The structure of nectaries in the genus Strombocactus (Cactaceae)

Background: Floral nectar is offered by the plant to its pollinators to promote cross-fertilization. Questions: Are floral nectaries morpho-anatomically similar among the taxa of Strombocactus? What sugars do nectaries offer their pollinators? Studied species: Strombocactus disciformis subsp. disciformis, S. disciformis subsp. esperanzae and S. corregidorae, during 2016-2018. Methods: Bud flowers and flowers in anthesis from each taxon of Strombocactus, were processed to be observed in SEM; other specimens were embedded in Paraplast and sectioned to histochemical tests. The concentration of nectar was measured using a refractometer. Results: The nectaries are similar among the three studied taxa, located below the stamens and reach the upper part of the ovary forming a ring in the hypanthium. An almost flat epidermis constitutes the nectary in S. disciformis subsp. esperanzae, although in S. disciformis subsp. disciformis is slightly bulked and papillae in S. corregidorae, a nectariferous parenchyma composed of metabolically very active cells, and a subnectariferous parenchyma of larger cells, associated with vascular bundles. The nectar is secreted through nectarostomata. The concentration of nectar was 8.65 +/- 3.98, 16.12 +/- 4.48 and 22.09 +/- 7.42 degrees Brix for S. disciformis subsp. esperanzae, S. corregidorae and S. disciformis subsp. disciformis, respectively. Conclusions: All taxa of Strombocactus have an annular-type nectary that secretes a low volume of nectar and together with pollen are offered as rewards. Based on other floral characteristics observed, such as the diurnal anthesis, the infundibuliform shape and the color of the flower, we propose that bees pollinate these species.