## 4 Response of Barley to Abiotic Stresses
### Waterlogging  
The bulk of grain-producing species' growing fields have soils with a good draining status. However, flooding due to higher rainfall volumes may be one of the most common natural disasters in terms of adverse crop-growing circumstances. The sluggish diffusion of oxygen via the water may cause a reduction in oxygen availability for the roots, which may impair plant development in many vegetal species [91]. The lack of oxygen caused by floods may affect physiologic characteristics, such as carbon assimilation, macronutrient absorption, and root respiration metabolism [154]. Soil flooding causes a decrease in seed production and damages the chemical composition, physical, and physiological integrity of plants [155]. Changes in the radicular system and a decrease in photo-assimilated translocation, as well as a decrease in plant metabolic activity [156], result in a decrease in chlorophyll content, and can interfere with the efficiency and conversion of luminous radiation to chemical energy [157]. [PERSON] and [PERSON] [158] discovered a significant decrease in photosynthetic activity of the 1,5-bisphosphate carboxylase/oxygenase (RuBPCO) in barley plants that had been subjected to soil flooding. As a consequence of waterlogging stress, oxidative stress may occur in cells and tissues. Increased production of ROSs is one of the plant's responses to oxygen deprivation [51; 159; 160]. These ROSs are able to oxidize and break down biological components, such as lipids, proteins, carbohydrates, and nucleic acids, as well as affect enzymatic function [161]. ROS may be created by a variety of processes in plant roots under oxygen-limited circumstances, including plasma membrane (PM) NADPH, mitochondrial malfunction, and metal ion buildup [162]. Under O\({}_{2}\) deprivation, increased iron and copper activity (together with other transition metals) in ionic and catalytically active chelated forms is commonly thought to be a key cause of the ROS burst through the conversion of H\({}_{2}\)O\({}_{2}\) to very hazardous HO [162]. ROSs generated in oxygen-depleted environments also play an important part in plants' developmental and adaptive responses to waterlogging stress as signaling molecules. ROS generation by PM NADPH oxidase and/or mitochondria seems to govern plant adaptation responses under oxygen-limited circumstances, according to a large body of evidence [78; 163]. Unbalanced ROS production, on the other hand, may harm cellular components and cause them to malfunction. As a result of the detrimental effect of ROS overproduction in living tissues, the capacity of plants to generate antioxidant enzymes is often connected with vulnerability to environmental stressors, such as waterlogging [164]. Moreover, waterlogging causes anaerobic conditions in the root zone of plants. Climate change is increasing the occurrence of waterlogging, causing significant agricultural losses. Aerenchyma development, energy metabolism, and phytohormone signaling are all responses to waterlogging stress, as waterlogging affects biomass, photosynthesis, adventitious root growth, and aerenchyma formation [165] (Figure 5). [PERSON] et al. [165] examined how waterlogging affects physiological and genetic pathways in four important grain crops. They also presented the state of knowledge on waterlogging tolerance mechanisms, genes, and QTL linked with waterlogging tolerance, as well as traditional and contemporary breeding approaches. Aerenchyma production under waterlogging stress is one of the most efficient strategies to overcome stress-induced hypoxia. Aerenchyma and radial O\({}_{2}\) loss (ROL) barriers are examples of survival strategies of plants adapted to flooding stress. To protect plants against waterlogging stress, [PERSON] et al. [166] studied the effect of phytohormones such as ethylene and _ERF-VII_ transcription factors. Based on sequence similarity, _ERF-VII_s from major crops and many other _ERF-VII_s involved in plant flood tolerance were gathered and examined.