Due to its high stress tolerance, barley is distributed all over the world. Its growing areas extend from subtropical to temperate zones including North America, Europe, Northwestern Africa, Eastern Asia, Oceania and the Andeans countries

of South America (Fig. 2). However, as can be seen in Fig. 1 and Fig. 2, the intensive barley production areas are mainly non-acid soil regions of Europe, North America and Australia. In addition to natural soil acidity, many agricultural and industrial activities lead to increased soil acidity, including acid rainfall [16], fertilizer use, especially buy Natural Product Library acid-forming nitrogen fertilizers [17], and organic matter decay [18]. H+ ions in acid rain interact with soil cations and displace them from original binding sites; cation exchange capacity reduces and H+ concentrations in soil water increase, resulting in leaching [19]. When crops are harvested and removed from fields, some basic materials for balancing soil acidity are also lost, thus leading to increased soil acidity. Guo et al. [17] reported that intensive farming and overuse of N fertilizer contribute to soil acidification in China. Acid soil toxicity is caused by a combination of heavy metal toxicity, lack of essential nutrients and acidity

per se [20]. Large amounts of H+ ions have find more adverse effects on the availability of soil nutrients; availability decreases with falls in soil pH [2] and [21]. Low pH also increases the solubility of heavy metal elements, such

as iron (Fe), copper (Cu), manganese (Mn), zinc (Zn) and aluminum (Al) (Fig. 3). Only small amounts of these heavy metals are needed by plants and excessive amounts of soluble ions make them toxic to plant growth [22]. Aluminum, the third most common element in the earth’s crust, is one of the most toxic Isoconazole [23]. Above a soil pH of 6.0, aluminum forms non-soluble chemical components, with only a small proportion in soluble form in the rhizosphere (Fig. 3). When soil pH decreases, Al becomes soluble and causes deleterious effects [24]. A high concentration of H+ ions in acid soil is also toxic to higher plants, a feature that has been underestimated for several decades [26]. Acidity toxicity and Al toxicity cannot be separated since Al is only soluble in acid solution. Excessive H+ ions compete with other mineral elements such as phosphorus (P), magnesium (Mg), calcium (Ca), and Fe for plant absorption and disrupt transportation and uptake of other nutrients, resulting in reduced plant growth [27]. Kinraide [26] reported that H+ toxicity was dominant at low Al concentration. After screening different collections of the grasses Holcus lanatus L.