Wheat

1.1.1 History, production, price, yield and area

The genus Triticum (wheat) originated in the area that stretches from Syria to Kashmir, and southwards to Ethiopia. In the very distant past, wheats gradually evolved in this region from wild plants. Since the early 1900s, it has been known that the wheat species and indeed all members of the Triticeae tribe have a basic chromosome number of n = 7.

Among the top 10 wheat-producing countries, China contributed, during the period 2006–2010, 13.7 % of the world’s wheat production from 8.6 % of the world’s wheat-growing area, while India contributed 9.8 % of the production from 10.7 % of the area. China produces a larger amount of wheat than India (89.0 compared to 63.6 million tonnes per year, 2006–2010) but from 4 % less wheat cultivation area (18.9 compared to 23.5 million hectares per year, 2006–2010). This is mainly due to the high wheat yield registered in China (4.7 tonnes/ha), second only to Germany and France with 6.7 and 7.0 tonnes/ha, respectively (FAO/UN, 2012) (Table 1.2).Throughout the last 10 years, wheat production has gradually increased by approximately 10%, growing from 585.7 × 10⁶ to 653.7 × 10⁶ tonnes, mainly due to an improved yield that has been increased by ≈ 10%(Table 1.1).

As reported in Table 1.1, the last 10 years have seen wheat producer prices increase by 36.5%, moving from 179.7 US $/tonnes to 283.3 US $/tonnes in 2010. Among the top 10 producer countries, Turkey and Russian Federation showed the highest (328.4 US $/tonnes) and the lowest (134.4 US $/tonnes) producer price, respectively. For the leading producing country China, the producer price for 2009 was equal to 270.9 US $/tonnes (FAO/UN, 2012).

1.1.2 Phytology, classification and cultivation

Wheat is an annual grass belonging to the Poaceae (Gramineae) family, tribe Triticae (Zohary, 2000).The wheats currently cultivated are the diploid T. monococcum (Einkorn wheat; 2n = 14, genetically described as AA plants), the tetraploids T. dicoccum (emmer wheat) and T. durum (pasta wheat or hard wheat) (2n = 28, genetically described as AABB plants), and the hexaploids T. aestivum (soft wheat or bread wheat) and T. spelta (spelt) (2n = 42, genetically described as AABBDD plants). Currently, about 95 % of the wheat grown worldwide is bread wheat, with most of the remaining 5 % being pasta wheat. The latter is more adapted to the dry Mediterranean climate than bread wheat. Small amounts of other wheat species (einkorn, emmer, spelt) are still grown in some regions including Spain, Turkey, the Balkans and the Indian subcontinent. In Italy, these hulled wheats are together called farro (Szabó and Hammer, 1995), while spelt continues to be grown in Europe, particularly in Alpine areas (Fossati and Ingold, 2001).

Wheat for the purpose of trading is classified into distinct categories according to grain hardness (soft, medium-hard and hard) and colour (red, white and amber). It may be further subdivided into subclasses based on growing habit (spring or winter). Each wheat subclass may also be grouped into grades, which are generally used to adjust the basic price of a wheat stock by applying premiums or penalties. Wheat grades are indicators of the purity of a wheat class or subclass, the effects of external factors on grain soundness (rain, heat, frost, insect and mould damage) and the cleanliness (dockage and foreign material) of the wheat lot. Today, wheat is a major component of most diets of the world because of its high agronomic adaptability, nutritional quality, the fact that it can be stored effectively indefinitely before consumption (provided the water content is below about 15 % dry weight and pests are controlled) and the ability of its flour to produce a variety of satisfying, interesting and palatable foods.

Wheat and Its Cereal Relatives

Wheat is one of the three most important grain species in the world, based on annual volume of production (Table 1.1). It shares this distinction with corn (maize) and rice. The production figure for rice (Table 1.2) is for paddy rice, the form of rice as it is initially harvested; this figure should be reduced by about 20% to indicate the production of rice grain, after removal of the outer hulls. Volumes of production are much less for the wider range of agricultural seed crops (Table 1.2); the species of next significance are barley and soybeans. Of all these, wheat has the distinction of being the most important in world trade and also the grain for which quality specifications are the most critical.

The seed crops of economic value are divided into two major taxonomic groups, namely, the “monocots” and the “dicots” (Table 1.2), referring to the presence of one or two embryonic leaves (cotyledons) in the seed and young seedling. Although both groups are sometimes referred to as “grains,” this botanical term should strictly be applied only to cereals in which the grain is a single-seeded fruit called a “caryopsis” (see Chapter 3). Further taxonomic groupings lead down through order, family, and tribe to genus and species (Heywood 1993; Morrison and Wrigley 2004).

Wheat as a Grain Genus

The genus name for wheat, Triticum, comes from the Latin tero (I thresh). Triticum vulgare is the old (no longer accepted) species name for bread wheat, in which vulgare means “common.” The current binomial name, Triticum aestivum, refers to hexaploid bread wheat (genomes A, B, and D), distinguishing it from tetraploid macaroni wheat (Triticum durum) (genomes A and B), which is used primarily for pasta production. Most of the wheat grown worldwide (>90%) is the aestivum species; despite its being referred to as “bread” wheat, it is used for the full range of applications, even including pasta production in some regions. In addition, T. monococcum (including “small spelt” wheat as a subspecies) and T. timopheevii (including “Georgian” wheat) are cultivated to a limited extent, the former in Yugoslavia and Turkey, and the latter in the former Soviet Union (Feldman and Sears 1981). The main cultivated form of spelt is the hexaploid T. aestivum var. spelta, also classified as T. spelta (Fig. 1.5) (Morrison and Wrigley 2004).

