil, and more than 8.7% of the subsoil of the total land area is salt-affected [1]. Saline, sodic, and saline-sodic soils and any other subcategories of salt-affected soils contain too much soluble salts capable of causing an anomaly in various physiological processes in most cultivated plants [2, 3]. Soil salinity primarily provokes osmotic stress by lowering the soil water potentiality, thus reducing water uptake in plants. Whereas another effect of salinization is the imposition of ion toxicity, particularly due to excessive deposition of Na⁺ and chloride ions (Cl⁻) in the upper part of the plants, and also interferes with the accumulation of essential nutrients [4]. Interference of salt stress in plants is liable for the disruption of metabolic activities such as permeability, biosynthesis of photosynthetic pigments and induces photosystem (PS) inefficiency of plants [5]. Forthcoming salt stress inhibits cell division, hampers cell expansion, alters stomatal closing and opening, reduces turgor pressure, and causes an imbalance in ionic homeostasis [6].

Oxidative stress due to the overgeneration of oxygen radicals and their derivatives, which are called reactive oxygen species (ROS), is the secondary effect of salinity. These ROS could be hydrogen peroxide (H₂O₂), ozone (O₃), singlet oxygen (¹O₂), superoxide radicals (O₂^•−), organic hydroperoxide (ROOH), hydroxyl radicals (^•OH), perhydroxy radical (HO₂^•), and peroxyl (RO₂^•), etc. [7]. The generation of ROS is a general phenomenon of plants in a normal condition to regulate different biological processes such as growth, cell cycle, hormonal regulation, defensive responses against biotic and abiotic stresses, program cell death, and development [6]. But under stressful conditions, excessive generation of ROS leads to oxidative burst and causes damage to cellular components such as carbohydrates, lipids, proteins, and nucleic acids [8]. To combat the adverse effect of this ROS-induced oxidative stress, plants are intrinsically organized with antioxidant defense mechanisms where both non-enzymatic and enzymatic antioxidants work in a coordinated manner to detoxify the over-accumulated ROS. The non-enzymatic antioxidants include flavonoids, carotenoids, vitamins, ascorbate (AsA), and glutathione (GSH), and enzymatic antioxidants are ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR), superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), glutathione S-transferase (GST), etc., actively perform their role in quenching the ROS to protect the plants from the oxidative stress [7].

Brassica is placed third among different oilseed species after soybean and palm due to its considerable economic and nutritional value [9]. The genus Brassica belongs to Brassicaceae family, which has nearly 435 genera and 3675 species [10]. Having food value and economic importance, plants of Brassica genus are well known as edible oil, vegetables, and silage. Rapeseed (Brassica campestris L. and Brassica napus L.) and mustard (Brassica juncea L. [Czern. & Coss.] and Brassica carinata A.Br.) are the most cultivated oil-yielding plants of the genus Brassica. Europe, as well as North America, cultivates mostly B. napus and B. rapa. The species B. carinata is mostly cultivated in North Africa. B. juncea is popular in South Asian countries. Brassica nigra (L.) Koch and B. tournefortii Gouan are limited to very small area [11, 12]. It is clear that the oilseed Brassica plants are cultivated in different continents throughout the world including Europe, America, Asia, and Africa [12]. Studies also prove that salinity is a major problem in different countries of the world, and this salinity is creating difficulties in the proper growth and development of oilseed Brassica plants. This plant is more sensitive to salinity in germination, seedling, and reproductive stage. Salinity interrupts osmoregulation, hinders water uptake, reduces water content, causes ionic toxicity, reduces chlorophyll (Chl) content, alters stomatal conductance/movement, decreases enzymatic activity, alters transpiration and photosynthesis, disrupts the antioxidant defense system, and results in the oxidative burst [13, 14, 15]. Considering the detrimental effect of salt stress, it is crucial to understand the mechanism of salt-induced damage and tolerance in Brassica plants. Inter- and intraspecific differences evidently exist between and among species of Brassicaceae family plants for various stress tolerances including salinity. Their response to salinity and exogenous elicitors under salt stress differs [12, 16]. Understanding all of these is important for enhancing salt tolerance or developing salt-tolerant oilseed Brassica plant cultivars. Osmoregulation, hormonal regulation, antioxidant defense, and signaling function are some of the basic strategies that need to be understood for developing salt-tolerant cultivars. Various approaches such as agronomic practices, screening of salt tolerance traits among different Brassica plants, traditional breeding, biotechnological approaches, and microbe assistance are some of the approaches practiced for improving salt tolerance capacity of oilseed Brassica plants [17, 18, 19]. This review presents the previous findings and recent progress in some approaches for the development of salt tolerance in oilseed Brassica plants.

2. Salt stress responses in oilseed Brassica

2.1 Seed germination and seedling establishment

Poor seed germination, emergence, and seedling growth are among the earliest effects of salt stress on plants [20, 21]. Many pieces of research have revealed that halophytes such as Suaeda salsa and Salicornia europaea have a strong salt tolerance capacity during the germination stage. However, their germination rates are decreased with increased salinity levels [2, 22], while glycophytes are highly vulnerable to salt stress [23, 24]. Likewise, the salt tolerance capacity of B. napus is much lower than the euhalophytes [22, 25, 26]. While screening 549 inbred lines of B. napus, Wu et al. [27] found that in the presence of 200 mM NaCl, the seed germination rate of 15 randomly selected inbred lines was decreased.

The seed germination rate and germination percentage (GP) can also be reduced by salinity due to the ionic toxicity and imbalanced nutrient uptake potentiality of plants. Shahzad et al. [19] stated that under saline conditions, B. napus showed reduced GP, and the germination rate was slower than normal due to the ionic toxicity or unavailability/reduced nutrient (mainly K⁺) uptake ability. According to Damaris et al. [28], the seed germination process is mainly associated with two important enzymes such as α-amylase and protease. Therefore, the seed germination process is hampered due to the reduced activity of these two enzymes under saline conditions [29]. Tan et al. [30] conducted an experiment with 520 B. napus germplasms to evaluate their seed GP and germination index (GI) under salt stress and distilled water. There was a large variation seen in both GP and GI values, which were as follows: GP ranging from 26 to 100% and from 0 to 100%, GI ranging from 3 to 54 and from 0 to 25 in distilled water and salt stress, respectively. Another study by Li et al. [31] with canola (B. napus) seeds under three levels of salt stress (50, 100, and 150 mM NaCl) stated that seed germination rates were clearly decreased with the increasing levels of salt stress.

Wan et al. [20] experimented with 214 B. napus inbreed lines under different levels of NaCl concentration (40, 80, 120, 160, 200, 240, 280, and 320 mM) where the results showed that the germination was inhibited with the increased salt stress levels. Most importantly, a significant variation in germination was observed in 160 mM NaCl solution between different B. napus inbred lines. Ahmad et al. [32] observed that B. napus seeds germinated slowly after the addition of different NaCl concentrations (50 and 100 mM) into the germination medium. However, under 100 mM NaCl concentration, the germination rate was the lowest with about 30% decrease compared with control condition. The establishment and early growth of seedlings can be both promoted or hampered under salt stress. Fang et al. [33] showed that 25 mM of NaCl solution promoted B. napus seedling growth, but the growth was negatively affected with increasing NaCl concentrations (50 and 100 mM).

2.2 Plant growth

Abiotic stressors, such as increasing soil salinity, have been shown to have negative impacts on plant growth and development on many plants [34]. Similarly, the adverse effects of salt stress on the growth and development of oilseed Brassica have been widely documented [32]. The plant family Brassicaceae along with the other plants in general shows sensitivity to salt stress that declines the growth and biomass, while retaining a large biomass, indicating tolerance (Table 1). Long et al. [46] observed that salinity stress affects B. napus root growth at 12-h post-exposure. According to Ashraf et al. [47], increasing salinity slows down cell division and cell elongation because it reduces nutrient absorption, disrupts cell membranes, causes cells to lose their turgidity, and alters hormonal balance, all of which have an impact on plant growth and development. Under 100 and 200 mM NaCl stress, plant growth in terms of the fresh and dry weight of roots and shoots was lowered to a significant degree in B. napus plants; however, the reduction was more prominent under 200 mM NaCl [48].

