Barley: Production, Improvement, and Uses

By Steven E. Ullrich

Barley is one of the world's most important crops with uses ranging from food and feed production, malting and brewing to its use as a model organism in molecular research. The demand and uses of barley continue to grow and there is a need for an up-to-date comprehensive reference that looks at all aspects of the barley crop from taxonomy and morphology through to end use. Barley will fill this increasing void. Barley will stand as a must have reference for anyone researching, growing, or utilizing this important crop.

• Language: English
• Publisher: Wiley
• Release date: Dec 30, 2010
• ISBN: 9780470958629

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Significance, Adaptation, Production, and Trade of Barley

Steven E. Ullrich

SIGNIFICANCE OF BARLEY

Barley (Hordeum vulgare L.) is one of the most ancient crops, and it has played a role in the human development of agriculture, civilizations, and cultures and the sciences of agronomy, physiology, genetics, breeding, malting, and brewing. It is grown and/or used around the world. For many centuries, barley has fed livestock, poultry, people, and people’s spirit. Barley was among the first domesticates playing an important role during the hundreds or thousands of years of human transition from a hunting and gathering to agrarian lifestyle in the Fertile Crescent of the Near East starting at least 10,000 years ago. The Fertile Crescent is considered the first of at least seven centers of agriculture origin in the world (Smith 1998). Barley, along with wheat (Triticum spp.), pea (Pisum sativum L.), lentil (Lens culinaris L.), goat (Capra aegagrus hircus), sheep (Ovis aries), and cow (Bos taurus), set the stage for the evolution of agriculture in the Near East, which eventually spread to North Africa, further east and north in Asia, and to Europe (Smith 1998). A concise history of the spread of barley cultivation is presented by Fischbeck (2002), and an update on the probable origin or origins of barley is presented in Chapter 2 of this book.

The prominence of barley can be seen from the interpretation of its genus name, Hordeum, which derives from the word by which Roman gladiators were known, "hordearii, or barley men, for eating barley to give them strength and stamina (Percival 1921). The English word barn derives from barley plus aern" or barley house/building (Webster’s Dictionary, various versions). Barley was presumably first used as human food, raw or roasted and in breads, porridges, and soups, but eventually evolved primarily into a feed, malting, brewing, and distilling grain. Barley’s decrease in prominence as a food grain was due in part to the rise in prominence of wheat and rice. In recent times, 55%–60% of the barley crop has been used for feed, 30%–40% for malt, 2%–3% for food, and about 5% for seed.

Barley is best known around the world today as a feed grain and as the premier malting and brewing grain. Barley varieties are quite variable in feed quality, and barley is often compared to maize or corn (Zea mays) and wheat in feed quality. Considerable research and debate about the attributes of each have ensued throughout modern times. The presence of a fibrous hull on barley grain generally puts barley at a disadvantage, especially for use by nonruminants, mainly swine and poultry. However, the advantage of maize or wheat over barley is not clear-cut, and some studies have shown that barley can be of equal or greater quality compared with maize or wheat (Bowland 1974; Owens et al. 1995). Furthermore, hulless barley, due to removal of the hull with threshing, tends to be superior to hulled barley and more on par with maize and wheat (Joseph 1924; Mitchall et al. 1976; Bhatty et al. 1979). Given the adaptation of barley versus maize, barley is very important in areas where maize is not produced, especially where the climate is cool and/or dry, that is, in western North America, northern Europe, the Middle East, North Africa, and the Andean region of South America. Barley is utilized as forage as well as for grain. Details of barley feed use, characteristics, and value are given in Chapter 16 of this book.

When most people think of the composition of beer, they think of barley (but not necessarily malt). However, in much of Africa, sorghum (Sorghum bicolor), maize, and/or millet (various genera/species) beers abound and are part of cultural traditions. The history of alcoholic drinks including beer goes back thousands of years. The use of barley for beer likewise goes back thousands of years and dates from archeological evidence to at least 8000 years ago in the Middle East and in Egypt (Arnold 1911). The long history of brewing means that barley has long been selected for improvements in malting and brewing qualities. The traditions of using barley for brewing in the Middle East gradually migrated north into Europe where these traditions grew even stronger. Eventually, with the far reach of Europeans during the exploration and colonization of unknown and lesser known parts of the world (Americas, Africa, East Asia, and Australia), barley brewing traditions spread worldwide. Today, the sciences of malting and brewing are highly developed. Malting barley breeding is quite refined as well, with a host of barley and malt traits under consideration by industry. However, in spite of advanced technologies of analyses (e.g., near infrared) and breeding (marker-assisted selection), actual malting of grain and wet chemical analyses are still the principal procedures for analysis and selection. Major traits relate to the germination process and the physical and chemical composition of barley and malt including such things as kernel conformation, hull, carbohydrates, proteins, enzymes, and enzyme activity. Whereas the preponderance of malted barley is used for beer, some types are used for distilling (e.g., Scotch whisky and Irish whiskey) and for food applications. Details of traits and trait improvement can be found in Chapters 8 and 15 of this book.

Although barley utilization for food is relatively minor on a global basis today, throughout its history, barley has remained an important and major food source for some cultures principally in western and eastern Asia, as well as in the Himalayan nations and in northern and eastern Africa (Grando and Macpherson 2005; Newman and Newman 2006, 2008; Baik and Ullrich 2008). Furthermore, there has been a resurgence of interest and use of barley for food, primarily in the developed world due to an increasing emphasis on incorporating a diversity of whole grains in people’s diets for health benefits. In addition, in 2006, the U.S. Food and Drug Administration issued an endorsement of the benefits of foods containing barley principally due to its soluble fiber content (β-glucans), which has been shown to lower blood cholesterol levels with implications for heart health. Barley also seems to lower blood glucose levels (glycemic index) with implications for those suffering from diabetes. See Grando and Macpherson (2005), Newman and Newman (2008), and Chapter 17 of this book for an expansion of barley food topics.

