Abstract
Optimising Nitrogen fertiliser management and using cultivars with high Nitrogen uptake and utilisation efficiency is a good strategy to improve the Nitrogen use efficiency (NUE). This study investigates the determinants of genetic variation in NUE and develops associations with NAM - A1 and NAM - B1 alleles in Australian wheat cultivars grown under different N treatments in Westonia Australia. The functional NAM - B1 allele improves nitrogen remobilization from the leaf tissue, which led to increasing grain protein content but reduces the grain yield. NAM - A1a allele has the same functional of NAM - B1, but with more influence on the period of grain filling. The Nitrogen fertiliser treatments were applied at rates (0, 50, and 100 kg N ha −1) at three stages (Mid Tillering, Booting, and Flowering) of plant growth. The following parameters were analysed: grain yield, number of head plant - 1, number of seed head - 1, thousand grain weight, Root dry weight, total NAM gene expression, grain protein content, residual N in straw, and Nitrogen content in leaf tissue to determine the Nitrogen accumulation and translocation from the leaves to the grain, and overall NUE and its component. The increase of Nitrogen application also resulted in increased grain yield, number of head plant - 1, and greater root dry weight, which was highest at 100 kg N ha −1. The timing of Nitrogen application had no clear effect on either grain yield or grain protein content. Application of Nitrogen at the late stage (booting and flowering) increased the only number of seed head - 1 and thousand grain weight. The duration of grain filling was critical on Australian wheat cultivars and had a strong impact on grain yield and grain protein content. Our results indicated that NAM - A1 alleles are significantly related to the length of the grain filling period. NAM - A1 allele a is associated with early and early to mid grain filling duration, while NAM - A1 alleles c and d associated with mid and mid to late grain filling duration.