Species and Cultivars	Salinity doses and duration	Responses	Reference
B. juncea cv. CS-54	150 mM NaCl; 10 d	Total biomass accumulation was reduced by 1.69-fold	[35]
B. juncea cv. Pusa Jai Kisan	100 mM NaCl; 15 d	Reduced leaf area (LA) and dry mass	[36]
B. napus cv. Dwarf Essex	100 mM NaCl; once a week	Reduced root and shoot fresh weight (FW) and dry weight (DW)	[37]
B. juncea cv. Pusa Bold	50, 100, 150, 200, 250, 300 mM NaCl; 30 d	Reduced LA, shoot FW and DW, root FW and DW	[38]
B. juncea cvs. CS-52, Pusa Agrani, Pusa Vijay, Pusa Varuna, CS-54, Pusa Jai Kisan, Pusa Bahar, Pusa Kranti, Pusa Bold, Pusa Laxmi, Pusa Basant, ZEM-1, RC-781 and JM-1	50, 100, 150, and 200 mM NaCl; 10 d	Maximum biomass reduction in Pusa Agrani, minimum in CS-54 in a concentration-dependent manner	[39]
B. juncea cv. Varuna	200 mM NaCl; 90 d	Reduced shoot length (SL) by 59%, root length (RL) by 41% and DW by 53%	[40]
B. carinata cvs. Adet and Merawi	50, 100 and 150 mM NaCl; 28 d	Reduced RL (47 and 49%), SL (35 and 38%), leaf number (LN) (20 and 29%), LA (28 and 27%) in Adet and Merawi at 150 mM NaCl, respectively	[41]
B. juncea cvs. Varuna and RH-30	78, 117, and 156 mM NaCl; 60 d	Reduced root FW and DW, shoot FW and DW, LA; Varuna showed more tolerance than RH-30	[42]
B. napus cvs. Yangyoushuang2, Xiangyouza553	100 mM NaCl; until harvesting	Reduced RL (5%), SL (46%), LN (7%), LA (16%), root DW (23%), stem DW (36%) and leaf DW (18%)	[43]
B. juncea	8, and 12 dS m^−1 NaCl; 5 and 10 d	Reduced growth in a dose-dependent manner	[44]
B. juncea cv. Varuna	50 and 100 mM NaCl; 30 d	Reduced SL (54%), RL (54%), shoot FW (56%), root FW (53%), shoot DW (57%) and root DW (52%)	[15]
B. juncea cv. Varuna	100 mM NaCl; 15 d	LA and plant FW were decreased significantly under salt stress	[45]

Table 1.

Alteration in growth parameters of oilseed Brassica sp. under salt stress.

Mohamed et al. [43] showed that salinity affected B. napus root system and aboveground growth characteristics significantly. They estimated that the osmotic impact of NaCl stress increases growth inhibitors, decreases growth promoters, and disrupts the water balance of NaCl-stressed plants, which might cause these growth reductions. Salinity reduces some morphological attributes of plants, such as root length, shoot length, root fresh weight, root dry weight, shoot fresh weight, shoot dry weight, leaf number, leaf area, and leaf size. Lei et al. [49] indicated that under 100 mM salinity stress for 144 h, the overall growth rate of B. napus seedlings was reduced significantly. Moreover, Wani et al. [50] recorded some alteration in shoot length and leaf area under salt stress by 34% and 47%, respectively.

2.3 Nutrient imbalance

Salinity hampers plants’ normal growth environment by altering the nutrient status of the soil. Due to excessive accumulation of Na⁺, the uptake of macronutrients, for example, nitrogen (N), phosphorus (P), calcium (Ca), and potassium (K), and micronutrients, for example, zinc (Zn), iron (Fe), manganese (Mn), is affected. However, over-accumulation of Na⁺ highly changes the uptake of K⁺, causing changes in ion homeostasis and stomatal opening of the plant cells. Na⁺ transport is unregulated in most of the salt-sensitive species of oilseed Brassicaceae family [35]. As Na⁺ tends to accumulate more quickly to a harmful level than Cl⁻, most research has focused on Na⁺ exclusion and controlling Na⁺ transport within the plant cell [4]. Many of the experiments with the members of oilseed Brassicaceae family showed that salt stress potentially decreased essential macro- and micronutrients uptake. Iqbal et al. [51] experimented with B. juncea under 100 mM salt stress, where both the leaf N content and the activity of nitrate reductase (NR) enzyme, related with N-uptake and metabolism, were significantly reduced. Another study from Yousuf et al. [35] found that under 150 mM of NaCl concentration, B. juncea showed a considerable reduction (1.63-fold) in NR activity than the control plants. Therefore, it is evidently proved that the activity of nitrate influx is substantially reduced under extreme salinity stress.

Under salt stress, total N content in oilseed Brassica plant leaves was declined, as did the concentrations of essential micronutrients, such as Fe, Zn, and Mn in the root, stem, and leaves [3]. They did, however, reveal that tolerant genotypes were able to retain higher N and other micronutrient levels when stressed. Also, a comparative study from Singh et al. [52] showed changes in the macronutrient (K, Ca, Mg, P, and S) and micronutrient (B, Fe, Zn, Mn, Cu, and Co) concentrations under salinity stress (25 and 150 mM NaCl) in two cultivars (CS-52, and Ashirwad) of B. juncea. Both of the cultivars showed an increase in B, Mn, and Cu contents under salt stress, but Fe, Zn, and Co contents were dropped considerably. Salinity stress causes an increase in the accumulation of harmful ions, particularly Na⁺, resulting in ion imbalance and hyperosmosis in plants. Nazar et al. [36] found that B. juncea plants showed an increased level of Na⁺ and Cl⁻ ion content in the leaves. The physiochemical processes of plant cells are weakened as a result of this imbalance, which hindered plant growth. As a result of excessive Na⁺ concentration in cells, K⁺ uptake is inhibited, which results in an elevated Na⁺/K⁺ ratio [53].

El-Badri et al. [54] experimented with five cultivars of B. napus (Yangza 11, Zhongshuang 11, Huayouza 62, Fengyou 520, and Yangyou 9) under different salinity levels (50, 100, 150, and 200 mM NaCl). It showed that in the tolerant cultivar Yangyou 9, Na⁺ accumulation was lower than the sensitive cultivar Zhongshuang 11, which elevated the K⁺ uptake in the tolerant cultivar under stress condition. In Zhongshuang 11, Na⁺ content (49 mg g⁻¹) in the seedlings was higher and the K⁺ content was lower (5 mg g⁻¹). In comparison to Zhongshuang 11, the Na⁺/K⁺ ratio in Yangyou 9 shoots reduced by 36% (normal circumstances) and 56% (stress conditions). Goel and Singh [55] stated that under salt stress, genes such as nitrate transporter (NRT), ammonium transporter (AMT), NR, nitrite reductase (NiR), glutamine syntheatase (GA), glutamate dehydrogenase (GDH), and asparagines synthetase (ASN) were decreased in B. juncea plants.

2.4 Water relations

The water potential in plant is reduced under saline conditions, subsequently creating water shortage situations in plants [56]. Both in soil solution and in plant organelles, salinity causes an imbalanced solute concentration. Thus, osmotic stress occurs due to loss of plant cell turgidity [57]. Extreme salt stress inhibits the expression of tonoplast aquaporins in plant cells [58] and disrupts metabolic and physiological processes, such as cell meristematic activity and cell elongation. Leaf relative water content (RWC) has long been employed as a measure of a plant’s water balance, owing to the fact that it reflects the quantity of water required by the plant to achieve artificial full saturation [59]. It decreases under salinity stress conditions, leading to the loss of cell turgidity in plants. Upon exposure to different levels of salinity stress, different cultivars of oilseed Brassica sp. showed a varied reduction in leaf RWC (Table 2). An experiment from Mahmud et al. [65] showed that salinity adversely affected the water status of B. napus seedlings by reducing their leaf RWC. Under two different (100 and 150 mM NaCl) salinity levels, leaf RWC was reduced by 6% and 11% compared with the unstressed plants.

Species and cultivars	Salinity doses and duration	Response	Reference
B. juncea cv. Pusa Bold	50, and 250 mM NaCl; 10 d	Reduced leaf RWC	[38]
B. juncea cv. Pusa Jai kisan	50 to 200 mM NaCl; 30 d	RWC was increased by 1.4-fold	[60]
B. rapa cvs. Brown Sarson, Yellow Sarson, Toria	50, 100, and 150 mM NaCl; 30 d	Inhibition in RWC by several folds in all three genotypes	[61]
B. juncea cv. Varuna	200 mM NaCl; 90 d	RWC was increased by 40, 36, and 28%, respectively at 30, 60, 90 DAT	[53]
B. carinata cvs. Adet and Merawi	50, 100, and 150 mM NaCl; 35 d	Reduction in RWC by 26%	[41]
B. juncea cvs. CS-52 (tolerant), Ashirwad (sensitive)	25 to 150 mM NaCl; 15 d	Reduced RWC in CS-52 (14%) and Ashirwad (21%), increased water use efficiency in CS-52 (12%) and Ashirwad (55%)	[52]
B. juncea cv. Varuna	120 mM NaCl; 6 d	Declined RWC	[62]
B. juncea cv. Varuna	100 mM NaCl; 7 d	Decrease in RWC by 22%	[63]
B. campestris cvs. BJ-1603, BARI Sarisha-11 and BARI Sarisha-16 (tolerant), BARI Sarisha-14 (sensitive)	8, and 12 dS m^−1 NaCl;	Significant reduction in RWC in BARI Sarisha-14 under 12 dS m^−1	[13]
B. juncea	150 mM NaCl; 7 d	Significant decrease in RWC by 16%	[64]

Table 2.

Changes in water relation parameters of oilseed Brassica sp. under salinity stress.

Another experiment conducted by Fang et al. [33] with B. napus plants under different salinity levels (25, 50, and 100 mM of NaCl) showed that 25 mM salt stress had very little effect on root water content at the seedling stage. But under 50 and 100 mM NaCl, water content in their root was decreased than in the control plants. Also, the osmotic potential of plant leaves alters with the increasing salt concentration in soil. Under 200 mM NaCl stress, B. napus showed decreased osmotic potential of −1.82 MPa, whereas it also lowered the RWC of leaves [48]. Similarly, a recent study by Mohamed et al. [43] concluded that 100 mM NaCl solution reduced leaf RWC by 15% and 18% in two cultivars of B. napus L., namely Yangyoushuang2 and Xiangyouza553, respectively. Reduction in leaf water potential is also a common salt stress response in plants. According to Wani et al. [42], B. juncea showed significantly lower leaf water potential under three different levels of NaCl (78, 117, and 165 mM) concentrations, in a dose-dependent manner.

