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The biology of Triticum aestivum L. (wheat)

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Background

The Canadian Food Inspection Agency's Plant Biotechnology Risk Assessment (PBRA) unit is responsible for assessing the potential risk to the environment from the release of plants with novel traits (PNTs) into the Canadian environment.

Risk assessments conducted by the PBRA unit require biological information about the plant species being assessed. Therefore, these assessments can be done in conjunction with species-specific biology documents that provide the necessary biological information. When a PNT is assessed, these biology documents serve as companion documents to Dir 94-08: Assessment Criteria for Determining Environmental Safety of Plants with Novel Traits.

Scope

This document is intended to provide background information on the biology of wheat including:

Such information will be used during risk assessments conducted by the PBRA unit. Specifically, it may be used to characterize the potential risk from the release of the plant into the Canadian environment with regard to:

The biology of wheat

General description, cultivation and use as a crop plant

T. Aestivum L. belongs to the family Poaceae (grass family), subfamily Pooideae, tribe Triticeae, and genus Triticum. Of the cultivated wheats, T. Aestivum, is economically by far the most important. Common names for T. Aestivum include bread wheat, common wheat, and wheat (GRIN taxonomy, 2024; Knott, 1960). In this document, T. aestivum is referred to as 'wheat' or by its scientific name. Note that in the literature, 'wheat' is also used to refer to other species.

Wheat as described by Lersten (1987), is a mid-tall annual or winter annual grass with flat leaf blades and a terminal floral spike consisting of perfect flowers. The vegetative state of the plant is characterized by tillers bearing axillary leafy culms. Culms comprise five to seven nodes with three to four foliage leaves. The uppermost, or flag leaf, subtends the inflorescence. Each culm produces an inflorescence or composite spike, the basic unit of which is termed the spikelet. Spikelets are born on a main axis, or rachis, and are separated by short internodes. Each spikelet is a condensed reproductive shoot consisting of two subtending sterile bracts or glumes. The glumes enclose two to five florets which are born on a short axis, or rachilla. Wheat florets contain three stamens with large anthers and the pistil which comprises a single ovary, with a single ovule, two styles, and two branching plumose stigmas at the end of each style.

T. aestivum is hexaploid (AABBDD) with a total of 42 chromosomes (2n=42; six copies of each of its seven chromosomes). Similarly, the different wheat species also contain some multiple of the basic haploid set of seven chromosomes. Modern wheat cultivars are either tetraploid (durum, AABB) or hexaploid (wheat and club wheat (Triticum compactum), AABBDD).

Wheat probably derived from a wild form of diploid einkorn (Triticum monococcum sensu lato) in an area that bordered the countries of Iran, Iraq, Syria, and Türkiye (Feldman, 1976). Tetraploid species evolved first through a combination of hybridization and amphidiploidy between T. monococcum and T. searsii, where T. monococcum is the source of the "A" genome and T. searsii the source of the "B" genome. The result was the tetraploid T. turgidum (AABB) which later was domesticated as emmer wheat and gave rise to the modern durum wheat cultivars. Hexaploid cultivars originated through a cross between tetraploid T. turgidum and T. tauschii (source of the "D" genome). Following an amphidiploidy event, a new species, wheat (T. aestivum), arose with a genome complement of AABBDD.

The cultivation of wheat began with wild einkorn and emmer (Cook and Veseth, 1991). The earliest plant breeding efforts with these wheats probably gave rise to plants with heads that did not shatter to facilitate harvest. Also, hull-less types were selected by early farmers for ease of threshing. In terms of plant adaptation, hexaploid wheat cultivation was adapted to cool climates due to the contribution of winter hardiness traits present on the "D" genome. Wheat plants were further adapted for cultivation in different environments via flowering behaviour. Spring wheat is planted in locations with severe winters and flowers in the same year yielding grain in about 90 days. Winter wheat is grown in locations with less severe winters. Winter wheat will only head after it has received a cold treatment (vernalization) and is therefore planted in the fall and harvested in the summer of the following year. Wheat varieties were adapted for cultivation in dry climates through the introduction of dwarf traits resulting in small plants that required less water yet produced good grain yield. Modern wheat cultivars have been developed to resist various diseases such as rusts and smuts. In addition to disease resistance, wheat breeding also focuses on increasing overall grain yield as well as grain quality (protein and starch).

