Nitrogen fertilizer application remodels source−sink nitrogen allocation and enhances sorghum yield under low-light stress

This study investigated how nitrogen fertilizer application affects source-sink-flow dynamics and yield formation in sorghum under low-light stress. Conducted at Shenyang Agricultural University from 2023 to 2024, the experiment used a randomized block design with two factors: low-light stress (50% shading) and full light (no shading), combined with three nitrogen rates (150, 225, and 300 kg N ha −1 ). Results showed that low-light stress significantly impaired photosynthesis, carbon and nitrogen metabolism, assimilate transport, and grain yield. However, application of 225 kg N ha −1 (TN2) most effectively alleviated these limitations by maintaining higher photosynthetic activity, enhancing assimilate transport capacity, and strengthening carbon and nitrogen metabolism in both leaves and grains, particularly in basal kernels. In addition, 15 N isotope tracing revealed that TN2 promoted nitrogen translocation from vegetative organs to panicles during grain filling. In conclusion, application of 225 kg N ha −1 enhances sorghum yield under low-light stress by improving photosynthesis, regulating carbon and nitrogen metabolism, and optimizing assimilate transport, and improving nitrogen allocation. This study provides theoretical support for coping with low-light stress and guidance for precise nitrogen management in crop practices. • Nitrogen coordinates source–sink C–N metabolism to enhance sorghum low-light tolerance. • 225 kg N ha −1 promotes amino acid and sugar transport via panicle neck bleeding. • Basal grains dominate yield through prioritized carbon and nitrogen allocation. • 15 N tracing confirms TN2 drives nitrogen translocation from vegetative organs to panicles.