Physiological and Transcriptional Basis Underlying Yield and Quality Formation of Rice Under Different Nitrogen Management Regimes in Black Soil Region

Nitrogen (N) is a pivotal nutrient governing rice growth, yield, and grain quality. With increasing global emphasis on sustainable agriculture, optimizing nitrogen use efficiency (NUE) has become a primary research priority. In this study, the high-yielding japonica cultivar Longjing 31 was used to evaluate NUE under various nitrogen application rates (0N, LN, MN, and HN) and allocation regimes (4:3:3, 3:4:3, and 3:3:4 for basal, tillering, and panicle fertilizers). Agronomic traits (such as plant height and tiller dynamics) and physiological indicators [including non-structural carbohydrate (NSC) content and nitrogen-metabolizing enzyme activities] were systematically analyzed to establish optimal nitrogen management strategies in the black soil region. The results indicated that elevated N input enhanced plant height, productive tiller number, biomass accumulation, and spikelets per panicle. Carbohydrate content (soluble sugars and starch), total plant N accumulation, and activities of key N-metabolizing enzymes were concurrently enhanced, whereas NUE indices progressively declined with increasing N rates. Transcriptional responses of key sucrosetransport and nitrogenassimilation genes further revealed the underlying molecular basis, with expression trends consistent with enzyme activities and assimilate translocation. Postponing nitrogen application to later growth stages significantly improved both agronomic and physiological performance, thereby enhancing grain yield, milling quality, and NUE. Notably, the MN 3:3:4 fertilization regime achieved a favorable source-sink balance by moderating excessive vegetative growth, reducing unnecessary dry matter retention in vegetative organs, facilitating NSC remobilization to panicles, and promoting grain filling with elevated NUE. Overall, these findings provide theoretical and practical guidance for optimizing nitrogen fertilization in cold-region black soil rice cropping systems.