Ontogeny and developmental anatomy of the xylopodium in Styrax camporum uncovers its regenerative potential in fire-prone ecosystems

Xylopodia are lignified underground bud-bearing organs associated with plant persistence and post-fire resprouting in fire-prone savanna ecosystems. However, their morphological and developmental diversity has generated uncertainty in their delimitation from other woody underground structures. This is particularly evident in Styrax camporum Pohl. (Styracaceae), a common species of the Brazilian Cerrado, where clustered individuals are connected by an extensive underground woody system that supports vigorous post-fire shoot production, and whose stem-base swelling has been interpreted as either a xylopodium or a lignotuber. This study investigated the ontogenetic origin and anatomical development of this structure. Developmental analyses were conducted from seedlings to adult plants under greenhouse conditions and complemented by field observations, focusing on axis thickening, bud formation, reserve accumulation, and resprouting. The underground system developed following rapid primary root elongation and the formation of adventitious roots at the hypocotyl base. It originated from the combined secondary growth of the hypocotyl and basal primary root, forming a diffuse woody structure of mixed origin. Buds were produced throughout early development and increased after cotyledon abscission. Anatomically, the organ is characterized by extensive secondary xylem, starch-rich ray parenchyma, gelatinous fibers, and phenolic-containing cells. These results support a hypocotyl-root origin, confirming the developmental identity of the underground organ as a xylopodium. In the absence of fire, the stem base of adult individuals underwent marked morphological changes, developing an asymmetrical globose structure resembling a lignotuber. These findings highlight the importance of ontogenetic approaches for resolving morphological ambiguities among underground bud-bearing organs in fire-prone ecosystems.