Wheat is an important cereal grain for export and domestic consumption in many countries throughout the world. In 2023, there were about 8.5 million hectares of wheat planted in Canada (Statistics Canada, 2023). Canadian wheat is categorized as either Western Canadian or Eastern Canadian depending on the regions in which the varieties are grown. Wheat is further classified into classes based on its functional characteristics (for example, kernel hardness and dough strength) (Canadian Grain Commission, 2019). There are 10 Western and 7 Eastern official wheat classes.

Planting and harvest of a commercial wheat crop depends on the type of wheat grown. Due to the vernalization requirement of winter wheat, it is planted in the early fall (September and October) so that plants can emerge and develop sufficiently prior to onset of winter. During the winter months, winter wheat plants remain in a vegetative, dormant state. Once the temperature begins to rise, the winter wheat plant will resume growth and switch to a reproductive stage resulting in head development. In most areas of North America, a winter wheat crop will be ready for harvest by mid-July. Spring wheat plants do not enter a dormant state, therefore the crop requires approximately 90 days, from planting to harvest. Thus, most North American spring wheat crops are planted in mid- spring (April-May) and harvested in mid to late summer (August-September).

Wheat is cultivated in temperate climates. The minimal temperature for germination of wheat seeds is between 3 and 4°C. Flowering begins above 14°C. In North America wheat is grown to about 50° latitude. Within Canada, the primary production areas are the prairie provinces of Manitoba, Saskatchewan, and Alberta, although there are some production areas in the eastern provinces. Most wheat grown in the prairies is spring wheat. Winter wheat is produced in eastern provinces.

Wheat is the number one food grain consumed directly by humans. The principal use of wheat grain is the production of flour which, depending on the specific type of wheat, is used in many baked goods. Flour from hard red wheat is used to make bread dough while cakes, pastries, and crackers are made from soft red wheat flour. Flour from hard and soft white wheat is used in the production of oriental noodles. Additionally, hard white wheat flour is desired for making tortillas while soft white wheat flour has many uses including dough for cakes, crackers, cookies, pastries, and muffins. A significant amount of wheat is also used for animal feed; mainly varieties from the Canada Eastern Feed class and the Canada Eastern Other Wheat class (Canadian Grain Commission, 2019).

Breeding, seed production and agronomic practices

Modern wheat breeding programs focus on the improvement of agronomic and grain quality traits. Agronomic traits include winter hardiness, drought tolerance, disease and insect resistance, straw strength, plant height, resistance to shattering, grain yield, and harvest ability. Grain quality traits include seed shape, colour, test weight, protein concentration and type, starch concentration and type, and flour performance (Knott, 1987).

The majority of wheat varieties grown in North America are pure line, derived from inbreeding. The process of developing a new variety begins with the generation of F1 hybrids. Wheat breeders make many crosses each year in an effort to transfer traits between breeding lines and cultivars. The F2 generation, derived from self-pollinating the F1, exhibits a wide range of genetic differences based upon the genotypes of the parents. Selection of desirable individuals begins in the F2 generation and continues for at least two generations until individuals produce progeny that are genetically uniform. At that point, usually F6, selection for complex traits such as yield and grain quality will commence. Also, once a line is sufficiently uniform, performance data from small plots are generated for use in deciding which lines will be advanced. It should be noted that spring wheat breeding proceeds faster than winter wheat breeding due to the vernalization requirement of winter wheat. Since spring wheat does not require vernalization, breeders can achieve two to three generations per year using nurseries in greenhouses or fields in Southern regions (i.e. California, Arizona) or regions with opposite production seasons (i.e. New Zealand).

Based upon small plot performance data, wheat lines are chosen for pre-registration trials which are comprised of 10 to 20 locations over three years. The data from these trials is used to decide if the line is worthy of registration as a new cultivar. Based on the trial data, administrative groups (i.e. The Prairie Grain Development Committee) will decide whether to support the breeder's application for variety registration. Once a cultivar registration is approved, breeder's seed is distributed to seed growers for increase. Breeder seed is increased to foundation seed from which commercial production registered and/or certified seed will be derived (Anonymous, 1994). Genetic male sterility and fertility restoration systems and/or chemical gametocides are utilized to produce large amounts of F1 hybrid seed for commercial plantings. Several winter wheat hybrids have been commercialized for limited acreage in the United States and planting of a limited acreage of spring wheat hybrids is planned in 2024; no wheat hybrids are commercialized in Canada.

