Wheat - An Exceptional Crop

Botanical Features, Chemistry, Utilization, Nutritional and Health Aspects

Herbert Wieser, Peter Koehler, Katharina A. Scherf

• Publisher: Woodhead Publishing and AACC International Press
• Year: 2020
• ISBN:
  9780128217160
• Topic: Technology & Engineering
• Subtopic: Food Science
• Index: Technology & Engineering

Wheat - An Exceptional Crop: Botanical Features, Chemistry, Utilization, Nutritional and Health Aspects presents the exceptional position of wheat among food crops. The book demonstrates the benefits and drawbacks of wheat from a wheat science, nutrition and technology perspective. Organized into 13 chapters, chapters 1 - 3 present a basic overview of wheat; chapters 4 - 6 explore the overall benefits of wheat for the general population, and chapters 7 - 13 assess wheat-related disorders that affect a small portion of the population.

Wheat - An Exceptional Crop: Botanical Features, Chemistry, Utilization, Nutritional and Health Aspects is an exceptional reference for those working in and researching the fields of agronomy, food chemistry, food technology, nutrition, allergology and gastroenterology.

• Explores the botanical features of wheat, chemical composition of wheat grains, and the cultivation and milling of wheat
• Highlights wheat-based food and feed, wheat-based raw materials, and the nutritional value of wheat
• Discusses principles of wheat hypersensitivities and various wheat-related disorders

Chapter 1

Botanical features of wheat

Abstract

Wheat plants are grasses belonging to the monocot family Poaceae. Cultivated wheat consists of five species: diploid einkorn (genome AA), tetraploid emmer and durum wheat (AABB), and hexaploid common wheat and spelt (AABBDD). Triticale (AABBRR) is a man-made cross of durum wheat and rye. Wheat grains are produced as dry one-seeded fruits, botanically defined as caryopsis. They consist of five main compartments: Fruit coat (pericarp) (4–5% of grain weight) and seed coat (testa) (1%) are the outer layers and surround the whole grain. The inner compartments (endosperm) comprise the aleurone layer (6–9%) and the starchy endosperm (80–85%). The germ (3%) is located on the dorsal side of the caryopsis. Common wheat and durum wheat have been the target of intense breeding with a specific focus on high grain yield, resistance against diseases and pests, and high quality of end products. Most breeding programs include hybridization of two promising parents, but artificial genetic mutation by irradiation and chemicals or adding and knocking down specific genes or gene segments (genetic engineering) are also possible.

Keywords

Breeding; Genetic engineering; Grain structure; Hybridization; Phylogeny; Poaceae; Wheat species

Cereals belong to the most important agricultural crops cultivated by mankind. They are grown on nearly 60% of total farmland in the world and are widely used for human nutrition, animal feed, industrial products, and generation of renewable energy. The main cereals are wheat, corn, rice, barley, rye, sorghum, millet, and oats. Botanically, cereals are grasses and belong to the plant family Poaceae. Cultivated wheat comprises five species: einkorn, emmer, durum wheat, spelt, and common wheat. They are annual plants and produce dry, one-seeded fruits (grains) with a fairly uniform anatomy consisting of fruit and seed coat, aleurone layer, starchy endosperm, and germ. Grains represent the most valuable and nutritive parts of cereals. Their morphological structure fulfills all criteria for a successful reproduction of the plant. Common wheat and durum wheat, the economically most important species, have been the target of intense breeding. Systematic wheat breeding started in the late 19th century resulting in currently around 25,000 varieties worldwide. They have been bred to specific purposes such as high grain yield, resistance against diseases, and high quality of end products. The present chapter covers some basic aspects of the phylogeny, grain structure, and breeding strategies of wheat.

