Morphology, anatomy, and ultrastructure of the floral nectary of Alphitonia excelsa (Rhamnaceae) in relation to its taxonomic position

Alphitonia excelsa (Fenzl) Reissek ex Benth. is a tree endemic to Australia. The study of floral characteristics can provide information that helps clarify the taxonomic position of the genus at the tribal level within Rhamnaceae, since it has not yet been determined. In this study, the morphology, anatomy and ultrastructure of the floral nectary were carried out with bright field, scanning and transmission electron microscopes. Alphitonia excelsa presents a structural nectary, with well-located and histologically differentiated areas. The epidermis has a single layer of cells, with a cuticle with undulations and striations, and sunken nectarostomas. The nectariferous tissue is formed by several layers of isodiametric cells, with a dense cytoplasm, large nuclei, small vacuoles, presence of plastids with several starch grains, numerous mitochondria, dictyosomes and rough endoplasmic reticulum. Cytoplasmic density decreases at the end of anthesis until this tissue is no longer differentiated from the subnectariferous parenchyma in post-anthesis. The subnectariferous tissue is formed by the third and last stratum of the nectary and is vascularized by boundless with abundant phloem. It is concluded that the annular type nectary, with sunken stomata, striated cuticle and absence of trichomes could represent a synapomorphy between Alphitonia excelsa (Tribe unknown), Gouania ulmifolia (Gouanieae Tribe), Ziziphus mucronata and Ziziphus jujuba (Paliureae Tribe). More studies should be done in order to confirm this position between or within the Gouanieae or Paliureae tribes of the Ziziphoids clade.