In normal agricultural practice, wheat is generally used in a crop rotation schedule to prevent the buildup of diseases, insects, and weeds. In western Canada, a number of rotations are possible and may include barley, canola, or flax, depending on the type of soils, cultural practices, etc.

Reproductive biology

Reproduction of wheat is only known in the context of cultivation; wheat is largely dependent on humans to harvest and propagate its seed. Wheat is predominantly self-pollinating. In general, outcrossing rates in any species which is primarily selfing may be up to 10% or higher, where the rate varies between populations, genotypes and with different environmental conditions (Jain, 1975). Grass populations that normally have outcrossing rates of less than 1% have shown rates of 6.7% in some years (Adams and Allard, 1982). In wheat, Hucl (1996) found that the frequency of outcrossing for 10 Canadian spring wheat cultivars varied according to the genotype, where the frequency was always lower than 9 per cent. Outcrossing tended to be highest among cultivars with low pollen staining, spikes which tapered at the extremities and with greater spikelet opening at anthesis. Similar results were obtained by Lawrie et al. (2006) who evaluated 35 Canadian spring wheat cultivars over two years. Generally, outcrossing was under 3.5%; however, a few exceptions occurred where outcrossing was as high as 10.6%. Martin (1990) reported outcrossing rates of 0.1-5.6% among winter wheat cultivars and concluded that the semi-dwarf stature of plants did not affect these rates.

Isolation of wheat plants for crossing purposes within the context of plant breeding can be done with greaseproof paper, cellophane bags, or dialysis tubing. Modest spatial isolation (3 metres) is required to prevent outcrossing in the production of foundation seeds in Canada (Anonymous, 1994).

deVries (1971) reported the duration of time that wheat florets remain open ranged from 8-60 minutes depending on genotype and environmental conditions. Once the anthers dehisce, 5-7% of the pollen is shed on the stigma, 9-12% remains in the anther, and the remainder is dispersed. Following dehiscence, wheat pollen viability was observed to range between 15-30 minutes. After release, wheat pollen attaches to the stigma branches via a brief electrostatic force followed by absorption of water by the pollen grain through gaps in the stigma cuticle (Heslop-Harrison, 1979). This process enables the pollen tube to grow which in turn facilitates fertilization. The duration of wheat stigma receptivity depends on variety and environmental conditions; the general range is 6-13 days. In general, pollen tube growth is initiated 1-2 hours after pollination followed by fertilization after an additional 30-40 hours (deVries, 1971). However, pollen grains can germinate within minutes after landing on the stigmatic surface with fertilization taking place in less than one hour (personal communication, George Fedak, 1999). The first spikelet to flower is generally in the middle third of the spike and usually near the upper part of this section; the flowering progresses rather rapidly upwards, downwards a little slower. The primary florets of a spikelet flower first, then the secondary and so on. The stamens are smaller and produce fewer pollen grains (1000-3800 per anther; 450,000 per plant) compared to other cereal grasses. According to deVries (1971), this compares to 4 million for rye (Secale cereale L.) and 18 million for maize (Zea mays L.).

Centre of origin

Although, the origins of wheat is complicated by various taxonomic opinions most researchers consider that modern wheat cultivars were derived from einkorn (T. monococcum ssp. urartu) and emmer wheat (T. turgidum) (Feldman, 1976). Wild einkorn wheat originated in southeastern Türkiye where it still grows today. Wild emmer wheat has a similar distribution but also extends into the Mediterranean portions of the middle east. Emmer wheat is often found in mixtures with einkorn wheat. Durum wheat cultivars were derived from domesticated emmer, while common hexaploid wheat originated from a combination of emmer and the diploid T. tauschii (donor of the "D" genome). T. tauschii is believed to have originated in the northern regions of Mesopotamia thus explaining the evolution of the winter hardiness traits residing on the "D" genome.

