School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemIdentification of boron tolerance in Brassica rapa( 2006)There has been increasing interest in developing canola quality B. juncea for low rainfall areas across Australia over the past two decades. However, B. juncea genotypes are susceptible to high levels of boron in Western Victorian soils. An understanding of the genetics and the molecular basis of boron tolerance may enable fast and accurate tolerance selection and lead to improved boron tolerance. Being an allotetraploid species, B. juncea is difficult to understand at the genetic level because of chromosomal duplication and the potential presence of multiple copies of the loci of interest. Therefore, once the tolerance genes or chromosomal loci governing tolerance are identified in the diploid progenitor genomes, B. rapa and B. nigra, boron tolerant B. juncea lines may be resynthesized. Thus, as an initial step in this process, this thesis aimed to understand the physiological, genomic and molecular mechanisms involved in boron tolerance in B. rapa. Initially, B. rapa genotypes were screened for tolerance to boron toxicity using hydroponic and soil assays. On the basis of primary root length, severity of leaf toxicity symptoms, dry matter accumulation and shoot boron uptake, the B. rapa genotypes WWY Sarson and Local were identified as the most tolerant and the B. rapa genotypes Shillong and Kaga the most susceptible to toxic boron concentrations (1000 ?M B in hydroponic assay; 54 mg B kg-1 soil in soil assay). The main mechanism of tolerance to boron toxicity in B. rapa involved reduced net boron uptake by roots, with some boron accumulation in the tap roots and partial exclusion of boron from shoots. Furthermore, boron uptake was much lower in the WWY Sarson and Local genotypes than in the Shillong genotype, despite higher rates of transpiration. This implied that an active boron efflux mechanism may be operating in the tolerant genotypes. The inheritance pattern of tolerance to boron toxicity in B. rapa genotype, WWY Sarson best fitted a Mendelian model of two major dominant and epistatic genes. A B. rapa linkage map was constructed from an intraspecific F2 population (WWY Sarson X Shillong) with ISSR, RAPD, SRAP and SSR marker loci. The linkage map spanned a total length of 874.1 cM and contained 12 linkage groups. Chisquare analysis (P < 0.05) revealed 25 dominant markers that showed segregation distortion in the F2 progeny. QTL analysis using composite interval analysis identified three significant peaks on LG2 and LG8 that were associated with primary root length and which accounted for 17% of the trait variation. Differential transcript analysis of SRAP markers following exposure to a toxic boron concentration identified up-regulation of me4+em2570bp, me2+em2650bp, me2+em1 1600bp, me2+em1800bp and me4+em2500bp genes in Shillong and Kaga and down-regulation of me2+em2650bp, me2+em1 1600bp, me2+em1800bp and me1+em21200bp genes in WWY Sarson and Local. Of these, a UDP-glycosyltransferase gene (sharing 80% similarity to the Arabidopsis thaliana homolog) was highly transcribed only in the sensitive genotype, Shillong, and may be involved in excessive boron cross-linking to the glycosyl groups present in the cell walls and/or membranes eventually causing the observed reductions in shoot and root growth.
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ItemSpring water use in raised bed cropping( 2004)Cultivation of heavy clay soils with the application of gypsum is often used to improve root exploration of the soil profile and hence more efficient use of the soil resource to enable higher grain yields of cereal crops. Soils in south-western Victoria are derived from tertiary basalts with high clay content and often dispersive subsoil. Cereal crops grown on these soils are prone to waterlogging. Waterlogging has been overcome with the use of raised beds. The hypothesis was that the heavy subsoil restricted rooting depth and hence efficient water extraction from the soil profile, particularly in the grain filling period during spring. Deep cultivation of the soil was proposed to overcome subsoil limitations. This study has compared the use of deep ripping, with and without the use of gypsum, to the use of direct drill techniques. Soil water use and plant root density have been compared for the different cultivation treatments. Soil water use indicated that the use of direct drill, compared with deep ripping, was favoured during years with dry autumn or delayed autumn breaks. Surface soil water was conserved in the direct drill treatments. The use of deep ripping, with and without the use of gypsum did not significantly increase the rooting density to a greater depth than direct drill. Furthermore the deeper roots failed to access soil water to improve grain yield compared with direct drill treatments. An increase in grain yield, with the use of deep ripping, was recorded in a year of above average rainfall in the growing season. The addition of gypsum, when deep ripping, provided no additional benefit to grain yield in the above average rainfall year.