Fig. 1.5. Variations in the appearance of heads of wheat species, one of many morphological characteristics used for their taxonomic classification. The wheat species are (including their genome assignments and common names) a, Triticum boeoticum (2x: wild einkorn); b, T. monococcum (2x: einkorn); c, T. dicoccoides (4x: wild emmer); d, T. dicoccum (4x: emmer); e, T. durum (4x: macaroni wheat); f, T. carthlicum (4x: Persian wheat); g, T. turgidum (4x: rivet wheat); h, T. polonicum (4x: Polish wheat); i, T. timopheevii (4x: Timopheev's wheat); j, T. aestivum (6x: bread wheat); k, T. sphaerococcum (6x: shot wheat, Indian dwarf wheat); l, T. compactum (6x: club wheat); m, T. spelta (6x: spelt wheat); and n, T. macha (6x: macha wheat). The diploid A-genome species, T. urartu, is not shown here. 2x = diploid; 4x = tetraploid; 6x = hexaploid.

(Adapted from Mangelsdorf 1953)Copyright © 1953

The ancestral diploid wheat species are T. monococcum, Aegilops speltoides, and T. tauschii and a wild Aegilops species that is probably most closely related to the modern A. speltoides. Each of these species has seven pairs of chromosomes (2n = 14). T. durum (also named T. turgidum ssp. durum) is tetraploid (2n = 14), having been derived from the natural hybridization of T. monococcum (A genome) and the ancestral A. speltoides (B genome). Common bread wheat (AABBDD) is a hexaploid (2n = 42) resulting from the natural hybridization of Triticum dicoccoides (AABB) and T. tauschii (DD) (Mangelsdorf 1953, Feldman 2001, Shewry et al 2003). The diversity of head morphology for these and other ancestral wheats is shown in Figure 1.5. Although it is not clear from the head illustrations, these various primitive wheats differ greatly in the ease of threshing out grains from the heads, an important characteristic for successful cultivation and harvesting of any grain species.

Wheat and Its Ancestral Relatives

The origins of wheat and these hybridization events are believed to have occurred in the Middle Eastern region of Ancient Egypt, the Levant, and Mesopotamia, watered by the Nile, Jordan, Euphrates, and Tigris rivers. Wild emmer, the progenitor of cultivated wheats, was first known to the Western world from the work of the Austrian botanist T. Körnicke. In 1873, emmer was shown in the National Museum of Vienna among samples of wild barley collected by Körnicke in 1855 on the slopes of Mount Hermon, in southeastern Lebanon. It was not until 1889 that Körnicke reported this discovery to the Botanical Society of the Lower Rhine and Westphalia (Feldman 2001). Wild emmer carries the full scientific name of T. turgidum L. ssp. dicoccoides (Körn. ex Asch. and Graebn.) Thell., or just T. dicoccoides (Fig. 1.5). It was later found in many sites across Israel, Jordan, Lebanon, and Syria, thus confirming Körnicke's hypothesis that this wild progenitor still grows in the Near East. In fact, it grows in a wide range of conditions, from the Jordan Valley, 200 m below sea level, to the slopes of Mount Hermon, up to 1,600 m, and in a wide range of soil types. Stories about the discovery of other wheat progenitors in the Fertile Crescent and beyond are told by Feldman (2001). They have proved to be valuable sources of novel genes for use in the improvement of cultivated wheats.

Wheat remnants, unearthed in the Levant (southeastern Turkey south to the Jordan), have been dated to about 10,000 b.c.e. Neolithic humans are presumed to have cultivated emmer wheat, einkorn (T. monococcum ssp. aegilopoides), T. urartu, and T. timopheevii. Of these four ancestral wheats, all except T. urartu evolved into domesticated forms with nonbrittle spikes. This important characteristic meant that the head did not readily shatter and fall to the ground, making it very difficult to harvest. These nonbrittle species gradually spread throughout Southwest Asia (Feldman 2001).

Wild einkorn suffered from the disadvantage that, when threshed, the glumes (hulls) still remained attached. A free-threshing form of einkorn has been found in Central Asia; this would have been easier to harvest and thus it would have been propagated in preference to other forms. Similarly, the early wild heads of emmer were brittle, and it may have been 9,000 b.c.e. before nonbrittle forms appeared. In about 6,000 b.c.e., this grain spread to Egypt, India, and Central Asia. It is still cultivated to a small extent in parts of India, Iran, eastern Turkey, and the Balkans.

Durum wheat (T. turgidum ssp. durum), related to emmer (T. turgidum ssp. dicoccoides), has been found in pottery dated to about 7,000 b.c.e., but it does not appear to have been established as a major crop until about 2,000 b.c.e. This apparent delay in its adoption would not be expected, as durum has free-threshing large grains, but may have been because of the limited agronomic adaptability of durum wheat (which has a narrower genetic basis than bread wheats) and because of the greater difficulty in milling the very hard durum grain. Today, however, durum wheat is cultivated worldwide to the extent of about 10% of wheat production.

Common hexaploid wheat first appeared in about 9,000 b.c.e., presumably in northwestern Iran or northeastern Turkey, resulting from natural hybridizations between tetraploid wheat (probably not wild emmer) and the diploid species T. tauschii (A. tauschii) (Feldman 2001). This event probably occurred more than once (Morris and Sears 1967).

1.2.3.1 Origin and types

Both wheat and barley were first domesticated 10,000 to 11,000 years ago in the Fertile Crescent of the Middle East, which was described as surrounding the northern reaches of the Tigris and Euphrates rivers. Together with pulses such as lentils, wheat and barley were the earliest plants to be cultivated in the region.

