Unveiling two types of reproductive nectaries in Pseudobombax argentinum (Malvaceae-Bombacoideae).

Nectaries in Malvaceae are in general trichomatous glandular structures usually aggregated in cushions. However, little is known on the diversity of nectaries in the Bombacoideae subfamily. Pseudobombax argentinum presents two contrasting types of nectaries in the flower that were investigated for the first time using light, confocal laser and scanning electron microscopy, and field observations. Sepal nectaries are elevated glands on the abaxial side of the calyx tube. The exuded nectar was exclusively consumed by ants in the period between the flower bud development and the anthesis. The nectaries consist of secretory epidermis, nectary, and subnectary parenchyma. The two latter tissues are vascularized by phloem and xylem. Nectar release could occur both through the rupture of the cuticle or through the papillae. Histochemical tests indicated that secretions on the epidermis presented polysaccharides, pectins, and proteins. The second type of nectary corresponds to secreting areas on the external side of the staminal tube, which were active when the flower opening was initiated in the early evening and for 2 days. Bees and hummingbirds searched for this nectar in daylight hours. These nectaries are non-structural, with nectarostomata probably involved in the nectar release. Our findings suggest that the primary function of sepal nectaries is to attract and feed ants, while staminal nectaries play a role in rewarding pollinators. Both nectaries are non-trichomatous. The evaluation of variations in nectaries may be useful to determine evolutionary changes of this trait and its systematic importance. Thus, a comparison of nectary types across the Malvaceae groups is presented.