With the progress of society and the improvement of agricultural scientific technology, the single focus on high yield for rice production has gradually shifted to high quality. Coordinated development of grain yield and rice quality has become a core issue for researchers, and the underlying mechanisms remain to be solved. Two varieties, Zhongzheyou1 (ZZY1) and Zhongzheyou8 (ZZY8), were used as study materials under field conditions. The yield of ZZY1 was higher than that of ZZY8, which was mainly characterized by a higher seed-setting rate and grain weight. The rice quality of ZZY8 was better than that of ZZY1, primarily due to lower chalkiness and a higher head rice rate. The total dry matter weight of ZZY1 was lower than that of ZZY8, but the proportion of panicle dry matter weight or nonstructural carbohydrate to the total in the former was higher than that of the latter. The maximum grain-filling rate, average grain-filling rate, and key enzyme activities of ZZY1 were significantly higher than those of ZZY8, while the active grain-filling period was shorter than that of ZZY8. Furthermore, the ATP/ATPase content and energy charge values in the grains of ZZY1 were higher than those of ZZY8 at the early grain-filling stage. Transcriptome analysis showed that carbohydrate and energy metabolism were the main ways affecting the yield and quality of the two varieties. The energy production of ZZY1 was insufficient to simultaneously supply the needs thus leading to the discordant formation in its grain yield and rice quality formation.

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.