Uncovering the Neglected Floral Secretory Structures of Rhamnaceae and Their Functional and Systematic Significance

Abstract

Rhamnaceae flowers are notably recognized by their fleshy nectary. Other types of floral secretory structures have been scarcely reported for this family. Thus, the objective of the present study was to update the occurrence of these structures in the family and to contribute to the knowledge of their morphology and systematic significance. To this end, we carried out an extensive bibliographic search on the secretory structures of the family and obtained data for 257 taxa. Additionally, we presented here novel data (surface, anatomy, and ultrastructure) for six species belonging to the main clades within Rhamnaceae. The family has a wide diversity of types of mucilage-secreting structures: epidermis, hypodermis, idioblasts, cavities, and ducts. Mucilage and phenolic idioblasts are widely distributed among the floral organs. Colleters are present in all sampled species, and these are the first reports of their occurrence in floral organs of Rhamnaceae. The information obtained about the structure, secreted content, and occurrence of the secretory structures of Rhamnaceae helped us to understand the assertive folk use of its species. The absence of mucilage and the presence of resin or mucilage cavities and ducts in some taxa may have intrafamily systematic significance.

Diversity of Leaf Glands and Their Putative Functions in Rhamnaceae Species

“…The secretory cavities occur in the interveinal region, and the epithelium delimits a spherical, isodiametric lumen. Ducts and cavities have also been detected in the floral organs, in the pedicel (duct), in the gynoecium (cavity), and in the hypanthium [29]. Such floral and foliar glands have a similar content with a vibrant green color when stained with toluidine blue (pH 5.8), indicating the presence of resin and phenolic compounds, in addition to similar results for the detection of lipids.…”

“…The internal glands can be defined as oil-resin secreting ducts and cavities (Rhamnidium elaeocarpum, present study) and lysogenic mucilage reservoirs (many species [46,47,50]). This diversity is also seen in the flowers of the species of the family [29,30], indicating that this group is a promising model for studies of the morphology and evolution of secretory structures. Among these, the atypical structure and functioning of the EFNs of G. polygama stands out, certainly deserving detailed studies of ontogeny, dynamics of nectar secretion, and ecological interaction.…”

“…Cavities and/or ducts mainly secreting mucilage have been recorded in about 100 species of Rhamnaceae (see Table S1 by Ribeiro et al, 2021 [29]). However, these structures are not in fact ducts or cavities, considering Fahn's concept (1979), but mucilage reservoirs, formed by the lysis of the walls of several grouped mucilage cells, a condition commonly described in species of Rhamnaceae [50][51][52][53], including R. elaeocarpum [29]. Within the Rhamnoid clade, to which Rhamnidium belongs, among the many species with records of secretory structures, leaf ducts or cavities secreting mucilage were reported only for Rhamnus diffusa and Rhamnidium glabrum [54].…”

“…Most Rhamnaceae species exhibit some type of secretory structure both in the flower and leaf [13,[25][26][27][28][29][30][31], like nectaries, phenolic and mucilage idioblasts, oil resin cavities and ducts, mucilage cavities and ducts, and colleters and glandular trichomes. Although a large number of species appear in the excellent compendiums by Gemoll (1902) [25] and Solereder (1908) [26], the glands are not characterized in detail and, therefore, the definition of types is dubious.…”

Comment on Nekkaa et al. Rhamnus alaternus Plant: Extraction of Bioactive Fractions and Evaluation of Their Pharmacological and Phytochemical Properties. Antioxidants 2021, 10, 300

“…Reynosia is a neotropical genus comprising about 18 species, distributed in the Caribbean region and central America [2], while Rhamnus is a large paleotropical and temperate genus comprising about 200 species of the Northern hemisphere [3]. Even if genera Rhamnus and Reynosia belong to the same tribe (Rhamneae), the secretory structures are not always the same, with idioblasts containing mucilage mainly in the flower being the most common situation in Reynosia, and idioblasts and other structures localized in the leaf and containing tannins in Rhamnus [4], with evident consequences about their content in bioactive compounds.…”