A number of methods are used to generate new wheat lines through breeding. These include mutation breeding (Konzak 1987), hybrid wheat production using chemicals to induce male sterility (Lucken 1987) or mechanical methods to remove anthers (Simmonds 1989). The single seed descent method is also used for the rapid production of inbred lines (Lucken 1987; Knott 1987; Konzak 1987; Simmonds 1989).
Traditional production of hybrid wheats using manual male sterilisation usually occurs in the controlled environment of a glasshouse. The first step involves the removal of the awns from the developing wheat head followed by the removal of the anthers from the female parent plants. The anthers from the male parent plants can then be manually brushed against the stigmas of the receptive female parent plants producing the controlled generation of hybrid seed (Simmonds 1989).
Hybrid wheat seed can also be produced using wheat plants where the female parent has been treated with a chemical hybridizing agent before anthesis to generate male sterile plants. The male and female (chemically treated) parent plants can then be planted in alternating rows in the field and the female parents wind pollinated. The disadvantage of this method over using a genetic system to control male fertility is the need to apply a chemical agent in the field (Lucken 1987).
Mutation breeding is a complementary method to traditional wheat breeding techniques and utilises methods to induce mutations, usually in the seed. These include exposure of seeds to ionizing radiation, ultraviolet radiation or chemical mutagens (Konzak 1987).
Single seed descent is a method used to rapidly select inbred lines with desirable characteristics. A single seed is taken from each plant, usually starting at the F2 generation, and used to produce the next generation. It is reported that, while the distribution of traits across the lines varies greatly in the F2 generation, the lines become more similar by the F6 generation (Knott 1987). This method can be used by breeders to reduce the number of plants propagated in the early generations before testing of the lines begins.
Other characteristics selected for in wheat breeding include reduced height of plants, nuclear male sterility and other advantageous alterations to plant physiology (Konzak 1987). In Australia, wheat breeding has been focussed on the production of varieties which combine high yield, acceptable quality characteristics and resistance to diseases (Simmonds 1989). A 2006 report from the Australian Bureau of Statistics (ABS) has summarised wheat breeding in Australia (ABS 2006). Richards & colleagues also discuss the history of wheat breeding in Australia and some of the challenges and potential targets for wheat breeding to provide wheat cultivars adapted to Australian conditions into the future (Richards et al. 2014). Long-term agronomic challenges are key targets for Australian wheat breeding – yield, drought, frost, disease resistance and salinity (GRDC 2011).
Valuable genes for disease resistance have sometimes been derived from wild wheat species including rust resistance genes from wheatgrass (Thinopyrum elongatum (Host.) DR Dewey (=ponticum) (Agropyron elongatum) for a rust resistant variety from the United States (Smith et al. 1968). . The modified translocation, 3Ag#3, is present in Australian cultivar (cv.) 'Torres' (Mackay 1983), while another modified translocation, 3Ag#14, also providing rust resistance is present in the Australian cultivars 'Skua', 'Sundor' and 'Vasco' (Martin 1984; Ellison 1984; Brennan et al. 1987).
Comprehensive reviews of plant breeding methodologies, including for those used for wheat, are presented by Simmonds (1986) and also by Allard (1999). A collection of winter cereals, including wheat varieties and advanced breeding lines from Australian and international breeding programs, is held at the Australian Winter Cereals Collection, Tamworth, which will be incorporated into the Australian Grains Genebank in Horsham (Stoutjesdijk 2013). This collection also includes wild relatives of wheat. Further information regarding the varieties available for planting in Australia is available online through a number of sources (GRDC 2015a; WQA 2015; Trainor et al. 2015).
In Australia, limited and controlled releases of genetically modified wheat have been conducted since 2007. Modifications have included increased tolerance to abiotic stressors, altered composition, improved grain quality, yield stability, nutrient utilisation and disease resistance (see OGTR website for more information). Similarly, in Europe, Canada and the United States, wheat with modifications for increased herbicide tolerance, abiotic stressor tolerance, increased yields, pathogen resistance, and increased carbohydrate and protein content have been trialled (see the Canadian Food Inspection Agency website, the European Commission GMO Register or the United States Department of Agriculture Animal and Plant Health Inspection Service websites for more information).
Section 3 Morphology
A brief description of the morphology of the wheat plant is provided below. More detailed descriptions and diagrams are available (Bowden et al. 2008; Kirby 2002; Setter & Carlton 2000b).
3.1.1 The stem
The mature wheat plant consists of a central stem from which leaves emerge at opposite sides (Figure 3). It is made up of repeating segments, called phytomers, which contain a node, a hollow internode, a leaf and a tiller bud found in the axil of the leaf (Kirby 2002). The leaf sheath wraps around the stem providing support to the shoot (Setter & Carlton 2000b). The stem terminates in the ear of the wheat plant.
The leaf structure consists of the sheath and the leaf blade which form from separate meristems (Figure 3). At the base of the leaf blade, where it joins the sheath, are a membranous ligule and a pair of small hairy projections known as auricles, which are characteristic of cereal species (Kirby 2002). Leaves are produced on alternate sides of the stem (Setter & Carlton 2000b). The final leaf before the ear is called the flag leaf. In spring wheat varieties the length of leaves increases from the base until one or two leaves before the flag leaf (Kirby 2002).
The leaf tissue is made up of three tissue types. The cell types making up the epidermis differ on either side of the leaf with the epidermis on the underside of the leaf having fewer cells. Both epidermal layers are covered with an epicuticular wax. The mesophyll is enclosed by the epidermal layers and transected by the vascular tissue (Kirby 2002).
Figure 3: The stem and leaf structure of a mature wheat plant. Reproduced in original form with permission (Setter & Carlton 2000b).
Tillers are lateral branches which are produced off the main stem of the wheat plant (Kirby 2002). They produce leaves on opposite sides of their central stem in the same manner as the leaves of the main stem are produced and are also able to produce an ear at their terminal (Setter & Carlton 2000b). Not all tillers will survive and produce an ear and this is thought to be due to competition for light and nutrients (Kirby 2002).
3.1.4 The roots
A mature wheat plant has two distinct root types. The seminal roots develop from the root primordia contained within the grain and are the first root type to emerge (Kirby 2002; Setter & Carlton 2000b) . The nodal roots emerge at the same time that tiller development starts. The root system can grow 1-2 m deep, but most roots are concentrated in the top 30cm of soil (Kirby 2002).