Cultivated wheat as a volunteer weed

During the domestication of modern wheat, key traits were modified that benefited early farmers but greatly reduced the ability of the resulting wheat races to survive in the wild. Plants with heads that did not shatter were favoured due to easier harvest. While farmers benefited by harvesting heads full of grain instead of gathering grain from the ground, the trait placed the wheat plants at a competitive disadvantage to plants of other species which could more efficiently distribute seed. In addition, hull-less type-plants were easier to thresh but exposed the developing seed to environmental extremes.

Despite these disadvantages, plants of modern wheat cultivars are occasionally found in uncultivated fields and roadsides. These occurrences are usually associated with grain dropped during harvest or transport. Plants growing in these environments do not typically persist and are usually eliminated by mowing, cultivation, and/or herbicide application. Similarly, wheat plants can also grow as volunteers in a cultivated field following a wheat crop. Wheat has been reported to reseed and volunteer for 3 years or more in some cases when grown in the Canadian Prairies (Harker et al. 2005; De Corby, 2007; Seerey et al. 2011). These plants are usually eliminated from the crop via cultivation or the use of herbicides.

Manipulation of wheat genetics has led to ever increasing gains in yield and grain quality, while decreasing the ability of wheat to survive in the wild. In fact, after hundreds of years of cultivation in North America and throughout the world, there have been no reports of wheat becoming an invasive pest.

Close relatives of wheat

Inter-species / genus hybridization

Important in considering the potential environmental impact following the unconfined release of genetically modified wheat, is an understanding of the possible development of hybrids through interspecific and intergeneric crosses between the crop and related species. The development of hybrids could result in the introgression of the novel traits into related species resulting in:

Wheat is primarily a self-pollinated crop and therefore natural cross hybridization with related species and genera is reduced. Wheat pollen-mediated gene flow decreases exponentially with increasing distance from the pollen source, with 90% of pollen falling within 3 meters of the source plant (Hegde & Waines, 2004). Hybridization within the genus Triticum was reviewed by Kimber and Sears (1987). While hybridization between cultivated wheat and related Triticum species can occur, no known wild Triticum species exist in North America.

There are many examples of successful classical cross-breeding within the genome lineage of T. aestivum. Hybridization is possible between all members of the hexaploid lineage (T. aestivum ssp. vulgare, T. compactum, T. sphaerococcum, T. vavilovii, T. macha, and T. spelta), as these species all have the genome AABBDD (Körber-Grohne, 1988). The stability of the hexaploid genome of T. aestivum is a result of genes (i.e., pairing homeologous 1 (Ph1) locus and other genes) which suppress homoeologous pairing. Consequently, the deactivation of the Ph1 locus is an important tool for plant breeders performing interspecific and intergeneric crosses. This can be achieved by using mutants such as Ph3a, Ph3b etc.

A well-known intergeneric combination involving wheat is triticale (Lukaszewski and Gustafson, 1987) derived from crossing and amphidiploidy between wheat and rye (S. cereale). There have been no reports of triticale serving as a bridge for hybridization with other wild grass species; however, not all species have been tested and triticale has not been widely planted (Kavanagh et al., 2010). Some artificial hybrids between wheat and rye have been reported (Florell, 1931). There are also some reports of natural wheat × rye hybrids occurring in Canada but these have not been confirmed or reported in the peer-reviewed literature (Hedge and Waines, 2004).

Wheat has been the subject of considerable work involving wide crossing (Sharma and Gill, 1983). However, much of these works will have little relevance to the natural environment as only a few species related to wheat are native to Canada and techniques such as embryo rescue, hand pollination, and use of male sterile plants may be necessary to obtain viable progeny.

Potential for introgression of genes from wheat into relatives

The closest known relatives to wheat in North America are from the Aegilops genus.

Hybridization between wheat and Aegilops cylindrica Host (jointed goatgrass) is reported under natural field conditions (Donald and Ogg, 1991; Gaines et al., 2008; Gandhi et al., 2006; Hanson et al., 2005, Morrison et al., 2002, Seefeldt et al., 1998; Stone and Peeper, 2004, Zaharieva and Monneveux, 2006; Zemetra et al., Mallory-Smith, 1998; Perez-Jones et al., 2006; Perez-Jones, et al., 2010; Rehman et al., 2017). F1 hybrids are mostly sterile, however; introgression is possible. Wheat is allohexaploid (2n = AABBDD = 42) and jointed goatgrass is allotetraploid (2n = CCDD=28). Since these species share the D genome, the probability of gene transfer is higher for genes located on the D genome of wheat.