Barley has figured prominently on the frontiers of science in general, but especially in genetics. There is a long history of genetics research focused on trait inheritance and mapping in the conventional sense (Smith 1951; Nilan 1964; Barley Genetics Newsletter, 1971–2010), also on induced mutagenesis (Nilan 1981), and more recently on molecular and physical mapping and genetic analyses (e.g., Graner et al. 1991; Hayes et al. 1993; Kleinhofs et al. 1993; Yu et al. 2000; Kleinhofs and Han 2002; Caldwell et al. 2004; Close et al. 2004; Druka et al. 2006, 2008; Hayes and Szucs 2006; Kumlehn et al. 2006; Varshney et al. 2007; Xu and Jia 2007; Massman and Smith 2008; Potokina et al. 2008; Hamblin et al., 2010). Chapters 3–8 of this book focus on basic and applied molecular genetics and breeding advances in barley that are representative of the state of the science in crop species and plant species in general. As barley is one of the first domesticated crop species, much research has been done on the origins of barley and related small grains as well as phylogeny and systematics (see Chapter 2 of this book). Considerable research has been done with barley as a model in physiological and anatomical areas, especially of the grain (see Chapters 13 and 14).

ADAPTATION OF BARLEY

Barley has evolved to include several morphological and commercial forms, including winter, spring, two-row, six-row, awned, awnless, hooded, covered, naked, hulless, and malting, feed (grain and forage), and food types. Barley is arguably the most widely adapted cereal grain species with good drought, cold, and salt tolerance. It is generally produced in temperate (winter and/or spring planting) and semiarid subtropical (winter planting) climates. It does not tolerate highly humid warm climates. Grain production occurs at higher latitudes and altitudes and farther into deserts than any other cereal crop. For example, in the Nordic countries of Norway, Sweden, and Finland, six-row spring barley is grown further north (above 65°N lat.) than winter and spring two-row barley and spring wheat and oat. On the Altiplano of the Andean nations of Peru and Bolivia, barley is grown for grain at higher elevations (over 4500 m) than oat, wheat, and maize. In the North African country of Algeria, barley is grown further south toward the Sahara than the most drought-tolerant durum wheats (author, personal observation). Whereas barley can thrive and produce an acceptable crop at some of the earth’s agricultural margins, it does very well under well-drained loam soils, at moderate rainfall (400–800 mm) or under irrigation, and at moderate temperature regimes (15–30°C). Barley production and reactions to biotic and nonbiotic stresses are detailed in Chapters 9–12 of this book. Below are a few rough illustrations of barley adaptation based on estimated yields from the United Nations Food and Agriculture Organization (FAO) database (FAO 2009). The estimated average barley yield in the world in 2006 was 2497 kg/ha, and in Western Europe, with a nearly ideal climate for barley with relatively high inputs of fertilizer and pesticides, it was 5956 kg/ha or 238% of the world average. Moving to warmer and drier Southern Europe, the average yield in 2006 was 2715 kg/ha. Moving to cooler and wetter Northern Europe, it was 4253, and in the Nordic countries mentioned above, the yield was 3550 kg/ha. The average yield across high-altitude Bolivia and Peru was 1045 kg/ha. In the North African countries bordering the Sahara desert, the average yield was 1168 kg/ha. Of course, a number of factors affect yield besides the adaptation effects of climate, soil, and biotic factors (+ and −), including the level of farmer inputs of cultivar, fertilizer, pesticides, and irrigation. All these things are reflected in the yield numbers above, especially lower inputs toward the margins of adaptation in southern and northern Europe, the Andean nations in South America, and in the Sahara desert bordering nations in North Africa. The wide adaptation of barley and production around the world have stimulated much study on the reactions of barley to abiotic and biotic stresses (see Chapters 10–12 of this book), and research and development of best management practices for barley production (see Chapter 9 of this book).

GLOBAL PRODUCTION OF BARLEY

How does barley figure into the whole scheme of crop agriculture around the world? Barley in recent years has been the fifth most-produced crop in the world and the fourth most-produced cereal on an approximate dry weight basis (Table 1.1). The three major food cereal grains, in order of production, are maize, rice (Oryza sativa), and wheat, which lead annual world crop production with 2000–2007 averages of ∼600+ M t (millions of metric tons) each by a wide margin. Soybean (Glycine max), barley, sugarcane (Saccharum spp.), potato (Solanum tuberosum), and sorghum follow (196, 140, 93, 61, 58 M t, respectively). There have been some major shifts in production rank of the top crops over the last 20 years. Twenty years ago, the production rank from the top was wheat, maize, rice, barley, soybean, sugarcane, sorghum, and potato. The average rank so far in the twenty-first century is maize, rice, wheat, soybean, barley, sugarcane, potato, and sorghum. The shifts have involved greater surges in maize, rice, and soybean relative to the other crops (FAO 2009). In the mid-1980s, barley production was nearly twice that of soybean (160 vs. 88 M t). Table 1.2 complements Table 1.1 by reporting area and yield averages for the top six grain crops in the world. One can get a good impression of differences in biology, adaptation, and production conditions from the combination of data in these two tables, especially relative yield data. For example, maize and rice are mostly produced under much less water stress than wheat and barley. This is reflected in total production versus total area grown and in global 8-year average yield estimates of 4.7 and 4.1 versus 2.8 and 2.5 t/ha for maize and rice versus wheat and barley, respectively. The fact that maize outyields rice (and many other crops) is at least partially reflective of the more efficient C4 versus C3 photosynthetic system of maize versus rice. The gap may be less than expected under similar production conditions because globally, maize typically experiences more drought than rice. Patterns over time in the overall production of these crops in Table 1.1 are reflected in the patterns in hectarage and yield in Table 1.2. Most area and yield data have been relatively stable in the twenty-first century so far. Exceptions are the trends of increasing yield of maize and increasing hectarage of soybean. Barley production, area, and yield data have been relatively stable this century, but barley has decreased by about 12% in overall production in the past 20 years.

Table 1.1 Global production estimates for 8 years of the top eight crops expressed as is, in millions of metric tons (M t), except dry weight (dw) for sugarcane and potato

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Source: Food and Agriculture Organization of the United Nations (http://faostat.fao.org/site/567/default.aspx).

