School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemReproductive development of wheat under different thermal and photoperiodic environments( 1995)The overall objective of the thesis was to advance knowledge concerning phenological development in wheat. Specifically, it examines the variability of response to the main environmental factors. These are mean temperature, vernalising temperature, and photoperiod. Responses were examined by changing the environmental factors in various combinations, and the generality of the responses was gauged by including different cultivars in each study. The thesis includes some simple mathematical descriptions of the responses. The thesis has seven chapters describing and analysing specific experiments. Each chapter has its own introduction, results, discussion and conclusions. Particular chapters examine (i) if thermal amplitude affects wheat development independently of mean temperature, (ii) whether there is variability in the sensitivity to mean temperature among different cultivars and phenophases in relation to cardinal (base and optimum) temperatures, (iii) whether genetic variability in response to vernalisation and photoperiod can be described with numerical parameters, and whether these parameters change with development, (iv) whether rate of change of photoperiod can affect wheat development independently of absolute photoperiod, and finally (v) whether the interactions between temperature x photoperiod are important modifiers of development. The durations of the developmental phases the seedling stage (Haun stage < 1) to terminal spikelet initiation and from then to anthesis showed no evidence of systematic change due to thermal amplitude (ranging from 0 to 14 C, around an average temperature of 19 C) in any of four cultivars examined. Final leaf number and phyllochron were not significantly affected by thermal amplitude. The same four cultivars were then subjected to a range of average temperatures between 10 and 25 C. The duration of the stage from seedling growth to anthesis was reduced as temperature increased towards 19 C. Further increase in temperature did not alter duration in the cultivars Condor, Rosella and Cappelle Desprez, but increased duration in Sunset. Rate of development towards anthesis generally increased curvilinearly with temperature, so the response was reassessed in greater detail by subdividing the full period to anthesis into three phases. All responses in all cultivars could then be described numerically within the linear constraints of the thermal time concept. Base and optimum temperatures increased as development progressed towards anthesis. Averaging across cultivars, base temperature rose from -1.9 to +8.1 C for the phases before and after terminal spikelet initiation, respectively. Optimum temperature also increased. Cultivars differed substantially in each of these parameters. The progressive increase in optimum temperature with phasic development was apparently the main reason why linear fits for the three phases appear curvilinear for the full phase to anthesis. Final leaf number was negligibly changed by temperature, but phyllochron was significantly reduced as temperatures increased to 19 C. Cultivars differed in their base temperature for leaf appearance but had a similar optimum temperature of approximately 22 C. It is concluded that cardinal temperatures not only change with phase of development, and are specific for each genotype, but also that they can be different for developmental processes that are occurring at similar times. A model partitioning the response to vernalisation into three parameters, viz. optimum vernalisation (Vo), vernalisation sensitivity (Vs) and basic length (Lb) was proposed to analyse the responses to vernalisation in the cultivars Odin, Robin, Rosella and Condor. Vernalisation lasted from 0 to 70 d after seed imbibition and significantly reduced the time to anthesis in all cultivars, changing all three parameters in each of the pre-anthesis phenophases considered. All cultivars exhibited quantitative responses to all levels of vernalisation during the vegetative phenophase to double ridge. However, for the reproductive phases, Odin failed to reach anthesis if treated with less than 2 weeks vernalisation, indicating that vernalisation affects development beyond the vegetative phase. There were significant progressive reductions in final leaf number with longer periods of vernalisation. For the most sensitive cultivars, Rosella and Odin, the number of leaves appearing after double ridge was reduced by vernalisation. However, the number of leaves appearing after double ridge was only partially associated with the length of the reproductive phase. In the sensitive cultivars, phyllochron was shorter early in plant development than later, the change occurring at about leaf 6. In a parallel study, the vernalisation period was interrupted