## 4 Response of Barley to Abiotic Stresses
### Salinity
[66] looked at seed-germination-related parameters in 350 barley accessions under control and salt stress conditions. Marker-trait associations (MTA) and the underlying candidate genes for salt tolerance during germination were discovered with the use of a genome-wide association study that included over 24,000 genetic markers. Across all chromosomes, they found 19 loci containing 52 significant salt-tolerance-associated markers, as well as four genes belonging to four families with functions underpinning the anticipated MTAs. A population including 187 DH lines derived from a hybrid between TX9425 and Naso Nijo was tested for superoxide anion (O\({}_{2}\)\({}^{-}\)) and hydrogen peroxide (H\({}_{2}\)O\({}_{2}\))--two primary ROS species produced during hypoxic stress--to discover the QTL for ROS tolerance in barley [78]. The results demonstrate that measuring ROS concentration after 48 h of hypoxia provides a rapid and accurate way to identify waterlogging-tolerant barley genotypes. Both superoxide (_QSO.TxNn.2H_) and hydrogen peroxide (_QHP.TxNn.2H_) were found to have the same QTL on chromosome 2H. This QTL was found in the same location as the QTLs for total waterlogging and salt tolerance described in earlier research, explaining 23% and 24% of the phenotypic variance for O\({}^{2-}\) and H\({}_{2}\)O\({}_{2}\), respectively. Accordingly, ROS production was shown to be linked to both waterlogging and salt stress tolerance in the study. In a study conducted by [PERSON] et al. [79], a recombinant inbred line (RIL) population including 185 individuals was subjected to QTL mapping analysis for 12 early seedling vigor (ESV)-related characteristics. A genomic map (1075.1 cM) was created using DArT markers, with an average adjacent-marker distance of 3.28 cM. In total, 46 significant QTLs for ESV-related characteristics were found. Across all chromosomes, 14 QTLs for biomass yield were discovered, 2 of which colocalized with QTLs for grain yield on 1H. The first and second leaf lengths, as well as the dry weight of the second leaf, biomass production, and grain yield, all demonstrated a high heritability (>30%). Meanwhile, grain and biomass yield showed a strong association, providing a clear picture of these qualities in the selection process. [PERSON] et al. [79] showed that the DArT markers _bPb-9280_ and _bPb-9108_ on 1H were linked to a pleiotropic QTL related to the specific leaf area of the second leaf, biomass yield, and grain yield. These could be used to improve seed vigor through marker-assisted selection and facilitate future map-based cloning efforts. Under control conditions, as well as exposure to 75 mM and 150 mM NaCl, 50 wild barley (_Hordeum vulgare_ ssp. _spontaneum_, Hsp) introgression lines (ILs) were utilized to find QTL alleles that improved germination and seedling growth. Under salt stress, germination and seedling development features were diverse, and found to be highly heritable [80]. Furthermore, significant differences were found across treatments and for five salinity tolerance indicators. For the 10 phenotypes studied and the tolerance indices, a total of 90 and 35 significant QTL were found, respectively. _Hsp_ introgression alleles were implicated in enhancing salt tolerance in 40 (43.9%) of the 90 QTL studied, including introgression lines S42 IL-109 (2H), S42 IL-116 (4H), S42 IL-132 (6H), S42 IL-133 (7H), S42 IL-148 (6H), and S42 IL-176 (6H) (5H) [75]. Seven unusual QTL alleles, including S42 IL-127 (5H), 139 (7H), 125 (5H), 117 (4H), 118 (4H), 121 (4H), and 137 (4H), were effectively verified in wild barley ILs (7H). Photosynthesis and transpiration are controlled by stomata, and these activities are crucial for plant responses to abiotic stressors such as salt. To find QTLs linked with stomatal and photosynthetic features relevant to salinity tolerance, [PERSON] et al. [81] identified 22 significant QTLs in a population comprising 108 DH lines. These QTLs were found on all chromosomes except 5H, and account for 9.5-17.3% of the phenotypic variance.