Foliar nitrogen and phosphorus panicle fertilization coordinates rice yield and quality by regulating nutrient accumulation and starch synthesis under basal slow-release nitrogen application

Slow-release nitrogen fertilizer (SRF) provides a sustained nitrogen supply, but precise nutrient regulation during the reproductive stage remains critical for achieving both high yield and superior grain quality. This study evaluated the effects of foliar nitrogen (N) and phosphorus (P) supplementation under basal SRF application on yield formation, starch biosynthesis, and grain quality. A two-factor field experiment was conducted with three foliar N rates (N1: 15, N2: 45, and N3: 75 kg N ha −1 ) and three foliar P rates (P1: 0, P2: 15, and P3: 30 kg P 2 O 5 ha −1 ). Foliar N and P application significantly affected dry matter accumulation and translocation, nutrient accumulation, grain yield, and grain quality, with significant N × P interactions detected for several agronomic, physiological, and quality traits, although the N × P interaction for grain yield was not significant. Among the nine N–P combinations, N2P2 produced the highest grain yield, which was 18.30% greater than that of the lowest-input treatment (N1P1). N2P2 was also associated with higher ADPG, SSS, SBE, and DBE activities and lower GBSS activity, accompanied by higher total starch and amylopectin contents and lower amylose content. Moreover, N2P2 exhibited a higher head rice rate, peak viscosity, breakdown, and taste value, together with lower chalkiness and setback. Comprehensive evaluation ranked N2P2 as the best-performing treatment among the tested N–P combinations. Overall, foliar N and P supplementation under basal SRF application improved reproductive-stage nutrient accumulation and was associated with changes in starch synthesis-related enzyme activities and starch composition, contributing to higher grain yield and improved rice quality.