Contribution to the taxonomy of Mexican species of the genus Colubrina (Rhamnaceae) based on leaf architecture and epidermal surface characteristics

Nuevas Contribuciones a la Taxonomía de las especies mexicanas del género Colubrina (Rhamnaceae) basadas en la Arquitectura foliar y las superficies epidérmicas de las hojas: Colubrina

Abstract

This study describes the leaf architecture and epidermal surfaces of nine Colubrina species distributed across Mexico, encompassing two subgenera and three sections. The species are categorized as follows: Subgenus Colubrina Section Colubrina with C. arborescens; Section Cowaiana with C. elliptica, C. heteroneura, C. johnstonii, and C. viridis; Section Barcena with C. ehrenbergii and C. triflora; and Subgenus Serrataria with C. californica and C. celtidifolia. Taxa differentiation was achieved by examining venation types (craspedodromous, brochidodromous, and imperfect basal acrodromous), branching patterns of the first pair of lower secondary veins, the number of secondary veins, trichome types (with or without a radial base, or absent), the presence or absence of glands on the leaf blade, straight or wavy epidermal walls, the presence or absence of free veins in the third and fourth category veins, and stomatal types. A dichotomous key and a clustering analysis were performed using the UPGMA method and the Jaccard similarity index, and a corresponding dendrogram is provided. The results from seven previously published species of Subgenus Serrataria (C. stricta, C. celtidifolia, C. greggii, C. lanulosa, C. macrocarpa, C. texensis, and C. yucatanensis) were considered in the discussions and conclusions to infer the similarities and differences among the 16 Colubrina taxa found in Mexico. The study found that Serrataria species lack glands in their marginal teeth, a key characteristic in the literature, and that the tooth protuberances are due to feeding through a secondary or tertiary vein. This suggests the need for a redescription or reassignment of this subgenus. The dendrogram separates the taxa into three main groups: one comprising Sections Cowaiana and Barcena, a second grouping that includes C. arborescens (from Section Colubrina) along with the nine Serrataria species. The inclusion of C. arborescens in this group is likely due to significant differences observed in specimens from southern Mexico compared to Caribbean taxa, potentially indicating it as a distinct taxon. The third grouping further separates the Serrataria species based on their venation patterns.