School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemEpidemiological and physiological studies of the effects of peach rosette and decline disease on the peach, prunus persicae L. Batsch( 1975)The incidence in the field of the disease peach rosette and decline (PRD), which is of considerable economic importance in the Goulburn Valley, causing fruit loss and tree death, was shown to increase from 0.9 to 91.3% in an orchard of cv. Golden Queen in 10 years. Similar results were found with the cv. Pullars Cling, in which infection increased from 1.5 to 29.7% over five years. The pattern of spread was mainly from infected trees to contiguous uninfected trees. This is consistent with the view that the main causal agent, prune dwarf virus (PDV), is transmitted only via the transfer of infected pollen : a previous finding in cherries which was confirmed in peaches. Prunus necrotic ringspot virus (PNRV) is the other virus always present in the field in PRD-infected trees. The mode of spread of PNRV is also by pollen. Within the tree, PDV moved erratically from the first infected limb, via phloem but not xylem, into the other limbs well in advance of the appearance of symptoms. Three months after flowering, PDV was detected in 65% of main limbs adjacent to the first infected limbs but in only 30% of limbs more remotely positioned on the tree. However, removing infected limbs within four weeks of flowering, when the initial infection was presumed to have occurred, did not prevent the movement of PDV into the rest of the tree. Laterals from peach trees infected with PRD were tested for the presence of PDV, using woody virus indicators (cvs Golden Queen, Italian Prune and Elberta seedling). Golden Queen was found to be a more reliable indicator for detecting PDV than Italian prune, as the presence of PNRV with PDV killed 71% of the Italian prune buds compared to only 34% of the Golden Queen buds. Golden Queen also developed more obvious foliage symptoms of PDV infection than Elberta seedlings. The probability of failing to detect PDV in infected field trees, using all three indicator plants, was higher in the first year of infection. The rate of spread of PRD was reduced in the orchard by preventing infected trees from flowering, either by removing obviously infected trees or by deblossoming. Removing infected trees resulted in a three-fold reduction in the spread of the disease in two seasons. Removing the flowers from infected trees before pollination reduced the spread of the disease by about half. This, only partial, control of the spread of PRD by tree removal or deblossoming was attributed to the presence of up to 14.3% of trees without symptoms being latently infected with PDV. It was observed that deliberate infection with PDV by pollen also resulted in a slow expression of the symptoms of PRD. The effects of PRD on the growth of young peach trees was obvious in the first three months of growth. There were considerable varietal differences in the severity of this effect. Those varieties based on cvs. Golden Queen or Levis Cling were more severely affected than the variety Elberta. The results from shoot elongation measurements agreed with those obtained from conventional growth analysis methods. These latter experiments showed that, after three months, the dry weight and leaf area of infected Golden Queen plants were reduced by 94%. The fruit yield from mature PRD-free trees was three times that of trees infected for the first season, even though symptoms were apparent only on one limb; and six times that from chronically affected trees infected for two seasons. The effect of virus infection on the photosynthetic ability of single, attached peach leaves was studied under laboratory conditions using infra red gas analysis. The constants derived from the equations describing the relationship between net photosynthesis (Pn) and both irradiance and CO2 concentration were used to analyse the effects of infection by PRD on photosynthetic characteristics of the leaf. The asymptotic value of Pn (Pmax) in young leaves was reduced 15% by PRD-infection, mainly through an increase in the "residual resistance" to 002 diffusion and a decrease of 23% in the parameter indicating photochemical efficiency. There was also evidence that the gas phase resistance was higher in infected leaves at low levels of irradiance. Dark respiration was 51% higher in infected leaves, but this difference was not significant. PRD did not reduce Pn in 60-day-old leaves, normal leaf senescence having a predominant and greater effect. It was concluded that PRD infection had its large effects on growth via a reduction in leaf area; the effects on the photosynthetic capacity per unit leaf area being minor. An effect of PRD infection on the translocation of 14C-assimilatesout of leaves was also observed. Infected leaves retained twice the assimilates than did uninfected leaves. It is concluded that the most promising methods of control of PRD include removal of infected trees, deblossoming suspected infected trees until diagnosis is confirmed, use of virus-tested plants, the gradual destruction of infected orchards and protecting young, healthy orchards from infection either by barrier crops or deblossoming the young plants until they reach an economic bearing age.
