## 4 Response of Barley to Abiotic Stresses
### Drought  
The frequency and severity of hydric insufficiency are the most important variables in determining global output limits. Research has shown that this restriction is responsible for 60-70% of the variance in ultimate output. Since it is the most common source of yield losses throughout the world, developing more drought-resistant genotypes is critical for food security [88]. Indeed, drought tolerance is a major goal of breeding programs for semi-arid regions with rainfed conditions [89], as well as other areas with a history of poor water supply. To escape dryness, plants close their stomata, deepen their roots for water absorption, thicken their cell walls, and enhance the waxiness of their cuticles. These traits are genetically determined and vary between cultivars. Thus, the diversity among barley genotypes' sensitivity to stress can lead to difference in mortality [90; 91]. The productivity of barley is determined by genetic potential and environmental factors that interfere with plant function, particularly photosynthesis. The photosynthetic process is dependent on the capture of photosynthetically active radiation and its conversion to chemical energy. This process is impacted by a variety of abiotic and biotic stressors that can diminish leaf area at different phases of the crop's growth and development [92]. For example, inadequate soil drainage conditions can affect barley yield. Plant tolerance to water scarcity can take several forms, depending on the species, length and intensity of the drought, developmental stage of the plant, organ, and most importantly, its genotype. In a study of the effect of water limitation in barley accessions, [PERSON] et al. [93] found that drought stress had a distinct influence on the cereal's growth characteristics. Similarly, [PERSON] et al. [94] confirmed that the quantity of water in the plant decreased throughout the cycle under dry semi-arid environments, and that barley with genotypes for higher productive capacity retained reduced stomatal resistance and showed increased water-retaining potential. Drought stress has a substantial impact on grain production and all yield components, according to the findings of [PERSON] et al. [95]. The average rate of grain production loss across years was 8.8-16.3%, while under extreme drought stress, the decrease is expected to reach 49-87% [96]. Figure 3 summarizes some important effects of drought stress on plant growth, as well as some considerable responses of plants to this stress. Wild barley is a major source of genetic variation, and exploitation of this germplasm has been suggested as a means to increase drought tolerance. Since the root is the first organ to detect and react to water shortages [48], the capacity of plants to endure water shortages is largely determined by their extensive root systems [97]. Strong root system architecture (direction, depth, and branching) results in more contact between roots and soil, which improves water and nutrient absorption and has a direct, positive effect on yield performance [98; 99]. Seminal and nodal roots emerge from the embryo and the base of the main stem, respectively, to form a fibrous root system for barley [97]. Nutrient-absorbing root hairs form on the lateral roots (secondary and tertiary roots) that arise from the main roots. The root is the most important organ for stress adaption, anchoring, and resource acquisition. Therefore, studying root system differences and selecting traits for drought tolerance could help to improve future global food security. [PERSON] et al. [100] focused on root system properties and how they interact with shoot architectural elements to cope with water shortages and preserve yield. They also used a microsynteny system to collect root-related drought-sensitive QTL/genes in cereal crops, including interspecies linkages, to simplify comparative genomic analysis. Finally, they offer a strategy for rapidly selecting root features required to generate robust crop types.