Foliar ammonium nitrogen at booting stage enhances 2-acetyl-1-pyrroline, modulates functional compounds, antioxidant activity, and head rice yield in fragrant rice.

2-Acetyl-1-pyrroline (2-AP) accumulation and head rice yield are crucial indicators of fragrant rice quality. Although adequate nitrogen supply enhances 2-AP accumulation, the effects of different nitrogen forms on yield, aroma formation, and associated metabolic mechanisms remain unclear. Herein, one colored and two non-colored fragrant rice cultivars were treated with three nitrogen forms at the booting stage, including ammonium nitrogen (LK), amide-peptide nitrogen (NK), and nitrate nitrogen (XK). This study aimed to investigate the effects of different nitrogen forms on rice yield, 2-AP accumulation, and antioxidant activity, as well as the underlying potential metabolic mechanisms. The results showed that LK significantly increased 2-AP content and key aroma-related metabolites, including proline, Δ¹-pyrroline-5-carboxylic acid, and 1-pyrroline, while generally promoting head rice yield, although the response varied between years in TX9. All three nitrogen treatments significantly enhanced the antioxidant capacity of TX9, whereas their effects were relatively limited in SXF1 and SX172. Metabolomic analysis revealed similar metabolite response patterns between TX9 and SXF1 under LK treatment, with differential metabolites mainly enriched in arachidonic acid metabolism and diterpenoid biosynthesis. The common upregulation of metabolites such as Prostaglandin I2, Gibberellin A4, and Gibberellin A51 suggested that LK may influence lipid signaling and growth-related hormone metabolism. Overall, the LK treatment relatively consistently promoted the accumulation of 2-AP, proanthocyanidins, and total phenolics in the three fragrant rice cultivars, whereas the responses of antioxidant activity and head rice yield were influenced by cultivar and year.