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ItemSubsoil physicochemical constraints and growth of cereals on alkaline soils in the Victorian Mallee( 2003)Alkaline soils used for dryland cropping across the semi-arid regions of south=eastern Australia typically have high levels of salinity (ECe), sodicity (ESP) and soluble boron (B) in the subsoil. The current research was undertaken to improve our knowledge of the physicochemical characteristics of these alkaline soils and assess what impact they have on the growth and water use of cereals. A survey of representative alkaline soils of the southern Mallee and Wimmera regions of Victoria, comprising 140 Calcarosol profiles and 10 Vertosol profiles, revealed high correlation between exchangeable Na+ and both ESP (r = 0.96) and B (r = 0.88). ESP and ECe (r = 0.71) and B and pH1:5 (r = 0.70) were also highly correlated. Readily measured properties (field texture, pH1:5, ECe and exchangeable Na+) were found to provide good estimates of ESP and boron in these soils. Overall, ESP was best defined as 1.47 + 2.68 x Na+ (r2 = 93.9) and boron by 0.34 +3.93 x Na+ (r2 = 76.7). A break point values for pH was defined such that soils with pH 1:5 < 8.1 have low levels of soil boron that are not potentially toxic to cereal growth. The growth and water use of the boron tolerant wheat cultivar, Frame, was monitored on these soils during the 1999-growing season. A descriptive model that explained 54% of the variation in grain yield (range: 1.3 - 6.1 Mg/ha) was established using ridge regression analysis, which is not confounded by the correlation that exists between the physicochemical factors (collinearity). The statistical analysis identified rainfall around anthesis, available soil water in the 0.10-0.40 m layer at sowing, nitrate in the 0-0.10 m layer and salinity and sodicity in the 0.60-1.00 m layer as significant explanatory variables for grain yield and water use. Subsoil salinity (ECe) and sodicity (ESP) were effective surrogates for extractability of water in the deep subsoil. It is estimated that crops can make use of water at depth if subsoils have ECe <8 dS/m and ESP <19%. Levels of soluble soil boron (reaching concentrations of up to 52 mg/kg), were not significantly correlated with root growth, water uptake or yield of wheat. It is proposed that the boron tolerance of cv. Frame rendered high boron as non-limiting and that the high Na+ in these soils had an overriding effect in driving variation in crop yield. The impact of boron tolerance, watering regime and soil disturbance on the growth and water use of near-isogenic lines of wheat and barley was assessed using large intact soil cores (0.3 m diam. x 1.0 m height), containing an alkaline soil (Calcarosol) sampled from the southern Mallee. Within the subsoil (0.80-0.90 m) ECes, ESP and B was 8.1 dS/m, 29% and 31 mg/kg respectively. In the shallower (0.61-0.71 m) layer respective levels were 7.2 dS/m, 22% and 29 mg/kg. Crop root growth between these layers decreased significantly with depth and no net water extraction occurred beyond 0.80 m, irrespective of crop boron tolerance. As the concentration of soil boron was equivalent across these layers, it was discounted from being a constraint to water extraction by crop. Rather, the increase in EC0 and more so ESP suggests these factors were constraining water use at depth. Grain yield per unit of applied water for wheat and barley grown under low water supply was 1.5 times that of their high water counterparts, suggesting inefficiencies in water capture by crops under the high water regime. Deep ripping had no effect on grain yield. Importantly these results showed boron tolerance provided little benefit to cereals on soil where constraints in addition to high boron exist. A glasshouse trial was conducted to assess how interactive effects of boron and salinity and cultivar affected the early growth of wheat using a range of soil salinity and soluble boron levels observed in the field survey. The three cultivars, Frame, BT Schomburgk and Schomburgk, varied significantly in tolerance to boron, with critical soil concentrations were estimated at 53, 32 and 27 mg/kg respectively in the absence of salinity. These 3 varieties did not differ in tolerance to salt in the absence of high boron, where cultivars equally tolerated ECe = 9 dS/m. Boron and Na+ in shoot tissue could not be used to define critical concentration for toxicity. This trial demonstrates the value Of using genetic variation for adaptation of wheat to high levels of boron, but appears not the case for salt tolerance here. This thesis confirmed that, due to the strong intercorrelation that exists between the physicochemical factors, salinity, sodicity and soluble boron, that these factors are likely to operate simultaneously to reduce cereal growth. Adequate boron tolerance currently appears to exist in commercial wheat cultivars i.e. Frame, enabling these varieties to withstand the high boron levels encountered in the alkaline cropping soils of north-western Victoria. High. levels of salinity and sodicity, however, are more likely to be constraining growth through osmotic, toxic and physical impediment. This thesis indicates the need for improve tolerance of cereals to salinity and the need for pyramiding tolerances for crops targeted to alkaline soils where constraints exist together.
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ItemThe transport and fate of chlorsulfuron and atrazine in the soils of the Wimmera-Mallee and their potential containment with controlled-release technology( 2001)Field investigations of the transport and fate of the sulfonylurea herbicide, chlorsulfuron and the s-triazine herbicide atrazine were carried out in several cropping soils in the Wimmera-Mallee region of Victoria between 1991 and 1995. The containment of these herbicides within topsoil by the use of controlled-release formulations was also examined in field and laboratory experiments. Concentrations of atrazine and chlorsulfuron were measured for 11 and 17-month periods after the application of each respective herbicide, in a sandy loam at Walpeup in the Mallee. These results showed that the mobility of the two herbicides were different. Atrazine was mainly retained within topsoil while chlorsulfuron rapidly leached to the subsoil. Analyses of soil water fluxes and weather data for this period showed that the mobility of atrazine was governed by sorption while chlorsulfuron mobility was largely due to mass flow. The rapid leaching of chlorsulfuron was also confirmed in further studies in a Wimmera grey clay and a Kattyoong sand. While the leaching of atrazine was minimal, it was estimated that absolute residue levels that could reach groundwater depths would be higher than chlorsulfuron due to substantially higher rates of application of atrazine in commercial practice. The first order rates of degradation of atrazine and chlorsulfuron, within the rootzone of the Walpeup sandy loam, were calculated to be 62 and 75 days, respectively. The persistence of residues in the Walpeup sandy loam, at the phytotoxic threshold of several crops that are sensitive to each respective herbicide, were 11 and 17 months for atrazine and chlorsulfuron, respectively. The leaching of the herbicides was found to be an important determinant of their persistence. The relatively lower persistence of atrazine could be explained by its retention in topsoil where microbial and hydrolytic breakdown of the herbicide is at a maximum, whereas the rapid transport of chlorsulfuron to subsoil increased its persistence. Plant-back experiments for up to two years after application of chlorsulfuron in the Wimmera grey clay resulted in injury to annual medics suggesting that the productivity of medic leys in similar soils would be under threat for at least two years after the application of the herbicide. Investigation of the use of controlled-release formulations of the two herbicides showed they provided similar weed control to conventional formulations. This makes them viable for commercial practice. However, it was found that at the current stage of development of the controlled-release formulations, their reduced leaching characteristics were insufficient for total containment of the herbicides in topsoil.