Genetics
Wheat genetics is more complicated than that of most other domesticated species. Some wheat species are diploid, with two sets of chromosomes, but many are stable polyploids, with four sets of chromosomes (tetraploid) or six (hexaploid).
- Einkorn wheat (T. monococcum) is diploid .
- Most tetraploid wheats (e.g. emmer and durum wheat) are derived from wild emmer,. Wild emmer is itself the result of a hybridization between two diploid wild grasses and a wild goatgrass such as Aegilops searsii or Ae. speltoides. The hybridization that formed wild emmer (AABB) occurred in the wild, long before domestication and was driven by natural selection.
- Hexaploid wheats evolved in farmers’ fields. Either domesticated emmer or durum wheat hybridized with yet another wild diploid grass (Aegilops cylindrica) to make the hexaploid wheats, spelt wheat and bread wheat. These have three sets of paired chromosomes, three times as many as in diploid wheat.
The presence of certain versions of wheat genes has been important for crop yields. Apart from mutant versions of genes selected in antiquity during domestication, there has been more recent deliberate selection of alleles that affect growth characteristics.
Genes for the ‘dwarfing’ trait, first used by Japanese wheat breeders to produce short-stalked wheat, have had a huge effect on wheat yields world-wide, and were major factors in the success of the Green Revolution in Mexico and Asia. Dwarfing genes enable the carbon that is fixed in the plant during photosynthesis to be diverted towards seed production, and they also help prevent the problem of lodging. ‘Lodging’ occurs when a ear stalk falls over in the wind and rots on the ground, and heavy nitrogenous fertilization of wheat makes the grass grow taller and become more susceptible to this problem.
By 1997, 81% of the developing world’s wheat acreage was planted to semi-dwarf wheats, giving both increased yields and better response to nitrogenous fertilizer.
Wild grasses in the genus Triticum and related genera, and grasses such as rye have been a source of many disease-resistance traits for cultivated wheat breeding since the 1930s.
Heterosis, or hybrid vigor (as in the familiar F1 hybrids of maize), occurs in common wheat, but it is difficult to produce seed of hybrid cultivars on a commercial scale (as is done with maize) because wheat flowers are complete and normally self-pollinate.
Commercial hybrid wheat seed has been produced using chemical hybridizing agents; these chemicals selectively interfere with pollen development, or naturally occurring cytoplasmic male sterility systems. Hybrid wheat has been a limited commercial success in Europe (particularly France), the USA and South Africa.
F1 hybrid wheat cultivars should not be confused with the standard method of breeding inbred wheat cultivars by crossing two lines using hand emasculation, then selfing or inbreeding the progeny many (ten or more) generations before release selections are identified to be released as a variety or cultivar.
Synthetic hexaploids made by crossing the wild goatgrass wheat ancestor Aegilops tauschii and various durum wheats are now being deployed, and these increase the genetic diversity of cultivated wheat.
Stomata (or leaf pores) are involved in both uptake of carbon dioxide gas from the atmosphere and water vapor losses from the leaf due to water transpiration. Basic physiological investigation of these gas exchange processes has yielded valuable carbon isotope based methods that are used for breeding wheat varieties with improved water-use efficiency. These varieties can improve crop productivity in rain-fed dry-land wheat farms.
In 2010, a team of UK scientists announced they had decoded the wheat genome for the first time (95% of the genome of a variety of wheat known as Chinese Spring line 42). This was the expressed genome and was released in a basic format for scientists and plant breeders to use but was not a fully annotated sequence which was reported in some of the media.
Plant breeding
In traditional agricultural systems wheat populations often consist of landraces, informal farmer-maintained populations that often maintain high levels of morphological diversity.
Although landraces of wheat are no longer grown in Europe and North America, they continue to be important elsewhere. The origins of formal wheat breeding lie in the nineteenth century, when single line varieties were created through selection of seed from a single plant noted to have desired properties.
Modern wheat breeding developed in the first years of the twentieth century and was closely linked to the development of Mendelian genetics.
The standard method of breeding inbred wheat cultivars is by crossing two lines using hand emasculation, then selfing or inbreeding the progeny. Selections are identified (shown to have the genes responsible for the varietal differences) ten or more generations before release as a variety or cultivar.
F1 hybrid wheat cultivars should not be confused with wheat cultivars deriving from standard plant breeding. Heterosis or hybrid vigor (as in the familiar F1 hybrids of maize) occurs in common wheat, but it is difficult to produce seed of hybrid cultivars on a commercial scale as is done with maize because wheat flowers are complete and normally self-pollinate.
Commercial hybrid wheat seed has been produced using chemical hybridizing agents, plant growth regulators that selectively interfere with pollen development, or naturally occurring cytoplasmic male sterility systems. Hybrid wheat has been a limited commercial success in Europe (particularly France), the United States and South Africa.
The major breeding objectives include high grain yield, good quality, disease and insect resistance and tolerance to abiotic stresses include mineral, moisture and heat tolerance.
Farming techniques
Technological advances in soil preparation and seed placement at planting time, use of crop rotation and fertilizers to improve plant growth and advances in harvesting methods have all combined to promote wheat as a viable crop.
Agricultural cultivation using horse collar leveraged plows (at about 3000 BCE) was one of the first innovations that increased productivity. Much later, when the use of seed drills replaced broadcasting sowing of seed in the 18th century, another great increase in productivity occurred.
Yields of wheat per unit area increased as methods of crop rotation were applied to long cultivated land, and the use of fertilizers became widespread.
Improved agricultural husbandry has more recently included threshing machines and reaping machines (the ‘combine harvester’), tractor-drawn cultivators and planters, and better varieties. Great expansions of wheat production occurred as new arable land was farmed in the Americas and Australia in the 19th and 20th centuries.
