Reducing nitrogen with dense planting increase sweetpotato storage roots quantity via favoring starch synthesis over lignification

Excessive nitrogen (N) inhibits crop development, yet the synergistic effects of reducing nitrogen with dense planting on sweetpotato storage root (SR) formation remain unclear. This study evaluated four N levels (0, 23, 46, and 69 mg N kg^-1 soil) and three planting spacings (0.25, 0.20, and 0.15 m). Results indicated that medium density with medium N (MDMN) significantly outperformed the traditional low-density high-N (LDHN) treatment. From 15 to 30 days after planting (DAP), MDMN enhanced root number, length, and volume, while by 45 DAP it increased SR number, root activity, and N accumulation. MDMN upregulated <i>IbKNOX1</i> and <i>IbARF5</i>, elevating ZR and IAA levels while reducing GA₃, thereby promoting starch accumulation via increased <i>IbSUS</i> and <i>IbAGP</i> expression. Concurrently, lignin biosynthesis was suppressed through downregulation of <i>IbPOD</i> and lignin-related genes. Collectively, MDMN promotes SR formation by optimizing hormonal balance to favor starch synthesis over lignification.