Integrated morphological analysis of floral nectaries and androecia in the tribe Phaseoleae (Leguminosae, Papilionoideae)

Flowers of Leguminosae, the third largest angiosperm family, display outstanding diversity and are adapted to different modes of pollination. However, floral nectar is their most common and important reward. The morphology and position of the specialized floral nectaries (FNs) have been described only in the minority of legumes to date, so the investigation of these structures requires a further progress. The exact origin of these nectaries in legumes has been a matter of debate for more than a century. It was hypothesized that FNs are either of staminal origin or have no homology with any floral organs. This paper is devoted to the morphological study, using scanning electron microscopy (SEM), of the floral nectaries in seven taxa of the tribe Phaseoleae, with a focus on their localization, timing of their emergence, symmetry, along with a discussion of their possible identity. Some features of the androecium morphology potentially related to the interaction with pollinators were also studied. Although FNs have an annular habit in all studied species, they possess some features of monosymmetry, such as bilabiate shape or uneven distribution of secretory stomata. These FNs emerge late in ontogeny when all other floral parts are already differentiated and may be either free from other floral parts or adnate to the hypanthium. It is hypothesized that the leguminous FNs are not derived from stamens but represent structures sui generis although their spatial patterning is most probably dependent on some pentamerous floral whorl(s), i.e., a perianth or an androecium.