## 4 Response of Barley to Abiotic Stresses
### Salinity
The concentration of Na\({}^{+}\) was negatively correlated with the plant lifespan, while the leaf ratio of K\({}^{+}\)/Na\({}^{+}\) was strongly linked with the membrane stability index. Mechanisms of ion exclusion (Na\({}^{+}\)) are probable in wild barley, resulting in homeostasis of K\({}^{+}\)/Na\({}^{+}\), reduced root and Na\({}^{+}\) flare-up, and ultimately a higher survival rate. Figure 2: Effect of high salt concentration on various aspects of plant growth and developmental stages. It has been shown that a greater level of salt tolerance is associated with an efficient mechanism for selective uptake of K+ over Na+ [56]. The reduction in K+ concentration occurs due to the presence of excessive Na+ in the growth medium. In other words, a high Na+ concentration is known to have an adversary effect on K+ uptake in plants [57; 58]. Under salinity stress, this antagonism between the absorption of Na+ and K+ usually occurs at the root surface [59]. Salinity stress affected leaf growth of barley through changes in plasma membrane potential, osmolality, and transpiration [60]. Another consequence of high salinity on growth and development is its negative effect on stomata behavior and photosynthetic activities. Reduction in gas exchange due to photosynthetic processes could result from (i) toxic Na+ and Cl-, that reduce stomatal conductance and photosynthetic electron transport; (ii) reduced absorption and metabolism of carbon; and (iii) oxidative damage in the photosynthesis II apparatus (PSII) [61]. Moreover, salinity stress significantly increases the accumulation of reactive oxygen species (ROS) in plant cells. In turn, ROSs adversely affect chlorophyll membranes and ultimately decrease photosynthesis [62]. In a comparative study between wheat and barley, it was found that barley responded to salinity better than wheat, due to its high photosynthesis rate, chlorophyll content in their leaves, the capacity to alleviate the negative effects of ROS accumulation in cells, and other physiological mechanisms [63]. One of the main differences between barley and other crops is its germination potential and ability to establish seedling under severe salinity conditions. Understanding the genetic basis of seed germination under salt stress may improve barley plant growth, output, and even productivity. [PERSON] et al. [64] investigated salt stress tolerance in 121 spring barley accessions from across the globe by exposing them to 200 mM NaCl during seed germination and seedling growth. As expected, seed germination parameters and seedling-related traits were drastically lowered when salt stress was induced. Using 9K Single nucleotide polymorphisms (SNPs), a genome-wide association scan (GWAS) discovered numerous intriguing genomic areas (over 1500 potential genes), including 80 significant SNPs linked to the attributes investigated. Many genes encoding potassium channels, such as _AKT2_, were found on chromosome 1H. In addition, squamosa promoter-binding-like protein 6 at chromosome 5H was revealed to be associated with salinity stress tolerance during seedling developmental phase. The results showed that these genes may have a role in salt tolerance during the early phases of development, implying that early selection might increase salinity tolerance. Functional markers related to tolerance loci might aid in the development of barley cultivars that are more resistant to salt stress. At the seedling and germination stages, genotypic diversity and separate quantitative trait loci (QTLs) were found to be linked with salt tolerance. This was an important attempt to bridge the genetic gap by focusing on early seedling survival characteristics and seedling vigor indicators in barley following germination under salt stress [65]. Identifying the genetic loci and alleles in existing germplasm is required for breeding efforts aimed at increasing salt tolerance during germination. Using 103 doubled haploid (DH) lines of the CM72/Gairdner population, [PERSON] et al.