Table 1.2 Global grain production estimates for 8 years: first row in area harvested in millions of hectares (M ha), second row and grain yield in metric tons per hectare (t/ha)

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Source: Food and Agriculture Organization of the United Nations (http://faostat.fao.org/site/567/default.aspx).

The adaptation of barley described above can be seen in the regional distribution of barley production over the globe (Table 1.3). Thirty-eight countries in Europe (including the Russian Federation and Ukraine) produced 83 M t of barley in 2007, which is more than 60% of the world’s barley production. The 27 countries of the European Union (EU) produced over 40% of world production in 2007 and for the 5-year average of 2003–2007 (FAO 2009). Approximately 16% of the world’s barley in 2007 was produced in 34 countries in Asia with 45% of Asian barley produced in 16 western Asian countries. North America (Canada and the United States) grew 12.5% of global production, followed by Australia/New Zealand (4.4%), 16 countries (mostly North and East) in Africa (∼4%), and eight countries in South America (2%).

Table 1.3 World distribution of barley production based on estimates in millions of metric tons (M t), 2007

Source: Food and Agriculture Organization of the United Nations (http://faostat.fao.org/site/567/default.aspx).

Barley production (5 year means, 2003–2007) by country is detailed in Table 1.4. These data again generally illustrate the adaptation of barley and complement the data in Table 1.3. The top 10 countries in descending order are the Russian Federation, Canada, Germany, France, Spain, Turkey, Ukraine, Australia, the United Kingdom, and the United States, and they produced approximately 67% of the world’s barley over this 5-year period (94.4/140.8 M t). Seven of the top 10 countries are considered in Europe including the Russian Federation, Turkey, and Ukraine. Of these seven countries, four are currently (2009) in the EU. Of the 27 countries listed in Table 1.4, 15 are in Europe and 12 of these are currently in the EU with Turkey’s membership pending. Although European countries dominate global barley production, each continent and subcontinent listed in Table 1.3 is represented by at least one country in Table 1.4. Country yield averages range from about 2 t/ha (Russian Federation, Australia, and Iran) to ≥7 t/ha (Germany, France, and the United Kingdom; Table 1.4). The country yield averages, hectarage, and total production reflect relative growing conditions (mainly precipitation) and management technology (mainly soil fertility and pest management). For example, the most influential factors in the countries with the lowest yields are climate for Australia and climate and management technology for the Russian Federation and Iran. At the high end of the yield spectrum, Germany, France, and the United Kingdom all have a favorable climate and a high level of management technology.

Table 1.4 Barley production estimates by country—5-year averages (2003–2007)

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aEuropean Union member countries.

Source: Food and Agriculture Organization of the United Nations (http://faostat.fao.org/site/567/default.aspx).

Table 1.5 World trade estimates of barley and barley products

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G, billions; M, millions; K, thousands; t, metric tons.

Source: Food and Agriculture Organization of the United Nations (http://faostat.fao.org/site/535/default.aspx#ancor).

GLOBAL TRADE OF BARLEY

Most but not all barley-producing countries utilize the bulk of their production domestically. There is considerable trade of barley and barley products. FAO estimates of global trade of barley and barley products are summarized in Table 1.5, where import and export figures are presented and compared for the years 2000 and 2005. Over 20 M t of barley grain have typically been exported and imported annually this century, globally generating about US$3 billion per year. The estimated amount, value, and price of barley grain exports and imports rose between 2000 and 2005 by about 10%, 32%, and 22%, respectively. Barley malt trade has amounted to an estimated 5–6 M t/year with modest increases between 2000 and 2005 (exports: 5.5–6.2 M t [13%]; imports: 5.2–5.7 M t [10%]). The value of malt exports and imports rose from an average of about US$1.35 billion in 2000 to about US$2.0 billion in 2005 (48%). The increase in the value of malt outpaced the increase in the quantity of malt traded during these years. The quantity and value of beer exports and imports grew very dramatically between these two years. The quantity of beer exported and imported rose from 6.2 to 9.8 M t and from 6.3 to 9.4 M t or by 49% and 59%, respectively. The value of beer exports rose from US$4.8 billion to US$8.2 billion (71%) between 2000 and 2005, and the value of beer imports rose from US$5.4 billion to US$7.9 billion (46%). Furthermore, the increase in the value of beer outpaced the increase in the value of malt exported between 2000 and 2005, but import value gains were about equal.

Other value-added barley products traded are malt extract, pearled barley, and barley flour and grits. Compared to trade of barley grain, malt, and beer, the trade of these commodities is relatively minor. Generally, dramatic increases occurred from 2000 to 2005 in the export and import quantity and value of these commodities, except for import numbers for pearled barley and barley flour and grits, which actually went down, and in some cases dramatically. Given that all the data presented by the FAO are estimates from various sources with variable accuracies, export and import numbers for the various commodities in Table 1.5 agree fairly well. However, there are major discrepancies, and therefore, inaccuracies in the numbers for malt extract, pearled barley, and barley flour/grits.

