## 4 Response of Barley to Abiotic Stresses
### Drought
In spring barley, the drought-adaptive QTLs were located around dehydrin genes (_Dlnn4_, _Dlnn7_, _Dlnn8_, and _Dlnn9_) and aquaporins (e.g., _HvPIP1;5, HvPIP2;7_, and _HvTIP2;1_)--genes that are implicated in dehydration tolerance [107]. The relevance of QTL hotspots for seedling growth is shown by the colocalization of well-known key genes for barley development and flowering time. The role of _HvPpd-H1_ in the formation of the root system was discovered using association analysis [108]. Root QTL colocalization with _HERK2_, _HvARF04_, _HvEXPB1_, _PIN5_, _PIN7_, _PME5_, and _WOX5_ suggest further explorations of the functions of these genes in barley. As a result, the genes _HvHOX2_, _HsfA2b_, _HvHAK2_, and _Dlnn9_, which are known to be implicated in the abiotic stress response, were found within stress-specific QTL areas and will be validated in the future [108]. [PERSON] et al. [109] evaluated a nested association mapping population called HEB-25, which consists of 1420 BC1S3 lines developed by crossing 25 different wild barley accessions to the elite barley cultivar \"Barke\" under both the control and the drought-stressed conditions. Best linear unbiased estimators (BLUEs) for each trait were developed and utilized in the GWAS analysis. Many of the 14 quantitative trait loci (QTL) discovered had known inflorescance and developmental genes as colocalizers. The yield of cereal crops is determined by kernel weight (KW) and kernel number per unit area. To find loci regulating KW, [PERSON] et al. [127] employed two barley RIL populations, including Baudin/AWCS276 (BA) and Morex/AWCS276 (MA) with a common parent. A linkage map was provided for BA. In this population, many large effect QTLs influencing KWwere discovered. The genotyping by sequence (GBS) data of 201 RILs from this group was used to create a linkage map with 5273 makers forming 1454 clusters. Each cluster was represented by a single marker, and the linkage map created with these markers has a total length of 1022.4 cM, with an average distance between loci of around 0.7 cM. Three of the large-effect loci influencing KW found in the BA population (found on 2 HL, 6 HL, and 7 HL) were also found in the MA population under different environmental conditions. [PERSON] et al. [110] measured morphological and physiological responses to drought stress in a group of 209 spring barley genotypes, and used a GWAS study to determine QTL and allele contributions for each of the variables studied. Almost all characteristics showed substantial water deficiency effects, and 37 phenotypic variables showed significant genotype \(\times\) treatment (GT) interactions. During the vegetative stage experiment, GWAS found 77 significant loci related to 16 phenotypic variables, and a total of 85 significant loci were associated with 13 phenotypic traits during the generative stage trial [110]. For 17 phenotypic features, more than 110 loci for GT interaction were discovered, accounting for more than half of the genetic variation in several instances. Among the spring barley accessions, the genetic regulation of the nodal-root architectural and anatomical response to water deficiency was examined by [PERSON] et al. [128]. The architectural and anatomical nodal-root response to water deprivation was linked to 11 QTL intervals in a GWAS analysis. Three and four QTL intervals, respectively, exhibited substantial impacts across seasons, and on root architectural and anatomical features. A total of 44 epistatically interacting SNP sites were discovered during a genome-wide epistasis analysis. Further research revealed that these QTL intervals include significant candidate genes, such as _ZIFL2_, _MATE_, and _PPIB_, whose activities have been linked to plant root adaptation responses to water shortage. Moreover, a set of polymorphic SSR markers were used to discover MTAs 48 barley accessions. The MLM discovered a total of 12 significant MTAs.