Other Aegilops species that are known weeds in California include: Ae. crassa; Ae. geniculata; Ae. ovata; and Ae. triuncialis. There are no reports of wheat outcrossing to these species.

Another relative is Elymus repens (formally Agropyron repens). Knobloch (1968) cited reports of hybrids between wheat and E. repens, however, such hybrids have been found to be difficult to reproduce by manual pollination. No subsequent reports were found in the literature.

Cultivated wheat can hybridize with other Agropyron species; however, no known naturally-occurring hybrids have been reported (Knott, 1960; Mujeeb-Kazi, 1995). Hybrids between T. aestivum and intermediate wheatgrass, A. intermedium, have been reported in Russia where fertile plants were successfully produced (Tsvelev, 1984). Similarly, Smith (1942) repeatedly obtained fertile progeny from hand pollinations between wheat and A. intermedium. These hybrids are of interest, as intermediate wheatgrass occurs in Western Canada for use as a range improvement forage grass and as an adventive weedy grass. Additionally, artificial hybrids have been made between wheat and A. curvifolium, A. distichum, A. junceum, A. cristatum, A. elongatum, and A. trichophorum (Smith, 1942).

Other weedy relatives native to North America include Elymus bakeri (Baker's Wheatgrass), Hordeum californicum, Hordeum jubatum (Squirrel-tail grass), Leymus angustus (Altai Wild Rye), Elymus canadensis (Canadian Wild Rye), and Elymus virginicus (Virginia Wild Rye). These species have formed hybrids with wheat using artificial methods (personal communication, George Fedak, 1999). There are no reports of natural hybrids.

The potential for interspecific and intergeneric gene flow between wheat and other species from the Triticeae tribe under natural conditions is unlikely in Canada. However, the numerous reports of hybridizations with wheat should be considered when evaluating the potential for the introgression of 'novel traits' from transgenic wheat into wild relatives.

Occurrence of related species of wheat in Canada

There are no wild Triticum species in Canada (Feldman, 1976; Hedge and Waines, 2004).

The weedy relative jointed goatgrass, Ae. cylindrica, is present in Ontario and British Columbia (Canadian Food Inspection Agency, 2023). Jointed goatgrass is a regulated pest and a prohibited noxious weed in Canada (Canadian Food Inspection Agency, 2023). The incursions of jointed goatgrass in Canada are small, localized populations under Canadian Food Inspection Agency regulatory control. Some of these localized populations have been declared eradicated, while others are currently under eradication. In the western United States, jointed goatgrass is a problem weed in hexaploid winter wheat production.

In Canada, the most common weedy relative is quack grass, E. repens, which is present in all provinces and territories of Canada (Crompton et al., 1988). E. repens is listed as a primary noxious weed in the Weed Seeds Order (2016). It is a perennial weedy grass common in the agricultural areas especially in grasslands, cultivated fields, gardens, roadsides, and waste places (Frankton and Mulligan, 1993; Alex and Switzer, 1976).

The following species are relatives of wheat from the Triticeae tribe and have been cited by Knobloch (1968) to produce artificial hybrids when crossed with wheat. They occur in Canada as naturalized and cultivated plants and are used as specialized forage crops or for soil stabilization purposes. These grass species are present in Canada and are known to colonize disturbed habitats such as uncultivated fields and roadside areas. It is improbable that hybrids between wheat and these relatives would occur in nature.

Species name Common name(s) Presence in Canada
Elymus dahuricus Turcz. ex Griseb. in Ledeb Dahurian wild rye cultivated/ introduced
Psathyrostachys juncea (Fisch.) Nevski Russian wild rye cultivated/ naturalized
Leymus arenarius (L.) Hochst Sea lyme grass, Strand-wheat naturalized
Thinopyrum intermedium (Host) Barkworth & D.R. Dewey Intermediate wheatgrass cultivated/ naturalized
Elytrigia intermedia (Host) Nevski Pubescent wheatgrass cultivated/ naturalized
Agropyron elongatum (Host) Beauv. Tall wheatgrass cultivated/ naturalized
Agropyron cristatum (L.) Gaertn. Crested wheatgrass cultivated/ naturalized

Summary of the ecology of relatives of wheat

Ae. cylindrica and E. repens are the primary weedy relatives of T. aestivum present in Canada.

