Dissecting genetic variation for nitrogen use efficiency in wheat

Nitrogen (N) is essential for high grain yield (GY) in cereals. A major aim of breeding programs is to increase GY while minimising the level of external inputs, such as N fertilisation. Nitrogen Use Efficiency (NUE) is a complex trait controlled by both genetic and environmental factors resulting in variation depending on seasonal growth conditions. Only 30-50% of N supplied is actually taken up by the plants with the extra N lost through run-off, leaching, denitrification and gas emission. These losses have a negative environmental impact, leading to surface and underground water pollution, algae blooms and intensifying global warming. In addition, nitrogen (N) application is costly further emphasising the importance of NUE improvement to reduce the economic and environmental issues associated with N application. NUE of wheat is important in all production areas but little is known about genetic variation for NUE in low-yielding environments such the Mediterranean-type climate of Southern Australia with low rainfall and high temperatures during critical growth periods. Research described in this thesis evaluated variation in NUE in Australian wheat germplasm and then to identify loci regulating NUE traits in a bi-parental mapping population of RAC875/Kukri. Improvement in NUE will require the integration of physiological and molecular aspects of N status in plants under different growth conditions: the highly variable conditions of field trials and controlled environments such as under hydroponics. The assessment of NUE and N response under both field and controlled conditions could facilitate the identification of traits and QTL and lead to the discovery of candidate genes underlying the traits. The first step of this research involved NUE traits and N response assessment of Australian cultivars in different environments, with varying N input. Genetic variation for NUE was identified in Australian spring wheat cultivars, and the cultivars were ranked for their N-efficiency and responsiveness. The dissection of genetic variation for NUE was investigated in the RAC875/Kukri population across six field trials between 2011 and 2013 covering 16 environment by treatment combinations. Nitrogen responsiveness was compared with N efficiency and the genotypes were ranked for the consistency of a positive response and high efficiency of N use versus negative responsiveness and low efficiency. Quantitative Trait Loci (QTL) analysis identified the genome regions associated with GY, grain quality and responsiveness to N. In addition, specific-environment associated N QTL were identified. A QTL on chromosome 2A was detected for most of traits studied and across multiple environments. Further stable QTL were identified on chromosomes 1A, 1B, 2A, 3D, 7A and 7B for GY across environments. The physiological response to N was studied at the early stages of growth for selected lines in a hydroponics system that allowed the measurement of N uptake and utilisation. The aim of the experiments was to investigate the physiological basis for the effects seen in the field trials. However, no consistent response was seen in these studies suggesting that future work should focus on later growth stages. To conclude, the results showed significant genetic variation and transgressive segregation for NUE despite the complex nature of the effect of N on grain yield and quality traits. These genome regions can be used to support marker assistance selection (MAS) for improved NUE and for cloning genes underlying the loci affecting NUE in wheat. The results show that selection for improved NUE is possible and also provide a base for further molecular and physiological studies on efficient use of applied N.Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Agriculture, Food and Wine, 2015