## 4 Response of Barley to Abiotic Stresses
### Salinity  
An important environmental factor that affects the development of barley plants is salinity, which affects about 800 million hectares throughout the world [36]. Salinity causes significant losses in agricultural productivity, particularly in arid and semi-arid countries, where around 25% of irrigated land is salinized (FAO [37]). Several cations, such as sodium, calcium, magnesium, and anions, including chloride, sulfate, bicarbonate, and carbonate, make up the solute salts that effectively contribute to salinizing the soil [38]. It has been shown that soil salinity can reduce agricultural production by up to 50% in a wide range of economically important crops, including maize, rice, and barley, which are all susceptible to salt stress. Specifically, plants' cellular metabolism is affected by salinity as a result of osmotic stress [39]. The high concentration of solutes in the soil decreases the hydric and osmotic potential, causing soil to retain more water [40], as well as ionic stress, which can induce ionic toxicity and nutritional imbalance from the high levels of Na\({}^{+}\) and Cl\({}^{-}\)[39,41]. Compared with wheat, barley has a higher tolerance to salt, and a greater capacity to compartmentalize Na\({}^{+}\) into vacuoles and thus maintain a high K\({}^{+}\)/Na\({}^{+}\) ratio [42]. Although there are hints of more resistant cultivars, barley is nevertheless considered vulnerable to soil salinity, since it exhibits a gradual decline in growth as the concentration of salts in the environment increases [43]. The reduction in dry mass may represent the metabolic energy expenditure associated with adaptation to salinity and a reduction in carbon gain, which may be explained by the diversion of energy for the functional maintenance of tissues [44]. Since barley is the most salt-tolerant cereal [45], it can be regarded as a plant model species that is beneficial for understanding the complicated processes of salinity tolerance and resource use, which can then be applied towards increasing salinity tolerance of other cereal crops [46]. Although barley is known as a salt-tolerant crop, its growth and production are affected by ionic and osmotic potential in saline soils [47]. Moreover, it is worth noting that tolerance to salinity stress is a complex quantitative trait that is controlled by the interaction between physiological mechanisms and pathways of many genes [48]. Similar to other field crops, this stress negatively affects almost all aspects, i.e., phenology, morphology, physiology, and biochemistry processes, which ultimately reduces its economic value [49, 50, 51, 52]. Figure 2 illustrates the most important effects of this stress on various aspects of plant growth and development. As a consequence of domestication, selection, and breeding cycles [53], a wider genetic diversity (GD) is needed to reach the reproductive objectives, in order to compensate for the genetic degradation and loss of many alleles--not only in barley, but also in other crop species. The intrinsic ecological flexibility and stability of wild barley in highly varied settings may indicate that this is an excellent genetic resource for increased environmental and biotic stress tolerance in farmed barley [54]. To improve barley salinity tolerance in connection with climate change and genetic deterioration, it is essential to use GD available in the _H. spontaneum_ wild subspecies [55]. When compared with wild emmer (_T. turgidum_ ssp. _dicocoides_), wild barley produces higher yields in salt stressed conditions, perhaps due to reduced sodium contents in the shoots [54]. For instance, [PERSON] et al. [55] investigated 47 wild barley accessions and 6 tolerant check cultivars under salinity stress conditions (300 mM NaCl). They found that salinity increased malondialdehydedehyde (MAA) and proline, as well as Na\({}^{+}\) and Ca\({}^{+}\) in both the roots and the leaves. Moreover, they showed that _H. spontaneum_ genotypes have been less affected by salt than check cultivars (_H. vulgare_ L.).