Week 4 (unit 7)
Wheat - Triticum aestivum and Related Species
Origin, taxonomy, genetic and reproductive
Important wheat species
Wheat was domesticated in the Near East at least 9,000 years ago. It originated through hybridization and polyploidy of several species of the genus Triticum, all with a basic chromosome number of 7 (n=7). Some species of Triticum are cultivated and some are noncultivated, or rarely so. The wild species of wheat can still be found in northern Iraq, Iran, and Turkey. The most important contemporary Triticum species and subspecies are described in this section.
Common wheat - Triticum aestivum L.
Common bread wheat is an allohexaploid, with 2n=42 chromosomes that are thought to derive from three wild ancestors, each with a unique genome of n=7 chromosomes. The genome of wheat is designated 'ABD' to signify the ancestral sources of its 21 pairs of chromosomes.
Related species and wheat progenitors
Emmer / Polonicum - Triticum turgidum subsp. dicoccum
Emmer was the dominant type of wheat produced in the Near East and Europe between 10,000 and 4,000 BC. Like einkorn, it also has a brittle rachis, with the hull remaining attached to the grain during threshing. It is a tetraploid species (AABB) with 2n=14 chromosomes derived from hybridization of einkorn wheat and a type of goat grass. Formerly the various species of goat grasses were considered to be in the genus Aegilops, but now they are included in the genus Triticum. Limited amounts of emmer are grown today, primarily as feed for animals. Durum wheat is closely related to emmer, but the grain of Durum can be readily separated from the chaff.
Spelt - Triticum aestivum subsp. spelta
Spelt is thought to have originated from hybridization between emmer wheat and another species of goat grass (Triticum tauschii), followed by doubling of the chromosomes to produce a hexaploid (2n=42 with three genomes: ABD). Spelt was the dominant wheat grown throughout the Near East, Balkans and Europe during the Bronze age (4,000 to 1,000 BC). Some of the earliest references to spelt wheat are from the Bible.
Spelt is an alternative feed grain to oat and barley, with nutritional value similar to oats. Unlike modern wheat, the grain remains attached to the hull during threshing. The spelt hull has nearly as much feed value as the grain. Spelt can be used as food grain after removal of the hull and can be obtained through organic or health food outlets. There are a few hundred thousand acres of production in the US.
Spelt is thought to be easy to digest and to impart a unique taste in bread products. Spelt is sometimes suggested as an alternative for individuals with allergies to wheat or gluten proteins. However, for the majority of sufferers of Celiac disease, spelt is not an alternative to wheat, because it has gluten proteins similar to those of common wheat.
Polyploid origin and domestication of wheat
The diagram below provides a summary of important aspects of wheat domestication, highlighting the importance of polyploidy as a mechanism for crop evolution.
Cytogenetic and molecular studies have shown that there is a high level of homoeology among the three genomes. Corresponding chromosomes from the A,B, and D genomes have many of the same genes, and the genes tend to occur in a similar order along the chromosomes. This suggests that the three ancestral species also evolved from a common ancestor.
Why is polyploidy important?
Genetic redundancy (duplication of genes) permits wheat to tolerate major structural changes in chromosomes.
Cytogenetic relationships among the chromosomes were determined by Dr. Ernie Sears (University of Missouri) through his classic work on the development and analyses of aneuploid genetic stocks (aneuploids have one more or one less than the normal, euploid number of chromosomes.)
Genetic stocks for gene mapping
Difficulties in molecular genetic manipulations of wheat
Wheat is a cool season crop that grows best at temperatures around 60 °F. The minimum length of the growing season is about 100 days, depending on the cultivar and environmental conditions. From 15 to 20 or more inches of precipitation are necessary for annual cropping. In some dryer areas wheat is grown once every 2 years, with the land fallowed (kept free of vegetation) in alternate years to accumulate moisture in the soil.
Physiological traits of wheat
Growth stages of wheat
Review at least one of the sites below to learn about wheat morphology and growth stages:
The Extension Service at Kansas State University has developed a website called "Adopt a Wheat Field" that takes a step-by-step look at the growth and development of a wheat crop throughout the growing season in Kansas. Although intended for kids, the site provides a close-up view of the crop from a grower's perspective that can also be appreciated by adults: http://www.oznet.ksu.edu/pr_aawf/welcome.htm
For more technical information, see "The Growth and Development Guide for Spring Wheat" from the University of Minnesota Extension Service: http://www.extension.umn.edu/distribution/cropsystems/DC2547.html
Additional information on growth stages is available from the North Dakota
State University Extension Service:
Compensatory growth pattern - ‘plasticity’
Components of Yield:
The plant may compensate for a decrease in one component by increasing another yield component. For example, a plant that is stressed early in the season and produces few tillers may have larger kernels (higher seed weight). This plasticity contributes to yield stability over environments and facilitates response to changing environmental conditions during the growing season.
Major traits impacting adaptation
Photoperiod response - daylength
Abiotic stress tolerances
Earliness per se
The endosperm makes up about 83 percent of the kernel weight. It consists of starch granules embedded in a protein matrix and is the source of white flour. The greatest share of total grain protein and carbohydrates occur in the endosperm. The endosperm is also a good source of iron, and many B-complex vitamins, such as riboflavin, niacin, and thiamine.
The bran constitutes about 14.5 percent of the kernel weight. Bran is included in whole-wheat flour and is also available separately. Of the nutrients in whole wheat, the bran contains a small amount of protein, larger quantities of the B-complex vitamins listed above, trace minerals, and indigestible cellulose material also called dietary flour.
