Density-dependent regulation of rapeseed root development by long-petiole leaves: Mediated by sucrose partitioning and phytohormones

• The functional contribution of long-petiole leaves to root growth diminished with increasing planting density. • Density-dependent metabolic reprogramming and hormonal regulation was observed under LP treatment. • At low density (D3), LP enhanced SUSY activity to promote sucrose allocation to roots; at high density (D5), LP stimulated invertase and starch remobilization to sustain basal root growth. The morphological establishment and yield formation of rapeseed are fundamentally dependent on root system development. While long-petiole leaves constitute the earliest true leaves in rapeseed, their regulatory effects on root growth under high-density planting conditions remain unexplored. Field experiments were conducted with two planting densities (D3, 4.5×10 5 plants ha −1 ; D5, 7.5×10 5 plants ha −1 ) and two leaf treatments (CK: no leaf pruning; LP: removal of 50% long-petiole leaves). A continuous 13 C-CO 2 labeling experiment was implemented in pot-grown plants to track photoassimilate partitioning, with half of the long-petiole leaves receiving 13 C-CO 2 pulse labeling. The results indicate that compared with CK, LP treatment reduced per-plant leaf area and dry weight while increasing root-to-shoot ratio. From seedling to bolting stages, LP increased relative expansion rate of leaf area (RER LA ) by 56.01% (D3) and 19.87% (D5), but decreased relative expansion rates of root surface area (RER RSA ) and relative growth rates of root volume (RGR RV ) from seedling to flowering stages, with average annual reductions of 32.71% and 56.98% (D3), and 32.60% and 16.61% (D5), respectively. At the seedling stage, LP treatment enhanced root sucrose synthase (SUSY) activity and elevated sucrose, starch, and indole-3-acetic acid (IAA) concentrations, but reduced sucrose transporter (SUT) levels. By the flowering stage, cytokinin (CTK), abscisic acid (ABA), and SUT contents declined under D3 density, coinciding with inhibited lateral root growth. Furthermore, LP treatment increased invertase (INV) activity and sucrose content at both seedling and flowering stages but diminished starch reserves, SUT activity, and IAA levels, collectively impeding lateral root development. Notably, LP treatment significantly elevated ABA content under D5 density, which stimulated taproot elongation. Siliques exhibited the highest 13 C assimilation and distribution rates under both planting densities. Elevated density reduced 13 C-labeled photoassimilate accumulation in vegetative organs but enhanced their allocation to seeds and stems. Root 13 C distribution rates declined from 14.5% (D3) to 11.5% (D5), demonstrating that the contribution of long-petiole leaves to root carbon allocation diminished with increasing plant density. The regulatory influence of long-petiole leaves on root growth diminished with increasing planting density. At D3, reduced long-petiole leaf count enhanced sucrose translocation to roots, whereas at D5, starch remobilization in roots was prioritized to sustain basal root development. This study elucidates key mechanisms by which long-petiole leaves modulate root morphogenesis under varying densities and establishes a theoretical framework for optimizing root-shoot balance in high-density direct-seeded rapeseed cultivation systems.