Morphological and anatomical characterization of extrafloral nectaries of Opuntia streptacantha and Ferocactus recurvus (Cactaceae)

Mario A. Sandoval Molina , Simón Morales-Rodríguez, Mariusz Krzysztof Janczur

Background: The Cactaceae family displays remarkable diversity in the morphology of extrafloral nectaries (EFNs). Despite their taxonomic, ecological, and evolutionary significance, their anatomy and morphology are poorly understood.
Questions: How are the morphological and anatomical attributes of extrafloral nectaries in Opuntia streptacantha and Ferocactus recurvus?
Studied species: Opuntia streptacantha Lem. and Ferocactus recurvus (Mill.) Borg.
Study site and dates: Helia Bravo Hollis Botanical Garden, Zapotitlan Salinas, State of Puebla, México, during 2017.
Methods: EFNs samples were collected from the plants, fixed in glutaraldehyde, and processed for analysis using scanning electron microscopy and light microscopy.
Results: In both species, EFNs are modified spines adapted for nectar secretion. In F. recurvus, they are elongated and blunt, and epidermal cells are wrinkled, forming a lump at the tip. In O. streptacantha, EFNs possess an apical secretory cone where nectar is stored and exuded. This region has globular and imbricated, bag-shaped epidermal cells without stomata. We distinguished three regions in these nectaries: an apical secretory cone, a middle elongation section, and a basal meristematic region. The apical secretory cone has globular epidermal cells that surround a lignified region of the spine. We could not detect vascularization in the extrafloral nectaries of O. streptacantha.
Conclusions: This study reports, for the first time, the existence of EFNs in O. streptacantha and sheds light on the histological and morphological characteristics of EFNs in F. recurvus.
Keywords: microscopy, nectar secretion, nectaries, secretory spines.