Numerous examples of ancient wheat have been unearthed in archaeological investigations; the grains usually carbonized, although in some cases the anatomical structure is well preserved. The earliest cultivated wheats were both hulled species: einkorn Triticum monococcum ssp. monococcum (diploid, chromosome complement AA) and emmer Triticum turgidum ssp. dicoccum (tetraploid, AABB) both with grain tightly enclosed by tough husks (glumes: lemma and palea, see Chapter 3). These two species are cultivated as a staple only in mountainous regions including central and eastern Europe, but they are increasingly cultivated as ‘health foods’ elsewhere.

Naked or free-threshing forms (i.e., without adherent glumes) including the hexaploid bread, or common, wheat Triticum aestivum ssp. aestivum and tetraploid durum, or macaroni, wheat T. turgidum ssp. durum, evolved about 9000 years ago. Common or bread wheat (AABBDD) is a hexaploid allopolyploid; with three genomes, each corresponding to a normal diploid set of chromosomes, it resulted from accidental hybridization of emmer or durum wheat with the wild grass Aegilops tauschii.

There are about 16 cultivated species but the two most important are bread wheat and durum wheat. Each of these species has thousands of cultivars and landraces.

By far the most important form of common or bread wheat is T. aestivum ssp. aestivum, but T. aestivum ssp. compactum, known as club wheat, is also grown commercially in a limited area in the western states of North America, and T. aestivum ssp. sphaerococcum, known as Indian dwarf or shot wheat, is found in northwest India and Iran.

Strength

This property refers not to the physical strength of the endosperm but to the characteristics of the protein present. In fact, it refers to ‘baking’ strength, or the degree of suitability to provide flour capable of making good leavened bread.

A ‘strong’ wheat variety produces a flour with a protein complement that, on hydration and mixing, produces a gluten that can form films capable of containing expanding gases. Leavened bread made from strong wheat flours has a relatively large volume and a soft texture. Strong wheats have a relatively high-protein content but the qualities of the protein are also essential for the criterion of ‘strength’ to be met.

Spring wheats have a shorter growing season than winter wheats and accumulate less starch. They do however accumulate similar quantities of protein to winter varieties and consequently protein contributes a higher proportion. This, coupled with protein strength conferred by selective breeding, has established many spring wheats, including those of North America, as the best wheats for bread-flour production. Because of their high quality many countries whose climates are less suited to production of strong spring wheats have imported such grains. The United Kingdom is an example of a country with a long history of importing bread wheat, but in the 1970s, in pursuit of self-sufficiency and to avoid levies on third-country imports imposed by the European Economic Community (now the EU), a focused breeding programme led to the introduction of (mainly winter) varieties with sufficient strength to replace imported types. At the same time bread-making techniques were developed that allowed acceptable quality to be achieved from lower protein flours (Chamberlain et al., 1962). A further facilitating factor was the coproduction of starch and vital gluten from wheat, making the latter available for inclusion in flour produced for bread making. The United Kingdom is now around 90% self-sufficient in bread-making wheat. Even where high-quality spring wheats are available, they may not be milled alone. Instead a ‘grist’ is made in which they are supplemented by ‘filler’ wheats with lower bread-making properties.

Although strength is a desirable characteristic of flours destined for bread production, there are many applications of flours for which ‘weakness’ is the desirable property. In these uses, such as biscuit (cookie) and confectionery applications, starch is the more important structural component. Because neither hardness nor strength can be measured in authentic fundamental units, empirical methods of measurement have been devised. In the case of hardness, several methods exist, but they do not necessarily rank all samples in the same order. A popular method is particle size index, whereby a sample ground by a standard method is sieved. Values may be expressed as the proportion passing through the sieve. Measuring the energy required to grind a sample is an alternative approach.

The relationship between protein content and particle size index has been explored and suitability of types based on these characteristics has been determined. Results are shown in Fig. 1.37.

Figure 1.37. Indication of protein content and grain hardness requirements for a range of baked products.

Reproduced from O’Brien, L., Blakeney, A.B., 1985. A Census of Methodology Used in Wheat Variety Development in Australia. Cereal Chemistry Division, Royal Australian Chemical Institute, Melbourne. with permission.

Many countries sort their wheats and other cereals into grades depending on quality, purity and cleanliness before sale to buyers at home or abroad. In almost all systems the properties of varieties are established and only those varieties that have been shown to conform to certain standards are permitted within the more demanding classes. In the United Kingdom, where no official grading system exists, a Wheat Guide is published by the National Association of British and Irish Millers following annual assessment. In it, varieties are grouped according to suitability for end use. Group 1 varieties are most suited to bread making and therefore attract a premium. At the other extreme, Group 4 varieties are suitable mainly for feed, with a possibility of inclusion as ‘fillers’ in other grists. Most feed wheats are excluded from the grading system (http://www.nabim.org.uk/).

1.2.3.2 Cultivation

T. aestivum and Triticum durum wheats exist as winter and spring varieties. Cultivated varieties, which are of widely differing pedigree and are grown under varied conditions of soil and climate, show wide variations in characteristics. The climatic features in countries where spring wheat is grown – maximum rainfall in spring and early summer, and maximum temperature in the mid- and late-summer – favour production of rapidly maturing grain with endosperm of vitreous texture and high-protein content, traditionally suitable for bread making. Winter wheat, grown in a climate of relatively even temperature and rainfall, matures more slowly, producing a crop of higher yield and lower nitrogen content, better suited for biscuit and cake making than for bread, although in the United Kingdom, where winter wheat comprises about 96% of the total, winter wheat is used for bread making.

The yield of durum wheat, which is grown in drier areas, is lower than that of bread wheat.

1.2.3.3 Diseases and pests

Several rusts infect wheat. They include stem rust (Puccinia graminis), leaf rust (Puccinia recondita) and stripe or yellow rust (Puccinia striiformis). There are also several blotch diseases such as spot blotch (Bipolaris soroboniana), speckled leaf blotch (Septoria tritici) and glume blotch (Septoria nodorum). Other fungal diseases are head scab and foot/root rots (Fusarium spp.) Rhizoctonia root rot (Rhizoctonia spp.), loose smut (Ustilago tritici), common bunt or stinking smut (Tilletia tritici and other Tilletia spp.), karnal bunt (Tilletia indica) and powdery mildew (Erysiphe graminis).

