## 4 Response of Barley to Abiotic Stresses
### Drought
In a study of 72 DH lines and a sensitive variety, [PERSON] et al. [102] found two QTL for drought tolerance (leaf wilting) and one for salinity tolerance (plant survival). In three independent experiments, the QTL on 2H for drought tolerance explained 42% of the phenotypic variation. The drought tolerance QTL on 5H was less affected by agronomic traits and can be used in breeding programs. The draft barley genome sequence indicated that a candidate gene for this QTL was located on 5H. To investigate the relationships between DArTseq and SNP markers, as well as key physiological parameters associated with plant responses to drought, [PERSON] et al. [104] employed 109 spring barley genotypes with high or poor drought tolerance. Drought tolerance in four-leaf seedlings was phenotyped under controlled settings (growth chambers). There were 58 novel connections discovered, including 34 new markers (16 DArTseq and 18 SNP markers). Three of the markers yielded findings that were similar to those reported in a previous conventional biparental QTL mapping analysis. The greatest number of significant MTAs were found in the areas around markers on linkage group 2H. Additionally, five markers associated with photosystem II photosynthetic activity (PSII) were discovered on chromosome 4H. The sequences next to DArT markers on linkage group 6H had the fewest number of connections. In both biparental QTL and association studies, a chromosome 3H region associated with water usage efficiency and net photosynthetic rate may be especially useful, since these metrics correlate with plants' capacity to stay highly productive under water deficiency stress. [PERSON] and [PERSON] [105] used a GWAS analysis to identify SNP-trait relationships and potentially elucidate the complex genetic architecture of seven essential spring barley attributes. They revealed significant connections, some of which were shared with the single-SNP strategy. Their findings explained higher phenotypic variation (R\({}^{2}\)), which better suited the genetic model [Bayesian information criterion (BIC)]. The cumulative R\({}^{2}\) and BIC of the single-SNP and haplotype-based techniques were comparable to the multi-SNP approach, with the haplotype-based approach improving. Despite low overlap, each strategy revealed previously verified QTLs, indicating that each approach may identify a distinct group of QTLs. A combined single-locus and multilocus GWAS technique may increase the ability to find significant QTLs and offer a more comprehensive picture of the genetic architecture of complex characteristics in barley. Another study used a 9 K SNP chip to genotype 183 spring barley gene bank accessions from 23 countries to discover MTAs and candidate genes influencing agronomic and seed-associated attributes during post-anthesis drought stress [106]. The results of GWAS identified 97 significant MTAs related to kernel number and kernel weight per spike. [PERSON] et al. (2016) discovered SNP markers related to 15 complex traits in a breeding population of barley assessed under differing water availability conditions in central Chile's Mediterranean climate. Given the significant segregation distortion of markers, a QTL mapping mixed model was utilized to account for the genetic relatedness between genotypes. In rain-fed environments, 57 QTL were found, accounting for 5-22% of the phenotypic variance. In full irrigation, 84 SNPs were related with various characteristics, explaining 5-35% of the phenotypic variance. Most QTLs were found on chromosomes 2H and 3H; 12 of the 15 characteristics assessed had environment-specific genetic regions. Most QTL-trait relationships were environment- and trait-specific, although several were significant and persistent. Full irrigation revealed a significant genomic area on chromosome 1H that accounted for between 27% (_SNP2711-234_) and 35% (_SNP1923-265_) of the behavioral variance. In both rain-fed and irrigated conditions, the locus 1923-265_ was discovered for grain yield, accounting for 9% and 18%, respectively.