School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemNitrogen use efficiency (NUE) in wheat and its relationship to root traits and grain quality under elevated CO2( 2017)Plants grown under elevated [CO2] (e[CO2]) generally produce greater biomass and higher grain yields, but exhibit lower grain nitrogen (N) concentrations. There is a need to find strategies to maintain grain protein in future high [CO2] environments. For example, how to better utilise available N in the soil through improved root traits. The aim of this PhD project was to investigate intra-specific differences in root traits and distribution in the soil profile, and to better understand the Nitrogen Use Efficiency (NUE) of wheat cultivars, particularly the capture of N via enhanced root traits under e[CO2]. Overall, four experiments were conducted in glasshouse and field conditions. The first experiment was conducted under controlled glasshouse conditions (ambient CO2 treatment) to investigate the intra-specific variation in root dry weight (RDW) of 20 wheat cultivars/lines in response to different N rates. Two wheat cultivars – ‘Scout’ and ‘Yitpi’ – were chosen for their contrasting RDW and similar genetic background for use in future experiments in the field and glasshouse. The second experiment, a field study, investigated the intra-specific variability in root traits and their relationship to N Uptake Efficiency (NUpE), N Utilisation Efficiency (NUtE), and consequently NUE in two contrasting wheat cultivars under different concentrations of [CO2] and N supply. Responses of root traits to e[CO2] varied with cultivar and environmental conditions. Scout had thicker but shorter roots than Yitpi, whereas Yitpi had thinner and longer roots than Scout. Greater root length and RDW in Yitpi resulted in greater N uptake and therefore an improved ability to maintain grain protein under e[CO2]. The third experiment, a field study, aimed to test the following hypotheses for the observed N/protein decline under e[CO2]; (i) accumulation of unassimilated nitrate in leaves and (ii) slower N uptake by roots. My results showed that nitrate in leaves, but not changes in root growth, are associated with decreased grain protein in wheat under e[CO2] in a dryland agro-ecosystem. The accumulation of unassimilated nitrate to total N in leaves in Scout had a strong correlation to N/protein decline under e[CO2]. The fourth experiment, a glasshouse (with ambient and elevated CO2 concentrations) study, aimed to investigate the contribution of different root layers to N acquisition, accumulation and remobilisation in two contrasting wheat cultivars under e[CO2]. The roots of both cultivars in 20-60 cm of column depth had a greater N uptake from fertiliser, and therefore made the greatest contribution to grain N concentration. The findings of this research suggest that intra-specific differences in root traits and their relationship to NUpE, NUtE and therefore NUE are largely dependent on environmental (N and water supply) conditions. Yitpi showed a consistently greater RDW than Scout, and it could better uptake N from soil and therefore maintain grain protein under e[CO2], depending on environmental conditions. Results from this study indicate that there is significant variation in the key root traits that contribute to differences in NUE amongst wheat germplasm. These differences can provide the basis for wheat breeders to develop new varieties that can maintain high quality grain without sacrificing increased grain yield resulting from the CO2 fertilisation effect, or significantly increasing the amount of N applied to crops.
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ItemWheat grain quality dynamics under elevated atmospheric CO2 concentration in Mediterranean climate conditions( 2013)Since 1959, carbon dioxide concentration [CO2] in the atmosphere increased from 315 µmol mol-1 to approximately 389 µmol mol-1 by 2009 in a rate of 1.5 µmol mol-1 per year. Within the next 50 years, atmospheric [CO2] will likely to rise to 550 µmol mol-1. Carbon dioxide is a greenhouse gas and a major factor that contributes to global warming. In parallel, global temperature is predicted to increase by an average of 1.5-4.5 ºC with more frequent occurrences of extreme climatic events such as heat waves and/or drought by the mid of this century. There is a limited understanding on the impact of elevated atmospheric [CO2] (e[CO2]) on wheat grain quality in semi-arid and Mediterranean cropping systems. The research reported in this thesis investigated the effects of e[CO2] on wheat grain physical, chemical, flour rheological properties under two main climate conditions: semi-arid and Mediterranean which represent the water-limited “mega-environment 4”, larger wheat grown area in the world as defined for wheat (Braun et al., 1996). The experiments were carried out using state art technology of free- air CO2 enrichment (FACE) facilities located in Walpeup and Horsham, Victoria, Australia. (See thesis for full abstract)