by a 3 d period of 18 C to investigate whether a moderate temperature can produce devernalisation. Partial devernalisation occurred in Rosella and Odin. In a field experiment, photoperiod was extended artificially in five treatments up to terminal spikelet initiation viz.; natural photoperiod (rate of change of photoperiod=2.3 min d-1 ), two faster rates of change (9.8 and 13.1 min d-1 ) and two constant photoperiods of 14.0 and 15.5 h. After terminal spikelet initiation, the two constant photoperiods were extended to 15.0 and 16.5 h, respectively, and treatments were randomly re-allocated. The rate of development from seedling emergence to terminal spikelet initiation responded to increases in photoperiod in both cultivars but there was no effect of rate of change of photoperiod. Phyllochron did not alter during plant development or in response to the photoperiod regimes. Finally, the effects on development of photoperiod (9, 12, 15, 17, 19 and 21 h) and temperature (21/17 and 16/12 C) in combination were studied. Again, four cultivars (a non-segregating awned selection of Sunset, Sunsetaw, Condor, Rosella and Cappelle Desprez) were used. Increases in both photoperiod and temperature always reduced the time to heading, but genotypes differed substantially in the magnitude of their responses to the individual environmental variables, and also in their responses to the different combinations. The interaction effects were sometimes greater than the individual effects. A model of the response of wheat development to temperature was proposed which includes the effects of photoperiod not only on thermal time but also on base temperature. Differential responses to short photoperiods were evident amongst genotypes, indicating that more than one degree of sensitivity to photoperiod might be possible for a single cultivar. Final leaf number on the main culm increased with shortening photoperiod, but was unaffected by temperature as observed previously. Although time to heading was always linearly related to final leaf number, the results suggest that photoperiod acted at least partially independently on the timing of heading and on final leaf number. The responses to photoperiod x temperature during three phenophases (pre-double ridge, from then to terminal spikelet initiation, and from then to heading) were assessed using a mathematical description which partitioned the response of each cultivar and phenophase into one or two photoperiodic sensitivities (Ps and Ps2), an actual maximum length (Lma) of the phase, which occurs at the critical photoperiod (Pc), a potential maximum length (Lmp) and a basic length (Lb) of the phase that occurs at the optimum (Po) or longer photoperiods. The duration of the early phase to double ridge was quantitatively affected by photoperiod and could be described by a single sensitivity value (Ps) which differed in magnitude between cultivars. The Po also differed amongst cultivars, and was longer at the higher temperature, while Lb during this phase showed a significant cultivar x temperature interaction. The duration of the phase from double ridge to terminal spikelet initiation was quantitatively responsive to photoperiod in all cultivars, and the response was affected by temperature. However, the responses of these two phases were different, as judged by their parameters. In this phase, Condor, Rosella and Cappelle Desprez showed a 3 to 5 fold greater sensitivity to very short photoperiods (Ps2) than to longer photoperiods (Ps). The response to photoperiod between terminal spikelet initiation and heading was also significantly affected by photoperiod, but its magnitude was different amongst cultivars. Sunsetaw showed a simple quantitative trend, while Condor and Rosella, which also had quantitative responses, responded in a more complex fashion with a much stronger sensitivity to very short photoperiods (< 12 h, Ps2) than to longer photoperiods (Ps). Cappelle Desprez had a qualitative response for very short photoperiods. It was concluded that (i) differences among cultivars in response to . photoperiod can be conveniently partitioned into different parameters for describing photoperiodic sensitivity, (ii) these parameters appear to be unrelated, allowing for speculation that plant breeders could manipulate them independently for customising cultivars for particular environments, (iii) the parameters were sensitive to temperature, suggesting that it would be inappropriate to extrapolate the response to photoperiod from one thermal environment to another, and (iv) the length of the late reproductive phase from terminal spikelet initiation to heading was not only significantly affected by photoperiod, but was even more sensitive to photoperiod than the early phase to double ridge. This thesis concludes with a chapter that discusses the relationships between the results from individual studies and identifies avenues for future work.
