Understanding Nitrogen Uptake, Partitioning and Remobilization to Improve Grain Protein Content in Wheat

Abstract

In cereals production, nitrogen (N) fertiliser management is necessary to maximise yield and grain protein content (GPC). Unfortunately, uptake of N fertilisers is less than 50% in wheat, necessitating the focus on improving N uptake/utilization and better N fertiliser management. Another major problem is the negative correlation between grain yield and GPC. The physiological traits correlating to this negative relationship are not yet understood hence, this makes it difficult for breeders to select for high yield and high GPC wheat simultaneously. Therefore, there is a need to identify the physiological traits that are responsible for the negative correlation. The first aim of this research was to understand the effects of foliar N application applied post-anthesis to improve GPC as an N management practice. N applied in split dosage, on the soil at sowing, followed by foliar application after anthesis was aimed at improving grain quality. Two Australian varieties of bread wheat, Gregory (low GPC) and Spitfire (high GPC) were used in field-like and controlled conditions. Foliar N was applied at different growth stages. There was a significant increase in Gregory GPC following foliar N application at heading and seven days-post-anthesis (7 DPA). These findings indicate the effectiveness of foliar N application at specific growth stages to increase GPC in low GPC wheat. The second aim was to investigate the leaf surface structures that could be correlated to efficient foliar N uptake including, wax shape and the wax chemical compositions in four bread wheat cultivars Spitfire, Gregory Kukri and RAC875 at stem elongation and 7 DPA. Trichome density, primary alcohols and alkanes were correlated to foliar N uptake. Foliar N uptake was also characterised by identifying the time taken for foliar N entry, the forms of N (nitrate, ammonium or urea) preferentially taken and the N transporters involved. Here, ¹⁵N isotope labelling was used in Gregory and Spitfire using single and combined N forms, and accumulation into the grain and other plant parts assessed. There was maximum foliar N accumulation in shoots of Gregory 2 h post-treatment. For the N form preferentially taken up, urea and urea ammonium nitrate (UAN) had high accumulation in the grain tissues. The N transporters genes investigated for their involvement in foliar N uptake included TaNRT1.1, TaNRT2.1, TaAMT1.1, TaAMT2.1 and TaDUR3. Their expression pattern was either upregulated or downregulated after foliar N treatment. The third aim was to identify feedback effects of foliar N on root N uptake and accumulation, and on root-expressed nitrate transporter gene expression. In this experiment, labelled nitrate (¹⁵NO₃⁻) was used in a flux experiment to quantify root N accumulation after a foliar UAN pre-treatment. Root N accumulation was lower in foliar treated plants, suggesting negative feedback regulation on root uptake. For the root nitrate transporters, the expression of TaNRT1.1 and TaNRT2.1 was downregulated by foliar treatment in the early time points and upregulated at later time points. The fourth aim was to decipher the negative correlation between grain yield and grain protein content by identifying the physiological traits associated with the two important agronomic traits. 15 genotypes from a genetic diversity panel contrasting for GPC/GPD were grown in field condition, and in a controlled semi-hydroponics set up with different N treatments. Several physiological traits associated with grain yield and grain N concentration under various N treatments were identified.