School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemThe role of early vigour in the adaptation of canola to a Mediterranean climate( 2002)Dryland crop production in the Victorian Wimmera is restricted by water availability, particularly after flowering, as generally occurs in a Mediterranean climate. Vigorous early crop growth has been suggested as an important trait for crop adaptation to this climate. Field experiments, simulation modelling and pot experiments were carried out to study the importance of early vigour (EV) for canola growth and yield in a Mediterranean environment. The field experiments demonstrated that EV influences subsequent crop growth. In the first field experiment comparing twelve canola cultivars, EV was significantly correlated to shoot dry matter at the beginning of flowering. Similarly, the difference in EV between a vigorous (Dunkeld) and a less vigorous (Pinnacle) cultivar was maintained throughout the growing season in three other field experiments. Differences in EV due to increased density also influenced crop growth, with the exception of high density, which had higher growth early in the season but was equalled at later growing stages by normal density. Differences in EV also affected the competitive ability of canola. The study has shown that there are various options available to improve the competitive ability of a crop. These are selecting a more vigorous cultivar or adjusting management by increasing the sowing density or by applying nitrogen. Differences in EV due to cultivar or density differences did not result in different water use at any of the growing stages in the field experiments. The simulations also showed that until the beginning of flowering there were no differences in total water use between crops of vigorous and less vigorous cultivars. Thus, this study has found that vigorous cultivars did not deplete the soil to a greater extent up to flowering than less vigorous cultivars. As Dunkeld produced greater shoot dry matter than Pinnacle, it had greater water-use efficiency. This was the consequence of greater transpiration efficiency as well as lower proportion of evaporation in evapotranspiration of Dunkeld, as shown in the simulations. Dunkeld was able to compensate for low density because it was vigorous from early in the growing season. Therefore, differences in EV due to management (sowing density, nitrogen fertilizer) did not influence yield. For Pinnacle, however, which had much slower growth, differences in EV did influence seed yield. High EV due to genotype resulted in greater or similar yield than low EV in the experiments and simulations. Differences in EV in an indeterminate crop such as canola seem to influence yield less than in a determinate crop such as wheat in a Mediterranean climate. Although the simulations have shown that low EV crops can have similar yields than high EV crops under the current management practices used, there were no yield reductions found due to vigorous early growth. Pot experiments showed that differences between Dunkeld and Pinnacle at the very early stages can be attributed to differences in radiation-use efficiency alone. Later, when competition between plants commences, both specific leaf area and partitioning of biomass seem to take effect adding to the existing differences in shoot dry matter per unit area and leaf area index between the two cultivars.
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ItemResynthesis of Brassica napus to provide useful traits for canola breeding( 2002)Canola is an important oilseed crop in Australia. Lack of genetic variation in canola for economically important traits hampers efforts to breed for elite lines competitive on the world markets. An alternative approach for improvement of canola is to tap the genetic variation present in the progenitor species of B. napus and transfer novel genes into canola lines. In this study, the progenitors of B. napus (B. rapa and B. oleracea) were screened for tolerance to manganese toxicity, shatter resistance and blackleg resistance with the aim to transfer these traits through resynthesis of B. napus. Genotypes of B. rapa displayed the best tolerance to manganese toxicity in a hydroponics sand culture bioassay. The inheritance pattern of tolerance to manganese toxicity in B. rapa strongly suggested the presence of a single gene control for this trait with incomplete dominance. In the studies to screen for siliqua shatter resistance, increased energy levels of siliqua shatter were observed in B. oleracea genotypes compared to B. napus. To identify molecular markers linked to shatter resistance in B. rapa, a segregating population derived form the cross DS-17-D by Torch, was screened using RAPD, ISSR and AFLP techniques. Two AFLP markers were identified that showed strong linkage to shatter resistance. To identify new sources of blackleg resistance, several B. oleracea and B. rapa accessions were screened in field trials. B. oleracea genotypes were more resistant to blackleg showing a high level of survival (30-80%) compared to the genotypes of B. rapa. Interspecific crosses were made between selected genotypes of B. oleracea and B. rapa, and hybrid embryos were rescued using ovary culture. The genotypic profiles of hybrids and the parental lines were ascertained with RAPD primers and flow cytometry. Synthetic B. napus lines were backcrossed to canola to begin the introgression of specific traits of interest into canola germplasm. Preliminary screening of synthetic hybrids derived from interspecific crosses between B. rapa and B. oleracea showed the same degree of manganese tolerance and L. maculans resistance as the resistant parental genotypes. BCi to B. napus retained the transferred resistance and tolerance traits of the synthetic hybrids. The presence of SCX-7 marker linked to one of the shatter resistance genes sh2 in the synthetic hybrid SH2 indicated the successful transfer of shatter resistance from B. rapa as this marker expressed itself in B. rapa genotype Chitose which is one of the parents of synthetic hybrid SH2. Results of this study show that synthetic alloploids are of great practical value as they provide an extended range of genetic variability combining the variations available in diploid parents.
