The biology of Triticum turgidum ssp. durum (durum wheat)

On this page

• Background
• Scope
• The biology of durum wheat
  • General description, cultivation and use as a crop plant
  • Breeding, seed production and agronomic practices for durum wheat
  • Reproductive biology
  • Centre of origin
  • Cultivated durum wheat as a volunteer weed
• The close relatives of durum wheat
  • Inter-species and genus hybridization
  • Potential for introgression of genes from durum wheat into relatives
  • Occurrence of related species of durum wheat in Canada
  • Summary of the ecology of relatives
• Potential interactions of durum wheat with other life forms
• Bibliography

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 D-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 Triticum turgidum ssp. durum including:

• its identity
• geographical distribution
• reproductive biology
• related species
• the potential for gene introgression from Triticum turgidum ssp. durum into relatives
• details of the life forms with which it has the potential to interact

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:

• weediness/invasiveness
• gene flow
• plant pest properties
• impacts on other organisms
• impact on biodiversity

The biology of durum wheat

General description, cultivation and use as a crop plant

Durum wheat (Triticum turgidum ssp. durum) is a monocotyledonous plant of the Gramineae family, and of the Triticeae tribe and belongs to the genus Triticum. For commercial production and human consumption, durum wheat is the second most important Triticum species, next to common wheat (Triticum aestivum L.).

Durum is a mid-tall annual grass with flat leaf blades and a terminal floral spike consisting of perfect flowers (Bozzini, 1988). As with common wheat, there are durum wheat varieties that have a semi-dwarf stature. The root system is composed of seminal roots which are produced by the young plant during germination and adventitious roots, which arise later from the basal nodes of the plant to become the permanent root system. The stem is cylindrical, erect, usually hollow, and subdivided into internodes. Some durum wheats have solid stems (Clarke et al., 2002).

Culms (tillers) arise from auxiliary buds at the basal nodes of the main stem. The number of culms formed depends on the variety, growing conditions, and planting density. Under usual field conditions, a plant may produce a total of three culms in addition to the main shoot, although not all will necessarily produce grain (Bozzini, 1988). As with other grasses, durum wheat leaves are composed of a basal portion (the leaf sheath) which envelops the stem, and a terminal portion, which is linear with parallel veins and an acute apex. At the attachment of the leaf sheath is a thin, transparent membrane (ligule) with two small lateral appendices called auricles. The main stem and each culm produce a terminal inflorescence.

The inflorescence of durum wheat is a spike with a rachis bearing spikelets separated by short internodes (Bozzini, 1988). Each spikelet consists of two glumes (bracts) enclosing two to five florets, all borne distichously on a rachilla. Each floret is enclosed by bract like structures called the lemma and the palea. Each floret is a perfect flower, containing three stamens with bilocular anthers and a pistil bearing two styles with feathery stigmas. Mature pollen is fusiform, normally containing three nuclei. Each floret has the potential to produce a one-seeded fruit called a caryopsis. Each seed contains a large endosperm and a flattened embryo located at the apex of the seed and close to the base of the floret.

Durum wheat is best adapted to regions having a relatively dry climate, with hot days and cool nights during the growing season, typical of Mediterranean and temperate climates. Seed germination will occur as low as 2°C, but the optimal temperature is 15°C (Bozzini, 1988). Most durum wheat produced in the world is of spring growth habit; however, durum wheat lines with winter habit (requires vernalization to initiate the transition from vegetative growth to reproductive growth) have been evaluated for production in the USA and Canada (Donmez et al., 2000; Schilling et al., 2003; Tamburic-Ilincic et al., 2012).

Major producers of durum wheat include the European Union (mainly Italy, Spain, and Greece), Canada, Türkiye, and the USA (International Grains Council, 2002). In Canada, durum wheat production primarily occurs in the drier, south central regions of the prairie provinces of Manitoba (<1% of Canadian production), Saskatchewan (approximately 79% of Canadian production), and Alberta (approximately 21% of Canadian production) (based on Statistics Canada production data from 2020-2023).