Bacterial leaf streak (Xanthomonas campestris) is one of the bacterial diseases, and barley yellow dwarf virus BYDV is one of the viruses infecting wheat crops.

Aphids, termites, grasshoppers and leafhoppers, bugs, thrips, and sawflies are among the pests. In some years, orange wheat blossom midge (Sitodiplosis mosellana) can cause damage to yields and quality. Wheat bulb fly (Delia coarctata) is a pest of mainly winter crops in England.

There are many grasses and broad-leaved species that compete with wheat crops; a widespread weed Phalaris minor (littleseed canarygrass) is widely distributed but it is particularly troublesome where rice-wheat cropping systems are employed. Blackgrass (Alopecurus myosuroides) is also widespread and difficult to control but it is considered invasive only in western Europe.

1.2.3.4 Uses

The main food use of bread wheat (T. aestivum) is as a source of flour for production of baked products. Different types are suited to different products and the flours with highest values are milled from wheats with high levels of protein with appropriate qualities for making leavened and unleavened breads. Flours milled from types with lower protein contents and different characteristics are used for noodles, confectionery, biscuit (cookie) production and for household uses. Types with protein contents and quality unsuited for milling into flour are used for animal feed. Industrial uses similar those applicable to maize are also increasing. The main use of durum wheats (T. durum) is production of semolina or flour for pasta manufacture.

By-products of milling of all types of wheat are suitable for use as feed.

1.2.3.5 Global production

As with rice it is Asia which dominates wheat production although the degree of dominance is far less than in the case of rice (Fig. 1.38). Nevertheless, it is in Asia that the most consistent increase in production has occurred (Fig. 1.39).

Figure 1.38. Proportional contributions to world wheat production by continental regions (mean of 2009–13 values).

Based on data from FAOSTAT.

Figure 1.39. Variation in wheat production in continental regions (tonnes).

Based on data from FAOSTAT.

5.2.1 Taxonomy

Over 90% of the wheat grown worldwide is T. aestivum. It is used for a wide range of applications, including bread, cakes, pastries, biscuits, puddings, thickeners and noodles. T. durum (durum wheat) is mainly used for pasta production. In addition, small amounts of a few specialty wheats are grown: spelt wheat (a hexaploid wheat—T. aestivum var. spelta), Triticum monococcum (small spelt wheat) and T. timopheevii (Georgian wheat) (see Chapter 14).

T. monococcum, Aegilops speltoides, T. tauschii and the wild Aegilops species, which is closely related to the modern Ae. speltoides, are ancestral diploid wheat species having seven pairs of chromosomes (2n = 14). The tetraploid T. durum is derived from the natural hybridisation of T. monococcum (A genome) and the ancestral Ae. speltoides (B genome) (Wrigley, 2009; Morrison and Wrigley, 2016). Hexaploid bread wheat (2n = 42) is a result of the natural hybridisation of T. dicoccoides (AABB) and T. tauschii (DD) (Mangelsdorf, 1953; Shewry et al., 2003; Gustafson et al., 2009).

The common wheats of today are divided into red and white wheats (based on the intensity of red pigmentation in the seed coat), hard and soft wheats (based on the resistance of the seed when it is crushed), and winter and spring types (differing in their requirement for a cold period to hasten or permit normal development towards reproductive development) (Gooding, 2009) (see Chapter 18).

5.2.2 Origins

Man assumed control over his own food production during the Neolithic Revolution. The pre-agricultural hunter–gatherer became familiar with nature’s periodicity and with the life cycle of the dominant plants in his environment, finally succeeding to domesticate many of them. Wheat is one of the world’s most important food crops to be domesticated. It is believed that wild relatives of wheat first grew in the Middle East, in the ‘Fertile Crescent’ (Feldman, 2001). Wheat was one of the first plants to be cultivated. It was grown about 11,000-years ago. Enormous changes in people’s lives occurred because of wheat being grown. People began growing their own food, no longer needing to wander in search of food. Permanent settlements were established because wheat provided people with a stable food supply. Soon they grew enough wheat to feed others from surrounding regions.

Once there was extra wheat available, trade between various cultures developed. By 4000 BC, wheat farming had spread to Asia, Europe and North Africa. New species of wheat developed because early farmers selected kernels from their best wheat plants to use as seeds for planting the following year’s crop. In that way, only the best wheat qualities were passed from one generation to the next. Soon wheat became an important worldwide crop.

Three main theories concerning the site of wheat domestication were proposed in the 19th century. According to the first theory, wheat was domesticated in the Near East. The evidence was from the Chaldean priest, Berosus, who mentioned that wild wheat occurred in Mesopotamia at about 2700 BP (Syncellus, Frag. Hist. Graec., vol. 2, p. 416). Another theory in 1899 proposed that the cultivation of wheat started in central Asia. The third theory proposed in 1908 suggested that wheat grew wild in Europe and was later domesticated there (Feldman, 2001; Morrison, 2016).

5.2.3 Genetic Constitution

Genetically, there are three main groups of cultivated wheat, namely, diploid, tetraploid and hexaploid wheats (McIntosh, 2016). The wild progenitor of cultivated diploid wheat einkorn was discovered in the Middle East, the progenitor for tetraploid wheat emmer and durum was discovered in the beginning of the 20th century. There are no equivalent progenitors for hexaploid wheat and it is clear that hexaploid wheats arose from hybridisations between cultivated tetraploid wheat and wild diploid species.

