## 4 Response of Barley to Abiotic Stresses
### Temperature  
Inadequate temperature is another environmental issue that has a detrimental impact on crops, especially when combined with another stressor, such as water scarcity. Indeed,water scarcity and heat are two of the most significant abiotic stressors limiting crop growth and production across the world [91]. As shown in sorghum [130], wheat, and barley [131], the combined impacts of heat and drought on yield are more detrimental than the effects of each stress separately [132]. Thermal factors influence a wide range of plant life activities, from seed germination and seedling emergence via soil temperature to vegetative growth and development of the entire plant via soil and air temperatures [133]. High temperatures can cause changes in cell membrane function, affecting metabolic pathways such as respiration and photosynthesis. This environmental component can alter the physicochemical characteristics of thylakoids, impairing electron transport, causing ionic imbalance, and causing ROSs. High temperature stress can affect the amount of carbon molecules produced by photosynthesis in plants with the C3 cycle, such as barley, which exhibits apparent photorespiration. Lower assimilate production tends to limit their allocation in seeds, resulting in thinner seeds with fewer stores intended for embryo development restart. Furthermore, it causes a high incidence of seedlings with poor development, which in the field increases the chance of creating a dominant plant within the population [134]. Detailed information regarding the effects of both high- and low-temperature stresses on plants is presented in Figure 4.  
**Temperature stress**  
**Figure 4. The effects of high- and low-temperature stress on the growth and productivity of plants. **According to [PERSON] and [PERSON] [135], an increase in temperature to 37 \({}^{\circ}\)C may result in decreased stomatal conductance. Leaf photosynthesis, being the most heat-sensitive metabolic activity, has been shown to be disrupted by high temperatures [136]. Thermal stress on the plant photosynthetic system primarily affects the PSII and Rubisco's carbon fixation [137; 138]. Furthermore, chlorophyll fluorescence has been discovered to be a valid physiological measure for assessing damage to the photosynthetic machinery caused by heat stress [34]. The maximum quantum yield of PSII photochemistry (Fv/Fm) has long been used as a criterion for selecting heat-tolerant genotypes in plants. High temperature during the grain-filling stage, according to [PERSON] et al. [139], reduces photosynthetic ability, chlorophyll content, and grain production in wheat genotypes. Proline accumulation is an important adaptive response in crops exposed to high temperatures [140]. It is involved in cellular osmoregulation and protection of cellular structures against increased temperatures through cell water balance conservation and stabilization of biological membranes. The response of _H. vulgar_ ssp. _spontaneum_ genotypes to terminal heat stress was assessed by [PERSON] et al. [141] using gas-exchange parameters, photosystem efficiency, proline buildup, cell membrane leakage, and grain yield characteristics. They reported that the adapted wild genotypes showed physiological mechanisms capable of retaining their production capacity and plasticity flow across time, which might be used to introduce heat tolerance by crossing with cultivated barley. In this section, we highlight some important studies that reported the key QTLs and genes related to cold tolerance in barley (Table S3) [126; 142; 143; 144; 145; 146; 147; 148; 149; 150]. The \"Nure \(\times\) Tremois\" mapping population (NT) linkage map demonstrated the benefits of having numerous informative markers for linkage map development. Marker order and distance are preserved compared with published maps. The NT population is segregating for both vernalization need and low-temperature tolerance, while the \"Dicktoo \(\times\) Morex\" population is segregating for photoperiod sensitivity and low-temperature tolerance [151].