Comparative morphological, anatomical and palynological study of Galium peplidifolium Boiss. and G. penduliflorum Boiss. (Rubiaceae) from Türkiye: diagnostic characters and taxonomic implications

This study presents a comparative morphological, micromorphological, anatomical, and palynological investigation of two closely related species, Galium peplidifolium and G. penduliflorum (Rubiaceae), belonging to sect. Kolgyda in Türkiye. Although morphological features of these species have previously been reported, comprehensive comparative data, particularly on micromorphology and palynology, remain limited. The results revealed several potentially diagnostic characters useful for distinguishing the two species. Galium peplidifolium was characterized by curved or geniculate stems with a partially purplish lower portion, divaricate pedicels, and a cup-shaped corolla with acute to acuminate lobes lacking blackish points. In contrast, G. penduliflorum possessed nearly erect stems, usually nodding pedicels, and a subrotate corolla with long-aristate lobes and blackish points. The mericarp surface was folded and distinctly striate in G. peplidifolium, whereas it was partially collapsed with fissures and faintly granulate—striate ornamentation in G. penduliflorum. Both species possessed quadrangular stems and bifacial, amphistomatic leaves, but differed in several anatomical features, including cortical organization, pith structure, and supporting tissues associated with the leaf midrib, while sand crystals were observed only in G. peplidifolium. Pollen grains were predominantly heptacolpate in G. peplidifolium and exhibited both hexacolpate and octacolpate forms in G. penduliflorum, whereas both species showed prolate pollen and microechinate—perforate exine ornamentation. These findings demonstrate that the combined evaluation of morphological, micromorphological, anatomical, and palynological characters provides complementary evidence for distinguishing the two species and contributes to their taxonomic characterization within sect. Kolgyda.