## 4 Response of Barley to Abiotic Stresses
### Toxicity
Increased ion (manganese and iron) toxicity caused by changes in the soil redox potential under hypoxic conditions causes waterlogging stress, which disrupts plant metabolism. In a recent study, [PERSON] et al. [199] discovered a link between tolerance to Mn\({}^{2+}\) toxicity and waterlogging stress in barley, suggesting that enhancing Mn\({}^{2+}\) toxicity tolerance might also improve waterlogging tolerance. Based on chlorophyll content and plant survival as selection criteria, a DH population was utilized to find QTL underlying tolerance to Mn\({}^{2+}\) toxicity. At the seedling stage, this population had four significant QTL for plant survival under Mn\({}^{2+}\) stress (\(QSur.f1H\), \(QSur.f3H\), \(QSur.f4H\), and \(QSur.f6H\)). On chromosomes 3H and 6H, two significant QTL governing leaf chlorosis under Mn\({}^{2+}\) stress were discovered near _QSur.yf.3H_ and _QSur.yf.6H_. The main QTL, _QSur.yf.3H_, which was found at the marker Bmag0013, was responsible for 21% of the phenotypic variance. Soil elemental toxicity is also a result of floods. Excess water causes a gradual drop in soil redox potential, raising the concentration of Mn\({}^{2+}\), which may be hazardous to plants if it exceeds a certain level. Using barley as a case study, this research intended to assess the proportional contribution of Mn\({}^{2+}\) toxicity to waterlogging stress tolerance [175]. Twenty barley genotypes with varying levels of waterlogging stress tolerance were tested in the root rhizosphere for their capacity to deal with toxic (1 mM) quantities of Mn\({}^{2+}\). Most waterlogging-tolerant genotypes (TX9425, Yerong, CPI-71284-48, and CM72) maintained chlorophyll content above 60% of the control value under Mn\({}^{2+}\) toxicity, while sensitive genotypes (Franklin and Naso Nijo) exhibited 35% less chlorophyll than 35% of controls. Manganese content in leaves was not associated with visual Mn\({}^{2+}\) poisoning symptoms, indicating that alternative Mn\({}^{2+}\) tolerance mechanisms, such as avoidance (instead of tissue tolerance), may act in different tolerant genotypes [175]. The overall considerable (\(r=0.60\)) connection between Mn\({}^{2+}\) toxicity tolerance and waterlogging in barley showed that, at least in this species, plant breeding for waterlogging tolerance characteristics might be aided by targeting pathways involved in Mn\({}^{2+}\) toxicity tolerance. [PERSON] et al. [200] used a spring barley population including 148 genotypes for whole genome association mapping under Mn stress. They used a mixed linear model (MLM with K + Q) to examine marker-phenotypic trait associations. In both non-stress and stress conditions, 39 significant MTAs were found. These markers were located on chromosomes 1H-5H. Many of the related markers were found in QTL-rich areas. Three important markers were found on chromosomes 3H and 5H, linked to MSL and awn length (AL), suggesting that these two traits are controlled by the same genetic regions. The findings showed that Mn tolerance was quantitatively inherited, and the identified QTLs might be useful for marker-assisted selection and gene discovery in barley. Acid soil is a major issue in agricultural development across the globe. Toxic Al cations reduce root development and impairs yield in acidic soil. Although an acid soil tolerance gene has been discovered, it has not been exploited in malting barley breeding, since this gene has been connected to poor malting quality features [203]. Br2, a Brazilian malting barley variety, has been shown to be tolerant to acidic soil. [PERSON] et al. [203] created a DH population from a hybrid between Br2 and the acid-sensitive Australian cultivar Hamelin. The DH population was genotyped using SSR, DArT, and gene-specific markers, and evaluated for acid soil tolerance in native acid soil and a hydroponic system with pH 4.2, pH 4.2 + Al, or pH 6.5. For all factors relevant to acid soil tolerance, a single QTL was discovered on chromosome 4H near the _HvMATE_ gene.