Barley malt production data by country is difficult to obtain. Perhaps the reason is that maltsters guard production information for business purposes. Malt production capacity and pro­duction information is scattered across various Web sites. According to Worldmalt Statistics (http://www.coceral.com/), world malt production capacity is estimated at about 22 M t/year with actual annual production ranging from 18 to 22 M t in recent years. The EU countries typically produce 8–9 M t/year, about 42% of the world total. Typically, the top five EU malt producers in descending order are Germany, the United Kingdom, France, each with over a million metric tons, followed by Belgium (∼700,000 t), and Spain and/or the Czech Republic (∼500,000 t each). Furthermore, this Web site indicates that of the approximate 20 M t of malt used per year globally, about 94% is used for beer, 4% for distillation, and 2% for food. Other estimates of annual malt production/capacity include a 5-year (2002–2006) average production of approximately 1.8 M t in the United States (calculated from net import/export data from the United States Department of Agriculture [USDA] Foreign Agriculture Service [http://www.fas.usda.gov/ustrade/] and malt used in breweries from the U.S. Department of Treasury Alcohol and Tobacco Tax and Trade Bureau [http://www.ttb.gov/beer/beer-stats.shtml]), approximately 1.2 M t capacity for Canada (http://www.wheatgrowers.ca/), about 700,000 t production for Australia (http://www.barleyaustralia.com/), and 4+ M t capacity for China (Bormann 2007). China is capable of producing more malt than any other country in the world based on the data presented above. The surging economy in China in almost all sectors including the agricultural sector is well-known, so it is not surprising to see China’s large capacity to produce malt. According to Bormann (2007), beer consumption has risen dramatically, driving an increasing demand for malt. In 1989, per capita beer consumption in China was ∼5 L; in 1999, it was ∼15 L, and the projection for 2009 is for over 25 L. In 1990, the demand for malt by Chinese brewers was 0.8 M t, and in 2000, it was 2.7 M t. By 2004, 200 maltsters had developed the capacity to produce 4.3 M t of malt. Whereas China annually has imported about 2 M t of barley in recent years, some of which is malting barley, it imports very little higher-cost malt, only about 3000–4000 t/year (FAO 2009). China, which has roughly the same amount of crop land and 4.4 times the population of the United States, has surged to the forefront in the production of a number of agricultural commodities. It is the number 1 producer, globally, of rice, wheat, potato, sweet potato (Ipomoea batatas), groundnut (Arachis hypogaea), cotton (Gossypium spp.), rapeseed (Brassica spp.), squash (Cucurbita spp.), peach (Prunus persica), apple (Pyrus malus), tobacco (Nicotiana tabacum), and cabbage (Brassica oleracea/B. chinensis). China is number 2 in maize, number 3 in banana (Musa spp.), and number 4 in soybean production in the world (FAO 2009). China, the world’s most populous nation with 1.4 billion people estimated in 2008, is an agricultural giant.

A breakdown of leading exporting and importing countries of barley and malt by quantity traded is presented in Table 1.6. Data are presented for the years 2000 and 2005 to illustrate the dynamics of the trade. The EU dominates the export trade of both barley and malt. Three of the top five exporters of barley and malt in 2000 were EU countries (barley: Germany, France, the United Kingdom; malt: France, Belgium, Germany), and in 2005, France and Germany were among the top five barley exporters, and France, Belgium, and Germany were among the top five exporters of malt. Overall, EU countries exported 66% and 50% of the barley in 2000 and 2005, respectively, and 67% and 69% of the malt (FAO 2009). Australia and Canada, large countries with relatively small populations and high production, were among the top five barley and malt exporters in both years.

Table 1.6 Leading barley, malt, and beer exporting and importing countries based on quantity estimates (millions of metric tons [M t])

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Source: Food and Agriculture Organization of the United Nations (http://faostat.fao.org/site/535/default.aspx#ancor).

There tends to be greater diversification among the top barley and malt importing countries (Table 1.6). The consistently largest barley importer by far is Saudi Arabia, taking about 25% of the trade. Crop production is a relatively small component of Saudi Arabia’s agriculture compared with animal production, hence, the emphasis on importing feed stocks like barley. China, Japan, and Belgium were among the top five importing countries in both 2000 and 2005. Japan, Brazil, the Russian Federation, and Venezuela were among the largest malt importers in both 2000 and 2005. Although the EU dominates the export market, it is a prominent importer of barley and malt as well, importing 25%–30% of the barley and about 25% of the malt traded in the world (FAO 2009).

Most of the world’s people on every continent associate barley malt with beer. However, in rural areas in Africa, sorghum, maize, and millet beers are very important in local cultures. Unless otherwise indicated, the term beer in this section refers to beer made from malted barley. The average annual estimated global production of barley malt beer over the 3-year period of 2005–2007 was 165 M t. During this period, production increased rather sharply (11.5%) from 156 M t in 2005 to 165 M t in 2006, and to 174 M t in 2007 (Table 1.7). Beer production in 2007 was 128% of the beer production in 2000 (136 vs. 174 M t). By continent, in 2007, Europe produced the most beer, an estimated 57 M t (EU with 40 M t) followed by Asia at 53 M t, North America at 35 M t, South America at 18 M t, Africa at 8 M t, and Australia and New Zealand at 2 M t (FAO 2009).

Table 1.7 Leading beer-producing countries based on estimates of quantity produced (millions of metric tons [M t])

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Source: Food and Agriculture Organization of the United Nations (http://faostat.fao.org/site/535/default.aspx#ancor).

With 1.4 billion people, it is not surprising that China is now the world’s largest beer producer with an estimated 40 M t in 2007 (Table 1.7). The United States is a distant number 2 with an approximate production of 23 M t, and the Russian Federation is a distant number 3 with approximately 11.5 M t. Germany, Brazil, and Mexico closely follow at numbers 4, 5, and 6, respectively. The top 10 is rounded out with the United Kingdom, Japan, Spain, and Poland. The dynamics in beer production over the 3-year period of 2005–2007 are also depicted in Table 1.7. Among the top 10 producers, China is rapidly increasing its beer production, while the Russian Federation, Brazil, Mexico, and Poland have been slowly increasing production, and the United States, Germany, the United Kingdom, Japan, and Spain have been rather static in production.

Seven of the top 10 beer exporting and importing countries based on quantity of trade in 2000 and 2005 were EU countries. The EU countries as a whole exported approximately 60%, and they imported about 40%, of the beer traded in the world both of these years (FAO 2009). The top five exporting and importing countries in 2000 and 2005 are depicted in Table 1.6 along with the quantities exported and imported. Mexico has consistently been the leading beer exporting country in the world. Besides the European countries listed, Canada ranked fifth in 2000 and sixth in 2005 (FAO 2009). The United States has consistently ranked first in the importation of beer in the world and by a very wide margin versus the second-ranked United Kingdom. As noted above, global beer production has risen sharply (2007 was 128% of 2000) since the beginning of the twenty-first century. The trade of beer has risen even more sharply with export quantity in 2006 175% of export quantity in 2000, and import quantity in 2006 165% of that in 2000 (FAO 2009).