2.5 Photosynthesis

Salinity hinders the photosynthesis process by limiting plants’ stomatal and/or non-stomatal activities to some extent [66]. Salt stress affects stomatal conductance (g_s) initially due to disrupted water relations and then later because of local abscisic acid (ABA) production [67]. Salinization caused some stomatal closure, although photosynthetic losses were predominantly non-stomatal in nature. According to several investigations done with various oilseed Brassica cultivars, the duration and doses of salt stress have a substantial impact on plant photosynthetic properties (Table 3). Salt stress also has a deleterious effect on photosynthesis because it reduces photosynthetic pigments, and causes considerable changes in photochemistry [5]. The study performed by Mahmud et al. [65] found that salt stress (100 and 150 mM of NaCl) adversely affected the levels of photosynthetic pigments in B. napus plants.

Species and cultivars	Salinity doses and duration	Response	Reference
B. juncea cv. Pusa Jai Kisan	100 mM NaCl; 15 d	Reduced net photosynthetic rate (Pn) by 40%, stomatal conductance (gs) by 26% and intercellular CO2 concentration (Ci) by 41%	[36]
B. juncea cv. Varuna	100, and 200 mM NaCl; 45 d	Decreased Chl a (52%)	[32]
B. napus cv. Zhongshuang 11	100, and 200 mM NaCl; 7 d	Decreased soil and plant analysis development (SPAD) value by 6–20%, Pn by 34–53%, photosystem II (PS II) quantum yield by 27–42%	[68]
B. juncea cv. Pusa Jai Kisan	100 mM NaCl; 30 d	Decreased Pn, gs, Ci	[69]
B. juncea cv. Varuna	100 and 200 mM; 90 d	Decreased total Chl content by 45%	[40]
B. juncea cv. Varuna	200 mM NaCl; 50 d	Decreased total Chl content, total carotenoids (Car) and gs	[70]
B. juncea cvs. CS 54, CS 52-1-2012, CS 614–4–1-4-100-13, Pusa bold	12 and 15 dS m^−1 NaCl; 52 d	Decreased Pn (96%), gs (86%), Ci (42%) in CS 614–4–1-4-100-13	[71]
B. juncea cvs. Varuna and RH-30	78, 117, and 156 mM NaCl; 60 d	Decreased SPAD Value (31% in Varuna and 37% in RH-30	[42]
B. juncea cv. Pusa Tarak	100 mM NaCl; 15 d	Reduced Pn (43%), Ci, gs (23%), total Chl content (31%)	[72]
B. campestris cvs. BJ-1603, BARI Sarisha-11 and BARI Sarisha-16, BARI Sarisha-14	8 and 12 dS m^−1 NaCl; 26–28 d	Decreased Pn, transpiration rate (Tr), gs and total Chl content	[13]
B. juncea cv. Varuna	120 mM NaCl; 6 d	Decreased Chl a and Chl b content	[62]
B. juncea	50, and 100 mM; 8 h	Reduced Chl content (42%), PS II efficiency (35%); Increased non-photochemical quenching (npq) by 36%	[14]
B. juncea cv. RGN-48	50 and 100 mM NaCl; 30 d	Decreased SPAD value, Pn (48%), gs (46%), Ci (41%) in a concentration-dependent manner	[15]
B. juncea cv. Pusa Tarak	100 mM NaCl; 15 d	Decreased Chl concentration, Pn, gs and Ci	[73]
B. juncea cv. Varuna	100 mM NaCl; 15 d	Reduced stomatal density (19%)	[45]
B. juncea cv. RH0–749	100 mM NaCl; 20 d	Decreased Chl content, Pn, gs and Ci	[74]

Table 3.

Impact of salinity stress on photosynthesis and associated parameters of oilseed Brassica sp.

Salt stress has an impact on cell organelles, such as the chloroplast, where most of the photosynthetic activities, such as photosystem I (PS I) and PS II, take place [75]. It reduced the density of active reaction centers and the structural performance of PSII photochemistry, owing to damage on the receptor side of PSII [17]. According to an experiment by El-Badri et al. [54] with five different B. napus cultivars, Yangyou 9 and Zhongshuang 11 responded differently under 150 mM NaCl regarding photosynthetic pigments, such as Chl a, Chl b, and carotenoids (Car). Under salt stress, Chl a content was decreased by 36% in Yangyou 9 cultivar and 39% in Zhongshuang 11 cultivar, whereas Chl b content was reduced by 39% and 40% in Yangyou 9 and Zhongshuang 11 cultivar, respectively. Also, total Chl was reduced by 38 and 39% in Yangyou 9 and Zhongshuang 11, respectively, under the same dose of salinity. Moreover, Car content in Yangyou 9 and Zhongshuang 11 was decreased by 41% and 35%, respectively, under stress.

2.6 Phenology

Among the other abiotic stresses, salinity has a significant impact on the phenological attributes of the plant family Brassicaceae. Salt stress alters the duration of several physiological stages of plants, from seedling emergence, leaf unfolding, the appearance of first flowering, siliqua formation, grain filling to leaf color changing, and leaf senescence in the end. According to Mohamed et al. [43], 100 mM NaCl showed a significant effect on the duration of the flowering stage of two B. napus cultivars (Yangyoushuang2 and Xiangyouza553). Salinity delayed the first appearance date of flowering from 139 d to 143 d and from 141 d to 147 d in Yangyoushuang2 and Xiangyouza553 cultivars, respectively. Moreover, salinity increased the days to 50% flowering in both of the cultivars from 142 d to 146 d and from 144 d to 150 d. Another study by Pandey et al. [76] stated that under 50 mM salt stress, the cotyledonary leaf emergence rate of B. juncea was reduced by 30% to the control plants. According to several findings, extreme saline circumstances may bring delayed seed germination [77] because of decreased hydrolytic enzyme activity and seed metabolite mobilization [78]. B. campestris seed germination was delayed beyond 24 h under 120 mM salt stress level, as reported by Siddikee et al. [79].

Leaf senescence is another age-dependent phenological characteristic of plants that is related with a wide range of biochemical and molecular changes inside plant organs. Due to the decreased biosynthesis and enhanced breakdown of Chl molecules under salt stress, premature or early leaf senescence occurs in the plant life cycle. Alamri et al. [62] stated that the seedlings of B. juncea grown under 120 mM NaCl concentration experienced early leaf senescence.

2.7 Reproductive development

Several additional reproductive stage characteristics, such as flower initiation, anther, and pollen grain development, fertilization, siliqua formation and development, and seed filling, are considerably influenced by the salinity in Brassicaceae plants. During the reproductive stage, B. napus seed production potential is indicated by pre-and post-flowering activities. It has been suggested that the reproductive stage is the most sensitive to stress [80]. Flowering and seed filling are the most vulnerable stages of the oilseed Brassicaceae family to extreme salinity stress than the earlier vegetative phases, such as seed germination and seedling growth. Salinity reduces plant fertility by affecting the development of male and female reproductive organs, which are very susceptible to stress [81]. The reproductive stage is intrinsically linked to seed production because fertilization and seed development occur during this stage. Therefore, tolerance to salt stress during this stage is crucial [82].

Arif et al. [83] experimented with BARI Sarisha-8 and BINA sharisha 5 cultivars of B. napus under 100 mM NaCl and observed that flowering and siliquae formation were significantly affected by salinity. Discoloration and rolling of the leaves and flowers, inhibition of new buds opening, and the death of young siliquae altogether, leaving the adult siliquae with wrinkled and immature growth. As a result, early maturity symptoms of the seed occurred. However, among two of those cultivars, BARI Sarisha-8 showed more sensitivity at the adult siliquae development stage, whereas the young leaves and siliquae were more vulnerable in BINA sharisha 5. Another experiment by Gyawali et al. [34] with 131 B. napus accession lines showed varied responses under different salinity levels (1.4, 5, 10, 15, 20, and 28 dS m⁻¹ of NaCl). The number of branches and siliquae were affected more in two of the genotypes (Kuju 29 and Kuju 32). Similarly, in DH12075, there was a failure in fertile branches and siliquae production found under salt stress of more than 10 dS m⁻¹. The number of fertile branches is also affected under salinity. A study from Chakraborty et al. [3] reported that with increasing salinity levels (1.65, 4.50, and 6.76 dS m⁻¹ of NaCl), the number of siliquae on primary branches of seven cultivars, for example, CS 52, CS 54, Varuna, Pusa Jagannath, Pusa Agrani and T 9, Sagam of B. juncea, and B. campestris, were reduced. The reduction was almost 50% under 1.65 dS m⁻¹, whereas, under 6.76 dS m⁻¹, it was one-third of the control plants. Not only the reduction in fertile branch numbers and siliquae but salinity also caused wilting of the reproductive parts (mature flowers and fruits) of B. napus [84].

