Increased panicle-grain nitrogen application improves inferior-grain filling in rice under reduced drip irrigation in association with coordinated hormonal responses

Panicle-grain nitrogen (PGN) supply effectively promotes rice grain filling and increases yield, but its effects on grain filling under reduced drip irrigation and the underlying physiological mechanisms are unclear. A three-year field experiment (2023–2025) evaluated the effects of two drip irrigation amounts (D 1 , 10,200 m³ ha⁻¹; D 2 , 8966 m³ ha⁻¹) and three PGN levels (N 60 , 60 kg ha⁻¹; N 120 (CK), 120 kg ha⁻¹ ; and N 180 , 180 kg ha⁻¹ ) on root-shoot endogenous hormone levels, grain-filling characteristics and yield in rice. Compared with those under D 1 , under D 2 , the abscisic acid (ABA) content increased by 10.7%, and the timing of peak ABA accumulation and maximum grain filling rate occurred earlier in the inferior grains; however, the active inferior-grain filling period was shortened by 2.8 days, and the yield decreased by 3.1–9.0% under D₂. Compared with N 60 , the N 120 and N 180 treatments increased spikelet-root activity (SRA) by 5.0–244.9% and indole-3-acetic acid (IAA) levels and IAA/ABA ratios in roots, leaves, and grains by 2.3%–242.6%, thereby increasing the mean grain-filling rate ( G mean ) by 0.8–9.5%. PLS-PM revealed that under D 2 , inferior grain filling was positively associated with yield. Notably, compared with D 1 N 120 , D 2 N 180 altered the IAA metabolic characteristics of inferior grains, increasing SRA (19.4%) and G mean (8.24%), while maintaining a relatively high grain yield. In conclusion, the application of 180 kg ha⁻¹ panicle-grain nitrogen fertilizer under reduced drip irrigation (8966 m³ ha⁻¹) improved inferior grain filling by increasing root physiological activity and regulating the IAA/ABA balance, thereby maintaining a higher yield.