During the 20th century, breeders have developed many new wheat genotypes. These new types of wheat can produce good yields of grain, with resistance to cold, disease, insects and other crop threats. As a result, wheat production around the world has risen dramatically. During the last 30 years, there has been a tremendous increase in new knowledge of the biochemistry, genetics and functional properties of wheat. More recently, molecular biology has helped to elucidate the functions of the genes that determine the great diversity of wheat phenotypes (Jones et al., 2009; Akhonov, 2016; Henry, 2014, 2016).

Common or bread wheat (T. aestivum) is hexaploid with 21 pairs (2n = 42) of chromosomes comprising three homoeologous (similar) genomes (AA, BB and DD) each of 7 pairs. Durum and emmer wheats are tetraploids (2n = 28) possessing genomes, AA and BB, similar to these genomes in hexaploid wheat. Einkorn wheat is diploid (AA genome), offering a range of practical uses in the human diet (Abdel-Aal and Hucl, 2014) (see Chapter 14).

5.2.4 Plant and Grain Morphology

An understanding of plant, head and grain morphology (appearance, see Chapter 4) is useful for managing the stages of grain production, and also for the assessment of grain quality to the extent that morphology can be used for varietal identification—variety being a critical aspect of grain quality. Plant characteristics are an important part of the system for variety registration conducted by UPOV, the International Union for the Protection of New Varieties of Plants, for the purpose of enforcing Plant Breeders’ Rights. Registration requires that a new variety should exhibit the attributes of distinctness, uniformity and stability (DUS), mainly with respect to morphology.

The procedures and attributes for DUS testing of wheat are provided by Cook and Wrigley (2004) and on the UPOV website (www.upov.int). Several books and review articles are available concerning the morphological characteristics that are useful for variety identification, some of them accompanied by identification descriptions and keys for the specific varieties in national variety collections as listed in Chapter 4, which provides a general description of morphology comparing the range of cereal species, accentuating wheat.

The wheat grain is botanically a single-seeded fruit, called a ‘caryopsis’ or ‘kernel’, developing within the glumes, which are modified leaves. The shape and dimensions of the grain have the potential to provide evidence for distinguishing among varieties, as described further in Chapter 4. It is the inner endosperm of the wheat grain that is the source of the flour produced by milling (see Chapter 22) to provide the multitude of wheat-based foods (see Chapter 24).

4.2.1 Taxonomy

Over 90% of the wheat grown worldwide is T. aestivum. It is used for a wide range of applications, including bread, cakes, pastries, biscuits, puddings, thickeners and noodles. T. durum (durum wheat) is mainly used for pasta production. In addition, small amounts of a few specialty wheats are grown: spelt wheat (a hexaploid wheat – T. aestivum var. spelta), Triticum monococcum (small spelt wheat) and T. timopheevii (Georgian wheat).

T. monococcum, Aegilops speltoides, T. tauschii and the wild Aegilops species, which is closely related to the modern Ae. speltoides, are ancestral diploid wheat species having seven pairs of chromosomes (2n = 14). The tetraploid T. durum is derived from the natural hybridisation of T. monococcum (A genome) and the ancestral Ae. speltoides (B genome) (Wrigley, 2009). Hexaploid bread wheat (2n = 42) is a result from natural hybridisation of T. dicoccoides (AABB) and T. tauschii (DD), as stated by Mangelsdorf (1953), Shewry et al. (2003) and Gustafson et al. (2009).

The common wheats of today are divided into red and white wheats (based on the intensity of red pigmentation in the seed coat), hard and soft wheats (based on the resistance of the seed when it is crushed), and winter and spring types (differing in their requirement for a cold period to hasten or permit normal development towards reproductive development) (Gooding, 2009).

4.2.2 Origins

Man assumed control over his own food production during the Neolithic Revolution. The pre-agricultural hunter-gatherer became familiar with nature's periodicity and the life cycle of the dominant plants in his environment, and finally succeeding to domesticate many of them. Wheat is one of the world's most important food crops to be domesticated. It is believed that wild relatives of wheat first grew in the Middle East, in the ‘Fertile Crescent’ (Feldman, 2001). Wheat was one of the first plants to be cultivated. It was grown about 11000 years ago. Enormous changes in people’s lives occurred because of wheat being grown. People began growing their own food and no longer needed to wander in search of food. Permanent settlements were established because wheat provided people with a stable food supply. Soon they grew enough wheat to feed others from surrounding regions. Once there was extra wheat available, trade between various cultures developed. By 4000 bc, wheat farming had spread to Asia, Europe and North Africa. New species of wheat developed because early farmers selected kernels from their best wheat plants to use as seeds for planting the following year’s crop. In that way, only the best wheat qualities were passed from one generation to the next. Soon wheat became an important world-wide crop.

Three main theories concerning the site of wheat domestication were proposed in the nineteenth century. According to the first theory, wheat was domesticated in the Near East. The evidence was from the Chaldean priest, Berosus, who mentioned that wild wheat occurred in Mesopotamia at about 2700 BP (Syncellus, Frag. Hist. Graec, vol. 2, p. 416). Another theory in 1899 proposed that cultivation of wheat started in central Asia. The third theory proposed in 1908 suggested that wheat grew wild in Europe and was later domesticated there.

4.2.3 Genetic constitution

Genetically, there are three main groups of cultivated wheat, namely, diploid, tetraploid and hexaploid wheats. The wild progenitor of cultivated diploid wheat einkorn was discovered in the Middle East, the progenitor for tetraploid wheat emmer and durum was discovered in the beginning of the twentieth century. There are no equivalent progenitors for hexaploid wheat and it is clear that hexaploid wheats arose from hybridisations between cultivated tetraploid wheat and wild diploid species. During the twentieth century, breeders have developed many new wheat genotypes. These new types of wheat can produce good yields of grain, with resistance to cold, disease, insects, and other crop threats. As a result, wheat production around the world has risen dramatically. During the last 30 years, there has been a tremendous increase in new knowledge of the biochemistry, genetics and functional properties of wheat. More recently, molecular biology had helped to elucidate the functions of the genes that determine the great diversity of wheat phenotypes (Jones et al., 2009).

