Objective To investigate the comprehensive effects of nitrogen application level on rice yield and quality, and identify the key factors influencing the synergistic improvement of rice yield and quality.

Method Using japonica rice cultivars ‘Jiyu Japonica’ and ‘Wuyou Rice No. 4’ as test materials from 2021 to 2022, three nitrogen application levels were set at 0, 150, and 300 kg/hm². The effects of nitrogen application levels on rice yield, yield components, and quality indicators were analyzed.

Result Nitrogen application increased grain protein content and yield, but reduced amylose content, taste value and gel consistency. Appropriate nitrogen application enhanced the activities of ADP glucose pyrophosphorylase (AGP) , starch synthase, starch branching enzyme, and sucrose synthase during the grain filling stage, increased total starch content in grains, raised setback viscosity and pasting temperature, as well as decreased peak viscosity, final viscosity and breakdown viscosity. Compared with non-nitrogen treatment, the 150 kg/hm² nitrogen application level treatment obviously increased the activities of AGP, starch branching enzyme, starch synthase and sucrose synthase, raised total starch content by an average of 18.21%, and increased yield by 32.50%. A comprehensive grain yield and quality index (GYQI) was derived through principal component analysis, with the treatments ranked as 150 kg/hm² > 300 kg/hm² > 0 kg/hm² in terms of GYQI. Further random forest analysis indicated that grains per panicle, starch synthase activity, and peak viscosity contributed the most to GYQI, followed by yield and glutelin content. Although the 150 kg/hm² treatment reduced taste value, it significantly increased grains per panicle, starch synthase activity, and glutelin content, which had higher contribution weights. The positive effects of these three factors far outweighed the negative impact of taste value reduction, ultimately resulting in the highest GYQI under the 150 kg/hm² condition.

Conclusion Grains per panicle, starch synthase activity, and peak viscosity are the dominant factors influencing GYQI. A nitrogen application rate of 150 kg/hm² achieves the maximum grains per panicle, the strongest starch synthase activity, and a relatively high peak viscosity, and is identified as the optimal level for the synergistic improvement of rice yield and quality. Therefore, in practical production, key indicators such as grains per panicle, starch synthase activity, and peak viscosity should be prioritized for optimization to achieve coordinated enhancement of both yield and quality.