Pollen Morphology of <scp> <i>Crucianella</i> </scp> L. (Rubiaceae) Based on Light and Scanning Electron Microscope Data and Implications for Taxonomy

Crucianella L. (Rubiaceae) is represented by 31 species worldwide and 14 taxa in Türkiye. Although taxonomic delimitation has traditionally been based on morphological characteristics, the importance of pollen morphology in species differentiation is not yet fully understood. This study was conducted to examine the pollen morphology of Crucianella species in detail using LM and SEM techniques and to assess the taxonomic significance of these details. The pollen grains are stephanocolpate, isopolar, radially symmetrical, and have a circular amb shape. The pollen surface is covered with pollenkitt, and the pollen grains exhibit microreticulate ornamentation. Differences among investigated species were observed in terms of pollen size, exine sculpturing, and aperture characteristics. The number of colpi varies between 6 and 13. Differences in the number of apertures were found among subspecies. Ornamentation was identified as microreticulate. Microechini, perforations and microverrucae were observed on the muri. Pollen size falls within the small class, with the polar axis ranging from 11.41 to 24.85 μm and the equatorial axis ranging from 9.96 to 24.87 μm. The smallest average pollen size was observed in C. kurdistanica (P = 14.90 ± 2.30 μm; E = 13.46 ± 2.11 μm), while the largest was recorded in C. bithynica (p = 21.92 ± 1.86 μm; E = 20.99 ± 2.54 μm). Exine thickness ranged from 0.54 to 1.15 μm, and intine thickness ranged from 0.33 to 0.59 μm. This study demonstrated that palynological characters within the genus Crucianella especially pollen size, aperture details, and the thickness of exine and intine, can provide valuable contributions to the systematic differentiation of closely related species and subspecies.