In contrast to the 165 M t of beer brewed with barley malt in the world in 2006, there was an estimated 6.9, 2.5, and 1.5 M t of beer brewed with malt of sorghum, maize, and millet, respectively (Table 1.7). Almost all of nonbarley malt beer is brewed in Africa. Canada (for maize beer) is the only country outside Africa among the leading producers of these beers. The leading countries are spread throughout sub-Saharan Africa. From the author’s own experience living in Malawi in southern Africa, msese, a maize beer, brewed in 55-gal oil drums, was available in rural villages, and chibuku, another maize beer, commercially produced and packaged in paperboard cartons, was available in stores. Both types of beer are opaque with much sediment, even chunks, typically filtered out of msese, but not chibuku. The chibuku brand was Shake Shake, probably to admonish the consumer to mix the contents thoroughly to get the full benefit (nutritional?) of the beer. Because considerable amounts of beer are brewed locally noncommercially in villages, the FAO production data are probably substantially underestimates. Nevertheless, these beers are produced on a much reduced scale compared with barley malt beer and are important mostly on a local or regional basis.

CONCLUDING REMARKS

This introductory chapter sets the stage for understanding the importance of barley as a major global crop. The following chapters will expand on some of the topics briefly discussed here. Barley has played a major role from the era of hunting and gathering, through the transition to agriculture, up to the present era. Barley was one of the first plants domesticated in the first agricultural region of the world, and it has maintained its prominence in the world for over 10,000 years in agriculture; human and animal food, feed, and nutrition; alcoholic beverage production and consumption; and in the continuing development of the biological sciences. Barley has played an important role in plant genetics and breeding, plant physiology, agronomy, cereal chemistry, human and animal nutrition, plant pathology, and entomology. Barley, as an experimental organism, has contributed to the development of scientific knowledge, and science has contributed to the improvement of barley as a crop. Barley, as the fifth most-produced crop in the world today, involves massive amounts of resources and people working in production agriculture; commodity transportation and trade; processing and end use product manufacture, transportation, marketing, and consumption, as well as, research and development for the improved production and use of the crop.

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Chapter 2

Barley Origin and Related Species

Roland von Bothmer and Takao Komatsuda

INTRODUCTION

The barley genus Hordeum was formerly considered a nonhomogeneous group with basically a number of unrelated but morphologically rather similar entities, thus not comprising a natural phylogenetic group. Today, there are new, powerful techniques available that have substantially increased our knowledge on evolutionary age, migration patterns, and differentiation of new species groups. Recent research has convincingly showed that Hordeum is indeed a monophyletic group with a common origin. All species, even cultivated barley, are thus related, some of them more distantly so. Hordeum is evidently an ancient genus, splitting from the wheat species some 13 million years ago (mya).

The stories of domestication of our cultivated plants are fascinating, combining biological and agricultural knowledge with archaeology and other sciences. Our view on the evolutionary history of domesticated barley and its relatives has been significantly altered in recent years. The former theory of a single domestication event of barley is now abandoned in favor of a view of at least two events, separated in time and place. The current research on the domestication process is largely directed toward understanding the mechanisms and pathways at the molecular level of the successive changes in the gene systems regulating traits responsible for major domestication syndromes.

Knowledge of the potential for utilization of the wild species has increased. The ancestral form of barley, Hordeum vulgare subsp. spontaneum (wild barley), despite belonging to the primary gene pool of barley, was earlier considered as an exotic germplasm and, as such, of limited value as a gene source in practical breeding programs. This view of wild barley has changed considerably. Today, the potential for practical utilization is highlighted for base broadening as well as for incorporation of individual, interesting genes regulating agronomic traits. Wild barley has also developed into a model organism in biological research on genetic diversity, differentiation in populations, disease resistance, and other important mechanisms.

Other wild species have lately become more in focus for utilitarian purposes. The secondary gene pool, Hordeum bulbosum (bulbous barley), has gained a renewed interest. Thirty years ago, it attracted attention for production of doubled haploids in wheat and barley, but now it has attained importance as a gene source particularly for incorporation of disease resistance. The South American Hordeum chilense is the target for creation of an entirely new crop, tritordeum, as a result of intergeneric hybridization with wheat. It has a number of promising characters, such as improved baking quality, stress tolerance, and disease resistance. Other species show potential for stress tolerance, for example, Hordeum marinum (sea barley) with a particularly high salt tolerance.

Some of the wild, perennial Hordeum species are important as components in natural pastures used for grazing in South America and in central Asia. Attempts are being made to develop pastures for grazing in stressful areas, for example, in the Middle East, by introducing drought-tolerant annual species with a rapid development, such as Hordeum murinum (wall barley) and wild barley.

Some species have negative effects on the environment by successful, opportunistic behavior. They occur as serious weeds, such as Hordeum jubatum (squirreltail barley), in many parts of the world. Other species may cause severe problems in cultivation by acting as hosts for pests and diseases from which a pathogen may spread to the cereal crops. Irrespective of the fact that wild species may have a positive or a negative influence on agriculture or of being a real or potential gene source for crop improvement, it is important to increase knowledge of their biological systems, gene contents, and ecological preferences.

This chapter presents a review of the genus Hordeum and its species. It aims to give a comprehensive survey of the present knowledge of biological diversity and agricultural potential. Descriptions of the species together with outlooks on agricultural potentials are reviewed and, in particular, the current views on evolution and phylogeny are elucidated. The domestication process is mirrored by description of changes in the major gene systems.

