We verified the taxonomic importance of pollen grain characteristics, based on pollen morphology of 44 populations of 36 Onobrychis Miller taxa belonging to two subgenera and six sections, chiefly for delimitation of taxa, sections, and subgenera. We evaluated 1100 pollen grains (25 per taxon) and tested them by 13 qualitative and quantitative characteristics using scanning electron microscopy and light microscopy. For 19 Onobrychis species, pollen descriptions are presented for the first time. We used Image Tools, MVSP, and SPSS software for statistical analyses. Pollen grains of all the taxa were tricolpate, single, radially symmetrical, and isopolar. We identified three shape classes according to the polar/equatorial axes length ratio: prolate (the dominant), perprolate, and subprolate. The outline shapes in polar view were almost identical, while the taxa were different in the equatorial view of pollen grains. We found the following three exine sculpture types occurred in the species studied: reticulate, suprareticulate, and microreticulate. Our results suggest that Onobrychis pollen morphological characters were relatively homogeneous, which agreed with previous investigations. Nevertheless, our analyses revealed significant variation in pollen characters among the studied taxa, whereas the outputs of principal components analysis and hierarchical clustering vaguely confirmed the arrangement of the Onobrychis species into subgenera and sections according to Flora Iranica. Therefore, the morphological features of pollen grains in Onobrychis taxa have limited taxonomic importance for delimitation of subgenera, sections and species.
