Delayed Panicle Nitrogen Application Enhances Stem Nonstructural Carbohydrate Accumulation in Large-Panicle Rice Through the Sucrose–Starch Metabolic Network

Accumulation of stem non-structural carbohydrates (NSC) at heading is crucial for mitigating grain-setting defects in large-panicle rice. While traditional panicle nitrogen fertilizer application at the emergence of the fourth leaf from the flag leaf stage (TL4) may weaken stem sink strength, delaying application to the emergence of the third leaf from the flag leaf stage (TL3) significantly enhances NSC accumulation. This study aimed to elucidate the molecular mechanisms through which TL3 remodels stem sink strength to promote NSC storage. Using two large-panicle rice varieties (Huiliangyou 280 and Yangliangyou 228), we compared stem NSC dynamics under TL4 and TL3 treatments and integrated sugar-related metabolite profiling with transcriptome analysis during the critical NSC accumulation phase. The results showed that TL3 treatment significantly increased stem NSC content and NSC per spikelet at heading, leading to a higher percentage of filled grains. The period from 5 days before heading (DBH) to heading showed the highest NSC accumulation rate. At the molecular level, TL3 treatment specifically up-regulated eight key genes in the sucrose–starch metabolism pathway, increasing the activities of sucrose phosphate synthase, sucrose synthase, and ADP–glucose pyrophosphorylase, and thereby promoting the accumulation of sucrose, trehalose, and D-fructose. In summary, delaying panicle nitrogen application to TL3 enhances stem NSC storage by remodeling sink strength via coordinated regulation of the sucrose–starch metabolic network.