Abstract: Aims To investigate the effects of nitrate concentration on anthocyanin accumulation and spatial distribution in Mikania micrantha, and to reveal molecular mechanisms of adaptation to nitrogen fluctuations through secondary metabolic regulation. Methods Controlled experiments with six nitrate concentrations (0, 0.5, 5, 10, 20, 40 mmol L–1) were conducted to measure anthocyanin content and spatial distribution in main stems and branches, combined with transcriptome analysis of key metabolic pathway genes and transcription factors. Important findings The results show that under low nitrogen conditions (0–0.5 mmol L–1 NO₃⁻-N), both the main stems and branches accumulated substantial anthocyanins to enhance stress resistance. By contrast, in treatments with medium-to-high nitrogen (5–40 mmol L–1 NO₃⁻-N), anthocyanin content in the main stems significantly decreased (by 43.1%–73.7%), turning them light green, while the branches remained red but with reduced anthocyanin levels, indicating a "main stem growth-branch defense" allocation strategy. Transcriptomic analysis revealed that 5 mmol L–1 NO₃⁻-N suppressed the expression of key anthocyanin biosynthesis genes (PAL, CHS, CHI, F3H, ANS, UGFT) in the main stems (downregulated by 39.2%–99.9%), while upregulating lignin synthesis genes (HCT1, CCoAOMT) and terpenoid metabolism genes (TPS11, TPS7). Additionally, nitrate modulated the expression of transcription factors (MYB [HHO2], bHLH [PIF3]) and sucrose metabolism genes (SPS1), influencing the secondary metabolic network. This study demonstrates that M. micrantha dynamically regulates anthocyanin allocation between main stems and branches, potentially to balance growth and stress resistance. This adaptive strategy might be a key mechanism contributing to its successful invasion in environments with fluctuating resource availability.

Key words: Mikania micrantha, nitrate, anthocyanin, transcriptional regulation, invasion mechanism.