In cereals production, nitrogen fertilizer management is necessary to maximize yield and grain protein content (GPC).

Unfortunately, uptake of N fertilizers is less than 50% in most plants, including cereals, necessitating better management. Foliar application of nutrients is a valuable complement to root application. N can be applied in split dosage: in soil at sowing, followed by foliar N after anthesis to improve grain quality. We investigated the effects of post-anthesis foliar N application on grain N in wheat and characterised uptake of different N forms through leaf.

Two Australian varieties of bread wheat (Triticum aestivum), Gregory (low GPC) and Spitfire (high GPC), were grown in field and greenhouse conditions. Granular fertilizer (urea 46%N w/w) was applied at sowing, and subsequently foliar fertilizer (UAN 32% w/w). In field conditions, this experiment was conducted in two consecutive years (2016 and 2017) and foliar N applied at seven or fourteen days after anthesis. In the greenhouse, foliar N was applied at five growth stages (awn emergence, heading, anthesis, and seven and fourteen days-post-anthesis). Plants were harvested at maturity and processed for yield components and grain N. In a separate experiment, 15N isotope labelling was used to identify the form of N (nitrate, ammonium or urea) preferentially taken up through foliar and translocated to grain and other plant parts. 15N label of the different N forms was applied at seven days-post-anthesis to the flag leaf of cv. Gregory, on plants grown at two basal N rates (low and high). In field conditions, there was a significant increase (p–value 0.01) in cv. Gregory grain N following foliar N application at seven days-post-anthesis. Spitfire showed no grain N increase, but maintained high GPC. Similarly, in greenhouse conditions, grain N in cv. Gregory increased following foliar N application at heading (pvalue0.01) and seven days-post-anthesis (p-value0.001). Urea had the highest uptake and translocation. Partitioning of the translocated 15N was high in grain, while stem, leaves and chaff had low 15N. Differences in uptake and translocation of the N forms could be due to electrochemical penetration potential of urea a non-ionic molecule; ions require an additional electrical potential gradient. The plant cuticle is negatively charged making it a greater barrier to NO3- than to NH4+.

These results could have important implications on fertilizer management to improve grain-quality and optimization of foliar N formulations to increase uptake and translocation.