**2.5. Cellular damage**

Inconsistent growth and improper uptake of water and nutrients ultimately result in deterioration of cell membrane properties of plants. Lipid peroxidation, accumulation of hydrogen peroxide (H2 O2 ), and increased membrane permeability are some common phenomenon of wheat seedlings under salt stress. Mandhania et al. [37] reported higher damage of cellular membranes of salt-sensitive cultivar due to higher H2 O2 accumulation and lipid peroxidation which enhanced the electrolyte leakage compared to the tolerant one. Higher accumulation of H2 O2 in salt-stressed wheat seedlings was also proved by Wahid et al. [35] which was responsible for the increased relative membrane permeability. Lipid peroxidation increased by 68% under NaCl treatment of 100 mM for 10 d compared to control [34].

#### **2.6. Ion uptake**

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].

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+

experiment conducted by Arfan et al. [19], exposure to salt stress reduced the transpiration

both cultivars. Similarly, net photosynthetic rate, transpiration rate, stomatal conductance,

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

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

Inconsistent growth and improper uptake of water and nutrients ultimately result in deterioration of cell membrane properties of plants. Lipid peroxidation, accumulation of hydrogen

wheat seedlings under salt stress. Mandhania et al. [37] reported higher damage of cellular

which enhanced the electrolyte leakage compared to the tolerant one. Higher accumulation of

 in salt-stressed wheat seedlings was also proved by Wahid et al. [35] which was responsible for the increased relative membrane permeability. Lipid peroxidation increased by 68%

), and increased membrane permeability are some common phenomenon of

O2

accumulation and lipid peroxidation

assimilation rate, stomatal conductance, and substomatal CO2

(chl) content was recorded in wheat seedlings at 100 mM NaCl, for 10 d [34].

ions in the chloroplast, the rate of photosynthesis gets inhibited [26]. According to the

concentration were decreased significantly at 150 mM NaCl stress [35].

concentration of

**2.3. Photosynthesis**

156 Wheat Improvement, Management and Utilization

and Cl<sup>−</sup>

rate, net CO2

and substomatal CO2

**2.4. Water relation**

NaCl.

**2.5. Cellular damage**

O2

membranes of salt-sensitive cultivar due to higher H2

under NaCl treatment of 100 mM for 10 d compared to control [34].

peroxide (H2

H2 O2 Higher accumulation of Na+ and Cl<sup>−</sup> ions interferes with the uptake of other necessary ions which disturbs plant processes. Salt-sensitive cultivars tend to uptake more Na+ compared to the tolerant one and this uptake rate increases with increasing concentration of salt [37]. Lower accumulation of NO3 − and PO4 3− ions were recorded by Wahid et al. [35]. They also reported higher uptake of Na+ and Cl<sup>−</sup> , and reduced uptake of K+ and Ca2+ by salt stressed wheat seedlings. Similar results were published by Afzal et al. [28] in wheat seedlings exposed to 125 mM of NaCl stress for 7 d. But, Jamal et al. [32] reported increased uptake of Na+ and K+ both ions, and decreased K+ /Na+ ratio in wheat shoots when exposed to 120 mM of NaCl. On the other hand, both Asgari et al. [40] and Afzal et al. [41] recorded significant decrease of K+ uptake under saline condition (15–16 dS m−1). Under medium salinity, higher accumulation of both Na+ and Cl<sup>−</sup> , and lower uptake of K+ , Ca2+, and Zn2+ ions were reported by Guo et al. [33].

#### **2.7. Yield**

All the above mentioned factors are responsible directly or indirectly for the subsequent yield reduction of wheat plants. Yield of almost all crops, except some halophytes, is reduced under salt stress. The amount of yield reduction may vary upon the sensitivity and tolerance of the wheat cultivars. Chinnusamy et al. [42] indicated that above the threshold level of salinity of 6 dS m−1, wheat yield can reduce at a rate of 7.1% per dS m−1 increase of salinity. Asgari et al. [40] reported that the spikes number per plant, spike length, number of spikelets per spike, straw weight, grain yield, 1000-grain weight, and harvest index declined with the increasing level of salinity, which ultimately caused yield loss. A significant decrease in number of grains per spike, 1000-grain weight, and grain yield were reported in both tolerant and sensitive cultivars of wheat seedlings under 15 dS m−1 salinity [41].