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ItemThe ecology and physiology of two species of Carduus as weeds of pastures in Victoria( 1977)Slender thistles (Carduus pycnocephalus and C. tenuiflorus) were introduced to Australia about the 1880s. They are now important weeds of pastures in much of southern Australia and are difficult to control with present methods. This study was undertaken to investigate several aspects of the ecology and physiology of the plants with the belief that a knowledge of some of these aspects, particularly of seed germination and seedling establishment, might disclose some "weakness" in the plants' growth which could be exploited to improve control measures. Because of confusion over differences between the two species which occur in Australia the initial step was to evaluate the morphological features which have been used to distinguish between the two species. Although they are very similar morphologically, cytological evidence confirmed that the two species were quite distinct and, in fact, had quite different evolutionary origins. Germination of seeds of slender thistles is controlled by three separate forms of dormancy; these are known as innate, induced and enforced dormancy. Dormancy ensures that the plants will survive in a Mediterranean-type climate and also colonize areas with quite different climates and, most importantly, survive natural catastrophes such as drought, fire, and flood. The germination of slender thistles in the field is confined to a very short period (about 6 weeks) after the autumn break in any year. This is a "weakness" in the plants' survival mechanism because they are vulnerable in that year, at least, to any treatment which can kill seedlings. The herbicide, diquat, was found to kill young seedlings of slender thistles and not affect seedlings of desirable pasture plants associated with the thistles in southern Australia. This treatment is economical and leads not only to a reduction in thistles but an increase of about 30% in pasture production. Several other aspects of the plants' growth were investigated. Slender thistles have early growth characters which give them advantages over more desirable components of pastures. They are more competitive than subterranean clover and ryegrass over a wide range of levels of nutrients, and the traditional approach to pasture improvement in southern Australia of applying superphosphate and sowing subterranean clover will encourage, not suppress, slender thistles. Since grazing animals generally do not eat slender thistles the presence of thistles in pastures at densities commonly occurring in Victoria considerably reduces pasture production.
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ItemThe water and nitrogen dynamics of a lucerne-based farming system in the Victorian Wimmera( 1998)The water and nitrogen (N) dynamics of a lucerne-based farming system (grazed lucerne-annual medic-ryegrass pastures grown in rotation with crops) was compared to continuous cropping (cereal, pulse and oilseed crops) in the Victorian Wimmera. The growth dynamics and CO2-exchange behaviour of lucerne in the pasture phase was also investigated. Soil profiles under lucerne-based pastures remained consistently drier during the year as compared to annual cropping. The amount of plant-available soil water (0.0 to 2.0 m) after 3 to 4 years of pasture was on average 48 mm less than after annual crops (wheat, field pea), most of which (81%) was extracted at depth (1.0 to 2.0 m). In the field, crop yields (canola, wheat) after lucerne were not reduced because water use by these crops was predominantly in the top 1.0 m of the soil profile. A wheat simulation study predicted that a small median yield loss of 0.4 t ha-1 (15%) could be expected for the first wheat crop grown after lucerne, although this yield penalty varied from 0 to 0.87 t ha-1 depending upon seasonal rainfall. The risk of a large yield penalty (>0.8 t ha-1) was low (5 years in 100). From simulation studies, the time taken to fully recharge the soil profile after lucerne to levels equivalent to that under continuous cropping was estimated to occur within 4 to 5 years. The contributions of N2 fixation by the legumes (lucerne, annual medic, field pea) to the N economy of the farming systems in this study depended upon the amount of dry matter production. N2 fixation by field pea (121-175 kg N ha-1 yr-1) was greater than pasture legumes (40-95 kg N ha-1 yr-1), although a large amount of N was removed in grain at harvest (115-151 kg N ha-1 yr-1). N2 fixation by lucerne (19-90 kg N ha-1 yr-1) was consistently greater than annual medic (2-56 kg N ha-1 because the effects of seasonal rainfall patterns on dry matter production were more pronounced for annual medic. Winter-cleaning of ryegrass in the pasture before cropping resulted in both a high legume content (85%) and generally increased N2 fixation (up to 55 kg N ha-1 yr-1 ). Despite some benefits in N fertility, large responses to N fertiliser were still observed in crops following pastures; in grain yield (increases of 0.33-0.55 t ha-1 for canola, 1.0 t ha-1 for wheat), protein (0.7-2.3% for canola, 1.3% for wheat) and oil yield in canola (124-205 kg ha-1). The growth pattern of lucerne was similar to that of annual species (annual medic, ryegrass) contained in the pasture, with the majority (70%) of growth occurring between July and November. The small amount of lucerne growth from summer to early autumn (December to March) was due to the small supply of water (rainfall and stored in soil) during this period. A more detailed study of two lucerne pastures during summer revealed that the plant was under considerable water stress; leaf:stem ratios increased (from 0.9-1.6 to 2.6-3.2), leaf folding and paraheliotropic movement decreased the amount of leaf area exposed to incoming radiation in the middle of the day (by 14-29%), and it was estimated that the some 75-83% of assimilated carbon was partitioned below-ground to roots and crowns.
