Floral Nectaries in Leguminosae: Structure, Diversity, and Possible Evolution

Leguminosae, the third largest angiosperm family, is a taxon displaying the exceptional diversity of flowers and having great agricultural significance. Despite the broad range of reproductive strategies in this group, floral nectar is a key reward of legumes. The paper aims to summarize the available data on structure, ontogeny, regulation, and possible evolution of the leguminous floral nectaries. To date, detailed characterization of ultrastructure and mode of secretion is available only for a few representatives of the family whereas the majority remains understudied from this perspective. In most cases, regardless of flower symmetry, nectaries are localized between stamens and a carpel. The nectar is usually exuded from modified stomata although the exact mode of secretion by specialized parenchyma may differ between taxa. The leguminous floral nectaries often have certain features of monosymmetry with preferentially abaxial development. Nectaries were independently lost in several lineages, and equally recurrent is the emergence of substitutive, i.e. evolutionary innovative, nectar-producing structures. Floral nectaries possess a certain degree of evolutionary inertia, i.e. they remain stable even in lineages in which flower morphology underwent significant changes compared with an ancestral plan, such as shifts in merism, symmetry, reduction or polymerization of organs and alike. Due to their evolutionary stability, floral nectaries are rarely used in the taxonomy of Leguminosae.