2.8 Oxidative stress

Salt stress leads to ionic toxicity due to higher accumulation of Na⁺ and Cl⁻ ions and depleted potassium ion pool in plants. Disruption in ion homeostasis leads to stomatal closure by hampering the functioning of guard cells, which in turn decreased carbon fixation due to insufficient CO₂ supply in the leaves, and accelerates the generation of the ROS such as H₂O₂, ¹O₂, O₂^•−, HO₂^•, RO^•, and ^•OH [7]. Another effect of salinity is to create drought-like conditions in plants due to lower water potentiality and is also responsible for the increase of ROS generation by disrupting photosynthetic activities [56]. Though ROS is beneficial for activating the stress signaling molecules at a certain level, after that it becomes phytotoxic and disrupts metabolic activities and also accountable for the breakdown of different cellular components, namely proteins, lipids, carbohydrates, and nucleic acids [85]. Thus, accelerated activities of ROS boosted protein denaturation, lipid peroxidation, and oxidation of carbohydrates and ultimately created oxidative stress in plants [7; Figure 1].

Figure 1.

Schematic representation of ROS-induced oxidative stress in plants and its consequences under salinity.

Salt induced oxidative stress due to higher accumulation of ROS observed in Brassica sp. However, the extent of salt-induced damage depends on the species, plant growth stage, ion strength, organ specificity, and the components of the salinizing solution [86; Table 4]. Sarwat et al. [95] observed that upon exposure to 100 and 200 mM NaCl, B. juncea plants resulted in upgraded levels of H₂O₂ content by 1.99- and 3.35-fold, respectively, with a maximum increase of malondialdehyde (MDA) content by 2.19-fold at 200 mM NaCl-treated plants. Salt-sensitive cultivar of B. carinata (cv. Adet) showed a higher accumulation of thiobarbituric acid reactive substances (TBARS) compared with the salt-tolerant one (cv. Merawi) in 150 mM NaCl-treated mustard plants [41]. So, it can be stated that degree of salt-induced oxidative damages depends on the cultivar type, dose, and duration of stressed period. Sami et al. [14] found elevated production of H₂O₂ (by 46%) and O₂^•− (by 47%) in 100 mM NaCl-treated B. juncea plants together with an augmented level of lipid peroxidation by 55% at a similar level of salinity.

Species and cultivars	Stress levels	Oxidative damage	References
B. campestris cv. Pusa Gold	40 and 80 mM NaCl; 30 d	Augmented TBARS (129 and 365%) and H2O2 (150 and 531%)	[87]
B. juncea cvs. Alankar (Tolerant) and PBM16 (Sensitive)	50 mM NaCl; 15 d	Increased H2O2, TBARS, and electrolyte leakage (EL)	[88]
B. juncea cvs. Varuna, RH-30, and Rohini	100 and 200 mM NaCl; 45 d	Enhanced MDA, H2O2, and EL	[21]
B. juncea cv. Varuna	150 mM NaCl; 24 h	H2O2 and MDA were increased by 2-folds	[89]
B. campestris cv. Shampad	150 mM NaCl; 48 h	Increased H2O2 (42%) and MDA (46%)	[90]
B. juncea cv. Varuna	100 mM NaCl; 30 d	Augmented H2O2 and TBARS	[91]
B. juncea cvs. Pusa Jai Kisan, Basanti, Rohini, and RH30	100 mM NaCl; 30 d	Elevated TBARS and H2O2	[92]
B. juncea cv. Pusa Jai Kisan	100 mM NaCl; 10 d	Increased H2O2 and TBARS	[93]
B. juncea cv. Varuna	100 and 200 mM; 45 d	Increased H2O2 (60 and 69%) and MDA (20 and 37%)	[32]
B. juncea cv. Pusa Jai Kisan	100 mM NaCl; 30 d	Incremented H2O2 and TBARS	[51]
B. juncea cv. Pusa Jai Kisan	100 mM NaCl; 30 d	Upgraded TBARS and H2O2	[36]
B. juncea cv. Pusa bold	50 and 250 mM NaCl; 5 d	Increased MDA, EL and H2O2	[38]
B. juncea cv. Varuna	100 mM NaCl; 30 d	Enhanced TBARS, H2O2, and O2^•−	[94]
B. juncea cv. Varuna	100 and 200 mM NaCl; 90 d	Augmented MDA, H2O2 and EL	[40]
B. juncea cv. Pusa Jai Kisan	100 mM NaCl; 30 d	Increased H2O2 and TBARS	[69]
B. juncea cv. Pusa Tarak	100 mM NaCl; 25 d	Elevation of TBARS (105%) and H2O2 (90%)	[72]
B napus cv. BARI Sarisha-13	100 and 150 mM NaCl; 48 h	Augmented H2O2 (33 and 50%) and MDA (26 and 60%)	[65]
B. juncea cv. RGN-48	50 and 100 mM NaCl; 30 d	Increased H2O2, O2^•− and MDA	[15]

Table 4.

Salt-induced oxidative damages in oilseed Brassica sp.

2.9 Yield and quality

Plants exposed to salt stress undergo morphological, physiological, and biochemical changes. It leads to a deleterious influence on reproductive characteristics and ultimate yield reduction in plants [7]. As water is the key element for flowering and siliquae formation in oilseed Brassica plants, the accelerated water loss from plant cells induced by salt stress has a significant influence on the reproductive stages. Salinity shows a negative impact on the growth attributes, such as SL, RL, stem diameter, FW, DW, LN, leaf size, LA, and branch number, of oilseed Brassica plants. Similarly, it also affects the yield contributing attributes, such as the number of flowers, number of siliquae, seed yield, 1000-seed weight, and oil content of mustard. Moreover, salinity enforces osmotic stress on plants that adversely affect the water conductance status by altering the whole nutritional status of plants [4]. Thus, it causes growth retardation, ionic and nutritional imbalance, disrupted water relations, and photosynthetic inhibition, which subsequently affects the yield attributes in oilseed Brassicaceae family. While experimenting with seven cultivars (CS 52, CS 54, Varuna, Pusa Jagannath, Pusa Agrani, and T 9, Sagam) from B. juncea and B. campestris, Chakraborty et al. [3] found a significant reduction in seed yield (SY) and oil content in all the seven cultivars. They also reported a great extent of interspecific variation in response to salinity stress in oilseed Brassica plants [3, 83].

Likewise, in different studies, a wide range of oilseed Brassica cultivars were used to investigate their varied responses in yield attributes under salinity stress (Table 5). Reduction in oil content and oil quality is also a common response to salt stress in oilseed Brassica plants. Because of the smaller seed size and reduced cellular metabolic activities, total oil content, as well as lipid, protein, and fatty acid content in the oil, is also hampered (Table 5). For a balanced osmotic pressure in plant cells, soluble sugar and soluble protein contents play a very significant role. Upon exposure to 100 mM NaCl concentration, two B. napus cultivars, Yangyoushuang2 and Xiangyouza553, showed reduced crude oil percentage by 22% and 30%, respectively. Whereas the crude protein percentage was increased by 44% and 24% for both cultivars under the saline condition. Also, seed moisture content, saturated (palmitic and arachidic acids) and unsaturated fatty acid concentrations, glucosinolate in both cultivars were greatly influenced under salt stress. But the oleic acid concentration in Xiangyouza553 cultivar remained unchanged under stress [43]. Six ecotypes of B. napus were tested under 50, 100, and 150 mM NaCl stress, which showed a similar response in yield attributing characters. The ecotypes were Super, Sandal, Faisal, CON-111, AC Excel, and Punjab, among which the number of pods per plant was reduced significantly under 150 mM NaCl in Punjab cultivar. Also, a similar trend in reduction was observed for 1000-seed weight in the same cultivar with increasing salt concentration, while the other varieties showed little to no response regarding those attributes [98].

Species and cultivars	Salinity doses and duration	Response	Reference
B. napus cv. RBN-3060	100 and 150 mM NaCl; 21 d	Significant reduction in SY, no. of seeds plant^−1 and 100-seed weight (SW)	[96]
B. juncea cvs. Varuna and RH-30	78, 117, and 156 mM NaCl; 60 d	Decreased no. of pods plant^−1, no. of seeds pod^−1, 100-SW and SW plant^−1 in both the cultivars at 156 mM NaCl	[42]
B. juncea cvs. CS 54, CS 614–4–1-4-100-13 (highly salt sensitive) CS 52-SPS-1-2012 (saline tolerant) and Pusa Bold	12 and 15 dS m^−1 NaCl; 52 d	Decreased SY in CS 614–4–1-4-100-13 and CS 52-SPS-1-2012 by 86% and 60%, respectively.	[71]
B. napus cvs. Yangyoushuang2 and Xiangyouza553	100 mM NaCl; until final harvest	Reduced total no. of effective siliquae (TES), biomass of effective siliquae (ESB), no. of green siliquae, SY and 1000-SW	[43]
B. napus cv. Okapi	50, and 100 mM NaCl; at flowering stage	Reduced oil content by 11% and SW by 35% under 100 mM NaCl	[97]

Table 5.

Changes in yield and quality traits of oilseed Brassica sp. upon salinity stress.

3. Mechanisms of salt tolerance in oilseed Brassica

To overcome salinity-induced osmotic and ionic stress, plants evolve avoidance or tolerance mechanisms in order to protect the cellular components from sustaining the growth and development. Ion homeostasis, vacuolar compartmentalization, accumulation of secondary metabolites, hormonal regulation, osmolytes production, and by activating defensive responses, plants endure the salinity-induced damages and enhance the stress tolerance.