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ItemPhysiological aspects of root growth of Eucalyptus pauciflora, subsp. pauciflora and Eucalyptus nitens( 1997)This thesis examined i) morphological and physiological effects of low soil temperatures on root growth of subalpine Eucalyptus pauciflora Sieber ex Sprengel subsp. pauciflora and montane Eucalyptus nitens (Deane & Maiden) Maiden, ii) determined the variability, and in particular the day/night variability, in root elongation, and iii) explored the physiological basis for such variability. A series of experiments were undertaken with seedlings of E. pauciflora and E. nitens grown at soil temperatures of 3, 7, and 13C, and where seedlings were transferred from one temperature to another. E. nitens grew faster than E. pauciflora at 7 and 13C, but E. pauciflora grew faster than E. nitens at 3C. E. pauciflora always produced greater total and white root length than E. nitens. E. nitens roots browned faster in response to lowering soil temperatures than E. pauciflora. The osmotic potential of the roots decreased with soil temperature, but more so in E. pauciflora than E. nitens. Proline was a prominent osmoregulant in roots of E. pauciflora and arginine in E. nitens roots. It is suggested that E. pauciflora is better adapted than E. nitens to root growth at low soil temperatures because it can keep roots white longer and can maintain lower root osmotic potentials. Root growth of E. pauciflora was examined for 31 months (December 1992 - June 1995) in a mature stand at an elevation of 1545 m on Mt Stirling, Victoria, Australia. Greater night than day root elongation was recorded from eight in situ rhizotrons during the summer and early autumn of 1993. Shoot growth was also monitored during part of this study (April 1994 - June 1995). It was found that root growth commenced in the spring at soil temperatures 5 1.5C, under 550 mm of snow, at least one month before the onset of shoot growth and continued at least two months longer that shoot growth during the autumn. A period of root dormancy for at least one month a year occurred in roots of E. pauciflora. The seasonal variability in root numbers of E. pauciflora appeared to be related mainly to soil temperature and to a lesser extent to soil water content. Moreover, there appeared to be some internal periodicity in root growth which was independent of the external environment on Mt Stirling. Greater night than day root elongation was recorded in seedlings of both eucalypts in a glasshouse. Root elongation rates were greatest in E. nitens, and root elongation of both eucalypt seedlings were greater than that of the mature E. pauciflora on Mt Stirling. The zones of day and night elongation were determined in root marking experiments. Histological studies of the zone of elongation showed that cell division occurred mainly during the day and cell elongation mainly at night. Night root elongation rates were increased by increasing day-time air temperatures, light-period, and light intensity; and by decreasing water stress during the night. The turgor pressure of the root tips was greater during the night than the day. It is suggested that the amount of root growth during the night is determined directly by turgor pressure during the night and indirectly by processes during the day (light duration and intensity, and temperature during the light period) which determine the extent of cell division during the day. A greater rate of cell division during the day will be translated into a greater rate of root elongation, especially in the night.
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ItemA marketing approach to recreation management in national parks( 1995)Recreation managers have always attempted to ensure that resources are used effectively in planning and delivering outdoor recreation opportunities. However the lack of a sound theoretical understanding of management's' role and function in the provision of these opportunities limits the ability of managers to meaningfully evaluate the effectiveness of their actions. Many authors believe that the challenge facing recreation research is the development of a model to provide this much needed theoretical base (Allen 1988, Watkins 1990). Using data derived from a visitor survey conducted at Wilsons Promontory National Park, this study examines the potential of defining and evaluating the role of outdoor recreation management from a market-oriented perspective. The concept of voluntary exchange underpins this perspective. Fundamentally, approaching recreation management from this perspective suggests that effectively providing opportunities for outdoor recreation is not necessarily contingent on fully understanding participants recreation experience outcomes. Rather, what is important, is understanding the nature of the exchange between management and participants and how well management performs in this exchange. The visitor survey principally contained two research instruments, importance-performance analysis (Martilla and James 1977), and recreation experience preference scales (Driver 1977, Driver 1983). This not only enabled the nature of the exchange between management and participants to be investigated, but also provided a means to compare a market oriented approach to outdoor recreation management with the more traditional approaches based on the recreation demand hierarchy. The results identified 13 general dimensions to the recreation opportunity provided at Wilsons Promontory National Park. Three visitor segments were identified in the Park visitor population. Analysis of the components of the recreation opportunity important in producing satisfaction for each of these segments suggests that the role of visitor management at the Park is to deliver recreation opportunities which primarily differentiate along a facility based comfort/asceticism continuum. The study also shows that visitor perceptions of management's performance in the voluntary exchange process can be used to help management ensure resources are used effectively in the delivery of recreation opportunities. The study concludes that a market-oriented approach to visitor management in national parks and outdoor recreation areas has considerable potential. However, further research is required to refine the methodology and techniques explored in this study.
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