## 4 Response of Barley to Abiotic Stresses
### Waterlogging
Water supply impacts barley more than other crops, resulting in loss of output from 40 to 79% based on genotype, stages of growth, and duration of water supply [167]. One significant mechanism for water logging is the development of adventitious roots (ARs), as they help water and the absorption of nutrients under hypoxic stress [168]. Barley genotypes have significantly higher root porosity and faster development of aerenchyma in adventitious roots when exposed to water [169]. [PERSON] et al. [170] reviewed seven different genotype barley physiological and morpho-anatomical responses to waterlogging stress. Slight reductions in plant height and SPAD (soil/plant development analysis), tillers, and shoot and root biomasses were observed in the tolerant varieties compared to sensitive varieties. The number of adventitious roots of tolerant genotypes was likewise much greater than in sensitive genotypes under water stress. Because of the greater ethylene content, lower ABA level, and less O2 buildup, the leaves of the waterlogging resistant cultivars exhibited larger intercellular gaps and better integrated chloroplast membrane structures [170]. The impact of waterlogging timing and duration on yield is uncertain. To examine this, [PERSON] et al. [171] used a variety of waterlogging treatments on barley genotypes with varying waterlogging tolerances. One experiment included four waterlogging treatments: ZS 12.5 for 1 or 2 months (WL1 and WL2), ZS 15 for 2 months (WL3), and 1 day before travelling for 15 days (WL4). Another experiment comprised only WL2 treatment. The reduction in the number of spikes per m\({}^{2}\) and kernels per spike lowered the yield by 35% in W1 and  
Figure 5: Important effects of waterlogging stress, and plants’ responses to it. by 52% in WL3. This was attributable to WL4's influence on spikelet fertility and grain filling. The waterlogging susceptible cultivar showed the highest delays (1-8 ZS), whereas waterlogging resistant genotypes capable of aerenchyma production showed the least delays (0-4 ZS). Early phenological waterlogging prevented 23% yield loss in genotypes with aerenchyma development QTL. Waterlogging in late developmental stages lowered average final grain production by 70%. Similar to studies on other abiotic stresses, there are many genetic studies on the discovery of genomic regions associated with waterlogging tolerance (Table S4) [78; 172; 173; 174; 175; 176; 177; 178]. Waterlogging tolerance has been linked to a number of QTLs based on agronomic, physiological, and anatomical features. Several features such as plant height [179], grain yield [180], plant survival [181], leaf chlorosis [87], and plant biomass [182] under waterlogging stress have been used to conduct QTL analysis for waterlogging tolerance in barley. Since these QTLs have been found on all seven chromosomes, their practical use has been limited. Furthermore, the majority of this research has relied on quantitative traits, which might change across contexts. For example, a QTL discovered in one environment may not be detected in another [183]. [PERSON] et al. [78] evaluated 187 barley DH lines under waterlogging and salinity stresses. They provided the first report of a large QTL mapping of waterlogging tolerance for both O\({}_{2}\) and H\({}_{2}\)O\({}_{2}\) based on ROS accumulation on chromosome 2H. The QTL for ROS was found in the same location as the QTLs for waterlogging and salinity tolerance. Using this characteristic as a physiological marker, the one QTL discovered permits precision mapping of the gene responsible for waterlogging and salinity tolerance. The presence of QTL for many abiotic stressors at this location on chromosome 2H suggests a particular mechanism for various stress tolerances, such as waterlogging and salt tolerance. [PERSON] et al. [172] created a composite map using SSRs, RFLP, and DArT markers to discover QTLs linked with waterlogging tolerance in barley. Twenty QTLs for waterlogging tolerance were discovered in two barley DH populations.