THE TRIBE TRITICEAE

Barley belongs to one of the economically most important plant groups in the world, the Triticeae, which is a tribus (tribe) in the grass family, Poaceae. It comprises the major temperate cereals: wheat (Triticum, several species), rye (Secale cereale), barley (H. vulgare), and the man-made crop rye wheat or triticale (Triticosecale). There are other species of value as forage grasses, such as the crested wheatgrasses (Agropyron cristatum and related species) and Russian wildrye (Psathyrostachys juncea). Some species have gained attention, for example, as sand binders in eroded areas, such as lyme grass on Iceland (Leymus arenarius). Many Triticeae species are noxious weeds, such as quack grass (Elymus repens, formerly Agropyron or Elytrigia repens), mouse barley (H. murinum subsp. leporinum), and foxtail barley (H. jubatum).

All species in Triticeae have spikes (sessile florets); the basic chromosome number is x = 7, and they have large chromosomes reflecting large genomes. The tribe has a worldwide distribution occurring on all continents in most cold and warm temperate areas, and it even has some subtropical representatives.

The diversity centers of the tribe, defined as number of species, are in remote areas, such as in the mountainous regions of central Asia, where little material for research has been available and new species are still to be found. Thus, taxonomy at the species level is far from satisfactorily solved. No comprehensive monograph or review of the whole tribe is available, and there is still a great uncertainty about the number of species, ranging from ca. 325 (Dewey 1984) to ca. 500 (Löve 1984). More recently, Barkworth et al. (2007) estimated the number to be between 400 and 500 species.

The circumscription of genera, likewise, are in disagreement. Löve (1982, 1984), by applying a strict concept for generic delimitation based on genomes, recognized 38 genera, whereas Stebbins (1956) argued that all species should be lumped into one genus due to a high degree of cross­ability. Neither of these views has been widely accepted. Today, an intermediate number of genera are accepted, but still there is no consensus about the generic delimitations (Kellogg 1989; Barkworth 1992; Watson and Dallwitz 1992; Yen et al. 2005; Barkworth and von Bothmer 2009).

Triticeae is a very good example of a successful, widespread plant group with large diversity and high versatility in biological characters. It represents very complex modes of speciation, including polyploidy, and a high degree of interspecific and intergeneric hybridization leading to a reticulate pattern of evolution. A typical example of this phenomenon is the widespread I genome in Hordeum (see below concerning genome designations). This genome is also occurring as one component in polyploid species of Elymus and Hystrix, in combination with different other genomes, such as Y, St, P, and W (Mason-Gamer 2008; Zhang et al. 2008). The large number of species, the large diversity, and the reticulate pattern of relationships makes the whole tribe a model group for studies of evolution and phylogeny as well as an interesting gene source for cereal and forage grass breeding.

Even though the phylogeny and evolutionary pattern in Triticeae is complex, phylogenetic studies of the grass family indicate that the tribe is monophyletic and that it constitutes an evolutionary homogeneous group (Catalan et al. 1997; Seberg and Frederiksen 2001; Petersen and Seberg 2005). Estimations have shown that the Triticeae diverged from tribus Avenae (oats) ca. 25 mya and that wheat (Triticum) and barley (Hordeum) diverged around 13 mya (Gaut 2002).

Species of the grass family differ in basic chromosome numbers and show large variation in genome size. All members of the Triticeae have a basic chromosome number of x = 7 and have the largest genome size of all grasses, where Psathyrostachys fragilis has the largest size of all with 17.9 pg (cf. Gaut 2002). The genome size is mainly a function of repetitive DNA. The complements of low-copy genes are, however, similar in rice and barley, but there is a 12-fold difference in DNA content (Saghai-Maroof et al. 1996; Gaut 2002). There is no evidence that annual, self-pollinated species should have smaller genomes than perennial, cross-pollinating species in the Triticeae (Eilam et al. 2007). During the 77 million years of grass evolution, there have been dramatic genome changes, but the major chromosomal organization has been surprisingly stable with conserved linkage blocks and gene orders (Devos and Gale 2000; Qi et al. 2006; Stein 2007 for references; Stein et al. 2007; Cuadrado et al. 2008).

The closest related genera to Hordeum in Triticeae are Psathyrastachys, which is an Asiatic group of tufted perennials growing in harsh, dry steppe environments, and Taeniatherum, an annual genus originally of Mediterranean origin (Frederiksen and von Bothmer 1989). Hordeum, together with Psathyrostachys, constitute a monophyletic group clearly separated from the rest of the Triticeae (Hsiao et al. 1995; Seberg and Frederiksen 2001). The relationships to the monotypic genus Hordelymus with the single species Hordelymus europaeus is not clarified (Ellneskog-Staam et al. 2006; Petersen and Seberg 2008). Significantly, newer data have revealed that the barley genus, Hordeum, is monophyletic; thus, the former splitting of the species into different genera (primarily Hordeum and Critesion) has no phylogenetic basis (Kellogg 1989; Petersen and Seberg 1997; Seberg and Frederiksen 2001).

THE GENUS HORDEUM

Distinguishing Hordeum species from all other Triticeae species are a few distinct characteristics. The major trait is the typical triplet, which are three one-flowered (one seeded) spikelets at each rachis node. In the wild species the lateral spikelets are always stalked, whereas they are sessile in H. vulgare (both in cultivated barley and its wild progenitor). The lateral spikelets in the triplets may be fertile and seed setting as in six-rowed barley, whereas the laterals are sterile in two-rowed barley. Despite the basic features characterizing the genus Hordeum, the morphological and genetic variation within and between species is considerable. A prominent feature is the large plasticity in morphological traits found particularly in some of the annuals. Under unfavorable stress conditions caused by drought, heat, salinity, or flooding, the plants may be slender with a single, short culm with a minute spike, with a low but secured seed set. Under favorable conditions, the same genotype may be luxuriant with a height of 1 m, with several culms and large spikes and florets. A good example of the extreme plasticity is H. murinum.