### **3. Salt-induced oxidative stress in wheat**

Salt stress can lead to stomatal closure, which reduces CO2 availability in the leaves and inhibits carbon fixation, exposing chloroplasts to excessive excitation energy which in turn increase the generation of reactive oxygen species (ROS) such as superoxide (O2 •–), H2 O2 , hydroxyl radical (OH•), and singlet oxygen (1 O2 ) [26, 48, 49] (**Figure 2**). Since, salt stress is complex and imposes a water deficit because of osmotic effects on a wide variety of metabolic activities [50]; this water deficit leads to the formation of ROS that are highly reactive and may cause cellular damage through oxidation of lipids, proteins, and nucleic acids [51]. If there is a serious imbalance in any cellular compartment between the production of ROS and antioxidant defense, oxidative stress and damage occur [52] (**Figure 2**). Enhanced production of ROS under salinity stress induces phytotoxic reactions such as lipid peroxidation, protein degradation, and DNA mutation [53]. When a plant faces harsh conditions, ROS production overcomes scavenging systems and oxidative stress will burst. In many plant studies, it was observed that production of ROS increased under saline conditions [54] and ROS-mediated membrane damage has been demonstrated to be a major cause of the cellular toxicity by salinity in different crop plants ([49]; **Table 2**). Long-term salinity treatments (5.4 and 10.6 dS m−1, 60 d) caused significant increase in H<sup>2</sup> O2 and lipid peroxidation in wheat seedlings, which were higher in salt-sensitive cultivar than salt-tolerant cultivar [55]. Increased lipid peroxidation and H2 O2 levels with increased salinity stress in *T. aestivum* were observed in our study [24]. Wheat seedlings exposed to 300 mM NaCl resulted in 60 and 73% increase in H2 O2 and MDA contents. Salt stress also decreased ascorbic acid (AsA) content by 52%. According to Zou et al. [34], *T. aestivum* leaves showed 35% increase in MDA content upon 100 mM NaCl treatment for 5 d which further increased by 68% after 10 d of treatments. Rao et al. [56] observed dose dependent increase in lipid peroxidation in wheat exposed to salt (2, 4, 8, and 16 EC) and these effects were variable among the cultivars. They found increased MDA content in cultivars, ZARDANA (55.9%), ROHTAS-90 (42.26%), SAUGHAT-90 (51%), and SHAHEEN-94 (52%), and hence they were designated as salt sensitive, whereas PUNJAB-85 (33%), BHAKAR 2002 (35%), PIRSBAK-05 (31%), and AUQAB (28%) showed decreased levels of lipid peroxidation and were categorized as salt tolerant [57].

 **Figure 2.** Generalized scheme of salt-induced oxidative stress in plants.


**Table 2.** Salt-induced oxidative stress in *T. aestivum* compared to control.

and ROS-mediated membrane damage has been demonstrated to be a major cause of the cellular toxicity by salinity in different crop plants ([49]; **Table 2**). Long-term salinity treat-

in wheat seedlings, which were higher in salt-sensitive cultivar than salt-tolerant cultivar

*vum* were observed in our study [24]. Wheat seedlings exposed to 300 mM NaCl resulted

(AsA) content by 52%. According to Zou et al. [34], *T. aestivum* leaves showed 35% increase in MDA content upon 100 mM NaCl treatment for 5 d which further increased by 68% after 10 d of treatments. Rao et al. [56] observed dose dependent increase in lipid peroxidation in wheat exposed to salt (2, 4, 8, and 16 EC) and these effects were variable among the cultivars. They found increased MDA content in cultivars, ZARDANA (55.9%), ROHTAS-90 (42.26%), SAUGHAT-90 (51%), and SHAHEEN-94 (52%), and hence they were designated as salt sensitive, whereas PUNJAB-85 (33%), BHAKAR 2002 (35%), PIRSBAK-05 (31%), and AUQAB (28%) showed decreased levels of lipid peroxidation and were categorized as salt

O2

O2

levels with increased salinity stress in *T. aesti-*

and MDA contents. Salt stress also decreased ascorbic acid

and lipid peroxidation

ments (5.4 and 10.6 dS m−1, 60 d) caused significant increase in H<sup>2</sup>

O2

 **Figure 2.** Generalized scheme of salt-induced oxidative stress in plants.

[55]. Increased lipid peroxidation and H2

158 Wheat Improvement, Management and Utilization

in 60 and 73% increase in H2

tolerant [57].

Plants have antioxidative mechanism to fight against stress under adverse conditions. So, they naturally produce higher amount of antioxidant enzymes, e.g., CAT, GR, SOD, APX, POD, and DHAR, etc. to minimize the damage due to stress. Mandhania et al. [37] reported that the activities of CAT, GR, SOD, APX, and POD enzymes increased with the increasing concentration of salt irrespective to tolerance or sensitivity of the cultivar. In another experiment with sensitive and tolerant type of cultivars, ascorbic acid (AsA) content and activities of SOD, CAT, and POD also increased in both under salt stress [20]. But, in another experiment by Singh et al. [45], SOD activity was recorded to decrease with the increasing concentration of salt in a salt-sensitive cultivar named HD2329; while activities of POD, APX, CAT, and GR increased with the same treatments. Significantly, higher activities of SOD and POD were presented by Zou et al. [34] with NaCl treatment of 100 mM for 10d, but they showed insignificant increase of CAT and APX activities, and significant decrease of GR and DHAR activities under same treatment. The activities of SOD and POD were increased with increasing the levels of salt concentrations in *T. monococcum* seedlings [39].
