Improve Nitrogen use efficiency of wheat cultivars under Western Australian conditions

Nitrogen use efficiency (NUE) of wheat cultivars can be improved by modifying the onset of senescence to optimise sink-source balance for target environments. The NUE can be divided into two main components: nitrogen uptake efficiency (NUpE), shows the capacity of the plant to absorb N from the soil; and nitrogen utilisation efficiency (NUtE), shows the ability to use available N within plant to produce grain. Therefore, understanding the mechanism regulating NUE and its components is required to produce high yielding cultivars with minimum N application. The objective of this study was to investigate the impact of No Apical Meristem gene (NAM) on the senescence in relation to NUE in Australian wheat cultivars. An additional aim was to identify the critical traits associated with NUE and its components under low and high N conditions. Nineteen cultivars of bread wheat (Triticum aestivum L.) carrying different combination of NAM gene were studied under three levels of N fertilisation applied at two development stages in two-year field trials at Western Australia. Further detailed investigation was carried out through two glasshouse experiments using four cultivars with significant differences in nutrient remobilisation, which were selected based on the results of field trials with different N treatments. In general, NUE showed stronger association with NUpE rather than NUtE. The traits correlated to high NUE include, greater above-ground biomass, robust root system and stay-green attribute to increase pre-anthesis N uptake and post-anthesis N remobilisation. Cultivars carrying functional allele of NAM-B1 or/and NAM-A1a allele accelerated the onset of senescence, while the non-functional allele of NAM-B1 or/and NAM-A1c and d alleles delayed the onset of senescence. The complete absence of the functional NAM-B1 gene is a common phenomenon in Australian wheat cultivars. However, NAM-A1, a gene with a similar function to NAM-B1 involved in remobilising nutrients and accelerating senescence, is characteristic of Australian cultivars. Accelerating the onset of senescence results in a short grain filling phase reaching to mature grain before the unfavourable summer conditions with dry season end and promote better remobilisation of stored carbon and nitrogen at Western Australia conditions. As concluded, NUE could be improved by selecting specific combinations of NAM gene alleles for target environments by fine-tuning the duration of growth phases influencing the sink-source relationships. Furthermore, selecting genotypes with high NUE require identifying the key traits under low nitrogen conditions, as well as, under high nitrogen conditions