**2. Wheat responses to salt stress**

Salinity is one of the most devastating abiotic stresses having enormous negative effects on morphological, physiological, and biochemical attributes of plant including germination, growth, water uptake, photosynthesis, nutrient uptake, enzymatic activities, and yield. A number of studies revealing the effects of salt stress on different wheat cultivars, among which some are tolerant and some susceptible to salt stress. Higher salinity causes lower germination rate, photosynthesis, transpiration, and higher accumulation of Na+ and Cl<sup>−</sup> ions which disturb the normal metabolic processes of wheat plants (**Table 1**; **Figure 1**).


**1. Introduction**

152 Wheat Improvement, Management and Utilization

Na+

, cytosolic K+

phytoprotectants.

**2. Wheat responses to salt stress**

development of plants under salt stress [10–12].

Worldwide, more than 20% of the cultivable land is affected by salinity. Due to climate change and anthropogenic activities, the salt affected area is tended to increase day by day [1]. Abiotic stresses (including salinity) are accountable for more than 50% yield reduction [2]. In contrary, due to rapid increase of global population, food production should be increased by more than 70% by 2050 [3]. Wheat (*Triticum* spp.) ranks first in the world's grain production. Wheat is consumed as staple food by more than 36% of world population. Wheat provides nearly 55% of the carbohydrates and 20% of the food calories consumed globally [4, 5]. The productivity of wheat is often adversely affected by salt stress. The yield of wheat starts to decline at 6–8 dS m−1 [6]. Under salt stress, hyperosmotic and hyperionic (ion toxicity) stresses occur due to low water potential of soil and excess sodium ion accumulation within the plant. Ionic stress is also associated with nutritional imbalance [7, 8]. Decreased germination percentage, reduced growth, altered reproductive behavior, and reduced yield are general effects on plants under salt stress [9]. Altered enzymatic activity, disrupted photosynthesis, oxidative stress, disrupted biomembrane structure and function, damage of ultrastructural cellular components, and hormonal imbalance are some reasons for decreasing overall growth and

Salt stress tolerance is a polygenic trait regulated by multiple factors/genes. Exclusion of

transpiration efficiency, and enhanced antioxidant defense system are vital for better plant performance under salt stress [13–15]. Different approaches have been adopted to improve plant performance under salt stress; introduction of genes, screening of better performing genotypes, and crop improvement through conventional breeding methods which are often not so successful and not suitable due to time consuming or reduction of plant vigor with the succession of time. Uses of exogenous phytoprotectants, seed priming, nutrient management, and application of plant hormones are convenient for improving plant performances. These approaches are being also popular for stress management practices including the salt stress [16–25]. In this chapter, we will review the recent research works on different approaches of salt stress tolerance in wheat plants emphasizing the use of exogenous

Salinity is one of the most devastating abiotic stresses having enormous negative effects on morphological, physiological, and biochemical attributes of plant including germination, growth, water uptake, photosynthesis, nutrient uptake, enzymatic activities, and yield. A number of studies revealing the effects of salt stress on different wheat cultivars, among which some are tolerant and some susceptible to salt stress. Higher salinity causes lower

germination rate, photosynthesis, transpiration, and higher accumulation of Na+

which disturb the normal metabolic processes of wheat plants (**Table 1**; **Figure 1**).

/Na+

homeostasis, osmotic adjustment,

and Cl<sup>−</sup>

ions

retention and maintenance of K+


**Table 1.** Responses of *T. aestivum* plants to salt stress.

#### **2.1. Germination**

Germination is one of the most important and vital processes of plant life cycle. It is the determinant of the subsequent growth and yield characteristics of plants. Available literature showed that wheat seeds tended to germinate at a lower rate and consumed longer time when exposed to salt stress. The reasons underlying this fact are higher concentrations of salt create lower osmotic potential of germination media which hampers the imbibition of water by seed, creates an imbalance in the normal activities of enzymes responsible for nucleic acid and protein metabolism, causes hormonal imbalance, and deteriorates the food reserves of seed [26]. But, there are many other factors related to the plant and environment which also have effects on germination process. These include age of seed, seed dormancy, seed coat hardness, seed polymorphism, vigority of seedling, moisture, temperature, gasses, and light, etc. [27]. Germination also varies with different cultivars considering whether tolerant or susceptible type. Afzal et al. [28] reported that under saline condition (125 mM NaCl), wheat seeds required longer time for germination than seeds under nonsaline condition. Similar results were presented by Ghiyasi et al. [29] with upto 16 dS m−1 salinity levels. Mean germination time increased and germination rate and germination index decreased with increasing levels of salinity. Akbarimoghaddam et al. [30] reported delayed and decreased wheat seed germination at 12.5 dS m−1 salinity. Fuller et al. [31] also showed a distinct relationship of the decreasing germination percentage with increasing salinity levels (upto 200 mM NaCl).

