Micromorphological and cytological comparisons between Youngia japonica and Youngia longiflora using light and scanning electron microscopy.

The genus Youngia Cass. is taxonomically complex and widely distributed in East Asia. Despite the high diversity of species, only two species, Youngia japonica (L.) DC. and Youngia longiflora (Babc. & Stebbins) C.Shih, are currently found in Korea. Among them, Y. longiflora was once recognized as a subspecies of Y. japonica. However, based on chloroplast and nuclear DNA sequences, as well as morphology, Y. longiflora was found to be a distinct species. Here, the two Korean Youngia species were analyzed using scanning electron microscope and light microscope for chromosome number, pollen, cypselae, and leaf micromorphological characters to evaluate if the examined characters are informative for taxonomy. Pollen morphology is described, and the micromorphological characters of Y. longiflora are described for the first time. Cypselae and pollen surface ornamentation, as well as trichome morphology and stomatal size varied between the two species. Y. japonica had longer chromosomes than Y. longiflora, while other chromosomal characters including the localization of the NOR (nucleolar organizer region) were similar. The present micromorphological and cytological results are of great taxonomic value for Youngia species identification in Korea.