## 4 Response of Barley to Abiotic Stresses
### Salinity
[65] applied QTL mapping to explore seedling vigor characteristics; 85 barley germplasms were used to validate the phenotypic response. They found that 150 mM NaCl stress considerably lowered all the observed behavioral characteristics. Several characters, such as seedling vigor index, root length, and root dry weight, showed a lesser decrease in DH populations and diversity barley germplasms compared with those that were exposed to 150 mM NaCl. A total of 6 QTLs consisting of 13 markers on chromosome 1H (3 markers), 3H (8 markers), and 4H (2 markers) were found in the DH population [65]. The regions of high potential for candidate genes were 3 QTLs on 1H and 2 QTLs on 3H with significantly associated markers (_BmAC0032_, _bPb-9418_, _bPb-4741_, _bPb-45046_, _bPb-9624_, _bPb-3623_, _bPb-5666_, and _bPb-6383_). On chromosome 3H, a QTL (flanked by markers _bPb-4576_ and _bPb-9624_) was associated with multiple salinity survival characteristics. This finding serves as the basis for future studies in selection genetic analysis [65]. [PERSON] et al. [66] investigated the characteristics of germinating seeds under controlled and salt stress conditions in 350 barley accessions. They found that multiple genes at distinct loci affected salt tolerance during germination and seedling development. This discovery aligns with the findings of [PERSON] [67] and [PERSON] et al. [68], who also revealed that salt tolerance in barley is governed by separate genetic processes during the germination and seedling stages. [PERSON] and [PERSON] [69] linked the \(S\) gene (rachilla hair length) to a genetic component determining salt tolerance during germination. Short-haired rachilla was shown to be substantially more tolerant of salt than long-haired rachilla in the Step-toe/Morex DH lines. [PERSON] et al. [70] also utilized the Steptoe/Morex DH lines to locate the QTL linked with seed dormancy on the long arm of chromosome 5H. In general, detecting significant salt tolerance genes in seedlings or using markers associated with salt tolerance genes may provide accurate and consistent approaches in plant breeding. Recently, [PERSON] and [PERSON] [71] have reviewed the advances in QTLs or genetic loci (and their causal genes, if applicable) on salinity tolerance, with a specific focus on the essential functions of roots, such as ion homeostasis mediated by membrane transporters and signaling molecules under salinity stress. Our review focusses on relevant literature for developing barriers against apoplastic ion flow, such as Na\({}^{+}\), as well as understanding the roles and components of the barrier structures under salt stress. For example, a set of 149 DH lines produced from a hybrid between Clipper (salt sensitive) and Sahara 3771 (salt resistant) barley genotypes were tested under natural saline-stress and non-stress settings; 14 characteristics were measured to identify the QTLs related with agronomic and physiological variables [72]. The genetic linkage map, which consisted of 517 molecular markers spanning a total of 1502.4 cM, was used to conduct QTL analysis using the composite interval mapping approach. Several traits, including days to heading, relative water content, chlorophyll content, plant height, spike length, days to maturity, biomass, grain yield, harvest index, grain number per spike, 1000-seed weight, Na\({}^{+}\) and K\({}^{+}\) concentrations, and K\({}^{+}\)/Na\({}^{+}\) ratio were found to have 40 and 38 QTLs in normal and salinity environments, respectively. Most of the QTLs discovered were on chromosome 2H. Individual QTLs explained between 3.3 and 68.6% of the phenotypic variance. Under both environments, a significant QTL around the _Vrs1_ gene on chromosome 2H was found to be associated with biomass, grain number per spike, 1000-seed weight, plant height, and grain yield [72]. This QTL might be valuable in barley breeding efforts for marker-assisted selection to improve salt tolerance.