Floral vascularization aiding in unraveling morphologically complex flowers of Apocynaceae.

Apocynaceae exhibit remarkable floral morphological diversity, primarily through increased synorganization between the androecium and the gynoecium. However, internal structures such as floral vascularization remain underexplored. This study investigated vascular patterns in different morphological groups within the family, focusing on progressive adnation between stamens and carpels to better understand morphological variation and floral evolution. Floral buds and open flowers from five Apocynaceae species were analyzed using histological techniques, 3D reconstructions via X-ray microcomputed tomography, and ancestral character estimation. All species exhibited pedicel traces arranged in an eustele pattern, with more than ten vascular bundles. Each sepal was supplied by three traces, while petals and stamens received a single trace each, and the gynoecium was vascularized by the remaining bundles. A consistent petal-stamen vascular ring was observed, even in flowers without epipetalous stamens, suggesting an evolutionary remnant of a petal-stamen tube. In Asclepias curassavica, staminal traces contributed lateral bundles to the petals. Staminal coronas in Asclepiadoideae varied in their vascularization. Analyses indicated that non-vascularized coronas, the presence of petal vascularization derived from staminal traces, and unbranched stamen traces represent ancestral conditions. These vascular patterns reveal complex evolutionary trajectories and support the hypothesis of an ancestral floral tube. The results contribute to a deeper understanding of structural and evolutionary aspects of floral diversity in Apocynaceae.