## 4 Response of Barley to Abiotic Stresses
### Waterlogging
Several of these QTLs were verified by cross-season replication or population colocation. _Qwt4-1_, for example, contributed to both lowering barley leaf chlorosis and boosting plant biomass under waterlogging stress, whereas other QTLs regulated both leaf chlorosis and plant survival. Comparatively, improving waterlogging tolerance in barley is still in its infancy. Using the identified QTLs along with conventional field selection, waterlogging tolerance in barley can be improved. In addition, there may be a homoeologous link between QTLs influencing waterlogging tolerance in barley and in other crops [172]. Under waterlogging conditions, [PERSON] et al. [184] used a panel of 110 individuals of an F\({}_{2}\) population to investigate QTLs affecting adventitious root development (ARF) on the soil surface. ARF QTLs were found on chromosomes 1H, 3H, and 7H. These genomic regions were similar to those identified in a teosinte population (_Zea mays_ ssp. huehuetenangensis). [PERSON] et al. [174] used a DH population to study the adventitious root porosity (gas-filled volume per unit root volume). On chromosome 4H, a single QTL for root porosity explained 35.7% of the phenotypic variance in aerated and oxygen-deficient environments, close to _EBmac0701_. When the population was screened in an earlier independent soil waterlogging experiment, this QTL contributed to waterlogging tolerance. Indeed, this QTL is syntenic with the _Quer1.02-3 QTL_ in maize and the _Sub1A-1_ gene in rice, both of which are involved in aerenchyma development. The capacity of plants to retain extremely negative membrane potential (MP) is related to their tolerance to both salt and waterlogging stress. However, few efforts have been made to discover QTL underlying this feature. The plasma membrane potential of epidermal root cells from 150 DH lines was measured using noninvasive microelectrode ion flux measurements (MIFEs) technique by [PERSON] et al. [185]. A significant MP QTL was found in hypoxia-exposed epidermal root cells. This QTL was found on 2H, among the QTLs for waterlogging and salinity tolerance. Furthermore, it has been revealed that MP has a key role in waterlogging and salinity tolerance. The fact that MP is regulated by a single main QTL shows the efficiency of single-cell phenotyping, and paves the way for fine mapping this QTL for better marker-aided selection [185]. In a comprehensive study, [PERSON] et al. [177] analyzed 632 QTL for drought, salinity, and waterlogging tolerance in barley. Only 195 significant QTL were utilized for meta-analysis to refine QTL locations for MAS. On the barley physical map, the meta-analysis highlighted significant QTL for drought, salinity, and waterlogging tolerance from multiple mapping populations. The meta-QTL (MQTL) locations were then utilized to identify candidate genes for drought, salinity, and waterlogging tolerance. For example, _MQTL3H.4_ and _MQTL6H.2_ regulate drought tolerance in barley. In addition, the salinity tolerance QTLs _HoNax4_ and _HoNax3_ were verified on _MQTL1H.4_ and _MQTL7H.2_, respectively. _MQTL2H.1_ and _MQTL5H.3_ were also targeted for enhancing salt tolerance in barley. _MQTL4H.4_ is the primary area that determines waterlogging tolerance in barley, including fine-mapped QTL for aerenchyma development. Hence, finding and refining MQTLs and candidate genes is critical for future MAS breeding success. [PERSON] et al. [176] genotyped 247 global spring barley genotypes using 35926 SNP markers. Biomass, spikes per plant, grains per plant, kernel weight per plant, height of plant, and chlorophyll content all showed significant phenotypic variation in the test barley panel. A GWAS for waterlogging tolerance revealed 17 genomic areas with 51 significant waterlogging tolerance-associated markers, accounting for 5.8-11.5% of the phenotypic variance; most QTL were located on chromosomes 1H, 2H, 4H, and 5H. Six new QTL were discovered, and eight candidate genes for abiotic stress responses were found near QTL for waterlogging tolerance.