Common or bread wheat (Triticum aestivum) is hexaploid with 21 pairs (2n = 42) of chromosomes comprising three homoeologous (similar) genomes (AA, BB and DD) each of 7 pairs. Durum and emmer wheats are tetraploids (2n = 28) possessing genomes, AA and BB, similar to these genomes in hexaploid wheat. Einkorn wheat is diploid (AA genome).

4.2.4 Plant and grain morphology

An understanding of plant, head and grain morphology (appearance) is useful for managing the stages of grain production, and also for the assessment of grain quality to the extent that morphology can be used for varietal identification – variety being an important aspect of grain quality. Plant characteristics are an important part of the system for variety registration conducted by UPOV, the International Union for the Protection of New Varieties of Plants, for the purpose of enforcing Plant Breeders’ Rights. Registration requires that a new variety should exhibit the attributes of distinctness, uniformity and stability (DUS), mainly with respect to morphology. The procedures and attributes for DUS testing of wheat are provided by Jarman (1995), Mauria (2000), Cooke and Wrigley (2004) and on the UPOV web site (www.upov.int). Several books and review articles are available concerning the morphological characteristics that are useful for variety identification, some of them accompanied by identification descriptions and keys for the specific varieties in national variety collections; for example, Ferns et al. (1975) and various editions through to Fitzsimmons et al. (1985); also Jarman (1995) and Agrawal (1997).

Plants

While a wheat grain is seen by us as a food source, it is really the means by which a wheat plant perpetuates itself and by which it is multiplied. For the wheat plant, the bulk of the grain is a repository of nutrients stored to benefit the new plant until its leaves are developed enough to provide its own energy needs via the processes of photosynthesis. The first stage (germination, Gooding, 2009) is initiated by moisture, involving the swelling of the germ (embryo), followed by the splitting of the germ cover to permit the growth of the roots and coleoptile. Continued growth of the coleoptile leads to the appearance of the first green leaf, followed in turn by successive leaves unfolding from the leaf sheath (stem), each clasping to the stem by an auricle. The plant shown in Fig. 4.1 shows only a single stem for simplicity of illustration, but most wheat genotypes produce multiple stems (tillers), which arise from the base of the plant.

Fig. 4.1. The wheat plant, showing only one stem, near to maturity with the stem fully elongated, carrying the head at its tip.

Reproduced with permission from Ferns et al. (1975).

Wheat varieties differ in various aspects of plant morphology throughout the stages of growth, permitting limited opportunity for variety identification. For example, in the early stages of growth, when there are several leaves, growth habit may be classed as prostrate (lying almost flat against the ground) for some varieties, or as erect for others (the leaves standing reasonably upright). At various growth stages, the abundance of short hairs on the leaf surface (leaf pubescence) is another attribute to use in variety identification, as are also leaf dimensions (length and width), but the latter are also dependent on growth conditions.

Heads

As the stem elongates, the grain43earing head (ear, spike) develops inside the upper leaf sheath. Eventually the last leaf (the flag leaf) unfolds and the head appears, and flowering occurs. Wheat is almost exclusively self pollinating, with little out-crossing so pollen is not shared with neighbouring flowers. This means that wheat remains true to genotype during propagation once its genetic constitution has become stabilised. Following flowering, the fertilised embryo develops inside the protecting glumes. For the purposes of distinguishing between varieties, it is useful to note plant height at maturity and straw strength (both dependent on growth conditions). Time to maturity is an important genetic attribute as it is interactive with time of planting and expected time for harvest. In many wheat-growing regions, the timing of these events is critical. The time for sowing must coincide with an unpredictable rain event to promote uniform germination, flowering must be timed to occur after the last frost, but the later stages of grain filling should not be so late as to occur when daily temperature maxima are excessive (e.g., over 35 °C).

Several morphological characteristics of the wheat head are useful for variety identification. The most obvious characteristic is the presence or absence of awns (beard), the long hair-like extension of the lemma glumes (Fig. 4.2). This distinction is so clear that it can be seen in a wheat crop when driving past, provided it is sufficiently mature for the heads to be out of the flag leaf. Head colour (also glume colour) at maturity is another obvious distinguishing attribute – either white or brown. A third obvious characteristic is the club head, for which the florets are closely spaced on the rachis. The club head is the extreme case (ten internodes occupying less than 4 cm of head) for the character known as head density. At the other extreme is the lax head, with ten internodes occupying more than 5 cm of head (Ferns et al., 1975). An additional distinguishing characteristic is head cross-section (either square or flattened when seen from above). The durum head is generally much wider when viewed from the side (looking between the spikelets) than in the face view (the dorso-ventral view, as seen with the spikelets in full face; the view in Fig. 4.3). Head length may also be useful for identification, but it is partly dependent on growth conditions.

Fig. 4.2. A single spikelet of wheat, with other florets removed to reveal the rachis, which is the ‘backbone’ of the head. The lemma and palea have been pulled apart to show the grain resting in the lemma.

Reproduced with permission from Ferns et al. (1975).

Fig. 4.3. Heads, seen in dorso-ventral view, of three wheat varieties that differ in awnedness.

From left to right, these heads are classed as unawned, half awned and fully awned. Reproduced with permission from Ferns et al, (1975).