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ItemShelterbelt effects on microclimate and cereal crops( 1993)The 1980's saw a rapidly expanding community interest in re-establishing trees and other natural vegetation in the largely cleared rural environments in Australia. This culminated in Prime Minister Bob Hawkes' pledge to establish one billion trees in Australia by the year 2000. Tree growing became a major focus for many of the hundreds of Landcare groups which established in this period. The Landcare movement, which has at its foundation the objective of achieving sustainable land use, grew rapidly. Landcare encompasses a broad range of land management issues including soil erosion and salinity, pest plants and animals, declining soil fertility and nature conservation. Tree planting became one of the most popular landcare activities as trees are directly or indirectly related to the majority of landcare issues. Involvement in tree planting can lead an individual into a myriad of more complex aspects of land management. Indeed, many of today's Landcare groups started as Farm Tree groups. The motivation for people to re-establish trees varies greatly and reflects the heterogeneity of the population (such as their needs, their level of understanding and their values) and the multiple benefits that trees can provide. Some are motivated by what they see as the tragedy of the insidious loss, due to tree decline, of quintessential Australian rural landscapes which have resulted from the attractive combination of remnant eucalypts, acacias and other trees and shrubs with agricultural pursuits. Others recognise the importance of rural vegetation in maintaining the biodiversity of Australia's flora and fauna. There is increasing evidence that deep rooted perennial vegetation is important for sustainable agricultural production from many agro-ecosystems. Tree-establishment, in addition to the adoption of other conservation farming practices, is prominent in many strategies to control land degradation. For example, salinity management plans in Victoria are proposing the establishment of trees at low and medium densities over tens of thousands of hectares of land with high ground water recharge to control dryland salinity (Goulburn Broken Salinity Pilot Program Advisory Council 1989). Well planned, integrated revegetation programs may indeed provide combinations of these and other benefits (Burke and Voul 1988). Revegetation projects are generally costly with many long term benefits which are often difficult to measure in monetary terms. In recognition of the public benefit flowing from rural tree growing projects, state and federal governments have provided modest financial incentives such as those under the Tree Victoria Program, and the One Billion Trees Program. These programs at their current levels however only fund a small percentage of the tree growing component of the strategy needed to seriously address some of the land degradation problems present. Most farms in south-eastern Australia are family-owned small businesses. Regardless of the motivation for undertaking tree growing projects, given the modest level of financial incentives and subsidies available, these must largely be funded from the income generated from the farm. There is increasing recognition that "landcare should be able to pay for itself'. For this reason, tree growing projects are particularly compelling which have, at least as one of their benefits, enhanced short-term farm productivity. Shelterbelts are generally rows of trees and shrubs established to ameliorate the effects of the wind. Increased farm productivity from the provision of shelter is widely cited as one of the major benefits of trees on farms in the media and popular literature. This study aims to investigate some of the effects that shelterbelts can have on cereal crop production in south-eastern Australia and thereby help determine the feasibility of some tree growing projects to be at least partially "self funding".
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ItemThe effects of cropping on physical and chemical properties of irrigated paddy soil in Lampung province, Indonesia( 1992)The effects of varying times of cropping on the physical and chemical properties of a Podzolic soil used mainly for paddy rice were studied. The different durations of cropping that had been given were 1, 3, 6, 15, 25, and 45 years. Properties of the soil that were measured were texture, bulk density, porosity, fast and slow drainage pores, saturated hydraulic conductivity, Atterberg limits, soil water retention, organic matter content, C/N ratio, total-N, NH4+ and NO3 - -N contents and extractable Fe and Mn. Soil samples were taken from the 0-15 cm, 16-30 cm, and 31-60 cm depths and analysed in the laboratory. As the durations of cropping increased, the soil texture become coarser, and soil water retention capacity, Atterberg limits (liquid limit, plastic limit, and plasticity index), organic matter,total nitrogen and exchangeable ammonium decreased in samples taken from each of the 3 depths. In addition, the stability of aggregates from the 0-15 cm depth decreased as cropping continued. Also, there were marked increases in bulk density and extractable iron for soil at the 16-30 cm depth as cropping proceeded for 45 years. Similarly, higher bulk densities were recorded for soil at the 31-60 cm depth where cropping had continued for 45 years. Rapid decreases in values for organic matter and aggregate stability for soil from the 0-15 cm depth occurred with the first 3 years of paddy rice and associated crops. Similarly, rapid decreases in clay percentage, saturated hydraulic conductivity and total nitrogen for soil from the 16-30 cm depth occurred during this initial cropping phase. Formation of a "plough pan" at the 16-30 cm depth was indicated by relatively high bulk density and extractable iron, and relatively low organic matter, total nitrogen, aggregate stability, clay percentage and saturated hydraulic conductivity when this Indonesian Podzolic soil was used for paddy rice and associated crops. Formation of this "pan" was desirable, as it prevented loss of irrigation water by movement through the soil profile. Its formation was also rapid, as some of its properties were evident after 3 years of cropping. Results for nitrogen analyses suggest that if maximum plant growth is to occur on this soil after it has been cropped for more than 3 years, some addition of fertilizer nitrogen may be required.