Distribution

Hordeum has a wide distribution and occurs in most temperate areas in the world. Southwest Asia is probably the original area for the genus, and the first representatives of Hordeum have been estimated to date from ca. 12 mya (Blattner 2004, 2006; Jakob and Blattner 2006; see under phylogeny). One group of perennial species is today present in central Asia (Hordeum roshevitzii, Hordeum bogdanii, and Hordeum brevisubulatum). From this original Asian distribution, migration to North America occurred ca. 4 mya and some perennial species developed here (Hordeum brachyantherum and H. jubatum). Probably as a result of long-distance migration by birds, Hordeum spread from North to South America, where further speciation took place, and today, South America is the diversity center of the genus, defined as containing the highest number of species (18), including both diploid and polyploids. Annuality was developed here (Hordeum euclaston), and a secondary dispersal took place from South to North America where the two diploid annuals Hordeum pusillum (widespread) and Hordeum intercedens (endemic to southwestern California) were developed (Blattner 2006). Later, the annual, allotetraploid Hordeum depressum was differentiated in the southwestern (present-day) United States. The single Central American species, Hordeum guatemalense, was established, also probably through bird migration from the perennial North American taxa. In the Mediterranean area, H. murinum is of ancient origin and separated from the Southwest Asian H. vulgare and H. bulbosum ca. 10 mya. The other Mediterranean annual H. marinum differentiated from the I genome group ca. 6 mya. Theses two, originally Mediterranean annual species, have become widespread as weeds during the last centuries in suitable, temperate areas of the world. Somewhat puzzling are the two remaining species: Hordeum secalinum, with a European and North African distribution, and Hordeum capense, native to South Africa. Both are distinct, tetraploid, perennial species, and both have the diploid cytotype of H. marinum as one of its ancestors. H. capense has probably differentiated from H. secalinum or from a common ancestor in South Africa.

Habitats

The habitat preferences for Hordeum comprise a wide range of ecological niches. The majority of the perennial species are confined to dry or, more often, wet pastures. Some species, such as H. bogdanii and H. depressum, grow in saline environments; H. marinum, in particular, can be classified as a halophyte tolerant to high salt concentrations (see under the species). Dry steppes, stony hillsides, and salt pans are preferred particularly by some of the species in South America, for example, Hordeum patagonicum, Hordeum comosum, and Hordeum tetraploidum. One extreme specialization is H. patagonicum subsp. magellanicum, which is mainly confined to sandy beaches on Tierra del Fuego and has adapted a dispersal mechanism specialized to this environment. The altitude range is large, from sea level where several species are found up to 4000–5000 m in the Himalayas (H. brevisubulatum subsp. nevskianum and subsp. turkestanicum) and in the Andes (Hordeum muticum, H. comosum, and Hordeum halophilum).

Life Forms, Reproduction, and Dispersal

Hordeum shows a large variation in reproductive pattern and in life forms. Most species are long- or short-lived perennials. Annuals have developed independently from perennials in different parts of the world through adaptation to particular ecological requirements, such as halophytic communities for H. marinum, summer or winter annuality in H. murinum, and adaptation to the vernal pool habitat for H. intercedens in California. The two species Hordeum flexuosum (South America) and Hordeum arizonicum (North America) have a versatile life form. They occur usually as short-lived perennials (often biennials) or as annuals, dying off under unfavorable conditions. The European species H. secalinum is very slow growing and it has a shallow and poorly developed root system. It grows in slightly saline environments, needs occasional flooding, and is very sensitive to competition from more aggressive species. Most perennial species are bunch grasses, but some species have a distinct vegetative reproduction by subterranean runners (rhizomes). This is particularly typical for H. brevisubulatum. The swollen shoot base (corm), looking like a bulb, is significant for H. bulbosum and contributes to vegetative reproduction.

The original sexual reproduction was a versatile system with a capacity for self-pollination as well as for cross-pollination, with partly open flowers and a not well-differentiated time lapse for pollen maturity and stigma receptivity. This is also the most common sexual reproduction system among present-day Hordeum species. However, specialization has been developed independently in various groups. Some of the annual species, such as H. murinum and H. intercedens, are more or less obligate inbreeders with cleistogamous flowers, short stigmas, small anthers with few pollen grains, and simultaneous development of stamens and stigma. They show generally a very high seed set, irrespective of external conditions. The ancestor of cultivated barley (H. vulgare subsp. spontaneum), in spite of being an annual, has a comparatively versatile system with often rather open flowers and thus with capacity for cross-pollination. The domestication process has led to a decreased outbreeding, and modern cultivars of barley are considered to be almost exclusively inbreeding. For the plant breeding process, barley is estimated to generally have x226A_EhrhardtMT_11n_000100 1% outbreeding. More primitive forms, such as older landraces, have a higher rate of outbreeding than modern cultivars. On the other extreme, there has been an evolution toward complete outbreeding. Some species, for example, H. secalinum and H. tetraploidum, are self-compatible but are mostly open flowering, with a rich pollen production and long exserted stigmas. The most extreme forms are H. bulbosum and H. brevisubulatum, which are both more or less obligate outbreeders and well adapted for wind pollination. They have both developed self-incompatibility mechanisms, which for H. bulbosum is a two-locus genetic system (Lundqvist 1962).

The seed dispersal mechanism is markedly differentiatied. Most species have a brittle rachis (main axis of the spike); that is, the spike disarticulates at maturity at each rachis node. The dispersal unit is a triplet; thus, a central spikelet together with the two lateral ones united with one internode of the spike. Only the two distantly related taxa H. vulgare subsp. vulgare and H. bogdanii have a tough spike where only the mature, central, seed-bearing spikelet disarticulates, leav­ing the main spike axis intact. The wild ancestor to barley, subsp. spontaneum, has a very fragile spike. The tough rachis in the cultivated form is a domestication syndrome and an advantage for easy and secure harvest. The genetic background for brittle versus tough rachis is fairly simple (two genes, see under Domestication of Barley). The genetic background for the tough rachis in H. bogdanii has not been investigated.

Most species have a rather unspecialized seed dispersal mechanism, but particularly, two types of specializations are evident. H. murinum and H. vulgare subsp. spontaneum have both long and tough bristles on awns, glumes, and on the edge of the rachis. These hairs are unilaterally directed, which means that the triplet, despite the comparatively large and heavy seeds, very easily attach to, for example, the furs of animals. This is an obvious adaptation to a zoochorous dispersal mechanism (see under H. murinum). The other extreme are those species adapted to wind (anemochorous) dispersal. They have small, light seeds and long, slender awns and glumes, which are spreading out (often 90°) at maturity. Several triplets are often attached to each other, forming a light flying apparatus, which is easily spread in the wind, sometimes for long distances. Typical representatives with wind dispersal are the species in section Critesion, for example, Hordeum lechleri in South America and H. jubatum in North America.

