Unraveling the development and secretory structures of the atypical flowers of Dalea purpurea (Leguminosae: Papilionoideae)

Abstract Dalea purpurea, an herbaceous papilionoid legume of the North American Great Plains, has floral characteristics that are highly atypical within subfamily Papilionoideae, such as organs reduction, low differentiation among petals, with keel absence, adnation between petals and stamens and presence of anther glands. This study aims to elucidate the ontogenetic processes responsible for its atypical architecture. Floral buds and flowers were processed by scanning electron microscope (SEM) and light microscope (LM). Five sepals initiate in a modified unidirectional order, followed by five petals simultaneously. The five antesepalous stamens initiate in a bidirectional order. No antepetalous stamens primordia were initiated, suggesting that the presumed evolutionary reduction from ten to five stamens in Dalea was due to ontogenetic pathway organ absence since the initiation. The single and central carpel is precocious, emerges concomitantly with the first stamen primordium. The hypanthium is short, and the ovary is superior, with a short stipe. The five anthers are dorsifixed and rimose, with elongated glandular dorsal and apical appendages. The petals are clawed, and positioned like a hood above the reproductive whorls during development. We found an adnation of the basal region of the filaments to the petal claws, forming a stemonozone. The structure, on the abaxial side, forms an elongated sheath through the adnation of the stamens to four weakly differentiated petals to the height of the calyx lacinia. The adaxial petal (banner) is free for the most part, with adnation only at the base of the stemonozone, forming the apron. The peculiar feature absence of the typical staminal tube seems to be circumvented by the adnation of the five stamens to the five petals and the position of the adaxial petal in D. purpurea, which looks like it performs a similar function to that of the diadelphous staminal tube of other Papilionoideae. The diversity and distribution of floral secretory structures indicate that this species, which lost part of its floral specialization, might be using chemical defenses as protection against the predation of the reproductive organs. Thus, we conclude that the atypical Dalea pupurea’s floral architecture is due to some ontogenetic events associated with evolutionary and environmental pressures.