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ItemDevelopment of molecular markers for the genetic improvement of lentil( 1999)Molecular markers were applied in this study to explore two essential aspects of cultivated lentil breeding programs: 1) the genetic resources available within the species and within the Lens genus, including the wild relative species and 2) the genetics of resistance to foliar infection by the fungal pathogen Ascochyta lentis, examining both the host resistance mechanism and the diversity of the pathogen. 1) The genetic diversity within the Australian lentil breeding program was shown to be limited, representing a potential major constraint to the genetic improvement of lentil in Australia (Ford et al., 1997). Wild relative germplasm may be a source of variability for the transfer of desirable traits into superior breeding lines. A phylogeny of the Lens genus, based on nucleotide data from the 5S rRNA mutligene family, showed that L. culinaris ssp. orientalis was the closest and L. odemensis was the next closest related taxa to the cultivated species. Due to their close genetic relationships and crossibility with the cultivated species, these (sub-) species may provide new sources of useful germplasm. 2) The current major limiting factor to lentil production in Australia is ascochyta blight disease caused by Ascochyta lentis. A major strategy for disease control is breeding for resistance. However, pathogen variability is a threat to resistance breeding in that the pathogen may quickly adapt to overcome host resistance. A. lentis isolates in Australia were found to be as genetically variable as A. lentis isolates originating outside Australia. The genetics of resistance to foliar infection of A. lentis within cultivar ILL5588 was determined to be controlled by a single major dominant gene (AbRI). Molecular markers linked to the AbRI gene locus were identified with bulked segregant analysis and random amplified polymorphic DNA analysis. The two closest flanking markers being 6 and 14 cM away from AbRI. These markers were investigated for the potential to produce stable sequence characterised amplified regions (SCARS) for the identification of the AbRI locus in other lentil germplasm and the consequent use of the AbRI locus in marker assisted selection.
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ItemThe development of molecular markers for shatter resistance in Brassica rapa L( 1999)Shattering is a major problem in canola production worldwide and especially in Australia because the crop matures in summer under hot and windy condition. Little variation for shatter resistance is available in Brassica napus, which is the major canola species. Considerable variation for shatter resistance is available in B. rapa, which is a relatively minor oilseed species. Shatter resistance is being transferred from B. rapa into B. napus by backcrossing. Screening for shatter resistance in canola breeding programs is currently based on mechanical tests of individual siliqua strength. The test, which is time consuming, can only take place with completely dry and mature siliquae. This study aimed to identify molecular markers linked to shatter resistance in a population segregating for this trait, and to develop specific and robust primers to be used in a routine test, for screening for shatter resistance in canola breeding programs. Crosses were made between the shatter-susceptible Canadian cv. Torch of B. rapa L. ssp. oleifera (Metzg.) Sinsk and a shatter-resistant Indian line, DS-17-D of B. rapa L. ssp. oleifera var. Brown Sarson (Singh) Prakash. Shatter resistance was found to be a recessive trait controlled by two major genes, designated sh1 and sh2. The phenotypic segregation in the F3 population of this cross fitted a Mendelian ratio of 12: 3: 1 for shatter-susceptible (S), intermediate-shattering (M) and shatter-resistant (R) respectively. Further evidence from the phenotypic segregation in 19 F3 families was in agreement with the FZ ratio of 12: 3: 1. Bulked segregant analysis coupled with random amplified polymorphic DNA (RAPD) analysis was applied to both F, and F3 populations of the cross. Three RAPD markers, designated RAC-3, RX-7 and SAC-20, were identified from the F3 population. RAC-3 and RX-7 appeared to be linked in coupling to sh1 and sh2 at approximate distances of 13 cM and 20 cM respectively, whereas SAC-20 appeared to be linked in repulsion to both of these alleles at approximately 20 cM. The linkage of SAC-20 to both loci supported a theory of chromosome duplication during evolution of the B. rapa genome. Three pairs of sequence characterized amplified region (SCAR) primers designated SCAC-3, SCX-7 and SCAC-20, 21 to 24 bp in length were designed based on the sequences of RAPD markers RAC-3, RX-7 and SAC-20 respectively. Only the SCX-7 primer produced polymorphic markers corresponding to RX-7 amplified from 'shatter-resistant' DNA individuals, while SCAC-3 and SCAC-20 produced amplified products corresponding to RAC-3 and SAC-20 to both shatter-resistant and shatter-susceptible DNA individuals. SCAR SCX-7 was subsequently tested with other Brassica populations including B. napus and BC,F, of B. napus introgressed with B. rapa to investigate the presence of the sh2 gene. All lines from the backcross populations possessed the expected SCX-7 marker with the exception of one line where the marker was absent. This indicated that shatter-resistant gene sh2 was present in all these populations except for the one line.
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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.