Structure and ultrastructure of floral nectaries of two Opuntia species (Cactaceae) in relation to their floral visitors

Opuntia is the most diverse and widespread genus in the cactus family. In spite of the strong relationship with biotic pollination, little is known about their floral nectaries. We present the first deep structural and ultrastructural description of the floral nectaries of two Opuntia species (Opuntia anacantha var. retrorsa; Opuntia elata var. elata), linking these attributes to their functioning. A characterization of their floral visitors is also made. Both nectaries are very similar and represent the hypanthial type, with the presence of surface undulations that favor nectar retention. In O. elata, the style dilatation forms a nectar chamber. Three tissues are distinguished in these nectaries: an epidermis consisting of a single layer of cells with a thin cuticle, a nectariferous parenchyma with numerous vascular bundles with xylem and phloem and, a subnectariferous parenchyma with numerous amyloplasts. Pre-nectar may come from the starch hydrolysis of these plastids as well as from other green tissues of the plant. The exudation occurs through stomata and cuticle microchannels. O. anacantha may be a melittophilous plant, while in O. elata the presence of a nectar chamber and the orange color of the perianth suggest an incipient ornithophily.