My Account: Log In | Join | Renew
Table of Contents
Select All Chapters
Human population projections for the remainder of this century are a grim reminder that world food production must continue to increase. Wheat (Triticum aestivum L.), as a dietary mainstay for approximately one-third of the 4.5 billion people in the world, figures prominently in food strategies for the future. Arable land most suited for crop production is already being farmed. New land that might be used for cropping generally is in regions where there are powerful production constraints and excessive development costs. Extensive loss of prime cropland to urban and industrial development, roads, erosion, and misuse in the USA and most developed countries continues to occur (Wortman, 1982). New cropland probably could do no more than compensate for such losses of existing cropland. If so, production increases to feed the more than 6 billion people projected for the year 2000 must come mainly from higher yields per unit area.
Cultivated wheat (Triticum L. species) are autogamous, disomic polyploids characterized by phenotypic buffering and tight linkage inside terminal chiasmata. However, wheat also has great evolutionary potential through diploidization of homomeric loci and alien gene transfers. Wheat improvement by conventional methods implies the search for a superior genotype or group of related genotypes for a given agroecological niche. This search may rely on the creation of genetic variation by means of simple, convergent, or composite crosses, or by the induction of mutations. Then, the outcome mainly depends on the efficiency or duration of the process of population management and selection. Alternatively, the ambition may be to direct variation by means of strict backcrossing rather than relying on selection. It should, however, be very clearly realized that most efficient breeding systems adapted for wheat will require alternating or simultaneous procedures for recombination and selection. The characteristic inheritance pattern of polyploid wheat and the impossibility of realizing recombination potentials for more complex goals, within reasonable plot or population size, calls for strategies accepting some kind of stepwise progress.
The novel approaches to wheat (Triticum aestivum L.) breeding that are discussed include wide crosses, somatic cell hybrids, doubled haploids, somaclonal variation, transformation, and N2 fixation. A large volume of work has been done on wide crosses in which the chromosomes of the alien species do not pair with wheat chromosomes. Many transfers have been made, particularly of disease resistance, but only a few have been used in commercial cultivars. Somatic cell hybrids have been limited to a relatively few species in which the hybrid cells can be induced to divide and regenerate into plants. Wheat has proven to be particularly difficult to work with. The most promising method of producing doubled haploids is through anther culture. However, the success rate is low and highly variable from cross to cross. Regenerated plants from wheat tissue culture show a remarkable range of somaclonal variants. While some are unstable, others are stable and have commercial potential. The use of the tumor-inducing (Ti) plasmid of crown gall bacterium (Agrobacterium tumefaciens) to introduce foreign DNA into a plant species has been studied extensively. Although it has been used successfully in dicots, the procedure has not yet been successful in wheat. Nitrogen fixation has been shown to result from associations between certain bacteria and wheat. Although some positive results have been reported, the value of field inoculation is still uncertain. Many exciting developments are occurring but in most cases their potential value in wheat breeding is still unclear.
Genetic diversity is the foundation of all plant improvement programs. The use of specific cultivars and genetic stocks can be directly associated with major contributions in wheat improvement. ‘Chinese Spring’ provided the opportunity to better understand the genetics and cytogenetics of the wheat plant, while traits like daylength insensitivity and semidwarf stature were contributed by cultivars such as Gabo and Norin 10-Brevor, respectively. Other major advances in wheat breeding cannot be identified with specific sources of germplasm; agronomically mediocre cultivars have combined to provide outstanding progeny. If future generations of scientists are to further improve the wheat plant they must have adequate sources of usable germplasm. Much diversity is being lost with the ever-increasing rate of genetic erosion. An obligation must be assumed by all associated with wheat research to collect, preserve, and evaluate existing germplasm and develop an effective means of disseminating information and genetic stocks on a worldwide basis.
Disease resistance stability is of paramount importance in striving for wheat (Triticum aestivum L.) production stability wherever high disease hazards exist. While classical genetical analysis for disease resistance is the standard system used to search for newer genes or gene combinations, two additional methods are presented herein. These are (i) multilocation testing for a particular disease and determining resistance on the basis of low average coefficients of infection (ACI), and (ii) choosing varieties for resistance breeding on the basis of the historical performance of durability. In addition, three standard systems for incorporating resistance genes are suggested. These are (i) breeding for broad-based disease resistance utilizing three-way and double crosses and the pedigree or bulk/ pedigree method of selection; (ii) breeding for dilatory resistance, using recurrent selection; and (iii) breeding for multiline varieties utilizing genetic diversity for resistance through backcrossing. Any of these breeding systems, in search for resistance, should result in stable disease resistance.
The increase in cereal grain yield due to breeding has amounted to only about 1% per year. A breakthrough was made after a radical reduction of plant height. Semidwarf and short straw genotypes are superior in grain yield to dwarf genotypes. The increase of harvest index should be approached at a higher level of biomass than the present one. To obtain high yields it is essential to secure an optimum number of stems or spikes/m2 for each genotype and environment. Usually this is >600 spikes/m2. Optimum leaf area index (LAI) usually exceeds 6 m2/m2. Short stem genotypes have more pronounced differences in green area of laminae and spike than in green area of stem and sheaths. As the stem is shorter, the spike, i.e., glumes and/or awns should be larger and longer. Too large an LAI may exhibit a negative direct effect on grain yield, while leaf area duration (LAD) has been found to be more positively correlated with yield than any other green area parameter. The effectiveness of LAD is particularly expressed after the milk stage of grain development. Further increases of genetic potential for yield may be sought in the prolongation of the grain-filling period. Greater changes are needed in spike structure, especially in the direction of more spikelets/spike and more fertile florets/spikelet. The potential productivity of wheat has not yet been reached and the breeding for new high-yielding cultivars suitable for particular agroecological conditions is both promising and necessary.
Research progress in breeding systems based on cytoplasmic male sterility-fertility restoration and cereal hybridizing agents has advanced the development of superior hybrid wheats (Triticum aestivum L.). Commercial and experimental hybrid wheats produced through both systems have been undergoing extensive yield testing to critically assess their yield potential and agronomic and processing attributes or deficiencies. Questions regarding economic success of hybrid wheat, therefore, are comparative: What are hybrid yields relative to those of conventional cultivars and what are the costs of producing hybrid seed relative to profit from growing hybrids? This chapter describes some of the genetic variation, selection methods, and breeding procedures currently used to improve hybrid yields and reduce the costs of producing hybrid seed. The topics discussed include: (i) the comparative effects of seeding rates and seed multiplication ratios on hybrid wheat breeding and hybrid seed production; (ii) the genetic components of cytoplasmic male sterility-male fertility restoration and the breeding of fertility restorer lines; (iii) breeding to improve the cross-pollination potential of hybrid wheat parents; (iv) cereal hybridizing agents; (v) the performance of hybrid wheats; and (vi) breeding considerations in evaluating combining ability and identifying superior parents.