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ItemStudies of the water relationships and productivity of crops : a collection of papers( 1991)The nature of crop growth Crops are complex systems whose basic ecological function is the collection of solar energy and its conversion to the chemical. energy of biomass. That biomass provides dietary energy for consumer organisms, including man, and ultimately for the decomposers that return its inorganic constituents to the soil and atmosphere for continuing productivity. Thus, crop growth requires a continuing supply of solar energy captured by the photolysis of water in pigment systems in leaves, carbon dioxide that diffuses there from the atmosphere to be photoreduced in the primary chemical steps of photosynthesis, and finally a set of essential inorganic nutrients that are adsorbed from the soil by the roots. The chemistry of those initial and subsequent steps of crop growth is complicated, proceeding from atoms to molecules and to the polymers that form the major chemical components of the plant body. The details of metabolism vary from species to species but a common feature is that the amount of carbon dioxide that is photoreduced in photosynthesis plays a major role in determining the amount of growth, i.e. crop productivity. An important aspect of crop metabolism is that it operates in, an aqueous phase. This has important consequences for the productivity of crops because in order to absorb carbon dioxide from the atmosphere there must be an open diffusion pathway from the wet insides of leaves to the dry atmosphere beyond. Uptake of carbon dioxide by photosynthesis inevitably involves simultaneous water loss by evaporation with the two gases diffusing in opposing directions through the stomatal apparatus of the leaves. In that sense, crop growth requires an inevitable exchange with the atmosphere of water for carbon dioxide and the practical result is that growing crops must absorb much more water from the soil than is needed to maintain the aqueous condition of their metabolism. Depending upon atmospheric conditions, as much as ninety nine percent of water adsorbed by crops can be lost directly to the atmosphere as transpiration. This brief picture of the nature of crop productivity reminds us of the great number of processes that operate at a range of physical, chemical and time scales. Specialist disciplines concentrate at various parts of this continuum. Physicists and chemists work with atoms, elements and compounds, biochemists work with molecules and membranes, physiologists with organs and whole plants, and agronomists with crops and their environmental interactions. The objective of agronomists is to apply understanding of the nature of crop growth to the formulation and selection of cultivar-management combinations that meet the needs of consumers. That requires improved efficiency in the use of environmental resources and usually greater productivity also. An appreciation of the nature, interactions and capacity for compensation between the underlying processes of crop growth can contribute greatly to this. The best performance will be achieved when it is possible to identify and select optimum combinations of crop traits. For example, crop productivity will be maximized with attention to the sequential steps of light interception, photosynthetic conversion efficiency and partitioning of biomass to yield organs. In recent years, agronomists have been assisted in this challenging task by the development of crop simulation models that provide explanatory links between process and outcome. They provide frameworks within which it is possible to arrange what is known about the nature of growth and its response to environment. They provide valuable links between specialist disciplines, allowing information to flow in structured ways between them. Some models are designed to aid researchers devise experiments, others are designed to help evaluate cultivar-management options. A particular strength of the modelling approach is its potential to include the response to variable environments, i.e. to interpret and use what in classical agronomic studies was seen only as a season-by-treatment interaction in analyses of variance. Developing better understanding of the nature of crop response to variable environments is essential to continuing improvement in the strategic and tactical management of crops. Content of thesis This collection of 28 papers deals with aspects of the productivity and water use of crops in response to environment and management. A complete list of the candidate's publications is presented as Appendix I. The investigations cover a range of crops and also a range of scales from seasonal and weekly analyses of growth and water use to daily and hourly measurements of photosynthesis and transpiration of canopies and individual leaves. In all cases, the primary interest lay in the agronomic performance of the crop and in the requirements for productivity and efficiency. The approach adopted in the research was to combine studies of the seasonal crop performance with measurements of status and process during the growth cycle. When undertaken, diurnal measurements of crop water status and leaf and canopy gas exchange assist the explanation of crop water balance and growth measured at say weekly intervals. In the same way, in any crop experiment, such intermediate harvests, preferably taken with attention to stages of crop phenological development, are invaluable to the explanation of the final yield outcome. The combined study of process and outcome is critical to progress crop agronomy. The major limitation of sow-and-harvest experimental programmes is that the same yield can be achieved by many combinations of responses. Understanding how yield is attained opens the possibility of devising new crop-management combinations for improved performance. Sequential harvests allow assessment of crop performance through growth analysis and consideration of components of yield. When measurements are made at the level of physiological process, interpretation through to crop performance necessitates the use of simulation models. The selection of papers includes descriptions and use of models of component processes and of crop performance. The papers are grouped by crop rather than by the hierarchical level at which individual papers were directed. This is consistent with the focus of the research on the nature and performance of crops because to do otherwise would separate the components of research sequences and integrated studies of individual crops. The thesis deals mainly with wheat and sunflower but includes the results of short periods spent with tobacco and cassava. The opportunities to work with those crops provided valuable comparisons and tests of technique for the more sustained work on sunflower and wheat. A short explanation introduces each of the four sections of the thesis.
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