Nitrogen regulates starch biosynthesis and multiscale structural properties of Sorghum under low-light stress

Global climate change has intensified low-light conditions, reducing photosynthetic efficiency and starch synthesis in crops like sorghum. While nitrogen (N) application can mitigate abiotic stress impacts, its role in regulating starch biosynthesis under low light remains unclear. This study investigated the effects of nitrogen on starch synthesis, physicochemical properties, and molecular mechanisms in sorghum under low-light stress. Field experiments were conducted during the 2023-2024 growing seasons using the sorghum cultivar 'Jiza 236', with plants subjected to full light (CK) and 50% shading (T), combined with three nitrogen application rates (N1:150 kg N ha^-1, N2: 225 kg N ha^-1, and N3: 300 kg N ha^-1). Low light stress reduced amylose, amylopectin content, granule size, crystallinity, and thermal stability, while increasing granule surface imperfections and solubility. However, the TN2 treatment (50% shading and 225 kg N ha^-1) mitigated these negative effects by improving photosynthetic efficiency and enhancing the activity of key starch-synthesizing enzymes (AGPase, GBSS, SS, and SBE). Transcriptomic analysis showed that TN2 markedly upregulated genes involved in starch and sucrose metabolism, glycolysis, and carbon metabolism, with a more pronounced response in apical kernels. Weighted gene co-expression network analysis (WGCNA) identified six nitrogen-regulated metabolic modules under low light, primarily involved in carbon metabolism and starch biosynthesis, highlighting nitrogen's coordinating role in starch formation. These findings provide a mechanistic framework for improving starch yield and quality in light-limited environments and offer insights for sustainable sorghum cultivation.