Durum wheat is a separate species from the other commercially grown wheat classes grown in Canada (which are almost entirely T. aestivum), and possesses unique quality characteristics that differentiates durum wheat from other classes of wheat. The principal use of durum wheat grain is the production of semolina for use in pasta products. However, in North Africa, durum is preferred for the production of couscous and bulgur. Traditional breads are also made with durum flour, particularly in Morocco. Durum (derived from the Latin word for hard) has the hardest kernel of all wheats. Durum wheat with high protein content and gluten strength is the preferred choice of processors for producing pasta products. Durum kernels are amber-colored and larger than those of other wheat classes. Also unique to durum is its yellow endosperm, which gives pasta its golden color. Durum wheat with strong gluten characteristics forms strong, non-sticky dough ideal for pasta production. Semolina with strong gluten properties also results in pasta products with superior cooking characteristics. In Canada, two sub-classes of durum wheat are recognized: conventional varieties with moderate gluten strength, and extra-strong varieties with extra-strong gluten properties similar to the USA desert durum (Clarke et al., 2005).

Breeding, seed production and agronomic practices for durum wheat

In Canada, durum wheat crops are planted in early spring (April-May) and harvested in mid to late summer (August-September). In normal agricultural practice, durum wheat is generally grown in an integrated crop rotation schedule to prevent the buildup of diseases, insects, and weeds. In western Canada, a number of rotations are possible and may include summer fallow, where fields are not planted to crops and weed growth is limited either through tillage, non-selective herbicide application, or a combination of both. Depending on cultural practices, cropping history, soil texture, available moisture and potential economic return, other crops are grown in rotation with durum wheat in Western Canada and can include barley, oat, canola, mustard, flax, pea, lentil, and chickpea.

The full range of genetic variation of traits related to grain composition and biotic/abiotic stress resistance has not been surveyed in Canadian germplasm, and there is undoubtedly considerable variation that is currently not being exploited. Durum wheat breeders focus on the simultaneous improvement of agronomic performance, disease resistance and grain quality traits. Agronomic traits include grain yield, drought tolerance, disease and insect resistance, straw strength, plant height, resistance to shattering, and harvest ability. Grain quality traits include physical quality traits such as test weight, seed size, and percentage hard vitreous kernels. Processing quality traits include, protein concentration, yellow pigment concentration, grain cadmium concentration, gluten strength, semolina milling properties, and pasta cooking quality and color. There is considerable variation in concentration of metal ions in the grain, which is currently being exploited to breed cultivars with low grain cadmium concentration (J. Clarke et al., 2002). Durum breeders are also breeding to maintain resistance to the wheat rusts, and continue efforts to incorporate stable resistance to the leaf-spotting diseases, seed borne diseases, and fusarium head blight. Resistance to pests such as the orange blossom wheat midge (Sitodiplosis mosellana) and the wheat stem sawfly (Cephus cinctus) and Hessian Fly (Mayetiola destructor) is available (Lamb et al., 2000; Lamb et al., 2001; Clarke et al., 2002) in Canadian durum wheat germplasm and is currently being utilized by Canadian durum wheat breeders in their breeding programs.

The majority of durum wheat varieties grown in North America are pure lines, derived either from repeated cycles of inbreeding, or through the use of double haploid technology (Knox et al., 2002). The process of developing a new variety begins with the generation of F1 hybrids by crossing two or more parents. Breeders must ensure that all parents used in a cross collectively possess the majority of desirable traits wanted in a new variety. In the case of inbreeding populations, the F2 generation is derived from self-pollinating the F1 and exhibits a wide range of genetic differences. Selection of desirable individuals can begin in the F2 generation and continues for a minimum of two generations until individuals produce progeny that are genetically uniform. Some plant breeders may chose not to make selections in the F2, and delay selection until the F3 or F4 generations. Selection at these early stages of the breeding program is usually for those traits with little environmental influence on the level of expression. Selection for complex traits such as grain yield and grain quality will usually commence in the F6 generation when a breeding line is sufficiently uniform. In the cases of doubled haploid technology, either the ovary or developing pollen grains from F1 plants are subjected to tissue culture techniques to develop many haploid plantlets. Following a chromosome doubling treatment with mitotic inhibitors like colchicine, breeding lines are developed that are 100% homozygous. After one or two cycles of seed increase and visual selection, doubled haploid lines are advanced to replicated field trials for assessment of yield and grain quality traits. Regardless of breeding method, performance data from small plots are generated for use in deciding which lines will be advanced. Many breeders use contra-season nurseries or greenhouses to achieve two generations per year to decrease the amount of time required to release a durum wheat variety. The use of molecular markers to assist in selection for difficult to measure traits is becoming a routine tool used by durum wheat breeders to increase the efficiency of selection.