1.1 Phylogeny and species

Botanically, wheat (Triticum) belongs to the monocot family Poaceae (“grass” family) and is closely related to rye (Secale) and barley (Hordeum) as members of the subfamily Pooideae and the tribe Triticeae (Fig. 1.1) [1]. The other well-known cereals are either distant relatives of wheat (oats) or show separate evolutionary lines (rice, corn, sorghum, and millet). Currently, cultivated wheat plants belong to five species with different numbers of subgenomes and chromosomes: diploid einkorn (Triticum monococcum L.), tetraploid emmer [Triticum dicoccum (Schrank) Schübler] and durum wheat (Triticum durum Desf.), and hexaploid spelt (Triticum spelta L.) and common wheat (Triticum aestivum L.) (Table 1.1). Traditionally, wheat has been classified into winter wheat sown in late summer or autumn and spring wheat sown in spring (see Section 3.1). The phylogenetic development of wheat from diploid einkorn to hexaploid spelt and common wheat occurred by natural hybridization and by human selection. With the beginning of the 20th century, intense breeding did not only lead to numerous different varieties within a species but also to a specific crossing of different species and even different cereals (see Section 1.3). Crossing of spelt and common wheat, for example, resulted in hybrids that combined the advantages of both species. Exchange of a wheat half chromosome by a rye half chromosome led to so-called wheat/rye translocation lines and these wheat hybrids have the disease resistance of rye. Furthermore, durum wheat and rye were crossed to obtain triticale (Triticum + Secale), a man-made hexaploid hybrid.

Fig. 1.1 Phylogeny of cereals belonging to the monocot family Poaceae.

Table 1.1

Classification of cultivated wheat [Triticum (T.)] species.

Species	Systematic name	Genome	Chromosomes	Husk
Einkorn	T. monococcum	AA	2 × 7 = 14	Hulled
Emmer	T. dicoccum	AABB	4 × 7 = 28	Hulled
Durum wheat	T. durum	AABB	4 × 7 = 28	Naked
Spelt	T. spelta	AABBDD	6 × 7 = 42	Hulled
Common wheat	T. aestivum	AABBDD	6 × 7 = 42	Naked

The botanical origin of the wheat plant is not known with certainty, but a good deal of evidence indicates that einkorn (genome AA) developed from a type of wild grass native to the arid lands of Asia Minor [2]. The first evolutionary event, leading to polyploid wheats, was the hybridization of two diploid wild grasses: Triticum urartu (genome AA), closely related to einkorn, and a yet unknown Aegilops species (genome BB) related to Aegilops speltoides (goat grass). Their crossing resulted in the tetraploid emmer (genome AABB). The cultivation of emmer provided a range of subspecies, some of which developed a naked, free-threshing tetraploid wheat known as durum wheat. When emmer reached the region south of the Caspian See around 5000 BC and hybridized with diploid Aegilops tauschii (genome DD), hexaploid common wheat (Triticum aestivum, genome AABBDD) developed [3]. The origin of hexaploid spelt is currently not clear. The hybridization of emmer and Aegilops tauschii might have taken place in Asia Minor around 6000 BC. The much later appearance of spelt in Europe probably was the result of the hybridization of emmer and common wheat. Thus it is unclear whether spelt has a single origin in Asia or two separate origins in Asia and Europe.

Einkorn (from German “Einkorn” = single grain) is a diploid wheat species carrying the genome AA with 14 chromosomes (Table 1.1). It was the first wheat to be domesticated around 10,000 years ago in the Middle East (in the “Fertile Crescent”). It was widely cultivated in the Neolithic Age but was gradually replaced first by emmer and then by common wheat and durum wheat. Today, einkorn is grown in small farmlands in Western Turkey, Balkan countries, Italy, Spain, and Central Europe. Einkorn belongs to the hulled wheats, i.e., grains do not break free from the husk with threshing and therefore it is difficult to remove the husk from the seed. Einkorn tolerates poor and dry soils and exhibits a pronounced resistance against cereal diseases like rust and mildew. However, low yields and poor dough and bread-making qualities have led to the almost complete disappearance of einkorn.

