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BackgroundThe inflorescences of the genus Vigna Savi have extrafloral nectaries (EFNs) among the flowers whose origin is still unknown. The disposition, anatomy and morphology, as well as the ontogeny of the extrafloral nectaries (EFNs) associated with the inflorescences of Vigna adenantha (G.F.W. Meyer) Maréchal, Mascherpa & Stainier (Leguminosae, Papilionoideae, Phaseolae) were studied. Besides, the ultrastructure of the secretory stage was described.ResultsThe inflorescence, a raceme, bears a brief globose secondary axis in each node with 2 flowers and 5–7 EFNs, which develop in acropetal direction. Each EFN originates from the abscission of a flower bud that interrupts its development, resulting in an elevated EFN. This secretory structure is formed by a ring of epidermal and parenchymatic cells surrounding a group of elongated central cells. The nectary is irrigated by phloem and xylem. Four developmental stages proceed; each one relates to a different embryological stage of the flowers in each secondary axis.ConclusionsThe first functional EFN of each secondary axis of the inflorescence reaches its maturity when both the pollen grains and the embryo sacs are completely developed and the flowers begin to open. The secretion is granulocrine. The following EFNs develop in the same way.Electronic supplementary materialThe online version of this article (doi:10.1186/s40529-014-0074-2) contains supplementary material, which is available to authorized users.

Four neotropical species of Vigna Savi (Leguminosae, Phaseolinae) have potential value as forage crops or ornamentals and could be cultivated in tropical or subtropical areas, even on floodplains. In order to obtain useful data for their culture and taxonomy, the seed morphology, germination pattern (hypogeal or epigeal) and seedling development were studied. The studied species belong to different sections of the genus:(Sect. Caracallae) and V. luteola (Jacq.) Benth. (Sect. Vigna). The seeds were collected during fieldwork conducted in northwestern and northeastern Argentina. The qualitative and quantitative characters of the seeds were registered, after which they were sown. The development of the emerged seedlings was followed, first in a greenhouse and thereafter in open field. We recorded the type of germination, the thigmotropic movements of the hypocotyl and of the stem, seedling architecture and plant longevity. These traits allowed us to differentiate the species and construct an identification key that could be useful for agronomic or floricultural purposes. The data obtained partially support the current taxonomic treatment of the genus.

To study the ontogeny of the extrafl oral nectaries present in the infl orescences of Vigna luteola (Jacq.) Benth (Leguminosae, Phaseolinae), the location, morphology, anatomy of the earliest stages, histology of the defi nitive structures and ultrastructure of the secretory stage were analyzed. The extrafl oral nectaries at different developmental stages were examined with light microscopy and scanning electron microscopy. The secretory stage was also examined with transmission electron microscopy. The racemose infl orescence of V. luteola has six nodes. At each node, a short globose secondary axis bears two fl owers and one to three extrafl oral nectaries. Each extrafl oral nectary originates from the abscission of a fl ower bud and is formed by two differentiated zones: a ring of epidermal cells surrounding a group of longitudinally enlarged papillose central cells, both with underlying secretory parenchyma. The primary secretory tissue consists of the central cells, while the ring contributes to secretion to a lesser degree. Secretion is granulocrine, by means of exocytotic vesicles and plasmalemma invaginations. Four developmental stages succeed; the third one being the secretory. The extrafl oral nectaries activity period starts when the fl owers of the same secondary axis open and ceases before fruit development.

Vigna candida (Vell.) Maréchal, Mascherpa and Stainier and V. adenantha (G.F.W. Meyer) Maréchal, Mascherpa and Stainier have potential as ornamentals and could be used as forage crops in warm areas. They are scandent, stolonate and pluriannual. In both species the flowering period begins in springtime and last until autumn. The flowers are flag blossoms, disposed in 5-10-flowered racemes of 10 cm long. The standard and the keel are white, the wings are white with violet nectar guides; a variation in the color pattern of these petals occur between the species.During the anthesis, that lasts 1-2 days, an anular nectary that surround the ovary secrete the nectar offered as reward. Legitimate flower visitors are species of Xylocopa, Centris and Bombus (Hymenoptera), because of only these individuals can trigger the system of pollen transfer; also, nectar and pollen thieves or robbers were detected but did not affect the reproductive success. In V. candida, aroma is also emitted as other secondary attractant. Experiments of controlled pollinations were carried out in order to know the reproductive system of each species. V. adenantha is self-compatible but the reproductive success reach the 100% when pollinators visit the flowers, oppositely, in V. candida, which is self-incompatible, if pollinators do not work, fruits and seeds cannot be produced. Because of the growing pattern, phenology, the beautiful and conspicuous flowers, and the nitrifying capacity, both species should be appreciated as ornamentals in warm or temperate areas, even in flooded land.