3.1 Screening of salt-tolerant traits

Screening of salt-tolerant cultivars of Brassica sp. is one of the most effective approaches to minimize the loss of yield; therefore, it has been attracted by many researchers and plant breeders. While working with 25 Indian B. juncea genotypes, Sharma et al. [99] reported that the lowest reduction of germination and speed of germination was found in the RB-10 and PR-2004-2 genotypes. Besides, the highest salt tolerance index for root growth was observed in six genotypes of mustard, such as RB-10, SKM-450, RK-05-02, JGM-03-02.RL-2047, and NRCD-509, and salt tolerance index for the dry matter was also highest in the RB-10 and PR-2004-2 genotypes. Finally, based on the results of germination, growth, and salt tolerance index, Sharma et al. [99] proposed that among the 25 genotypes, highly tolerant genotypes were RB-10 and PR-2004-2, whereas GN-48, JKMS-2, SKM −450, and CS-610-5-25P were categorized as tolerant and NDR-05-01, PBR 300, RK-05-01, NPJ-93, PDR-1188, and RGN-145 were moderately tolerant to salinity. Similarly, Yousuf et al. [39] experimented with 25 genotypes of B. juncea and reported among the 25 genotypes highest lipid peroxidation and lowest soluble protein content, antioxidant activities, and biomass accumulation were found in the Pusa Agrani genotype in a dose-dependent manner of salinity. Whereas, in CS-54 genotype salinity least affected the biomass accumulation, antioxidant activities together with minimal oxidative damages suggesting that CS-54 was more tolerant and the Pusa Agrani was sensitive genotype [39]. While working on the 21 genotypes of B. juncea, Prasad et al. [100] found the highest GP and vigor index in CS2009–347, followed by CS-52 genotype, and identified CS2009–347 and CS-52 as the most tolerant genotypes, whereas CS2009–256 and CS2009–145 genotypes were the susceptible genotypes under salinity. Previously, based on the P_n, g_s, T_r, water use efficiency, C_i and other physiological characters of 10 genotypes of B. juncea, Chapka Rohini was found to be the most susceptible to salinity, while Varuna was the most resistant genotype [101]. Moreover, Hossain et al. [13] studied the performance of four genotypes of B. campestris under salinity and found lower MDA, higher Pro, and antioxidants’ activities in the salt tolerant genotypes (BJ-1603, BARI Sarisha-11 and BARI Sarisha-16) in comparison with the salt-sensitive genotype (BARI Sarisha-14).

3.2 Osmoregulation

To negate the cellular dehydration, plants must retort the osmotic balance, so it activates its osmoregulation mechanism to enhance salt tolerance [102]. In order to accomplish this, plants synthesize different compatible solutes or osmoprotectants such as Pro, glycine betaine (GB), sugars, trehalose, polyamines, organic acids, and amino acids to maintain the osmotic balance under salt stress. Besides osmotic balance, osmolytes are also engaged in ROS-scavenging, protect photosynthetic apparatus, maintain membrane integrity and protein stabilization [103]. In salt-tolerant cultivars of mustard, higher accumulation Pro was observed compared with the salt-sensitive cultivar [13]. Similarly, Ghassemi-Golezani et al. [104] found that accumulation of Pro and soluble sugars increased in a dose-dependent manner. So, based on the available literature, it can be stated that the accumulation of osmolytes helps to maintain osmotic adjustment and also induce tolerance in the salt-stressed Brassica plants (Table 6).

Species and cultivars	Stress exposure	Osmolytes accumulation	References
B. juncea cvs. Varuna, RH-30, and Rohini	100 and 200 mM NaCl; 45 d	Proline (Pro) was increased in a dose-dependent manner	[21]
B. juncea cv. Varuna	150 mM NaCl; 24 h	Increased Pro and GB by 2- and 3-folds	[89]
B. juncea cvs. Alankar and Chutki	100 mM NaCl; 30 d	Higher accumulation of Pro in Alankar than Chutki	[105]
B. juncea cvs. Pusa Jai Kisan, Basanti, Rohini, and RH30	100 mM NaCl; 30 d	The highest Pro was in Pusa Jai Kisan and lowest was in RH30	[92]
B. juncea cv. Varuna	100 and 200 mM; 45 d	Pro was enhanced by 42 and 59%	[32]
B. juncea cv. Pusa Jai Kisan	100 mM NaCl; 30 d	Increased Pro content	[51]
B. juncea cv. Varuna	100 and 200 mM NaCl; 90 d	Pro was increased by 58%	[40]
B. carinata cvs. Adet and Merawi	50, 100 and 150 mM NaCl; 28 d	Lower accumulation of Pro in Merawi then Adet	[41]
B. napus cv. BARI Sarisha-13	100 and 150 mM NaCl; 48 h	Augmented Pro accumulation by 109 and 184%	[65]
B. juncea cv. Pusa Tarak	100 mM NaCl; 25 d	Pro was enhanced by 60%	[72]

Table 6.

Accumulation of osmolytes of oilseed Brassica sp. under salt stress.

3.3 Hormonal regulation

Hormones actively take part in the mediation and modulation of plant’s responses to varying environmental conditions. The regulations of plant hormones are prominent in salt-stressed condition, and it can induce plant-adaptive mechanisms to cope with the stressed condition [106; Table 7]. Abscisic acid is a well-known stress-responsive plant hormone that helps in the mitigation of salt stress through increasing concentration within the plant to control stomatal closure and ultimately initiates defense mechanisms. Upon exogenous application, ABA enhanced salt tolerance through attenuating the ionic and oxidative stress caused by salinity by lowering the accumulation of Na⁺ and Cl⁻, and reducing the overproduction of H₂O₂ and TBRAS contents in B. juncea [74]. Additionally, the antioxidant activities of B. juncea were recorded to be increased even under salt stress due to ABA application as a consequence of increased activities of APX, GR, and SOD. Similarly, auxin, particularly indole acetic acid (IAA), plays an important role in the regulation of salt stress in Brassica. Being growth-generating hormone, auxin has the capacity to stimulate the growth attributes of plants, and this phenomenon also took place under salt stress in Brassica crops. Besides improving growth and photosynthetic characteristics including the recovery of stomatal aperture, the link between auxin and ROS resulted in well adaptation of B. juncea against salt stress [107] through uplifting enzymatic (CAT, POD, and SOD) and non-enzymatic (Pro) antioxidant activities.

Species and cultivars	Hormones	NaCl levels	Protective effects	References
Brassica juncea cv. RH0–749	Abscisic acid (ABA)	100 mM; 20 d	Reduced Na^+, Cl^−, H2O2 and TBARS contents.; Increased photosynthetic parameters, Rubisco activity and antioxidant enzymatic activities.	[74]
B. juncea cv. Varuna	Auxin	100 mM; 38 d	Increased root and shoot length and dry mass.; Improved leaf characteristics.; Enhanced SOD, POD, CAT activities and Pro content.	[107]
B. napus cv. Talaye	Jasmonic acid (JA)	330 mM; 28 d	Ameliorated relative growth rate, net assimilation rate and RWC.; Increased photosynthesis with reduced respiration rate.; Decreased MDA content and LOX activity.	[108]
B. nigra	JA	80 mM; 21 d	Increased gs and quantum yield of PS II.; Lower transpiration rate with higher SPAD value.	[109]
B. juncea cv. Pusa Jai Kisan	Ethylene (ETH)	100 mM; 30 d	Downregulated ionic toxicity.; Upregulated SOD, APX and GR activities and redox state.; Increased Pn and Rubisco activity.	[69]
B. juncea cv. Varuna	ETH	100 mM; 30 d	Diminished Na^+ and Cl^− contents in roots and leaves.; Reduced H2O2 and TBARS contents and increased GSH/GSSG ratio.; Improved Pn and anthocyanin content.	[45]
B. napus cv. Okapi	Salicylic acid (SA)	100 mM; 45 d	Reduced Na^+ and increased K^+ level.; Improved plant nutrients, Chl contents and plant biomass.; Increased CAT, SOD and POD activities.	[110]
B. carinata cv. Adet and Merawi	SA	150 mM; 28 d	Increased plant height, LN and LA.; Improved plant biomass, Chl and Car contents.; Revived leaves functional attributes, antioxidant enzyme activities and reduced leaf TBARS content.	[41]
B. juncea cv. Varuna	Brassinosteroids (BRs)	100 mM; 38 d	Recovered plant growth in height and weight.; Improved guard cells, leaf area index and Chl contents.; Increased K^+/Na^+ ratio with decreased EL and MDA content.; Escalated level of N and declined ABA content.	[111]
B. juncea cv. NO BJ16–9	BRs	150 mM; 36 h	Reduced cell damage, electrical conductivity (%) and MDA content.; Ameliorated CAT, SOD and APX activities.	[112]
B. nigra cv. Black mustard	BRs	12 dS m^−1; at sowing time	Decreased root shoot Na^+ content with increased Ca^+ and K^+/Na^+ ratio.; Increased Chl, Car and anthocyanin contents.; Improved CAT, SOD and POD activities.	[104]
B. napus cv. Vestar	BRs	150 mM; 6 d	Reduced Na^+, Cl^− and TBARS contents.; Improved water content (%) and plant nutrients.	[113]
B. napus cv. Yangyoushuang2 and Xiangyouza553	Melatonin (MEL)	100 mM; 131 d	Improved K^+/Na^+ and Ca^2+/Na^+ ratio.; Increased Chl a, b, Car contents and gs.; Enhanced yield attributes and fatty acid composition.	[43]
B. juncea cv. Green mustard	MEL	150 mM; 38 d	Attenuated plant height, leaf growth and Chl concentration.; Improved photosynthesis, leaf RWC and antioxidant activities.	[64]

Table 7.

