A unique inflorescence in the tribe Mimoseae (Fabaceae, Caesalpinioideae): ontogeny and floral function division in Parkia platycephala, a native species of the Brazilian Cerrado

Abstract Background and Aims Parkia platycephala, a native Cerrado species, belongs to the tribe Mimoseae (Fabaceae) and is distinguished by its unique capitulum, which comprises two floral morphotypes: seed-producing flowers and nectariferous flowers. The functional roles of these morphotypes remain unclear, prompting us to investigate inflorescence and floral development to explore their evolutionary and ecological significance. Methods Flowers, inflorescences and floral buds at various developmental stages were collected and processed for surface analysis (scanning electron microscopy) and anatomical analysis (light microscopy). Key Results We observed the initiation of three floral unit meristems (FUMs), with abortion of the basal FUM probably due to spatial restriction. Floral buds exhibit a subtle developmental gradient, with seed flowers located in the basal and median regions and nectariferous flowers restricted to the apical region of the capitulum. Both morphotypes display an elliptical floral meristem. Sepals arise in a sequential order: petals, simultaneously; antesepalous and antepetalous stamens emerge unidirectionally; and the single carpel develops concurrently with the lateral antesepalous stamens. The stamens are basally fused and adnate to the petals, forming the stemonozone. Polyads in seed-producing flowers contain cytoplasmic content, unlike most polyads in nectariferous morphotype. Conclusions The spatial arrangement of floral morphotypes within the inflorescence defines its zonation, with each capitulum functioning as a pollination unit. The congested arrangement of floral buds and pressure exerted by bracts may contribute to developmental peculiarities, including fasciation and merism variation. We hypothesize that the seed-producing flowers function primarily in fertilization, whereas nectariferous flowers, the only morphotypes containing an intrastaminal nectary, serve as food reward for pollinators. Additionally, the type of initiation of sepals, reported exclusively in Parkia, underscores its unique floral developmental pattern. Our anatomical and developmental analyses elucidate the reproductive structures of P. platycephala and clarify the functional significance of its floral dimorphism.