Agriculture and Food Systems - Theses

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    Nitrogen fixation by Casuarina oligodon agroforestry in the Papua New Guinea central highlands
    Wemin, Johnny Minga ( 2006)
    Casuarina oligodon L. Johnson is a multipurpose tree species grown in the highlands of Papua New Guinea (PNG). The integration of C. oligodon into agricultural systems is seen by villagers as means of restoring soil fertility, controlling soil erosion, providing shade for crops and producing fuel wood and building materials. Biological nitrogen fixation by C. oligodon through symbiotic relationships with Frankia (micro-organism) under field conditions in short (5-10 years) and long (11-15 years+) fallows in the PNG central highlands was investigated using the 15N natural abundance technique. Results from the study showed that as much as 70% of N in C. oligodon was derived from the atmosphere. The rate of N2 fixation was relatively low in short fallows of casuarina and increased as the trees aged in the long fallows. A rate of N2 fixation up to a maximum of 36 kg N ha -1 year -1 was estimated based on commonly practiced tree stocking rates and field conditions in the PNG highland areas. Although casuarina fallows tend to accumulate higher total N and C compared with equivalent period of grass fallows, the amounts of N and C in the surface soils of all systems under the study showed no significant difference. The amounts of total N and C under long fallows of casuarina (11-15 years+) were generally greater than short fallows of casuarina (5- 10 years). A significant proportion of the total N was stored in the above ground biomass of trees that were more than 10 years of age. Management of the standing biomass, particularly when the fallow is converted back to the cropping phase, is therefore critical in ensuring that the farmers are able to gain maximum benefit from the fixed N. Whilst the removal of stem wood for use as fuel or building material may be an important product of the agroforestry system, retaining the foliage, small branches and bark on the site is vital in sustaining agricultural productivity.
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    Studies of Ascochyta rabiei in Australia
    Pradhan, Prashanti ( 2005)
    Ascochyta rabiei (teleomorph: Didymella rabiei) which causes ascochyta blight is the most serious disease of chickpea (Cicer arietinum) in Australia as it causes significant losses in crop yield and quality. Although A. rabiei is heterothallic and genetically diverse elsewhere in the world, a study carried out on Australian isolates collected between 1995 and 2000 identified only one mating type and a low level of genetic diversity within the Australian A. rabiei population. In 2002, ascospores of Didymella rabiei, the sexual state of A. rabiei, were trapped in a discharge chamber, from chickpea stubble naturally infected with ascochyta blight in Western Australia. Examination of the stubble revealed pseudothecia typical of Didymella rabiei. The reported presence of the teleomorph in Western Australia indicated that the second mating type had been introduced into Australia or that the pathogen was capable of a low level of homothallic compatibility, previously undetected. The aims of this research were, to undertake a new survey of Australian A. rabiei isolates, to test for the presence of the second mating type, to determine if there has been a change in the diversity of the Australian population and to investigate if pathogenic variability was displayed among isolates. Sixty-seven isolates collected from chickpea fields in South Australia, New South Wales, Queensland, Victoria and Western Australia during the 2003 cropping season were single spored and confirmed as A. rabiei using a PCR test. The isolates were typed for mating type using MAT gene specific PCR primers and compared with tester isolates from USA. This test revealed that all the 67 isolates belonged to mating type 2 (MAT 1-2), thus, the presence of mating type 1 (MAT 1-1) in Australia could not be confirmed. Sequence Tagged Micro Satellite (STMS) markers were used to examine the genetic diversity of the A. rabiei isolates. The isolates were assessed for allelic variation at 19 microsatellite loci, each of which amplified a single locus. Seven of the loci were polymorphic across all the 67 isolates, while the remaining twelve were monomorphic. Based on the allele profiles at the seven polymorphic loci, 19 distinct A. rabiei haplotypes were identified with a total of 33 alleles. One haplotype constituted 35.8 % of the population and was found in the collections from South Australia, New South Wales, Queensland and Victoria. Cluster analysis did not show a clear distinction between isolates based on the state from which they were collected. Polymorphism across the 19 microsatellite loci revealed a slight elevation in diversity in the 2003-2004 population (Ht = 0.07; compared to 0.02 in the 1995 to 2000 collection) and an increase in the number of haplotypes compared with that detected in the previous study of Australian isolates. To examine the pathogenic variability of the Australian population of A. rabiei, nine isolates were inoculated on five chickpea differentials, ranging from highly susceptible to resistant, under controlled conditions optimal for A. rabiei growth and infection. Eight of the isolates were virulent on the susceptible and intermediate chickpea cultivars but not the resistant cultivar and one isolate was only virulent on the susceptible cultivar. Based on these results the isolates were classified into two pathotype groups. The results obtained from the study of the population structure and the pathogenic variability of A. rabiei in Australia will enable the Australian chickpea breeders to understand the A. rabiei population better for formulating management and breeding strategies.
