## 4 Response of Barley to Abiotic Stresses
### Toxicity
cludes a list of useful QTLs linked with heavy metal toxicity tolerance in barley [1, 197, 198, 199, 200, 201, 202]. Cadmium's (Cd) high toxicity and easy absorption by plants has become a serious agricultural issue. Finding the gene(s) responsible for Cd accumulation/tolerance is critical for generating low-Cd agricultural cultivars. [PERSON] et al. [197] used the phenotypically polymorphic Oregon Wolfe Barley (OWB) mapping population to explore the genetic architecture and genetic control of Cd tolerance in barley. They identified one minor and one significant QTL on chromosomes 2H and 6H, respectively, after analyzing the Cd tolerance of 87 double haploid lines at the seedling stage. The significant QTL explained 47.24% of the phenotypic variation for chlorosis and 38.59% of the phenotypic variance for necrosis characteristics. Moreover, 16 Cd-tolerance-related candidate genes were discovered by combining the data from the identified QTLs with RNA-Seq analysis; 9 of these genes were metal ion transporters. [PERSON] et al. [1] created a DH population from a cross between Suyinmai 2 (Cd-sensitive) and Weisuobuzhi (Cd-tolerant) to map QTLs. They measured chlorophyll content, developmental characteristics, metal concentration, and antioxidant enzyme activity in DH population lines and parents under control and Cd stress. A single QTL, \(qShCd7H\), on chromosome 7H was connected to shoot Cd content, explaining 17% of the phenotypic variance. Gene silencing was employed to isolate, clone, and characterize the candidate gene. The identification of \(qShCd7H\) led to the discovery of a new gene, \(HvPAA1\). A sequence comparison revealed that \(HvPAA1\) has seven N-glycosylation domains. The amount of vital nutrients and chemically equivalent hazardous analogues accumulating in cereal grains has a significant influence on crop nutritional quality and safety. Introgression lines (ILs) may be used to take advantage of naturally existing genetic variation in order to create superior varieties. A collection of 54 ILs of the wild ancestor _H. vulgare_ ssp. _spontaneum_ in the cultivated variety (cv. Scarlett) were studied using multi-element analysis on vegetative leaves, senesced flag leaves, and mature grains. Plants were grown on anthropogenically heavy-metal-contaminated soil acquired from an agricultural area, enabling the simultaneous identification of QTLs for the accumulation of critical nutrients and harmful trace elements in barley [202]. [PERSON] et al. [202] found 25, 16, and 5 QTLs for the micronutrients Fe and Zn, and interfering toxin Cd, respectively. They linked QTL to candidate genes based on similarity to known metal homeostasis genes in Arabidopsis and rice by evaluating the gene content of the introgressions. Cu and Fe, rather than Cd, were remobilized preferentially from the flag leaf to growing grains, according to global comparative assessments. Grain micronutrient filling is a controlled and nutrient-specific mechanism that differs from vegetative micronutrient homoeostasis [202]. [PERSON] et al. [198] also used genome-wide association mapping to look for Cd accumulation in different plant organs. They reported a high degree of genotypic heterogeneity in Cd content in all organs. Moreover, their results indicated a strong link between solution shoot Cd and soil culture, as well as between shoot Cd and grain Cd, but no link between root Cd and grain Cd. They also identified nine QTLs for root Cd, 21 for shoot Cd, 14 for root-to-shoot translocation, and 15 for grain Cd. On chromosome 5H, a shared QTL for shoot Cd and root-to-shoot translocation was discovered at 132.6 cM. On chromosomes 2H and 5H, two significant QTL for grain Cd were discovered. Hence, future cloning of potential genes for Cd accumulation and breeding low-Cd barley cultivars will be aided by the genetic diversity in Cd accumulation and key QTLs discovered in different studies.