The germ contributes about 2.5 percent of the kernel weight. The germ is the embryo or sprouting section of the seed. Of the nutrients in whole wheat, the germ contains minimal quantities of protein, but a greater share of B-complex vitamins and trace minerals. Wheat germ can be purchased separately and is included in whole-wheat flour. However, the germ is usually separated because it is high in fat, which limits the keeping quality of flour.
Nutrient Comparison of Selected Wheat Foods*100 Grams Edible Portion (3.5 ounces)
Wheat is the only plant species to possess gluten proteins and the only species to have proteins with these unique physical properties.
The definitive trait of wheat is its ability to form a visco-elastic gluten network when hydrated in doughs.
Cystine disulfide bonds in endosperm storage proteins have the ability to form protein networks (polymers) in doughs. Proteins are linked via disulfide bonds to form a macro-polymer network which holds gas bubbles during fermentation and baking.
Gluten protein quality is defined by genes for:
High- and low- molecular weight glutenins
Gliadins (omega and gamma)
Ratios of HMW and LMW glutenins, gliadin, and non-gluten proteins are important to meet functionality and product quality requirements.
Milling is the separation of the bran and germ from the endosperm and the reduction of the endosperm to uniform particle size (flour). This is done by a sequence of breaking, grinding and separating operations.
Breaking involves passing the wheat through a series of grinding rolls which break the wheat up into a bran fraction (which is removed), large mainly bran-free endosperm chunks, and a small amount of flour. The endosperm chunks are then passed through a smooth set of rolls which reduce the endosperm to finer and finer particles. After each break and reduction roll, the ground material is sieved and free flour is removed, leaving only large particles to go forward into the next set of rollers, where they are further reduced to produce more flour. A typical flour mill will have up to four break rolls and twelve reduction rolls, which lead to the production of some 16 flour streams, a nearly pure bran stream, a germ stream and a bran/flour/germ wheat feed stream.
The milling process is common to the production of all flours. The quality
of the wheat going into the mill, e.g. protein content, will determine
the types of flour being produced. By blending together the many different
flour streams produced by the mill, a miller can create further variations
in features such as flour color. Very white flours would come from the
early streams only, while most streams produce brown products. Whole
is produced when all the streams, bran, germ and flours are blended back
together with nothing removed.
U.S. Wheat Market classes
Hard red winter - HRW
Hard red spring - HRS
Soft red winter - SRW
Soft white - SW
Hard white - HW
‘Industry acceptability’ is required.
‘Value-added’ quality traits desired for IP marketing (sale as patented varieties).
The production of new wheat varieties requires intensive testing of baking quality:
Hard wheats - breads, noodles
Minnesota Association of Wheat Growers. Wheat Facts. http://www.smallgrains.org/Toolshed/Wheat_Facts/wheat_facts.htm
United States Department of Agriculture
Winter wheat production is concentrated in the central Great Plains states and in the Pacific Northwest. These states have sufficient cold in the winter to satisfy vernalization requirements. Spring wheat production is concentrated in the northern Great Plains where temperatures are too low to sustain a crop through the winter. Soft white winter and soft red winter varieties are grown in areas with relatively high rainfall.
Major Wheat Producing Countries, 2002
Wheat Diseases and Pests: a guide for field identification
The rusts are obligate parasites, which means that they must have the appropriate host in order to survive. The sexual stage for Stem rust is known to occur on Barberry. The alternate hosts required for the sexual stage of Leaf rust do not occur in North America. There is no known alternative host or sexual stage for Stripe rust.
Host plant resistance is the most effective mode of control for the rusts. Major race changes occur frequently for leaf and stripe rust in response to resistance genes deployed in production areas.
Leaf spotting diseases
Wheat reproduction and breeding
New directions in variety development:
Review some of the web sites listed in the References section below. You will find useful information to reinforce material presented in this lecture.
Take the quiz on this Unit on the Blackboard.
Center for New Crops & Plant Products, Purdue University. 2003.
Curtis, B.C., S. Rajaram, and H. Gómez Macpherson. 2002. Bread
wheat: improvement and Production. FAO Plant Production and Protection
Series No. 30.
Fowler, D.B. 2002. Winter Cereal Production. University of Saskatchewan. http://www.usask.ca/agriculture/plantsci/winter_cereals/index.php
Kansas State University Research and Extension. Adopt a Wheat Field. http://www.oznet.ksu.edu/pr_aawf/welcome.htm
Kansas State University Research and Extension. 2005. Wheat Page.
Levetin, E. and K. McMahon. 2005. The grasses. Chapter 12 in Plants
and Society, 4th edition. McGraw-Hill, New York, NY. Additional on-line
notes and references:
Minnesota Association of Wheat Growers. 2004. Small Grains. The internet source for small grain growers. http://www.smallgrains.org/
Minnesota Association of Wheat Growers. 2004. Wheat Facts.
NABIM. Flour milling.
Oplinger, E.S., E.A. Oelke, A.R. Kaminski, K.A. Kelling, J.D. Doll, B.R.
Durgan, and R.T. Schuler. 2000. Spelt. In Alternative Field Crops
Simmons,S.R., E.A. Oelke, and P.M. Anderson. 1995. Growth and development
guide for spring wheat. University of Minnesota Extension Service.
Stallknecht, G.F., K.M. Gilbertson, and J.E. Ranney. 1996. Alternative
wheat cereals as food grains: Einkorn, emmer, spelt, kamut, and triticale.
p. 156-170. In: J. Janick (ed.), Progress in new crops. ASHS Press, Alexandria,
USDA. Wheat Diseases and Pests: a guide for field identification