## 4 Response of Barley to Abiotic Stresses
### Drought
Barley has a tremendous genetic variety, owing to its extensive geographical distribution and climatic range. Only recently has this regional landrace and wild accessions  
Figure 3: Schematic of the information flow of drought stress on various mechanisms of plant growth and developmental phases, and responses of plants to this environmental stress. diversity been studied using functional genomics [101]. Over the past two decades, many investigations identified QTLs linked with yield and its related features and physiological factors in scarcely cultivated crops under drought stress [33; 102; 103]. Table S2 lists some of the identified QTLs in different barley populations [33; 75; 103; 104; 105; 106; 107; 108; 109; 110; 111; 112; 113; 114; 115; 116; 117; 118; 119; 120; 121]. [PERSON] et al. [101] evaluated the collected data to identify potential genes involved in signaling and protective mechanisms that confer drought tolerance in barley germplasm lines. With this knowledge, wild and landrace barley populations may be improved for drought tolerance by enhancing transcription factor, heat shock or late embryogenesis protein (HSP) allelic variations, proline biosynthesis, or redox regulation [101]. With the capacity of bioinformatics tools and advances in sequencing and mass spectrometry, large-scale trials of genotypes may be planned to evaluate their physiological and biochemical responses to water deprivation while determining the genetic basis of their differences. Using such methods can help breeders find potential loci to improve drought tolerance in barley. Individual investigations have only found QTLs for water-soluble carbohydrates (WSC) [122]. Utilizing a comparative genomic approach, many QTLs related to drought tolerance in wheat were shown to colocalize with QTLs discovered in barley under drought conditions [123]. QTLs linked with drought tolerance, such as _QDT_ and _TxFr.2H_, have been identified. [PERSON] et al. [102] discovered the _TxFr.5H_ gene on chromosome 5H, as well as QTLs related to proline content, such as _QPCD.TxFr.3H_ and _QPC-S.TxFr.3H_, and QTLs for moisture content in barley drought-tolerant plants [102], which might be used for MAS in barley drought tolerance breeding programs. Drought tolerance without restricting yield potential is a major problem, and QTLs for stress-related characteristics that coincide with QTLs for productive potential should be prioritized when developing new cultivars [124]. Approximately 50 research studies have documented the complex genetic basis of drought tolerance and related characteristics in cereal crops using QTL approaches. In total, around 800 QTLs/MTAs (MTA) have been reported in these investigations. The QTLs are equally scattered on almost all chromosomes, most of which are physiologically characterized QTLs/MTAs (about 429), with agricultural (318) and root-architecture-related features (23) for cereal cultivations. Major QTLs (with 20% phenotypic variance) and stable QTLs (detected in more than 50% of settings) were among those detected [125]. Under drought conditions, stay-green (SG) is a very important feature. [PERSON] et al. [126] used a DH population to discover QTL related to SG expression. Their study phenotyped the spikes (S), flag leaf (FL), and first leaf beneath the flag leaf (FL-1) visually, and by using single-photon avalanche diode measurements. The plant's green leaf area was assessed visually by comparing the S and FL green areas. Composite interval mapping identified 10 SG QTL on chromosomes 3H, 4H, 5H, 6H, and 7H, 6 of which were linked to heat stress and 4 to water stress. Although these QTL were not colocated with previously described barley stress-response QTL, some SG QTL were mapped near previously described chromosomal locations. For example, two heat-stress QTL mapped to pPb-5529 on 5H, next to QTL for root length and root-to-shoot ratio. The identification of SG QTL in barley cultivated under heat and water stress conditions allows for high-throughput screening for these characteristics.