At maturity, the glumes dry out completely whilst still enclosing the grain. The position of the grain in a floret can be seen in Fig. 4.2. In this expanded view, the grain is resting inside the lemma with the palea above it. The lemma continues up and out of the head as the awn (if awns are present). Each floret of the rachilla has a lemma and a palea, but only the lower florets have the outer glumes, which are especially useful for variety identification. Distinctive shapes are shown in Fig. 4.4. Even when examining a grain sample for visual identification of variety, glume shape is often useful, as there may be glumes present that have not been winnowed away from the grain. Therefore, glumes are included in the illustrations of grain shape in Fig. 4.5, in which the differences in glume shape are very evident.

Fig. 4.4. Variations in glume shape. These are characteristic for wheat variety.

Reproduced with permission from Agrawal (1997).

Fig. 4.5. Grain and glumes of the Australian wheat varieties (a) Catcher, (b) Kite and (c) Olympic.

Adapted from Ferns et al. (1975).

Grain

The wheat grain is botanically a single-seeded fruit, called a ‘caryopsis’ or ‘kernel’, developing within the glumes, which are modified leaves. From the dorsal (top, non-crease) side, the grains of different varieties may appear to be oval, ovate, elliptical, elongated or truncated (see the shapes of the five grains at the upper left for each of the varieties in Fig. 4.5). These characteristics can be useful in attempting to identify varieties by aspects of grain shape (Fig. 4.5). Wheat kernels average 2.5-3.0 mm in thickness, 3.0-3.5 mm in width, 6.0-7.0 mm in length and 30-40 mg in weight. Wheat kernels are rounded on the dorsal side with a longitudinal crease running the full length of the ventral side (see the view of the grains in the top right for each variety in Fig. 4.5). The presence of this wide and deep crease is seen more clearly when the grain is cut in half, as in Fig. 4.6a. The presence of the crease is undesirable because, if extreme, it can contribute to a low bulk density (test weight). The main inner volume of the grain is occupied by the starchy endosperm which becomes the white flour when it is released and crushed to fine particles by the flour miller. The crease complicates the task of the miller as the bran of the crease, as well as the bran of the outer grain surface, must be separated from the endosperm during flour milling. The embryo is located on the dorsal side at the end of the grain that is attached to the head. At the opposite end of the grain (the apical end) is a tuft of hairs, known as the ‘brush’.

Fig. 4.6. The cut surface of a wheat grain shown (a) at low magnification, (b) at medium magnification with the crease space at left and (c) at highest magnification to reveal the endosperm cell walls and the two types of starch granule (large, A-type and small B-type granules). Scanning electron micrographs provided by W. Campbell.

Grain characteristics are probably the most important for variety identification, because wheat is usually in the form of a grain sample when identification is needed. Given the small size of the wheat grain and its general uniformity, the grain is also the most difficult part of the plant for identification. Grain hardness (hard or soft) is a primary distinguishing characteristic, for identification, for trade and for processing. Apart from instrumental means, hardness is best judged by cutting grains transversely across the length with a sharp blade. With experience, biting the grain is also effective. Grain texture is closely related to hardness, with soft grains having opaque endosperm, appearing white or floury when cut. On the other hand, grains of hard varieties have a vitreous interior, appearing horny and translucent when cut in half. The difference in texture is usually evident in the whole grain, but grain colour and weathering may confuse the recognition of grain texture when examining whole grain.

Grain colour is an important primary distinguishing characteristic of significance for identification, trade and processing. Wheat grain is classed as either red or white (depending on the presence or absence of genes on homoeologous group 3 chromosomes). However, the red/white terms do not adequately describe the actual colour of the relevant grains, as endosperm texture also affects the perception of grain colour. Ideally, grains are soaked in a 5% aqueous solution of sodium hydroxide to enhance the colour differences.

Other distinguishing characteristics are illustrated by comparing grains of three varieties in Fig. 4.5. Multiple grains are included for each variety in Fig. 4.5 because there is inevitably some variation in size and shape from one grain of the same variety to another. The aim of classification methods is to use characteristics that provide the greatest distinction between varieties and that vary least due to changes in growth conditions. Such characteristics are evident in a comparison of the three varieties in Fig. 4.5. For example, grain of the variety Gatcher is pointed at the brush end when viewed from either the dorsal (back) view or from the side. In contrast, the grains of Kite and Olympic appear blunt in these views, Kite being classed as a large, oval grain. The long brush hairs of Gatcher are also evident. The germ face of Olympic is steeper than that of Gatcher.

4.2.5 Grain ultrastructure

When a wheat grain is cut in half, at a right angle to the crease, the structure of the endosperm and bran layers are seen. Scanning electron microscopy (Fig. 4.6) shows the wrinkled outside surface of the grain and the structure of the crease (Fig. 4.6a). At higher magnification (Fig. 4.6b) the wrinkles of the outer bran layers are evident, especially near the innermost part of the crease where this image is taken. Beneath the bran layers is the single layer of aleurone cells. To the right of the aleurone cells in Fig. 4.6b is the bulk of the endosperm, appearing as a mass of oval starch granules embedded in the matrix of storage protein. Mainly the large (A-type) starch granules are seen in Fig. 4.6b, but at even larger magnification (Fig. 4.6c) the small (B-type) granules are also visible, and so are the cell walls separating the endosperm cells.

Milling

Knowledge of the physical structure of grain at macroscopic and microscopic levels has application in flour milling. During the overall milling process, the grain is crushed and mechanically fractionated into its various morphological components – mainly endosperm, bran and germ. The bran is made up of several layers that remain after serving their purposes in the developing grain. The pericarp (the ripened ovary wall) consists of the outer epidermis, hypodermis, parenchyma, intermediate cells, cross cells and tube cells (Bechtel et al., 2009). The pericarp tissues are lignified and devoid of cytoplasm. The seed coat (testa) and the pigmented strand provide a complete covering around the seed. In red wheats, red-brown pigmentation is found in the seed coat and the pigment strand, but the pigmentation is absent in white wheat. Between the seed coat and the endosperm is a single crushed layer of empty epidermal cells called the nucellus. The next layer is the aleurone layer (one cell-layer thick in wheat), which is classified as being part of the endosperm. The starchy endosperm represents more than 80% of the grain mass, although it is virtually impossible for the miller to achieve an 80% yield of white flour of high quality, based on colour or ash content.