Emmer is a tetraploid wheat with the genome AABB and 28 chromosomes. Along with einkorn it was one of the first crops cultivated in the Middle East, but today it is a marginal crop preferably grown in mountainous areas. Like einkorn, emmer is a hulled wheat and characterized by modesty regarding soil, climate, and fertilization as well as by highly expressed resistance to diseases. Due to economic reasons and poor product (pasta and bread) quality, emmer was widely replaced by durum wheat and common wheat.

Durum wheat is tetraploid like emmer with the genome AABB and 28 chromosomes. In contrast to emmer, durum wheat is a “naked” cereal, i.e., free-threshing. It was developed from domesticated emmer and is now produced in many parts of the world especially in warm areas with low precipitation. Among wheat species, durum wheat grains have the highest hardness (durum in Latin means “hard”). Due to this hardness, grains are mostly milled into semolina that is preferably used for making pasta (“pasta wheat”). Most durum varieties have been bred for a yellow endosperm that gives pasta its color. Khorasan wheat is a tetraploid hybrid derived from durum wheat and a wild wheat form (Triticum polonicum). One variety is marketed under the trademark “Kamut”. Khorasan wheat is quite resistant against diseases, has modest requirements regarding fertilization, and is therefore most suitable for extensive agriculture. It needs a warm and dry climate for cultivation and is mainly produced in North America and Southern Europe.

Spelt is a hulled hexaploid wheat with the genome AABBDD and 42 chromosomes. It was an important staple food in the Middle East and Europe from the Bronze Age to the Middle Age but was then successively replaced by common wheat that had higher yields, lower processing costs, and better baking performance. Only in some regions of Southern Germany, Austria, and Switzerland spelt remained the most important cereal until 100 years ago. Since some years, spelt is again increasingly cultivated notably in organic farming and water-protected areas because of its robustness, modesty with regard to climate and soil, and fewer requirements for the use of fertilizers and pesticides. The pleasant taste and flavor of spelt products and a possibly low potential to trigger hypersensitivities increased the popularity of spelt. To compensate for the disadvantages of spelt (e.g., increased height of plants, lower yield, and poorer bread-making quality compared to common wheat) and to preserve its desirable properties (e.g., resistances to disease and unfavorable soil and climate), cross-breeding of spelt and common wheat was started at the beginning of the 20th century. Since that time, spelt/wheat cross-breeds have been cultivated and processed into food products similar to pure spelt. The degree of common wheat crossing can be determined by the analysis of storage protein patterns using high-performance liquid chromatography for instance [4]. Unfortunately, information on the degree of crossing does not have to be labeled on spelt products, which would be of special interest for persons sensitive to common wheat and tolerant to spelt.

Common wheat, known as bread wheat, represents around 90% of cultivated wheat worldwide. Common wheat is hexaploid with the genome AABBDD and 42 chromosomes that carry around 100,000 genes. Using recent advances in sequencing techniques, the International Wheat Genome Sequencing Consortium (IWGSC) published a detailed description of the genome of common wheat, the result of 13 years of collaborative international research [5]. The research article, authored by more than 200 scientists from 73 research institutions in 20 countries, presents an annotated reference genome of the common wheat variety Chinese Spring. The 21 chromosome-like sequence assemblies contain both coding and noncoding elements across the A, B, and D subgenomes and give access to 107,891 high-confidence gen...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Author biographies
• Preface
• Frequent abbreviations
• Chapter 1: Botanical features of wheat
• Chapter 2: Chemical composition of wheat grains
• Chapter 3: Cultivation and milling of wheat
• Chapter 4: Wheat-based food and feed
• Chapter 5: Wheat-based raw materials
• Chapter 6: Nutritional value of wheat
• Chapter 7: Principles of wheat hypersensitivities
• Chapter 8: Celiac disease
• Chapter 9: Wheat allergy
• Chapter 10: Non-celiac gluten sensitivity
• Chapter 11: Irritable bowel syndrome
• Chapter 12: Treatment of dietary wheat hypersensitivities
• Chapter 13: Future research on wheat hypersensitivities
• Index