## 4 Response of Barley to Abiotic Stresses
### Toxicity  
Heavy metals are known to be naturally dispersed across the globe, but they are also introduced into the environment via human activities, such as mining, farming, sewage waste disposal, and industrial sludge [186, 187]. This is a worldwide issue that must be addressed. All heavy metals are potentially hazardous, with varying degrees of toxicity depending on their biologic availability and the susceptibility of organisms exposed to the heavy metal. Lead (Pb) is an example of a heavy metal that is a well-known quickly accumulating contaminant in soils and sediments [188]. Pb is regarded as a serious toxin, and agricultural yield suffers significantly in Pb-contaminated soils. Chlorosis, decreased growth, blackening of the root system, disturbances in mineral nutrition and water balance, changes in hormone status, and effects on the structure and permeability of membranes are all toxic consequences of lead on plants [188]. Moreover, researchers are increasingly focusing on the combined impacts of metals on plants. For example, copper (Cu) was shown to have an antagonistic impact when combined with other divalent metals, such as Cd, Ni, and Pb. Many plants have been harmed in recent years as a result of increased contamination of the ground by heavy metals, prompting researchers to become interested in soil improvement, as well as the development and usage of resistant variants. The majority of key agricultural qualities, such as yield, grain quality, and tolerance, as well as tolerance to biotic and abiotic pressures in nature, are qualitatively complex traits. [PERSON] et al. [189] investigated the combined toxicity of cobalt (Co) and Cu on two barley genotypes with different degrees of Co tolerance. This toxicity reduced mineral uptake (Mn, Zn, and K), resulting in a significant reduction in plant growth and an increase in oxidative stress (ROS and MDA), while concurrently boosting the capacity to scavenge active oxygen species (AOS). This process is shown by enhanced glutathione (GSH), oxidized GSH (GSSG) and phytochemicalin (PC) levels, as well as the related gene expression (_HoPCS1_ and _HoVGR1_) [189]. Figure 6 shows how this stress alters plant growth. Barley production is also limited in major agricultural areas across the world due to soil acidity and aluminum toxicity. Aluminum (Al) toxicity prevents root cell division and elongation, lowering water and nutrient absorption as well as grain yield [190]. In Brazil, hazardous amounts of Al are found in more than half of the agricultural regions, and this feature limits barley production. As a result, some amount of genetic tolerance to hazardous levels of Al is critical for barley production in Brazil and other acidic soil locations. Cadmium (Cd) has been shown to decrease chlorophyll concentration, change nitrogen metabolism, and inhibit the photosynthetic system in plants [191]. Leaf chlorosis is the most visible indication of Cd poisoning in plants, and when the period of treatment and/or the Cd content increases, the shift from leaf chlorosis to necrosis occurs [192]. Extrusion across the plasma membrane, chelation in the cytoplasm, and vacuolar sequestration are only some of the strategies that plants have developed to adapt to Cd-contaminated environments [193]. The efflux pumping of Cd at the plasma membrane is mediated by transporters with heavy metal binding domains, such as pleiotropic drug tolerance-type ATP binding (ABC) protein (PDR9) and multi-antimicrobial extrusion protein (MatE) [194]. The tonoplast-based heavy-metal-transporting ATPase 3 (HMA3) aids in the sequestration of Cd into vacuoles [195]. Zn transporters such as Zrt (zinc-regulated transporter)/IRT-like protein 1 (ZIP) and the cation efflux family (cation efflux) proteins have also been shown to transport Cd ions [196; 197]. In general, multiple QTLs underlie these qualities, which are heavily impacted by the plants' environment as well as the genes linked to heavy metal tolerance [188]. Table S5 in  
Figure 6: Heavy metal stress and its effects on plants.