Floral Development of Rhamnaceae and Origin of Its Unique Floral Features

Rhamnaceae flowers have a peculiar morphology, including keeled sepals, one stamen whorl closely related to the petals, and a broad perigynous hypanthium that supports a voluminous nectary. In the present investigation, we detailed the flower development of five Rhamnaceae species to understand the origin of such specific floral characteristics. Floral buds and flowers were processed for surface and histological analyses. The sepals emerge in sequential order and the other organs in simultaneous order. The development of the perigynous hypanthium renders the floral apex broad and concave. The sepals undergo abaxial thickening early on, forming a keel and strongly influencing the floral merosity. Petals and stamens appear close to each other on the same radius in a very short plastochron. The carpels unite soon after their emergence, forming a syncarpous ovary and free style branches. Differences in intercalary carpel growth promote the formation of inferior (Gouania virgata) and semi-inferior ovaries (Colubrina glandulosa, Hovenia dulcis, and Sarcomphalus joazeiro). Rhamnidium elaeocarpum does not undergo such growth, and the resulting ovary is superior. The keeled sepals promote the isolation of the petal–stamen pair inside the flower bud. The possibility of a common primordium that the originates petal and stamen is refuted. Comparisons with other Rosales families provide insights into the floral origin and diversification of Rhamnaceae.