Systematic palynology in Korean Piperales with special focus on its exine surface ornamentation and orbicule morphology

The pollen and orbicule morphology of the Korean Piperales (Aristolochia, Asarum, Houttuynia, Piper, and Saururus) were investigated via scanning electron microscopy. Piperales pollen is a monad, its size ranging from very small to large (P = 7.78–51.4 μm, E = 6.68–43.1 μm), and having a mainly circular to sub-circular shape. The aperture type is constant in the genus [inaperturate (Aristolochia), tri to pentaporate (Asarum), and monosulcate (Houttuynia, Piper, and Saururus)]. There are four distinct types of exine ornamentation: Fossulate with perforate, microreticulate with gemmae, microperforate with granula, and microechinate. The orbicules (minute sporopollenin granules) were observed in all studied taxa and thus, may be a possible symplesiomorphic characteristic of Piperales. Further, the observed orbicule surface ornamentation was similar to pollen exine patterns, for example muri, gemmae, or granula. This resemblance between orbicule and pollen exine ornamentation may imply a similar biosynthesis pattern of sporopollenin of pollen exine and orbicules. The phenogram resulting from a cluster analysis using palynological characters was generally consistent with the known molecular phylogeny of Piperales. This initial study will help understand the palynological diversity and provide detailed information of pollen and orbicule characteristics in Piperales.