Hormonal regulation in salt stress tolerance of oilseed Brassica sp.

Exogenous application of jasmonic acid (JA) increased ROS scavenging CAT, SOD, and POD activities with reduced TBARS content and thus, indicated amplification of salt tolerance of B. napus [110]. As a signaling molecule, ethylene (ETH) can modulate plant stress tolerance to some extent. Rasheed et al. [114] experimented that ethephon (an ETH-releasing compound) applied B. juncea plant resulted in better photosynthetic activities with improved stomatal behavior under salt-induced condition. With an increased APX, GR, and GSH activities and decreased H₂O₂ content within the plant, ethephon further strengthens the tolerance mechanism of B. juncea under the stressed condition imposed by salt. A similar role of ethephon was observed by Fatma et al. [45], whereas the elevated activity of AsA-GSH cycle to reduce the toxicity of H₂O₂ content in guard cells together with restricted ABA to initiate stomatal closure proved the salt tolerance mechanism of ethephon in B. juncea. Salicylic acid (SA) is widely used in enhancement of crop stress tolerance and effective against salt stress too. Besides improving the physiological attributes of B. carinata, such as g_s, P_n, T_r, and water use efficiency, SA can modulate TBARS and H₂O₂ contents to maintain the membrane stability under salt exposure [41]. Moreover, SA-treated Brassica plant can withstand salt-induced conditions due to the antioxidative mechanism of this hormone consisting of increased enzymatic (SOD, CAT, and POD) activities and ascorbate-glutathione pathway and that can ultimately maintain cell redox potential and ameliorate oxidative stress damage conferring salt tolerance ability of SA [41].

Brassinosteroids (BRs) can activate the stress-regulated genes and so take part in the stress amelioration of crops. In B. juncea, BRs in the form of 24-Epibrassinolide (24-EBL) showed better performance in bringing down the concentration of Na⁺, compliment with more K⁺ and therefore, give rise to an increased plant height with higher fresh and dry biomass and improved Chl contents against salt stress [111]. The abatement of salt toxicity was further proved in that experiment by reduced endogenous ABA accumulation, EL, and lipid peroxidation together with uplifted GK and PROX activities that increase the Pro biosynthesis to combat the stressful condition in B. juncea. Retarded growth and quality of B. nigra were attenuated upon 24-EBL application with better antioxidant activities as 24-EBL activated antioxidant enzymes (SOD, POD), controlled MDA content, and encouraged the generation of secondary metabolites (phenolic and flavonoid contents) as well as anthocyanin content [104]. Moreover, the resistance potentiality of B. juncea after applying BRs has been reported as a consequence of adjusted ROS contents, uplifted antioxidant enzyme activities, and increased transcript gene (BjAOX1a) that cause elevation of a cyanide-resistant respiratory activity, to enhance the tolerance mechanism of Brassica in salt-induced condition [112]. Recovery of salt-induced stress by melatonin (MEL) has been proved in many kinds of research and so in oilseed Brassica. Like other beneficial hormones, MEL is effective in amelioration of salt stress in B. napus growth, and in addition, MEL encourages the gene expression that linked with campesterol, JA, and GA hormones synthesis and properly regulates these hormones thus, ensured the salt tolerance mechanism of MEL [115]. Apart from this, root growth, which is the prime challenge under salt stress, was recorded to be uplifted after MEL treatment, resulted in increased root length, thickness, viability, and lateral root formation in B. napus due to the ability of MEL to impair the oxidative stress and maintain ion homeostasis [116].

3.4 Antioxidant defense

To protect the cellular organelles from ROS-induced damages, plants are furnished with defensive mechanisms containing non-enzymatic and enzymatic antioxidants. In plants, non-enzymatic antioxidants such as AsA, GSH, flavonoids, and tocopherols, and enzymatic antioxidants such as SOD, APX, DHAR, MDHAR, GR, GST, glutathione peroxidase (GPX), and POD work in a coordinated manner in order to detoxify ROS [7]. In plant cells, SOD first activates, which converts O₂^•− into H₂O₂, further transformation into less-reactive molecules take place in the presence of CAT, POD, GPX, or in the AsA-GSG cycle [117]. Under stressed conditions, the AsA-GSH cycle plays a crucial role in neutralizing H₂O₂ where AsA and GSH are accompanied by APX, DHAR, MDHAR, and GR in a cyclic manner [118]. Besides this, CAT, GST, GPX, polyphenols, and thioredoxins are also engaged in scavenging electrophilic substances, xenobiotics, and herbicides, and finally help in vacuolar transportation [119]. Plants are naturally equipped with the defensive mechanism to survive the stressed period by augmenting their activities. A number of papers have been published on the activities of antioxidant enzymes of Brassica sp. in salt-stressed conditions (Table 8). Upon exposure to salt stress (50 and 100 mM NaCl) to B. juncea cv. RGN-48, the activities of CAT, SOD, and POD are enhanced compared with the unstressed plants [15]. While working with four genotypes of B. napus (viz., BJ-1603, BARI Sarisha-11, BARI Sarisha-14, BARI Sarisha-16), Hossain et al. [13] found that activities of SOD, CAT, POD, and GPX were unchanged, whereas MDHAR and DHAR activities were decreased in the salt-sensitive cultivar (BARI Sarisha-14). On the contrary, antioxidant enzyme activities were increased in the salt-tolerant genotypes (BJ-1603, BARI Sarisha-11, BARI Sarisha-16) of mustard [13]. Another study from Husen et al. [41] found elevated activities of SOD, CAT, and POD in both cultivars (Adet and Merawi) of B. carinata upon exposure to salt stress, but in cv. Adet, the CAT and POD activities were higher, while activity SOD was more in cv. Merawi.

Species and cultivars	Stress exposure	Antioxidants name	Activity	Reference
B. campestris cv. Pusa; Gold	40 and 80 mM NaCl; 30 d	SOD, APX, GR	Increase	[87]
		AsA, GSH	Decrease
B. juncea cvs. Alankar (tolerant) and PBM16 (sensitive)	50 mM NaCl; 15 d	GSH, APX, GR, SOD	Increase	[88]
B. juncea cvs. Varuna, RH-30, and Rohini	100 and 200 mM NaCl; 45 d	SOD, CAT, APX, GR	Increase	[21]
B. juncea cv. Varuna	150 mM NaCl; 24 h	SOD, CAT, GR, APX	Increase	[89]
		POD	No change
B. campestris cv. Shampad	150 mM NaCl; 48 h	GSH, GSSG, APX, GST, GR	Increase	[90]
		CAT	Decrease
		AsA, MDHAR, DHAR, GPX	No change
B. juncea cv. Pusa Jai Kisan	100 mM NaCl; 10 d	GSH	Increase	[93]
B. juncea cv. Varuna	100 and 200 mM; 45 d	GSH, GSSG, SOD, POD, APX, GR, GST, GPX	Increase	[32]
		AsA, MDHAR, DHAR, CAT	Decrease
B. juncea cv. Pusa Jai Kisan	100 mM NaCl; 30 d	GSH, GSSG, DHA, APX, DHAR, GR	Increase	[36]
		AsA	Decrease
B. juncea cv. Varuna	100 mM NaCl; 30 d	GSH, APX, CAT	Increase	[94]
B. juncea cv. Varuna	100 and 200 mM NaCl; 90 d	GSH, GST, GR, APX, SOD	Increase	[40]
		AsA, CAT	Decrease
B. juncea cv. Pusa Jai Kisan	100 mM NaCl; 30 d	GSH, SOD, APX, GR	Increase	[69]
B. napus cv. BARI Sarisha-13	100 and 150 mM NaCl; 48 h	DHA, GSH, GSSG, APX, GR, SOD, GPX	Increase	[65]
		AsA, DHAR, MDHAR, CAT	Decrease
B. juncea cv. Pusa Tarak	100 mM NaCl; 25 d	SOD, CAT, APX, GR	Increase	[72]

Table 8.

Hormonal regulation in salt stress tolerance of oilseed Brassica sp.

3.5 Stress signaling

A complex array of mechanisms between different intracellular components is involved in stress signaling comprising reception, transduction, and induction of stimuli (Figure 2). ROS was previously believed as toxic molecule, but nowadays, ROS plays the role of signaling cascades. ROS can activate mitogen-activated protein kinase (MAPKs) pathway, which regulates the ionic homeostasis and osmotic adjustments [118]. In a well-organized and sequential pathway, MAPK cascades activated where phosphorylation of MAPK kinase kinase (MAPKKK) took place and transformed into MAPK kinases (MAPKKs) and MAPKs. Thus, the MAPK cascades transfer the stimuli of any environmental stresses to the target proteins and finally enhance gene expression and stress adaptation [120]. Thus, to maintain the osmotic adjustment, MAPK receives and transduces specific signals for the activation of genes to synthesize osmoprotectants, such as Pro, trehalose, and sugars, which are involved in ROS quenching, maintains membrane integrity, and stabilizes proteins by sustaining water transportation system [121].

Figure 2.

A schematic representation of mechanisms of stress signaling of plants under salinity. After sensing the stress stimuli, plants activate secondary messengers of mitogen activated protein kinase (MAPKs) pathway through perception and transduction, which regulates adaptive responses. Specific gene expression plays vital role in synthesizing osmolytes (proline, pro; trehalose; glucose), phytohormones (abscisic acid, ABA; salicylic acid, SA; gibberellins, GAs), regulates defensive responses of antioxidants (superoxide dismutase, SOD; ascorbate peroxidase, APX; monodehydroascorbate reductase, MDHAR; dehydroascorbate reductase, DHAR; glutathione reductase, GR), and transporters of salt overly sensitive (SOS) in inducing tolerance of plants against salinity.

