School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemA molecular genetic study of seed dormancy in aegilops tauschii and expression of sprouting resistance in common hexploid wheat( 2004)The wild wheat relative Aegilops tauschii, has been identified as a useful source of preharvest sprouting (PHS) resistance for hexaploid bread wheat. Seed dormancy, a major contributor to PHS resistance, was shown to be partly expressed in hexaploid wheat derived from direct hybridisation between Triticum aestivum and Ae. tauschii. The enhanced seed dormancy possessed by the Ae. tauschii derived direct-cross wheat lines was manifested by embryo and seedcoat related mechanisms. The embryo related mechanism could not confer full expression of dormancy without the presence of seedcoat related factors, suggesting that the two mechanisms may be independently inherited. The presence of seedcoat related dormancy however, was not associated with the red seedcoat phenotype, which has traditionally been associated with PHS resistance in wheat. Red pigmentation of the seedcoat is likely to be "involved in the extreme dormancy possessed by Ae. tauschii but does not preclude partial expression within a white seedcoat background. The ability of Ae. tauschii derived wheat lines to enhance seed dormancy may have potential economic benefit to breeding for PHS resistance in white wheat varieties. Presently, white wheat varieties grown in the sprouting susceptible regions of Australia possess inadequate protection, costing the industry up to $100M annually. Inheritance of seed dormancy in Ae. tauschii was found to be controlled by one or two major genes which were influenced by minor genes and/or environmental factors. These results are consistent with the findings of several previous reports. Inheritance was shown to be dominant at the F3 grain generation, consistent with the generally dominant nature of dormancy possessed by red seeded genotypes. However, preliminary assessment of individual F2 seeds indicated recessive control of dormancy. Because genes possessed by the maternal tissues of the seedcoat do not segregate until the F3 seed generation, the F2 recessive model may be indicative of separate genetic control for the embryo related dormancy mechanism(s). Based on the above inheritance information, a bulked segregant analysis approach was initially undertaken for the development of linked molecular markers for seed dormancy. One microsatellite marker on chromosome 1D produced polymorphism between resistant and susceptible DNA bulks. A mapping approach was subsequently undertaken, revealing two significant QTL mapping to chromosome 1D. The putative QTL for seed dormancy will relate to the embryo component of dormancy, as the trait data employed related to the F2 seed generation, which was segregating for embryo related genes. The D genome of hexaploid wheat presently possesses the fewest QTL for PHS resistance of the three contributing genomes. Within the D genome, chromosome 1D was poorly represented in the literature. As such, 4e. tauschii represents a potential to bolster numbers of QTL for sprouting resistance in hexaploid wheat. Given the homology between the D genomes of Ae. tauschii and T aestivum, the microsatellite markers identified, flanking the putative QTL, will likely be transferable to hexaploid bread wheat. Seed dormancy is strongly influenced by conditions during growth. As such, unambiguous selection through use of molecular markers will expedite the introgression of this economically important trait into elite wheat cultivars.
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ItemThe effects of post-anthesis heat stress on wheat yield and quality( 1996)Post-anthesis temperature is a major determinant of wheat yield and quality. Post-anthesis temperatures in the moderately high range (ca 25-32C) are known to reduce grain yield but increase bread wheat quality, whereas very high (>32C) temperatures are known to significantly reduce both yield and quality. In Mediterranean and continental climates, such as Australia and the US., wheat is exposed to moderately high temperatures throughout most of the grain filling period, and very high temperatures may occur for an average 8% of grain growth. This thesis is primarily concerned with examining the effects of very high temperature on wheat yield and quality. Specifically, the study was designed to: 1) quantify the effects of short (3-5 day) periods of very high temperature on wheat yield and quality; and 2) determine the extent of genotypic variation in response of wheat yield and quality to very high temperature. Two varieties of wheat differing widely in heat tolerance were selected from 75 cultivars of wheat that were screened for tolerance to very high temperature. These two varieties (Oxley and Egret, heat sensitive and heat tolerant, respectively) were exposed to a variety of heat treatments in order to determine whether varietal differences in heat tolerance were maintained for heat treatments occurring at 3) different stages of grain growth and for 4) varying durations of heat stress. The 5) interaction of moderately high and very high temperatures was examined in order to determine whether cool temperatures following severe heat stress could alleviate the deleterious effects of very high temperature on yield and quality. In order to 6) examine the importance of acclimation to heat stress and to 7) establish a repeatable selection methodology, the impact of sudden increases to a high