It is obvious that an inbreeding reproductive system, which guarantees a secure seed setting also under unfavorable conditions, together with rather unspecialized ecological requirements (preferably open, ephemeral habitats) and an effective dispersal mechanism (by wind or by animals), is an effective prerequisite for a weedy habit, like in H. murinum and H. jubatum.

Chromosomes and Genomes

The basic chromosome number is, like in all other Triticeae species, x = 7, and in the genus, both diploids (2n = 14) and polyploids (2n = 4x = 28 and 2n = 6x = 42) occur. The chromosomes are large and the karyotype is mainly symmetrical with 2–5 nuclear organizing regions (NORs). The C-banding as well as the fluorescent in situ hybridization (FISH)/genomic in situ hybridization (GISH) patterns are distinct, which partly are species specific and can be used for determination of evolutionary relationships and phylogeny (Linde-Laursen et al. 1986a,b, 1989, 1995; Linde-Laursen and von Bothmer 1989; de Bustos et al. 1996; Taketa et al. 1999, 2000, 2001, 2005). One particularly deviating species is the autopolyploid H. brevisubulatum complex, which shows a large cytological differentiation, from populations with minute, intercalary C-bands (subsp. brevisubulatum) to populations with large, distal constitutive heterochromatin similar to that in rye (subsp. violaceum; Linde-Laursen et al. 1980). All these distinct forms are, however, fully interfertile and with full meiotic pairing in the hybrids (Landström et al. 1984).

The concept of genomes was explored by Kihara (1940) in wheat and related species, and this concept has been widely accepted and used in several plant groups. The original definition of a genome is the degree of chromosomal pairing in the meiosis of interspecific and intergeneric hybrids. A high meiotic pairing rate in hybrids indicates a closer relationship between the parental species than a lower pairing rate. The conclusions of relationship may, however, be hampered by an autosyndetic pairing of chromosomes from each parent or by a meiotic regulation of pairing, which can both promote and reduce the chiasma frequency. Attempts, particularly in the Triticeae, have been made to use genome relationship as a basis for taxonomic delimitation, thus that only species containing identical haplomes (or combination of haplomes) should be combined in a genus. In the most ultimate form, such a taxonomy was proposed by Löve (1982, 1984) and was partly adopted by Dewey (1984), resulting in a large number of new and monotypic genera in Triticeae. This system has not been widely accepted, and the genome concept has been criticized as a basis for drawing phylogenetic conclusions and for taxonomic delimitations (Kellogg 1989; Seberg and Petersen 1998; Petersen and Seberg 2003). Nevertheless the genome concept based on a wider range of scientific data than only chromosomal meiotic pairing has shown to be a valuable tool for practical purposes as well as for considerations of relationships.

In Hordeum, the genome concept based on the traditional criteria, that is, the meiotic pairing in interspecific hybrids, was studied in a detail in the 1970s and 1980s (cf. von Bothmer et al. 1989c, 1995a, and references therein). The genomic content or delimitations in haplomes based on pairing behavior in the meiosis of hybrids have been widely verified in later studies including in situ hybridization and various nuclear and plastid molecular studies (see under Evolution and Phylogeny in Hordeum). The assembled data suggest that four basic haplomes occur in Hordeum, namely, the H haplome in H. vulgare and H. bulbosum, the Xa haplome in H. marinum, the Xu haplome in the H. murinum complex, and the I haplome or modifications thereof in all other Hordeum species. There is a differentiation within the haplome groups, so, for example, the H haplomes of H. vulgare and H. bulbosum are not identical. Within the large I haplome group, there is a tendency for differentiation between major distribution areas (Eurasia vs. South and North America).

The designations of the different genomes in Hordeum have obtained an unfortunate ambiguity. Formerly, when little data were available, all species of Hordeum, thus also including H. vulgare, were presumed to contain the same genome/haplome, named H. When more data became available, it was evident that at least two major genomic groups occur, namely, H. vulgare and H. bulbosum on the one hand, and most other Hordeum species on the other. At that time, the separate and distinct genomes of H. murinum (Xu) and H. marinum (Xa) were also identified. During the 1980s, it was internationally adopted that the haplome of the wild species should be named "H" and that of vulgare-bulbosum should be called "I." However, a working group assigned by the International Barley Genetics Symposium (IBGS) developed a classification system for chromosomes and genomes of cultivated barley. One major suggestion of the group was that the chromosome numbering in barley should follow the homoeologous wheat chromosome numbering, thus partly changing the chromosome numbering in barley. The group suggested further that the H. vulgare haplome should be named H (Linde-Laursen et al. 1997), ignoring that the haplome of the wild species had been named H for a number of years. This proposal was adopted at the IBGS and since then, both the chromosome numbering system and the barley haplome designation follow the recommendation of the working group. This has caused a considerable problem since scientists working with wild species have continued to use the designations in the opposite way, thus I for H. vulgare and H. bulbosum and H for the other wild species (apart from H. marinum Xa and H. murinum Xu). How this problem will be resolved is presently not clarified. In this presentation, the recommendation from the IBGS working group (Linde-Laursen et al. 1997) is followed.

Evolution and Phylogeny in Hordeum

Over the last 25 years, a number of new tools have been developed for studies of evolutionary pathways and phylogenetic patterns. For Hordeum, the new data have refined our view on relationships, phylogeny, routes of migration, and age of the various species or species groups. There is now a general and rather detailed picture of relationships, but depending on methods used and interpretation of data, details may differ between studies. In order to draw valid conclusions on general evolutionary or phylogenetic patterns, a complete set of species or at least a fair amount of accessions and species need to be included, but few investigations fulfill these requirements. Several studies include a restricted