Excessive Na⁺ elevates intracellular Ca²⁺ in the cytosol and activates Ca²⁺ signaling cascades. Calcium-permeable channel OSCA1 found in the plasma membrane as a putative osmosensor under osmotic stress due to the loss of function mutant osca1 thus enhanced Ca²⁺ signaling pathway. Besides this, antiporter KEA1/2 and KEA3 also augmented the osmotic-stress-induced Ca²⁺ signaling cascades by exchanging K⁺ in the plastids [122, 123]. Furthermore, ionic-stress-induced Ca²⁺ signaling mediated Na⁺-occupied calcium-permeable channel where the mutant of monocation-induced Ca²⁺ increases 1, moca1 hypersensitive to salinity and enhanced Ca²⁺ influx by controlling the Na⁺ transportation [124]. The plasma membrane receptor-like kinase FERONIA (FER), leucine-rich-repeat receptor kinase, and hydrogen-peroxide-induced Ca²⁺ increases 1 (HPCA1) also involved in the stabilization of plasma membrane by transmitting the stress stimuli, thus maintain cell wall integrity and stomatal closure under salinity [125, 126].

Along with the Ca²⁺ signaling cascades, the salt overly sensitive (SOS) pathway also plays a crucial role in alleviating the ionic toxicity through exporting Na⁺ from cytoplasm to apoplast. Under salt stress, Ca²⁺ sensors (SOS3/SCaBP8) received the signals and transferred them to the serine/threonine protein kinase (SOS2) where it is phosphorylated with the SOS1 and increased salt tolerance of plants through augmenting the Na⁺/H⁺ exchange capacity [127, 128]. Besides, ROS signaling activates defensive responses, which enhances antioxidant activities and scavenges ROS, thus helps in protecting the intracellular molecules through maintaining redox homeostasis [129].

Under salinity, osmoregulation is properly maintained as a result of ROS-induced activation of MAPK, and the Ca²⁺ signaling regulates the closing and opening of stomata. Besides this, H₂O₂ also played a crucial role in signaling cascades. Being a part of oxidative metabolism, H₂O₂ interplayed in between other biomolecules, such as ABA, ETH, SA, and NO at nontoxic level [130]. Thus, it helps to regulate the stressed period and enhance the tolerance capacity of the plants. Likewise, hydrogen sulfide (H₂S) is a small gaseous signaling molecule performed in traversing of the intra- and inter-cellular domains and regulates the redox homeostasis in plants under stress. Moreover, H₂S inhibits the dynamic synchronization of antioxidant enzymes and NADPH oxidase activity and induces the tolerance against stress [131]. Another molecule is nitric oxide (NO), which has capacity to modulate reactive nitrogen species (RNS), alter protein activity, metal nitrosylation, GSH biosynthesis, formation of tyrosine nitration/peroxynitrite, S-glutathionylation and S-nitrosylation to augment the stress endurance of plants [132]. Biosynthesis of PAs is regulated with the enhanced activity of NO, where NO reduces the PA oxidase activity, thus inhibiting the breakdown of PA and helping to resist the salt stress [133]. Furthermore, CO also increased salt tolerance through the NO-mediated signaling pathway, where plasma-membrane-localized proton pump (H⁺-ATPase) and antioxidant activities enhanced the stress tolerance capacity of plants [134]. Phytohormones have an indispensable role in regulating and enhancing stress tolerance of plants. Under salt stress, ABA activates kinase cascade pathway and regulates gene expression, thus increases endogenous ABA levels, which leads to stomatal closure to maintain the water balance in plants and also enhance selective absorption of ions for transferring Na⁺ from the cytoplasm to the vacuole [135]. Whereas, GAs works antagonistically with the ABA to regulate the germination of seeds by augmenting enzymes and H⁺-ATPase activity. Beside this, GAs also reduce g_s and increase transpiration and water use efficiency of plants to sustain the salt stress [136]. Cytokinins (CKs) perform differential role in the plant growth and development includes cell division, chloroplast biogenesis, apical dominance, leaf senescence, vascular differentiation, nutrient mobilization, anthocyanin production, and also known to induce salt tolerance in plants [137]. Synthesis of BRs under salinity enhanced the stress tolerances by regulating ionic homeostasis and osmoregulation and also responsible for the translational change of the stress-responsive proteins through expressing the stress-responsive genes to regulate Na⁺/H⁺ antiporters activity [138]. Further growth of plants is modulated by the enhanced activity of auxin, whereas ETH signaling, together with ROS, is liable for the AsA biosynthesis under salt stress [139]. There are many biomolecules involved in the stress signaling pathway to adapt to adversity. But the interaction of these biomolecules and cross talk among the components are complex and yet to be discovered to interpret the adaptation of plants under salinity.

3.6 Microbes-assisted salt tolerance

Microorganisms such as bacteria, mycorrhiza, and fungi are mostly used agents in mitigation of salt stress of Brassica inhabited in either the host plant or rhizosphere through promoting the growth of particular hosts [140; Table 9]. For instance, plant-growth-promoting bacteria Pseudomonas fluorescens inoculation in B. napus proved to be an effective approach in mitigation of salt stress in Brassica crops, as the inoculated seedling under salt stress gave higher plant biomass, RWC, and Pro content for better osmoregulation during stressful conditions [142]. In saline soil, recovery of damaged B. napus was recorded by Latef et al. [146] as a consequence of Azotobacter chroococcum inoculation, whereas microbes not only improved plant morphological and physiological characteristics but also reduced Pro, MDA, and H₂O₂ contents to protect salt-induced cell damage. Additionally, the activities of antioxidant enzymes, namely SOD, POD, APX, were also augmented after inoculation conferring salt tolerance while reducing Na⁺ level within the plants [146]. Rhizobium strains capable to produce ACC-deaminase improved nutrients (N, P and K) uptake of B. napus in salt-induced condition and thereby, growth parameters, such as plant height, DW, stem diameter, LN, and RWC, were also increased conforming salt tolerance to B. napus [143]. Inoculation of growth-promoting rhizobacterial strains Pseudomonas in regulation of salt mitigation activities of B. juncea has been confirmed by recording their positive role in increased GP, growth factors, ACC-deaminase activity, and aminolevulinic acid production even under high salt concentration [44]. Additionally, this microbe also takes part in auxin production, ETH reduction through ACC activity, and nutrient solubilization and so provides better establishment of B. juncea against extreme salt exposure [44].

Species and cultivars	Salinity levels	Microbial Inoculation	Tolerance traits	References
Brassica napus cv. Westar canola	250 mM NaCl; 4 d	Pseudomonas putida	Uplifted shoot FW and DW and more Chl content.; Declined ETH level, H2O2 content and Na^+ concentration.	[141]
B. juncea cv. Varuna	200 mM NaCl; 45 d	Trichoderma harzianum	Increased plant height, DW, Chl and oil contents.; Reduced Na^+, H2O2 and MDA contents.; Enhanced activities of SOD, POD, DHAR, MDHAR, APX, GR, AsA and GSH/GSSG ratio.	[32]
B. napus cv. Hyola308	150 mM NaCl; 21 d	P. fluorescens	Upregulation of plant FW and DW with increased relative water and proline contents.; Improved protein activity responsible for glycolysis, tricarboxylic acid cycle and metabolism of amino acid.	[142]
B. napus cv. Suyou No. 1	100 mM NaCl; 15 d	Enterobacter cloacae	Raised SL, RL, secondary roots number and Chl content.; Higher IAA content with lower ETH emission.; Increased SOD, POD and CAT activities with decreased MDA content.	[31]
B. napus cv. RGS003	50 mM NaCl and MgCl2; 121 d	Rhizobium legominozaroum	Elevated plant height, DW, stem diameter, LN and RWC.; More nutrient (N, P and K) uptake.	[143]
B. napus	300 mM NaCl; 42 d	P. sutzeri	Increased plant height, biomass, LN and Chl content.; Reduced lipid peroxidation and GSH content.; Enhanced cell number with more thickness.	[144]
B. juncea cv. Variety 749	12 dS m^−1 NaCl; 33 d	Pseudomonas azotoformans	Improved germination and seedling growth.; Increased root and shoot DW.; Uplifted IAA and aminolevulinic acid production with more P and K solubilization.	[44]
B. napus cv. Jura	200 mM NaCl; 112 d	T. parareesei	Increased crop productivity and gene expression for ETH pathway.	[145]
B. napus cv. Pactol	11.5 mmhos cm^−1 EC; 66 d	Azotobacter chroococcum	Uplifted growth attributes, photosynthetic pigments, soluble sugar and protein contents.; Alleviated antioxidant enzymatic activities and nutrient contents.	[146]
B. napus cv. SY Saveo	160 mM NaCl; 7 d	Arthrobacter globiformis	Improved seed germination (%), SL and RL.; Increased water content (%), Chl and CAR contents.; Elevated SOD activity and Pro content with reduced membrane injury index.	[147]

Table 9.

Microbes-assisted tolerance in Brassica under salt stress.