**Figure 1.** General scheme of salt stress responses and adaptation in plants.

#### **2.2. Growth**

**2.1. Germination**

Tajan, Rasoul, Atrak, and Kouhdasht

Dan-4589 80 mM (NaCl and Na2 SO4

154 Wheat Improvement, Management and Utilization

Jimai 22 100 mM NaCl,

10d

**Table 1.** Responses of *T. aestivum* plants to salt stress.

MH-97 and Inqlab-91

HD 2329 and Kharchia-65

Transgenic lines: T1, T4, and T6

Germination is one of the most important and vital processes of plant life cycle. It is the determinant of the subsequent growth and yield characteristics of plants. Available literature showed that wheat seeds tended to germinate at a lower rate and consumed longer time when exposed to salt stress. The reasons underlying this fact are higher concentrations of salt create lower osmotic potential of germination media which hampers the imbibition of water by seed, creates an imbalance in the normal activities of enzymes responsible for nucleic acid and protein metabolism, causes hormonal imbalance, and deteriorates the food reserves of seed [26]. But, there are many other factors related to the plant and environment which also have effects on germination process. These include age of seed, seed dormancy, seed coat hardness, seed polymorphism, vigority of seedling, moisture, temperature, gasses, and light, etc. [27]. Germination also varies with different cultivars considering whether tolerant or susceptible type. Afzal et al. [28] reported that under saline condition (125 mM NaCl), wheat seeds required longer time for germination than seeds under nonsaline condition. Similar results were presented by Ghiyasi et al. [29] with upto 16 dS m−1 salinity levels. Mean germination

**Cultivars Salinity level Effects Reference**

• Increased Na+

K+

15 dS m−1 • Decreased net CO2

at a molar

Yangmai 16 0.75% NaCl • Higher accumulation of Na+

ratio of 9:1)

16 dS m−1 • Decreased grain yield and 1000-grain weight

and Ca2+ contents

Caxton 200 mM NaCl, 8 d • Decreased germination percentage Fuller et al. [31]

• Decreased shoot FW

• Increased Na+

transpiration

concentration

tration in leaves

• Increased MDA content

Na+ ratio

200 mM NaCl, 9 d • Decreased activity of SOD and increased activities of POD, APX, CAT, and GR

200 mM NaCl, 4 d • Decreased net photosynthetic rate, stomatal con-

and Cl<sup>−</sup>

conductance, and transpiration rate

• Decreased rate of photosynthesis and

• Decreased chl content and intercellular CO2

ductance, and increased intercellular CO2

• Increased activities of SOD, POD, CAT, GR, APX, and dehydroascorbate reductase (DHAR)

• Decreased chl and carotenoid contents

contents, and decreased

assimilation rate, stomatal

content and decreased K+

Asgari et al. [40]

Iqbal and Ashraf

Guo et al. [33]

Singh et al. [45]

Tian et al. [46]

Zhang et al. [47]

Zou et al. [34]

[44]

content

concen-

/

and decreased K+

Salt stress affects the growth of wheat seedlings remarkably. Root and shoot length, plant height, leaf area, number of effective tillers, and number of spike, etc. are considered to be growth parameters. There are many reports that show the evidence of hampering these characters under saline condition. Moreover, in the seedling stage, plants are more sensitive to adverse environmental conditions. So, in this stage, high salinity may even cause death of seedlings. Fresh and dry mass of shoot, leaf area, etc. of both sensitive and tolerant cultivars declined under salt stress in wheat seedlings [19]. Length, fresh weight (FW), and dry weight (DW) of both root and shoot of wheat seedlings were negatively affected by different levels of salinity as 150 mM NaCl [20]; 125 mM NaCl [28], 16 dS m−1 salinity [29], and 120 mM NaCl [32]. Guo et al. [33] showed decreased growth of leaves of wheat seedlings and roots under salt stress, compared to the nonstressed control. Similarly, reduced shoot length, root length, wet weight, and DW after 10 d with 100 mM NaCl treatment were observed by Zou et al. [34].

### **2.3. Photosynthesis**

Photosynthesis is the major physiological process for plant survival and greatly influenced by environmental factors. As salinity reduces water potential and increases accumulation of Na+ and Cl<sup>−</sup> ions in the chloroplast, the rate of photosynthesis gets inhibited [26]. According to the experiment conducted by Arfan et al. [19], exposure to salt stress reduced the transpiration rate, net CO2 assimilation rate, stomatal conductance, and substomatal CO2 concentration of both cultivars. Similarly, net photosynthetic rate, transpiration rate, stomatal conductance, and substomatal CO2 concentration were decreased significantly at 150 mM NaCl stress [35]. Tammam et al. [36] reported that amount of photosynthetic pigments were significantly deceased in seedlings under 320 mM NaCl stress. Reduction of stomatal conductance and transpiration rate were also reported by Guo et al. [33]. Significant decrease of chlorophyll (chl) content was recorded in wheat seedlings at 100 mM NaCl, for 10 d [34].

#### **2.4. Water relation**

Availability of moisture in plants is a crucial factor for all physiological and metabolic processes of plants. Higher salt concentrations induce osmotic stress to plants, which ultimately causes low water potential. Relative water content (RWC) declined by 3.5 and 6.7%, compared to their controls in the salt-tolerant and salt-sensitive cultivars, respectively, after 6 d of 100 mM NaCl exposure [37]. They also reported lowering of osmotic potential with increasing salt concentrations. Arfan et al. [19] showed reduced water use efficiency (WUE) of both sensitive and tolerant cultivars under saline condition. Leaf water potential also decreased under salt stress of 150 mM NaCl [35] and 16 dS m−1 [38]. Percentage of water content decreased in root, but increased in shoot and spike of Banysoif 1 cultivar of wheat [36]. Lv et al. [39] recorded lower RWC in leaves of *T. monococcum* seedlings exposed to salt stress of 320 mM NaCl.
