Ontogenic configuration of stelar development in root orders of climbing Apocynaceae species, Vincetoxicum capparidifolium (Wight & Arn.) Kuntze

Root systems demonstrate peculiar anatomical plasticity, with structural traits altering predictably across root orders to optimize water uptake, transport proficiency, and mechanical assistance. This is the first comprehensive study to elucidate the anatomical structure and hydraulic specialization across five successive root orders of Vincetoxicum capparidifolium (Wight & Arn.) Kuntze, an endemic climbing species of the Western Ghats. Transverse sections uncovered progressive alterations in stelar architecture, cortical alignment, and vascular differentiation. Root order I featured a diarch, exarch siphonostele with a uniseriate epidermis, and abundant trichoblasts with 12–14 layered parenchymatous cortex. Order II developed a triarch stele with increased starch-rich cortical cells, while order III displayed a circular, polyarch, exarch stele sustaining the siphonostelic configuration, intersecting with lateral root primordial origination. Orders IV and V determined complex secondary growth, periderm maturation, and sclerenchymatous ring formation, whereas order V manifested a complete xylem ring. Quantitatively, root diameter expanded from 694 μm to 1085 μm (order I–V), stele diameter from 284 μm to 477 μm, and total conductive area from 1159 to 26,582 µm2. Protoxylem Wmax (CV = 67.5%) and Wmin (CV = 60.5%) were more variable, while primary metaxylem Wmin (CV = 9.7%) remained stable. Theoretical hydraulic conductivity increased in variance from 1 × 10− 16 m4 s− 1 Pa− 1 to 3.8 × 10− 16 m4 s− 1 Pa− 1 (order I–V), with specific conductivity peaking in order III (4.1 × 10− 9 m2 s− 1 Pa− 1). These findings reveal a conserved developmental trajectory marked by progressive xylem maturation, contributing to enhanced hydraulic efficiency and mechanical stability across the root hierarchy of V. capparidifolium. Graphical Abstract