Ae. cylindrica is a regulated pest and a prohibited noxious weed in Canada (Canadian Food Inspection Agency, 2016). It is an annual grass, reaching 40-60 cm tall and resembling wheat. Ae. cylindrica can be distinguished from wheat by its narrow cylindrical spikes and evenly spaced hairs extending from its leaf blades (CFIA, 2023).

E. repens is listed as a primary noxious weed in the Weed Seeds Order (2016) and is a troublesome weedy grass of agricultural areas throughout Canada. It is a perennial that can reproduce through seeds, which can remain viable in soil for up to 5 years, and by rhizomes (CFIA, 2017). It can be controlled in crops using selective herbicides.

Potential interactions of wheat with other life forms

Table 1 is intended to guide applicants in their considerations of potential impacts the release of the PNT in question may have on non-target organisms, but should not be considered as exhaustive. Where the impact of the PNT on another life form (target or non-target organism) is significant, secondary effects may also need to be considered.

Table 1.
Other life forms Interaction with wheat
(Pathogen; Symbiont or Beneficial Organism; Consumer; Gene transfer)
Pseudomonas syringae pv. atrofaciens
(Basal Glume Rot)		 Pathogen
Xanthamonas campestris pv. translucens
(Black Chaff)		 Pathogen
Erwinia rhapontici
(Pink Seed)		 Pathogen
Corynebacterium tritici
(Spike Blight)		 Pathogen
Colletotrichum graminicola
(Anthracnose)		 Pathogen
Ascochyta tritici (includes A. sorghi)
Ascochyta Leaf Spot		 Pathogen
Cephalosporium gramineum
(Cephalosporium Stripe)		 Pathogen
Tilletia caries
(Common Bunt)		 Pathogen
Bipolaris sorokinia (includes Helminthosporium sativum)
Common Root Rot		 Pathogen
Scleropthora macrospora
Downy Mildew		 Pathogen
Tilletia controversa
Dwarf Bunt		 Pathogen
Claviceps purpurea
Ergot		 Pathogen
Pseudocerosporella herpotrichoides
Eyespot, foot rot		 Pathogen
Puccinia spp.
Rust		 Pathogen
Monographella nivalis (includes Calonectria nivalis)
Pink Snow Mold		 Pathogen
Leptosphaeria herpotrichoides
Leptosphaeria Leaf Spot		 Pathogen
Sclerotinia borealis
Sclerotinia Snow Mold		 Pathogen
Erysiphe graminis
Powdery Mildew		 Pathogen
Septoria spp.
Speckled Leaf Blotch		 Pathogen
Typhula spp.
Speckled Snow Mold (Typhula blight)		 Pathogen
Gaeumannomyces graminis
Take All		 Pathogen
Pyrenophora trichostoma
Yellow Leaf Spot		 Pathogen
Heterodera avenae
Cereal Oat Cyst		 Consumer
Subanguina radicicola
Root Gall		 Consumer
Meloidogyne spp.
Root Knot		 Consumer
Paratrichodorus spp.
Stubby Root		 Consumer
Pratylenchus spp.
Root Lesion		 Consumer
Mayetiola destructor
Hessian fly		 Consumer
Midge Consumer
Diuraphis noxia
Russian Wheat Aphid		 Consumer
Beneficial Insects Symbiont or Beneficial Organism
Soil microbes Symbiont or Beneficial Organism
Earthworms Symbiont or Beneficial Organism
Soil Insects Symbiont or Beneficial Organism
Other T. aestivum Gene Transfer

Acknowledgments

This document was developed in collaboration with Cyanamid Crop Protection and Agriculture & Agri-Food Canada. Thanks to S. Darbyshire (AAFC-ECORC) for his assistance with researching and reviewing the document. In 2024, the CFIA updated this document.

Bibliography

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