To produce a high yield of white flour, the miller has to remove the embryo and the bran layers cleanly from the starchy endosperm. The aim is thus to remove all the endosperm from the inside of the bran particles, whilst keeping the bran and germ tissues as intact as possible, so that the non-endosperm fragments are large and thus easier to remove by sieving. One of the purposes of moistening the grain before milling (‘conditioning’) is to make the bran less brittle (friable) and thus less likely to break into small pieces during crushing. Nevertheless, small bran particles find their way into the final flour, with the potential to detract from its desired whiteness. The bran of red wheats is more likely to detract from whiteness than bran from white wheats. Therefore, 1-2% higher flour extraction might be expected from white wheats compared to red wheats, if the specifications for flour quality are based on flour colour. An alternative quality specification is flour ash content, which is another measure of the extent of contamination with non-endosperm tissues (Posner and Hibbs, 2005).

Grain hardness is an important factor affecting flour yield, the extent of starch damage and energy requirements in the mill. The miller must therefore achieve careful adjustment of the mill to optimise performance. During milling, the endosperm of a hard wheat breaks along the lines of the endosperm cell walls or through the cell, even across starch granules and embedded protein bodies, initially yielding semolina (coarse flour) rather than fine flour particles; further reduction milling is required to reduce particle size and achieve particle-size specifications. In contrast, when a soft wheat is milled, the endosperm fragments readily into fine particles, so that starch granules are released with little damage and largely free from the protein matrix. Starch granules from a soft wheat thus appear to have a smooth surface in contrast to starch from a hard wheat (Fig. 4.7). Grain softness is controlled by a dominant gene on the short arm of chromosome 5D. This gene determines milling characteristics and hence its suitability for the manufacture of specific products.

Fig. 4.7. Wheat starch granules at various stages of attack by α-amylase, (a) An intact A-type granule with a few B-type granules, (b) Initial attack by α-amylase at points of starch damage, (c) and (d) Progressively greater erosion of the starch granule by α-amylase. Scanning electron micrographs provided by W. Campbell.

Starch damage is an important quality specification for flour because it is one of the significant factors affecting water absorption. High starch damage is desirable for many bread making applications as it increases the moisture content of the final bread, but low starch damage is needed for biscuit (cookie) making because the moisture must be removed during baking. In addition, a significant degree of starch damage is desirable in flour to be used for leavened bread, to allow ready access by α-amylase to the inner part of the starch granule. The sequence of events in Fig. 4.7 illustrates how the surface of the starch granule provides a degree of protection from enzymic attack. In Fig. 4.7b, enzymic attack has started at places where there has been slight damage to the granule surface. Digestion of the starch has continued progressively (Fig. 4.7c and 4.7d). Figure 4.7d also shows the internal rings of crystalline starch, which differ in their susceptibility to enzymic attack (French, 1984).

Dough mixing

When water is added to the flour with mixing, the protein matrix becomes gradually hydrated, coming together to form the gluten strands visible in Fig. 4.8, while the individual starch granules (large and small) are trapped inside the gluten matrix. They are also visible on the surface of the dough in this scanning electron micrograph. As gas bubbles develop in the dough due to yeast action, the consistency of the dough must be elastic enough to trap the bubbles (preventing them just rising to the surface) and viscous enough to allow the gas bubbles to expand during baking, to provide a well expanded crumb and a large loaf volume.

Fig. 4.8. The surface of a dough piece, showing the continuous strands of gluten protein providing a matrix for the starch granules, both A-type large granules and B-type small ones. Scanning electron micrograph provided by W. Campbell.

Global impacts

CIMMYT has a global mandate for wheat improvement (bread and durum species).

In the late 1960s, about one-third of all wheat varieties in developing countries were CIMMYT crosses. By the 1990s, these numbers rose to about half from CIMMYT crosses and another quarter from varieties that had a CIMMYT parent. Worldwide, around 90% of all spring bread wheat releases had at least one CIMMYT ancestor; the percentage for durum wheat was even higher with nearly all spring durum wheat releases having a CIMMYT ancestor. In 1970, semi-dwarf wheat varieties were important only in Asia. By the 1990s, semi-dwarf wheat varieties covered 80% of world wheat area, with adoption rates of 90% and higher in Asia and Latin America. Wheat research has generated a total economic surplus of about $2.5 billion annually in developing countries, for total research costs that never exceeded $70 million annually.

Wheat improvement in West Asia and North Africa (WANA)

ICARDA's wheat improvement research focuses on spring bread and durum wheats in the WANA region. Per capita bread wheat consumption is highest in the WANA region (185 kg per year), and rising. Wheat is generally consumed as flat (Arabic) or leavened (French) bread.

ICARDA grain scientists have made major advances in identifying new sources of resistance to abiotic stresses (drought, heat, and cold) and biotic stresses (rusts, Septoria diseases, and Hessian fly), in broadening genetic diversity, and in selecting for grain quality. In 2001, 15 international durum wheat nurseries representing over 800 lines and 10 000 segregating populations suitable for three WANA environments (continental, temperate, and highland) were distributed to national programs for testing. Efforts to broaden the genetic base of durum wheat (that is notably poor in genetic diversity) involved crossing improved dryland genotypes with WANA landraces, wild relatives, and bread wheat. Wild relatives of durum (Triticum diccoides and T. monococcum) were used to improve grain quality and resistance to leaf rust, leaf blotch, and yellow rust.