Based upon small plot performance data in the F6 generation, wheat lines are chosen for pre-registration trials which are conducted, usually for two years, at 4-6 locations each year. Lines that perform equal to or better than current check varieties are then advanced to registration trials, which are conducted at 10-20 sites over three years. Based on registration trial data, administrative groups such as The Prairie Recommending Committee for Wheat, Rye and Triticale (PRCWRT) will decide whether to support the breeder's application for variety registration. Once a cultivar registration is approved, breeder 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).

Reproductive biology

Durum wheat is predominantly a self-pollinating species. During flowering, flowers generally remain closed (cleistogamous flowers), and the three anthers burst and release pollen (anthesis). Flowers may also open prior to pollen release. 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. Wheat pollen generally remains viable for 15-30 minutes. Flowering of the spike can last for three to six days, depending on weather conditions. Flowering generally begins in the basal florets just above the centre of the spike and proceeds towards the apex and base of the spike. Unfertilized florets usually open, exposing the receptive stigma to foreign pollen. The duration of wheat stigma receptivity depends on variety and environmental conditions, but ranges from 6 to 13 days (deVries, 1971). Once, fertilized, the ovary rapidly increases in size. Two to three weeks after fertilization, the embryo is physiologically functional, and able to generate a new seedling (Bozzini, 1988).

There are very few reports measuring outcrossing rates in durum wheat under natural conditions. Outcrossing rates in primarily self-pollinating species can be up as high as 10%, 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). Harrington (1932) measured outcrossing in durum wheat, and reported outcrossing rates no higher than 1.1%, but rates of 5% have been reported (Bozzini, 1988). In hexaploid wheat, average outcrossing rates range from 0-6.7%, depending on the cultivar (Martin, 1990; Hucl, 1996; Hucl and Matus-Cadiz, 2001). In hexaploid spring wheat, Hucl (1996) has shown that cultivars with higher outcrossing rates tended to have a greater degree of spikelet opening at anthesis. This is likely applicable to durum wheat as well. In durum wheat, outcrossing rates are also dependent on the environment (Loureiro et al., 2007). Humid conditions are favorable to self-pollination whereas outcrossing rates can increase during periods of drought (Bozzini, 1988).

Centre of origin

Wild tetraploid wheats were largely distributed in the Near East when humans started harvesting them in nature (Bozzini, 1988). The large spike and seed size compared to diploid wheats made them much more attractive for domestication. Durum wheat is thought to have originated in present day Türkiye, Syria, Iraq, and Iran (Feldman, 2001). Durum wheat is an allotetraploid (two genomes: AABB) with a total of 28 chromosomes (2n=4x=28), containing the full diploid complement of chromosomes from each of its progenitor species. As such, each chromosome pair in the A genome has a homoeologous chromosome pair in the B genome to which it is closely related. However, during meiosis, chromosome pairing is limited to homologous chromosomes by the genetic activity of inhibitor genes. A number of inhibitor genes have been identified, but the Ph1 gene located on the long arm of chromosome 5B is considered the critical inhibitor gene (Wall et al., 1971).

Based on cytological and molecular analysis, T. turgidum subspecies are believed to have originated from the natural hybridization of Triticum monococcum L. subsp. boeoticum (Boiss.) (synonym: Triticum urartu: AA) and an unknown diploid wheat species containing the B genome (Feldman, 1976). Kimber and Sears (1987) suggested that one or more of five diploid species in section Sitopsis of Triticum may have donated the B genome to the polyploid wheats. Molecular evidence suggests that the genome from T. speltoides is most related to the durum and common wheat B genome (Talbert et al., 1995; Khlestkina and Salina 2001). Furthermore, analysis of chloroplast DNA suggests that T. speltoides is likely the maternal donor of durum wheat (Wang et al. 1997). The result of this natural hybridization was wild emmer wheat (Triticum turgidum ssp. dicoccoides (Korn.) Thell) which was later domesticated as emmer wheat (Triticum turgidum ssp. dicoccum (Schrank) Thell). Emmer wheat spread from the Near East to large areas of the Mediterranean and Middle East, including Egypt and Ethiopia (Bozzini, 1988). Thousands of years of cultivation and selection have resulted in tremendous variability in the tetraploid wheats derived from wild emmer. As a result, a number of subspecies have been characterized, primarily based on morphological features (van Slageren, 1994): T. turgidum ssp. paleocolchicum, T. turgidum ssp. polonicum, T. turgidum ssp. turanicum, T. turgidum subsp. carthlicum, T. turgidum ssp. turgidum, and T. turgidum ssp. durum. Among all cultivated tetraploid wheats, T. turgidum ssp. durum is by far the most important.

