Nitrogen application at anthesis increases barley grain protein by enhancing phloem amino acid mobilisation

ABSTRACT Context. In the malting industry, the low protein content of barley grains is a major issue. A useful strategy to overcome this problem is to complement the initial fertilisation with a foliar application of nitrogen (N) near anthesis. Aims. This study aimed to advance knowledge of the metabolic adjustments displayed by plants grown under common management practices in response to foliar N fertilisation at anthesis. Methods. Field experiments with two different rates of foliar N application near anthesis were performed at three commercial sites under the common practice of each farmer. Key results. Grain protein content increased upon foliar fertilisation in all sites without affecting leaf senescence or N remobilisation efficiency. Barley plants also showed a rapid assimilation of the N applied at anthesis, increasing the leaf N organic pools and the global phloem amino acid mobilisation during grain filling. These results could be attributed to the increase in the gene expression of glutamine synthetase 1 and two amino acid transporters (HvAAP6 and HvAAP7). Conclusions. Foliar N applied was rapidly assimilated and exported with high efficiency to the grain with no negative impact over the contribution of N from vegetative organs. Furthermore, phloem N contribution was shown to be of major importance for grain protein content. Implications. Foliar fertilisation near anthesis is an efficient strategy to correct grain protein content to meet maltsters’ requirements because it did not affect pre-assimilated N remobilisation, and was useful under the different nutrient availabilities explored here (N sufficiency, N deficiency and sulfur deficiency).