The impaired growth of salt-induced B. napus can be ameliorated by applying mycorrhizae (Glomus macrocarpium), as inoculation of these microbes increases K⁺/Na⁺ in plant compared with salt-stressed plant alone, and so further abatement of salt damage was recorded in increased growth and yield parameters of B. napus with improved nutrient contents as well [148]. The amino acid and fatty acid profile also showed better performance in mycorrhizal inoculated plant under salt exposure in comparison with uninoculated stressed B. napus [148]. Moreover, proteins that are involved in cell function, leaf photosynthesis, redox potential, and amino acid metabolism are more prevalent in bacteria applied salt-stressed B. napus compared with salt-stressed seedling alone, and this is an important indication of salt tolerance mechanism induced by bacteria [142]. B. napus seed inoculation with Arthrobacter globiformis benefits higher GP with better seedling growth in high level of salt exposure, and this plant-microbe interaction also facilitates in higher phenolic compounds together with phenylalanine ammonia-lyase and SOD activities and more Pro accumulation as well to counteract salt-stressed damage in salt sensitive plants also [147].

4. Agronomic managements for salt tolerance in oilseed Brassica

Brassica adopts many intrinsic mechanisms to tolerate the salt stress through activating stress tolerance traits, and further enhancement of salt endurance capacity could be achieved by incorporating agronomic managements. These management practices include nutrient management, seed priming, application of hormones and other inorganic and organic elicitors. Some of them are presented in the Table 10.

Species and cultivars	Stress exposure	Elicitors and dose	Application method	Effects	References
B. juncea cv. Varuna	150 mM NaCl; 24 h	0.2 mM sodium nitroprusside (SNP)	Added with nutrient solution	Increased total Chl, leaf RWC, carbonic anhydrase (CA) and NR activity.; Reduced Na^+ and enhanced K^+ and Ca^2+.	[89]
B. napus cv. ZS 758	100 and 200 mM NaCl; 14 d	5-Aminolevulinic acid (ALA); 30 mg L^−1	Foliar spray	Enhanced SL, FW and DW.; Increased Chl a, Chl b, Car, and photosynthetic efficiency (Fv/Fm).; Decreased leaf and root Na^+ and K^+.	[149]
B. juncea cvs. Alankar and Chutki	100 mM NaCl; 30 d	N and/or S; 100 mg kg^−1 soil	Mixed with soil	Increased Fv/Fm and Chl.; Increased LA and DW.; Enhanced N content and NR activity.	[105]
B. napus cv. Westar bred	175 mM NaCl; 14 d	24-epibrassinolide (24-EBL); 10^−10 M	Added with nutrient solution	Increased SL, LA, FW and DW.; Increased RWC and reduced Pro.; Decreased MDA, phenolics and flavonoids.	[150]
B. juncea cv. Varuna	100 mM NaCl; 30 d	NO; 50, 100, and 150 μM SNP	Foliar spray	Decreased leaf Na^+ and Cl^−.; Increased Chl, Fv/Fm, Rubisco activity, Pn, LA and DW at 50 and 100 mM SNP.	[91]
B. juncea cv. Varuna	100 mM NaCl; 15 d	S; 100 and 200 mg kg^−1 soil	Mixed with soil	Reduced leaf Na^+ and Cl^−.; Increased ATP-sulfurylase activity and cysteine content.; Enhanced Rubisco and Pn activity.	[151]
B. juncea cv. Pusa Jai Kisan	100 mM NaCl; 30 d	SA; 0.5 mM	Foliar spray	Declined leaf Na^+ and Cl^−.; Upgraded ATP-sulfurylase serine acetyl and transferase activity; Increased Pn, gs, Ci, LA and DW.	[36]
B. napus cv. Sary	150 mM NaCl; 7 d	Lipoic acid; 0.1 mM	Foliar spray	Increased FW and DW of shoot and root.; Reduced MDA and Pro.; Decreased AsA and GSH.	[152]
B. napus cv. Suyou No. 1	100 mM NaCl; 2, 4, and 6 d	γ-glutamic acid; 20 mg L^−1	Added with nutrient solution	Increased DW.; Enhanced K^+/Na^+ ratio, Pro and antioxidant; enzymes activity.	[49]
B. juncea cv. Pusa Jai Kisan	100 mM NaCl; 30 d	Ethephon; 200 ml L^−1	Foliar spray	Reduced leaf Na^+ and Cl^−.; Increased N content and NR activity.; Declined 1-aminocyclopropane carboxylic acid synthase activity and ETH evolution.	[69]
B. carinata cv. Adet and Merawi	50, 100 and 150 mM NaCl; 28 d	SA; 0.5 mM	Foliar spray	Increased RWC, Fv/Fm, gs, Pn and Tr and NR activity.; Increased growth parameters, biomass and Chl content.	[41]
B. juncea cvs. Varuna and RH-30	78, 117, and 156 mM NaCl; 120 d	24-EBL; 10^−8 M	Foliar spray	Increased SL, FW, DW, and LA.; Reduced EL.; Enhanced RWC, SPAD, Pn, gs, Ci, and CA activity.; Increased seed yield.	[42]
		Pro; 20 mM
B. napus cv. BARI Sarisha-13	100 and 150 mM NaCl; 48 h	β-aminobutyric acid (BABA); 150 μM	Seedling pretreatment	Decreased Na^+ and enhanced K^+.; Improved plant height, FW, DW, RWC, Chl a and Chl b.	[65]

Table 10.

Supplementation of different chemical elicitors to inhibit the adversity of salt stress in Brassica sp.

Nutrient management in the salt-affected field could be an effective way to counteract the adversity of salinity in Brassica. Application of N, Zn, and Ca in salt-stressed Brassica plants significantly improved the stress tolerance [40, 51]. For instance, application of N (5, 10, and 20 mM) reduced the leaf Na⁺ and Cl⁻ contents, oxidative stress markers (H₂O₂ and TBARS) and helped to regulate osmotic balance in B. juncea under salinity [51]. Foliar spraying of 1 mM Zn improved growth and biomass of B. juncea in NaCl stress (100 and 200 mM) through reducing oxidative stress by augmenting antioxidants activities [40]. Sarkar and Kalita [153] applied selenium nanoparticles (SeNPs) in salt-stressed B. campestris plants and found that SeNPs (12.5 and 50 mg L⁻¹) improved GP, SL, RL, and Chl content and thus enhanced salt tolerance of the plants.

Seed invigoration through priming could be an effective tool to enhance germination of Brassica under salinity. Comparative study by using three different priming techniques, such as hydro-priming, chemo-priming (CaCl₂), and hormonal priming (ABA) in salt-stressed B. juncea, Srivastava et al. [154] reported that GP and the rate of germination were increased in the primed seeds than the non-primed. Seed priming with different concentrations of SA (1, 1.5, 2, and 5 mM) improved the GP and the average velocity of germination of B. napus under salt stress [84].

Application of SA (1 mM) and 24-EBL (0.1 μM) in salt-treated mustard improved the contents of anthocyanins, phenolics, flavonoids, Chl a, Chl b, and Car and also reduced lipid peroxidation by enhancing antioxidant activities. Whereas foliar spraying of GA₃ increased P_n, g_s in salt-stressed Brassica sp. [155]. Siddiqui et al. [155] also found reduced MDA content, EL and increased activity of NR and CA in the salt-stressed plants.

Other inorganic and organic chemical elicitors are also used to induce the salt tolerance in Brassica plants. Application of NO alleviates salt stress in Brassica crops through enhancing P_n, g_s, C_i, NR, and CA activity compared with the non-saline control plants [15]. Beside this, Sami et al. [14] also sprayed glucose (4%) to ameliorate the salt stress in Brassica sp. and found profound increase of growth, photosynthetic and antioxidant enzyme activities. Further, Xu et al. [156] reported that pretreatment with poly-γ-glutamic acid (γ-PGA) enhanced salt tolerance of B. napus by improving Pro accumulation and increased total antioxidant capacity. Application of γ-PGA also inhibited the content of the oxidative stress indicators (MDA and H₂O₂), thus enhancing growth and development of plants [156]. Thus, scientists have evolved plenty of ways to mitigate the adverse effect of salt stress in oilseed Brassica sp. cultivation and found some efficient approaches to increase the yield by minimizing the oxidative damages and improving defensive responses.

5. Concluding remarks

In the past few decades, salt stress in plants has been widely studied in many crops. As oilseed Brassica is an important oil crop, therefore, the production of this crops in the salt-affected areas should be explored. There are numerous genotypes of Brassica available globally. Therefore, screening the genotypes would be a very first line of work by breeders and plant biologists. As a model plant, Arabidopsis has been widely studied by researchers and many salt tolerance traits have been revealed. Moreover, genetic and molecular bases of salt stress tolerance is underway. Salt stress-induced overgeneration of ROS and subsequent oxidative stress is a common phenomenon in any plants. Understanding the basis of antioxidant defense including ROS signaling and other signaling cascade should be fine-tuned in the light of plant responses to salt stress. Moreover, the approaches should be applicable in the field. Recently, exogenous application of biostimulants, phytohormones, plant nutrients, and many stress elicitors has been researched and applied on the oilseed Brassica plants to enhance salt tolerance. However, their appropriate doses and application methods should be fine-tuned. An integrated approach involving agronomy, plant physiology, and genetics is needed to avail such outcomes.

Acknowledgments

We acknowledge Md. Asad, Farha Naz and Suparna Islam for providing some important literature.

Conflict of interest

The authors declare no conflict of interest.