Cultivated durum wheat as a volunteer weed

A weed may be defined as any plant growing where it is not wanted. Commercialized durum wheat cultivars are occasionally found in uncultivated fields and roadsides (Thomas et al., 1996). These occurrences usually result from grain dropped during harvest or transport. Plants growing in these environments do not persist and are usually eliminated by mowing, cultivation, and/or herbicide application. Similarly, durum wheat plants can also grow as volunteers in a cultivated field following durum wheat production. These plants are usually eliminated from the subsequent crop via cultivation or the use of grassy-weed herbicides. However, no studies were found investigating the persistence and volunteer potential of durum wheat. A related species, T. aestivum, has been reported to reseed and volunteer for more than 3 years in some cases when grown in the Canadian Prairies (Harker et al. 2005; De Corby, 2007). There are no reports of durum wheat becoming an invasive weed.

The close relatives of durum wheat

Inter-species and genus hybridization

An understanding of the possible development of hybrids through interspecific and intergeneric crosses between the crop and related species is important in considering the potential environmental impact following the unconfined release of genetically modified T. turgidum ssp. durum. The development of hybrids could result in the introgression of the novel traits into related species resulting in ecosystem disruption and the related species becoming more weedy.

Durum wheat and T. turgidum ssp. paleocolchicum, T. turgidum subsp. polonicum, T. turgidum subsp. turanicum, T. turgidum subsp. carthlicum, and T. turgidum subsp. turgidum are sexually compatible and produce fertile hybrids. While hybridization between durum wheat and related species can occur, no known wild Triticum species exist in North America. Hybridization within the genus Triticum was reviewed by Kimber and Sears (1987). No known wild Triticum species exist in North America. Hybridization within the genus Triticum was reviewed by Kimber and Sears (1987).

Hybridization between durum wheat and related crop species including T. aestivum, rye (Secale cereal L.), and triticale (× Triticale Wittm. ex A. Camus.) can occur; however, observed F1 hybrids are sterile (Hills et al., 2007).

Potential for introgression of genes from durum wheat into relatives

The closest known North American relatives of durum wheat are species within the Aegilops genus. Jointed goat grass (Aegilops cylindrica Host) is a problem weed in hexaploid winter wheat production in the western US states. In Canada, jointed goatgrass is found in Ontario and British Columbia where efforts to eradicate it are underway (Canadian Food Inspection Agency, 2023). There are no reports of durum wheat hybridizing with jointed goatgrass under field conditions. Other Aegilops species that are known weeds in California include: Ae. crassa; Ae. geniculata; Ae. ovata; and Ae. triuncialis. Ae. cylindrica is more closely related to T. aestivum (common bread wheat) than to T. turgidum spp. durum since it contains the D genome present in T. aestivum. Durum wheat does not contain the D genome of T. aestivum. Prazak (2001) reported hybridization between Ae. cylindrica and T. turgidum ssp. durum, but the resulting progeny were sterile. Natural hybridization between Ae. triuncialis and T. turgidum spp. durum has also been reported in Türkiye (Mamedov et al., 1996), but these results have not been confirmed.

Both hexaploid and tetraploid wheat have been the subject of considerable research involving interspecific and intergeneric crosses (Sharma and Gill, 1983). However, this work has little relevance to the natural environment as only a few species related to wheat are present in Canada. The stability of the durum wheat genome is the result of genes (Ph1 and other genes) which restricts chromosome pairing to homologous chromosomes. Furthermore, laboratory techniques such as hand pollination, embryo rescue, and artificial chromosome doubling using meiotic inhibitors are necessary to obtain fertile progeny. For example, under laboratory conditions hybrids between T. turgidum ssp. durum and Ae. cylindrica, Ae. triuncialis, Ae. crassa and Ae. ovata have been reported (Knobloch, 1968; Arzani et al., 2000; Benavente et al., 2001; Prazak, 2001).

