Floral Development of the Early-Branching Papilionoid LegumeAmburana cearensis(Leguminosae) Reveals Rare and Novel Characters

Abstract

Premise of research. We present a detailed analysis of the floral ontogeny of Amburana cearensis, which is an early-branching papilionoid legume with an unusual floral morphology. Especially the reduced one-petalled corolla is noteworthy and stimulates the question about the ontogenetic background of this reduction. Are all five petals initiated and later reduced, or are four petals completely lost (i.e., without any primordia evident)? Ontogenetic studies of early-branching papilionoids provide important insights for the understanding of relationships down to caesalpinioid legumes and up to derived papilionoid lineages.Methodology. We analyzed inflorescences, flowers, and flower buds with the SEM using standard procedures.Pivotal results. Sepals and petals are initiated bidirectionally, which is a rare character among papilionoid legumes. The reduced one-petalled corolla is the result of early suppression of four petal primordia. Stamen formation is in reversed unidirectional order starting from the adaxial side of the flower. This is a rare character among Leguminosae. Floral characters that could be important for floral ecology are the rapid growth of the adaxial petal, which participates in bud protection; a long hypanthium with nectariferous stomata in the lower half; two fenestrae at the left and right of the adaxial stamen, through which nectar is accessible; and reduced anthers in the inner stamen whorl (i.e., heteranthery), which might be related to the reduction of four petals.Conclusions. Our study highlights the importance of ontogenetic studies among the diverse early-branching papilionoid lineages. Floral ontogeny in Amburana deviates considerably from the unidirectional pathway that is a common pattern among more derived papilionoid lineages. Organ reduction is based on the abortion of initiated primordia. Our study contributes to a better understanding of early-branching papilionoids and their systematic relationships.