maximum temperature was compared with more gradual (6C h-1) rises to the same high temperature (40C). For each of the experiments 3 to 7 (above) results are presented for the effects of heat stress on: a) the accumulation of grain dry matter and water during grain growth; b) the accumulation during grain growth of total protein and its functionally-important fractions (SDS-soluble and SDS-insoluble polymer [glutenin], monomer [gliadin] and albumin/globulin), as determined by size-exclusion high-performance liquid chromatography and c) dough mixing behaviour using the 2-g mixograph. It is concluded that: 1) wheat genotypes vary widely in their responses of yield and quality to short periods of very high temperature; 2) the response to heat stress varies with the timing of stress: yield was reduced more by early than late-applied stress, whereas dough strength tended to decline most markedly in response to heat stress applied towards the end of grain filling; 3) both grain yield and dough strength declined linearly with increased duration of heat stress; 4) in a heat sensitive variety, moderately high and very high temperatures during grain filling each reduced grain yield and dough strength: cool temperatures following exposure to very high temperature did not reduce the effects of very high temperature on either yield or quality; 5) some varieties of wheat appear to acclimate rapidly to heat stress: a gradual (6C h-1) increase from ca 20-40C lessened the impact of heat stress on yield and quality when compared with a sudden increase over the same temperature range. These results are discussed with special reference to their implications for: 1) selecting and breeding for heat tolerance in wheat; 2) predictive modelling of the effects of very high temperature on wheat yield and particularly quality.
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ItemGenotype and environmental influences on phasic development in wheat (Triticum aestivum L.) and the expression of yield components, especially spikelet number per head( 1995)The variation in, and the influence of, certain environmental factors on preanthesis phases of development in wheat was examined with particular reference to the number of spikelets produced per head. When the pre-anthesis phase was divided into three phases; the Ieaf initiation, spikelet initiation and culm elongation phases, considerable cultivar variation was found in the durations and rates of the three phases. A cultivar was found that departed from the general negative correlation between rate and duration of spikelet initiation giving possible scope for breeding for increased spikelet number without altering the duration of spikelet initiation. Variation in the rate and duration of the three development phases was also found for a selection of diploid and tetraploid wheat. For these species, spikelet number was found to be more closely associated with the duration of spikelet initiation. This character could be of use in long-season wheat cultivars. When the effects of photoperiod and light intensity on wheat phasic development and spikelet number were compared, photoperiod was found to have more influence. The transfer of wheat cultivars between long and short photoperiods at double ridge and terminal spikelet determined that the rate of development was influenced by a "memory" effect; both prior and current photoperiods influenced the rate of development. It was also noted that initial exposure to long photoperiod could have a sustaining effect on wheat development. Subjecting wheat lines to increased temperature increased the durations of development, in terms of thermal time, indicating that the relationship was not linear. The durations of pre- and post-terminal spikelet phases were found to respond differently to temperature. A selection of 6 wheat cultivars, varying in time to anthesis were grown in the field and it was found that photoperiod responses exerted the major influence on the durations of development. Basic development responses and vernalisation were found to exert comparatively less influence on development. The importance of basic development responses were not discounted as a means for breeding wheat cultivars for specific environments.
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ItemCytology and fertility of wheat-rye (Secale) hybrids with induced pairing between homoeologous chromosomes( 1986)Studies were made of the influence of individual rye chromosomes and their arms on crossability, chromosome pairing and pollen and spikelet fertility in hybrids of hexaploid wheat cv. 'Chinese Spring' and its two homoeologous pairing mutants, ph2a and phIb , each with six wheat (cv. 'Holdfast') rye ('King II') chromosome addition lines and their telocentrics. Crosses were also made of the three 'Chinese Spring' parents each with seven rye (Secale) accessions, including different species. Studies were made of crossability, hybrid viability, pollen and spikelet fertility, and chromosome pairing in both the amphihaploids and amphidiploids from these crosses. Studies were also made of crossability, chromosome pairing and pollen and spikelet fertility in the F1 of crosses of the three 'Chinese Spring' wheats with both an octoploid and a hexaploid triticale. In these studies new information was being sought, both on genetic and evolutionary affinities between rye and hexaploid wheat and on approaches for the more efficient exploitation of genetic variation in the rye gene pool for wheat improvement.