In Canada, the most common weedy relative is quack grass Elymus repens (formally Agropyron repens), which is present in all provinces and territories of Canada (Crompton et al., 1988). 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). Tsitsin (1940) reported hybrids between durum wheat and E. repens, but this report is questionable. In laboratory settings, tetraploid wheat has been hybridized with other related species including E. repens, Agropyron elongatum, and Agropyron intermedium (Knobloch, 1968; Schulz-Schaeffer, 1969). Schulz-Schaeffer (1969) indicated that the chromosomes of Triticum spp. generally do not pair with the chromosomes of Agropyrum indicating that natural gene flow between the two genera is unlikely. Furthermore, no known naturally-occurring hybrids or species derived from hybridization of T. turgidum ssp. durum and Agropyron spp. have been reported (Knott, 1960). Other weedy relatives native to North America include A. bakeri (includes A. trachycaulum, Bakers Wheatgrass), Hordeum californicum, Hordeum jubatum (Squirreltail Grass), Elymus angustus (includes Leymus angustus, Altai Wild Rye), E. canadensis (Canadian Wild Rye) and E. virginicus (Virginia Wild Rye). There is a single report of development of a hybrid between A. trachycaulum and Triticum turgidum spp. durum in laboratory settings (Knobloch, 1968). However, there are no reports of natural hybrids in the literature. Furthermore, no reports of natural hybrids between Triticum turgidum spp. durum and H. californicum, H. jubatum, E. angustus, E. canadensis and E. virginicus have been reported.

Occurrence of related species of durum wheat in Canada

There are no wild Triticum species in Canada (Feldman, 1976). Of the genera most closely related to Triticum, only E. repens is widespread in Canada. Knobloch (1968) cited reports of hybrids between T. turgidum ssp. durum and E. repens, A. elongatum, and A. intermedium. However, such hybrids are difficult to reproduce, and require manual pollination, embryo rescue, and chromosome doubling to survive and produce viable progeny.

The weedy relative Ae. cylindrica (jointed goatgrass) 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.

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 durum.

• Agropyron intermedium (Host) Beauv. Intermediate Wheatgrass (naturalized/cultivated)
• Agropyron elongatum (Host) Beauv. Tall Wheatgrass (cultivated/naturalized)
• Agropyron dasystachyum (Hook.) Scribn. Northern Wheat Grass (cultivated/naturalized)
• Agropyron trachycaulum (Link) Malte. Slender Wheat Grass (cultivated/naturalized)
• Agropyron trichophorum (cultivated/naturalized)
• Agropyron cristatum (L.) Gaertn. Crested Wheatgrass (cultivated/naturalized)
• Leymus arenarius (L.) Hochst (Elymus arenarius L.) Sea Lyme Grass, Strand-Wheat (naturalized)
• Leymus mollis Trin (Elymus mollis Trin) Sea Lyme Grass, Strand-Wheat (native)

All of these species occur in Canada as naturalized and cultivated plants and, in some cases, are used as forage crops. They are adapted to Canada and are known to colonize disturbed habitats such as uncultivated fields and roadside areas. Since the chromosomes from Agropyron and Triticum do not pair easily, if at all (Schulz-Schaeffer, 1969), it is improbable that fertile hybrids between durum wheat and these relatives occur in nature.

Summary of the ecology of relatives

Ae. cylindrica and E. repens are weedy relatives of durum wheat. Ae. cylindrica is a regulated pest and a prohibited noxious weed in Canada (Canadian Food Inspection Agency, 2016). 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 relatively easy species to control in crops using selective herbicides.

Potential interactions of durum wheat with other life forms

Table 1 is intended to be used to guide applicants in their considerations of potential impacts of the release of a PNT on non-target organisms. The intention is not to require comparison data between the PNT and its T. turgidum ssp. durum counterpart(s) for all interactions. Depending on the novel trait, applicants might decide to submit data for only some of the interactions. Sound scientific rationale will be required to justify the decision that data would be irrelevant for the remaining interactions. For example, the applicant might choose not to provide data on the weediness potential of the PNT if it can be clearly shown that the novel trait will not affect reproductive or survival characteristics of T. turgidum ssp. durum, either directly or indirectly. Where the impact of the PNT on another life form (target or non-target organism) is significant, secondary effects may need to be considered.

Table 1.

"X" indicates the type of interaction between the listed organisms and T. turgidum ssp. durum (information requirements may be waived if valid scientific rationale is provided).

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