Fraser, Alison
(2021)
Improving nitrogen use efficiency of winter oilseed rape (Brasica napus L.).
MPhil thesis, University of Nottingham.
Abstract
Nitrogen (N) fertilisers are used widely to improve winter oilseed rape (WOSR) yields. However, N is an expensive input and has environmental effects including greenhouse gas (GHG) emissions and nitrate leaching. Significant genetic variation in N use efficiency (NUE) has been reported among WOSR genotypes, and this can potentially be exploited by plant breeders. OSR yields are highly variable. They are affected by genetics, environment and agronomic practice as well as the interactions between them. Trials comparing NUE of diverse genotypes have shown that cultivar yields are ranked differently at high N and low N rates. If the interaction between genotype and N supply is highly significant but the correlation between low N and high N yields is low then it is possible to select genotypes with adaptations to low N. Better management of N application, with precise application rates dependent on genotype and environment, has the potential to improve NUE. In order to calculate the optimum N rate it is necessary to know both yield potential of the site and N requirements of the genotype, assuming the intention is to reduce N applications whilst maintaining yield (or to increase yields whilst maintaining N application). Therefore, understanding genotype performance over multiple environmental and management conditions is essential.
A field trial has been conducted as part of the Renewable Industrial Product for Rape (RIPR) project. Data was collected over two sites and two years. The field study characterised the genetics of NUE traits of WOSR in a structured association panel, testing 84 genotypes types at low (60 kg N ha-1) and high (300 kg N ha-1) N rates. 32 traits were measured and analysed including: N tissue analysis; phenology dates; and yield. A physiological model of N-acquisition and N-utilisation in WOSR has been developed and validated using an older field data set. The model is physiologically accurate, describing measurable varietal traits and how they impact on dry-matter production, N remobilisation and yield potential. The model has been used to identify novel traits for use in breeding strategies to: improve NUE; inform the optimal use of N fertilisers; and identify crop traits and trait combinations for increasing seed yield at low N of WOSR. Associative transcriptomic analysis (Harper et al., 2012) was conducted with the RIPR field trial data to explore the relationship between genotype and phenotype for NUE and flowering score.
Field trial results from the RIPR WOSR diversity panel showed a large amount of genetic variation for traits, yields and response to N. Significant relationships have been seen between yield formation at low N with: early flowering; post-flowering N uptake; stem N remobilisation; mid-flowering N concentration; maturity chaff N concentration; seed N concentration; height; and establishment. However, these traits have not been seen to impact yield at every trial and a lot of variation was seen for traits within and between environments. The trials described in this thesis support previous evidence that high yielding with low N optimum genotypes exist and have great potential to increase NUE. However, our results show that NUE of WOSR is a complex trait affected by genotype, environment, management and their interactions. NUE can be improved through breeding NUE efficient genotypes and by more precise application of N through increasing knowledge of N optimum based on genotype and yield potential of the field environment. There are many practical issues linked to monitoring traits and calculating N optima of genotypes over different environments. However, improved NUE of UK WOSR would have a positive effect on GHG emissions, the aqueous environment and farm economics. A simple process based model was used to identify novel traits for use in breeding strategies to improve NUE and identify crop traits and trait combinations to increase seed yield at low N of WOSR. The model has been used to provide a good estimate of yield potential of ten varieties in low N supply trials. The model did not accurately predict yield across all environments tested. Sensitivity analysis showed agreement across important traits between the model and published field studies. The model simulations showed the most important varietal traits for either reducing N requirement whilst maintaining current commercial yields or increasing yield at low N supply are: greater post-anthesis uptake; efficiency of N acquisition from fertiliser; days for seed filling; and N remobilisation from pods. Several promising candidate genes, some previously shown to influence or respond to NUE and flowering score in Brassica napus, have been selected using associative transcriptomic analysis. The next step would be to conduct experiments in A. thaliana T-DNA knockouts to confirm the level of influence.
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