Wheat. Peter R. Shewry
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bulgar. Other free‐threshing variety groups are only cultivated to a limited extent. These minor groups include rivet wheat (var. turgidum), Polish wheat (var. polonicum), Persian wheat (var. carthalicum), and Khorasan wheat (var. turanicum). T. timopheevi is represented in both hulled (var. timopheevi) and naked forms (var. militinae) and is still cultivated in small areas in the Caucasus.
Cultivated einkorn and emmer clearly developed from the domestication of natural populations. By contrast, hexaploid wheat ( T. aestivum, genome formula AABBDD) has never existed as a wild species. It originated through hybridisation of cultivated emmer with the wild goat grass Aegilops tauschii (also called Triticum tauschii and Ae. squarrosa), which contributed the D genome (Figures 1.13 and 1.15). This hybridisation probably occurred several times, with farmers selecting hexaploid lines for their larger grains and higher yields. Modern wild populations of Ae. taushcii are found growing in the north and northeast of the Fertile Crescent and populations carrying genomes closely related to the D genome of hexaploid wheat occur in Transcaucasia, between the Black Sea and the Caspian Sea (particularly in modern‐day Armenia), and around the southwest of the Caspian Sea (modern‐day northern Iran) (Dvořák et al. 1998). The first hexaploid wheats may, therefore, have occurred around the south or west of the Caspian Sea about 8000 BP (Kilian et al. 2009).
1.3.4 The Spread of Wheat Cultivation
The cultivation of bread wheat spread via Anatolia to Greece, and then both northwards through the Balkans to the Danube (7000 BP), and westwards to Italy, France, and Spain (7000 BP), reaching the UK and Scandinavia by about 5000 BP. Similarly, wheat spread via Iran into central Asia, reaching China by about 3000 BP, and to Africa, initially via Egypt. It was introduced by the Spanish to Mexico in 1529 CE and by the British to Australia in 1788 CE. These migration routes have been described in detail by Feldman (2001).
This wide geographic expansion of wheat was undoubtedly facilitated by the allohexaploidy of T. aestivum . The environmental and geographic distributions of allopolyploids are often wider than for their diploid parents (Miller 1987). Furthermore, T. aestivum has adapted to a wider range of environments than the tetraploid T. turgidum (Dubcovsky and Dvorak 2007). Feldman and Levy (2005) list several mechanisms by which allopolyploidy has led to the rapid evolution of wheat genomes and gene expression. The multiple genomes would be expected to confer fixed hybrid vigour, genetic buffering, and evolutionary adaptability (Simmonds and Smartt 1999). Having multiple copies of similar genes means that mutations in one or two copies can occur without catastrophic effects. This can, for example, allow the development of intermediate types and better fine tuning of growth and development patterns to the environment. The addition of the D genome appears to have been particularly important given the different and wider geographic distribution and environmental adaptation of Ae. taushcii compared with the wild donors of the A and B genomes. Modern wild stands of Ae. taushcii extend beyond the Mediterranean zones into continental climates (Zohary et al. 1969).
Although T. aestivum is one of the youngest major crop species, it is now also one of the most diverse. Feldman (2001) estimated that about 25 000 genotypes were available globally at the turn of the millennium, but this is clearly an underestimate as material present in countries such as China was not readily available at the time of publication. Although several genotypes of Ae. tauschii contributed to the formation of hexaploid bread wheat, most of the diversity in bread wheat has clearly arisen since the initial hydridisation occurred. Dubcovsky and Dvorak (2007) suggest that this has several origins. Firstly, extensive gene flow has occurred between bread wheat and cultivated and wild forms of tetraploid wheat, particularly wild emmer (He et al. 2019; Allen et al. 2021), capturing diversity from these species. However, gene flow into the D genome has been more limited; it remains less diverse than the A and B genomes (Allen et al. 2021). Secondly, bread wheat has an unusually large genome, comprising about 17Gb of DNA, (about 40 times the size of the rice genome). About 85% of the genome is non‐coding repetitive DNA including mobile elements, which facilitates the generation of variation by gene duplication and deletion and by insertion into regulatory and coding sequences. These events are more likely to be tolerated due to the buffering capacity of the three genomes. Hence, the combination of gene flow from related wild species, polyploidy, and high genome plasticity have contributed to the variation which has developed over the past 10 000 years.
The dominant grouping of T. aestivum is modern, free‐threshing, bread wheat (var. aestivum). This form accounts for about 95% of current global wheat production and is widely used to make bread, other baked goods including cakes and biscuits, Asian noodles (but not pasta), and as an ingredient in food processing. Two additional naked forms are club wheat (var. compactum), which has highly compact heads and is grown principally in the USA (Pacific Northwest and California), and Indian shot wheat (var. sphaerococcum), which is characterized by rounded grains and is from northwest India and Iran. T. aestivum also occurs in three hulled forms, the most widely cultivated of which is spelt (var. spelta). The other recognised hulled variety groups are var. vavilovi from Armenia and var. macha, from Georgia.
1.3.5 Increases in Harvest Index
In addition to the characteristics of the classic domestication syndrome, modern cultivated wheats now differ in many ways from their wild progenitors. The increase in harvest index for grain has been of great significance, i.e. the increase in the ratio of the grain mass to the total crop biomass (but often neglecting the roots and fallen leaves) (Donald 1962). Austin et al. (1982) reported harvest indices of only 0.15, 0.05, and 0.16 for T. uratu, Ae. speltoides, and T. tauschii, respectively, while the harvest index of tetraploid wild emmer was a little higher at 0.28. Even at this level, however, it would be necessary to produce more than 3.5 t of crop biomass to yield each tonne of grain. Although farmers have long recognized that shorter wheats have higher harvest indices (Roberts 1847; Garnett 1883), cultivars released before 1900 still had indices below 0.35 (Austin et al. 1989). During the twentieth century, significant increases in harvest index were achieved, particularly through breeding programmes that incorporated dwarfing alleles from Japanese wheats, reducing crop heights by 10–20% (Borojevic and Borojevic 2005). By the mid‐1980s, the harvest index of UK wheats approached 0.55 (Austin et al. 1989), i.e. at harvest most of the crop biomass in the field was present in the grain. This progression was seen in wheats throughout the world and across ploidy levels (Evans 1993). The considerable implications of the adoption of high harvest index, shorter wheats on wheat agronomy, quality, and sustainability are covered in Chapter 6.
1.4 Wheat as Food
Although wheat still accounts for half of the total intake of calories in some countries, the contribution to nutrition in Europe in historical times was even greater. For example, the cost of bread was estimated to account for between a third and two thirds of the total budgets of working families (as opposed to landowners and gentry) in the UK in the period between 1760 and 1836, with the highest estimate being almost 90% for one labourer's family (Peterson 1995). Apparently, therefore, the coarse bread that was consumed at the time was able to meet most of the nutritional requirements of adults engaged in physical work.
The wider contributions of wheat and bread to health are often ignored, with both being widely regarded as little more than a source of energy. In fact, even in modern Europe wheat, and particularly bread, still provide surprisingly high contributions of a range of essential nutrients. For example, although bread only contributes 10–13% (depending on age and gender) of the daily intake of energy in the UK, it also contributes 10–12% of the daily intake of protein and between 10 and 20% of the daily intakes of minerals (iron, zinc, copper, magnesium, selenium, and calcium) and B vitamins (B1 thiamine, B3 niacin, B9 folate). More importantly, it is also a major source of dietary fibre, contributing about 20% of the daily intake (Bates