**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<sup>+</sup> ) 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].

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<sup>+</sup> , 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<sup>+</sup> , 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<sup>−</sup> , 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


#### **Table 1.**

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

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<sup>+</sup> 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<sup>+</sup> 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.

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.


#### **Table 2.**

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

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.
