Floral ontogeny and secretory structures of Discolobium pulchellum and Riedeliella graciliflora (Leguminosae: Papilionoideae: Dalbergieae): two closely related genera with diverging floral morphology

Non-papilionate and actinomorphic flowers have evolved independently in Papilionoideae, and few ontogeny studies are available about them. The objective of the present study was to better understand the ontogenetic processes that result in papilionate and non-papilionate flowers in a sister group of the Pterocarpus clade. Inflorescences, flower buds and flowers of Discolobium pulchellum and Riedeliella graciliflora were analyzed by scanning electron microscopy, light microscopy and histochemistry of the secretory structures of floral organs. The differentiation of the petals is initiated by the petal standard followed by the keel petals and finally by the wing petals in Discolobium, in contrast to a unidirectional development of the petals in the corolla of Riedeliella. The order of sepals and antesepalous stamen is unidirectional in D. pulchellum and modified unidirectional in R. graciliflora; however, the order of petal and antepetalous stamen initiation, stamen symmetry and early appearance of carpel are similar in both species. Hypanthium and floral nectary are present only in D. pulchellum and colleters in R. graciliflora. Glandular trichomes and idioblasts are present in both species. The presence of secretory structures is possibly related to the protection against herbivores, to pollination and pollinator’s attraction since different secondary compounds are present in reproductive structures. The development of the actinomorphic corolla of Riedeliella graciliflora has no similarity in terms of floral development to the other representatives of actinomorphic symmetry in Papilionoideae.