Barley (Hordeum vulgare)
Barley (Hordeum vulgare), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains, particularly in Eurasia as early as 10,000 years ago. Barley has been used as animal fodder, as a source of fermentable material for beer and certain distilled beverages, and as a component of various health foods. It is used in soups and stews, and in barley bread of various cultures. Barley grains are commonly made into malt in a traditional and ancient method of preparation.
In 2016, barley was ranked fourth among grains in quantity produced (141 million tonnes) behind maize, rice and wheat.
Scientific classification
Kingdom: Plantae
Clade: Angiosperms
Clade: Monocots
Clade: Commelinids
Order: Poales
Family: Poaceae
Subfamily: Pooideae
Genus: Hordeum
Species: H. vulgare
Biology
Barley is a member of the grass family. It is a self-pollinating, diploid species with 14 chromosomes. The wild ancestor of domesticated barley, Hordeum vulgare subsp. spontaneum, is abundant in grasslands and woodlands throughout the Fertile Crescent area of Western Asia and northeast Africa, and is abundant in disturbed habitats, roadsides and orchards. Outside this region, the wild barley is less common and is usually found in disturbed habitats. However, in a study of genome-wide diversity markers, Tibet was found to be an additional center of domestication of cultivated barley.
Domestication
Wild barley (H. spontaneum) is the ancestor of domestic barley (H. vulgare) and harbours distinctive genes, alleles and regulators with potential for resistance to abiotic or biotic stresses to cultivated barley and adaptation to climatic changes. Wild barley has a brittle spike; upon maturity, the spikelets separate, facilitating seed dispersal. Domesticated barley has nonshattering spikes, making it much easier to harvest the mature ears. The nonshattering condition is caused by a mutation in one of two tightly linked genes known as Bt1 and Bt2; many cultivars possess both mutations. The nonshattering condition is recessive, so varieties of barley that exhibit this condition are homozygous for the mutant allele.
Each plant gets a set of genes from both parents, so two copies of each gene are in every plant. If one gene copy is a nonworking mutant, but the other gene copy works, the mutation has no effect. Only when the plant is homozygous with both copies of the gene as nonworking mutants does the mutation show its effect by exhibiting the nonshattering condition.
Domestication in barley is followed by the change of key phenotypic traits at the genetic level. Little is known about the genetic variation among domesticated and wild genes in the chromosomal regions.
Two-row and six-row barley
Spikelets are arranged in triplets which alternate along the rachis. In wild barley (and other Old World species of Hordeum), only the central spikelet is fertile, while the other two are reduced. This condition is retained in certain cultivars known as two-row barleys. A pair of mutations (one dominant, the other recessive) result in fertile lateral spikelets to produce six-row barleys. Recent genetic studies have revealed that a mutation in one gene, vrs1, is responsible for the transition from two-row to six-row barley.
Two-row barley has a lower protein content than six-row barley, thus a more fermentable sugar content. High-protein barley is best suited for animal feed. Malting barley is usually lower protein ("low grain nitrogen", usually produced without a late fertilizer application) which shows more uniform germination, needs shorter steeping, and has less protein in the extract that can make beer cloudy. Two-row barley is traditionally used in English ale-style beers. Six-row barley is common in some American lager-style beers, especially when adjuncts such as corn and rice are used, whereas two-row malted summer barley is preferred for traditional German beers.
Hulless barley
Hulless or "naked" barley (Hordeum vulgare L. var. nudum Hook. f.) is a form of domesticated barley with an easier-to-remove hull. Naked barley is an ancient food crop, but a new industry has developed around uses of selected hulless barley to increase the digestible energy of the grain, especially for swine and poultry. Hulless barley has been investigated for several potential new applications as whole grain, and for its value-added products. These include bran and flour for multiple food applications.
Classification
In traditional classifications of barley, these morphological differences have led to different forms of barley being classified as different species. Under these classifications, two-row barley with shattering spikes (wild barley) is classified as Hordeum spontaneum K. Koch. Two-row barley with nonshattering spikes is classified as H. distichum L., six-row barley with nonshattering spikes as H. vulgare L. (or H. hexastichum L.), and six-row with shattering spikes as H. agriocrithon Åberg.
Because these differences were driven by single-gene mutations, coupled with cytological and molecular evidence, most recent classifications treat these forms as a single species, H. vulgare L.
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