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    Effects of organic applicants in a southern Victorian vineyard
    Lakey, Vincent G ( 2007)
    Mulch is a material applied to the surface of the soil to reduce weed growth and reduce soil moisture loss through evaporation from the soil surface. The use of organic mulches will alter the soil environment. This alteration may include reducing soil temperature fluctuations, increasing soil organic matter, increasing soil microflora populations modifying soil chemical properties and increasing soil moisture retention. An experiment was conducted to compare composted green waste mulch and barley straw mulch with herbicide as alternative means of maintaining the undervine strip in a cool climate vineyard. Plant and soil responses to the different undervine treatments were monitored. Grapevine budburst was retarded, however, by the fourth week of vine growth there were no observable differences in grapevine growth stage. Both mulches stimulated grapevine growth and increased yield, with the compost mulch increasing vegetative growth with respect to fruit yield. The fruit quality parameters juice pH and titratable acidity were not significantly altered by the different undervine treatments. In the second year of the experiment the juice soluble solids were lower on the straw mulched grapevines. The compost mulch increased soil pH and carbon levels. The straw mulch improved soil water retention and the mass of soil fungal hyphae. Both mulches increased soil cation exchange capacity. The straw mulch increased soil exchangeable Mg to a greater extent than was predicted from straw nutrient content. The significant variations seen in soil cation content under the mulch treatments were not observed in the tissue analysis. Mulch can be used in a cool climate vineyard to increased yield without deleterious side effects.
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    Investigation of boron toxicity in lentil
    Hobson, Kristy Bree ( 2007)
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    RNA-mediated virus resistance for carrot (Daucus carota var.sativum) and celery (Apium graveolens var.dulce)
    McCormick, Nina ( 2006)
    Carrot virus Y (CarVY) and Celery mosaic virus (CeMV) are potyviruses that infect carrot (Daucus carota var. sativum) and celery (Apium graveolens var. dulce). The viruses cause severe yield losses and cost the Australian horticulture industry millions of dollars each year. For the control of CarVY and CeMV, the Australian carrot and celery industries recommend planting resistant varieties together with cultural crop management measures. However, conventional breeding for resistance, involving the movement of resistant genes from a proven source to an elite background, is not possible due to a lack of naturally available resistance sources. Increased knowledge of the molecular genetics of plant virus invasion and natural host defence has resulted in the development of novel methods of control. In this study, RNA silencing was applied to engineer carrot and celery plants resistant to CarVY and CeMV respectively. Resistance transgenes were designed for each virus to produce a double-stranded, intron-containing hairpin RNA (ihpRNA) structure, containing 258 nucleotides of the 3' region of the CarVY nuclear inclusion (NI) 'a' protein and 313 nucleotides of the 5' end of the CeMV NIb protein. Subsequently, intron-containing hairpin RNA transgenes, incorporating CarVY NIa sequence, were expressed in six carrot lines of the commercial carrot cultivar 'Crusader'. Three of the lines contained single transgene integrations at a single locus. Attempts to integrate a CeMV-containing ihpRNA transgene into the celery genome were unsuccessful most probably due to the recalcitrant nature of celery tissue towards Agrobacterium-mediated transformation.
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    The 'Boxing Day' tsunami - effects and responses among farmers in Maldives
    Shabau, Ibrahim ( 2006)
    The `Boxing Day' tsunami of 26 December 2004 devastated coastal areas of eight countries within and surrounding the Indian Ocean, including Maldives. More than 200,000 coastal inhabitants in the affected countries lost their lives; those remaining in the affected areas faced severe disruptions and hardships. Maldives is an atoll island nation in the Indian Ocean. The exclusively low-lying nature of the islands of Maldives make them vulnerable to natural hazards such as tsunamis, wave surges from tropical storms and potential sea-level rise. Agriculture plays an important role as a source of income for the rural population of Maldives. It is carried out in mixed home gardens, settled rain-fed cropping fields, bush fallow shifting cultivation areas and year-round horticultural production fields across agricultural islands of Maldives. Waves of over four metres in height, resulting from the tsunami, swept across the atolls and islands of Maldives. The islands suffered massive physical destruction to structures and facilities. Farming communities, in particular, lost homestead trees, crops, buildings and infrastructure through wave action and inundation. Agricultural lands were inundated, and inland soils and water reservoirs were contaminated with seawater and detritus. This study, undertaken a year after the tsunami, sets out to determine the main effects of the tsunami on the livelihoods of the farming communities of Maldives. Social research methods were used to catalogue the immediate and long-term strategies that affected farmers adopted to deal with losses of food, income and productivity of lands. Case studies of three affected islands were used to explore the effects the tsunami had on livelihoods of farmers on these islands, according to the residents. Assessments were carried out on soils and groundwaters of case study islands to measure the possible long-term effect of the tsunami on farming areas. The study revealed that continued disaster aid, and improvement of people's awareness of natural hazards and disaster relief processes among farmers and agency staff are important for the sustainability of agricultural livelihoods in Maldives. Building institutional capacity in monitoring and assessment of soil and groundwater quality were also noted to be essential. Suggestions for assisting recovery of farming households are discussed, based on views of farmers, and these could be of use to international development organizations and local institutions, to assist in future planning and development of areas that are vulnerable to natural hazards in Maldives.
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