Reynoutria species are medicinal plants that belong to the family Polygonaceae and are widely distributed in eastern Asia, North America and Europe. Although the phylogeny and morphological and anatomical studies of some species in Korea have been previously reported, there are no discriminative anatomical and chemical data available. Therefore, anatomical characterization of the leaf, stem and root, and high performance liquid chromatography-diode array detector (HPLC-DAD) analyses were carried out to assess the differences in anatomical and chemical profiles among the Reynoutria plants in Korea, i.e., R. japonica, R. sachalinensis, R. forbesii and R. japonica for. elata. The anatomical evaluation showed discriminative characteristics, such as the shape of the stomata and the stomatal index of the lower leaf surface; the ratio of the adaxial/abaxial height, the size of the vascular bundles and the frequency of druse in the midrib, petiole, and stem; and the pericycle number in the root. For the HPLC analysis, ten compounds corresponding to each major peak were isolated from R. japonica roots and their structures were identified by comprehensive spectroscopic studies. Samples collected before the flowering season showed higher contents of these ten major compounds than those collected after the flowering season. The orthogonal projections to latent structures-discrimination analysis (OPLS-DA) with the inner morphological and HPLC quantification results, clearly discriminated these plants. These results provide anatomical parameters and HPLC profiling that can be used to distinguish the four Reynoutria plants, which supports quality control for their precise identification.Plants 2020, 9, 222 2 of 13 and the young stems have been used as a foodstuff [10]. R. sachalinensis and R. japonica for. elata are endemic plants on the Ulleung and Jeju islands of Korea, respectively [11]. R. sachalinensis, with its characteristic large leaves, is known to have antioxidant and anti-inflammatory activities [12,13]. The outer morphology of R. japonica for. elata is similar to R. japonica. The only differences are its smaller size, red flower and seeds [11]. Early works have reported many phytochemical constituents from the roots and leaves of Reynoutria species, including stilbenes (resveratrol and polydatin), flavonoids (rutin, apigenin, quercetin, quercitrin, isoquercitrin, hyperoside, reynoutrin and kaempferol), anthraquinones (emodin, citreorosein, physcion, fallacinol, chrysophanol, phylloquinones B and C, and anthraglycosides A and B), coumarins, essential oils, and others (lapathoside, 8-hydroxycalamenene, oleanolic acid, chlorogenic acid, protocatechuic acid, gallic acid, tachioside etc) [1][2][3][4][8][9][10][14][15][16].Although Reynoutria species are used as a folk medicine, their exact botanical origin is unclear. Previous studies have reported certain Reynoutria species with great ambiguity in their taxonomic classification, because of variabilities in their chromosome number and morphology [17]. In addition, taxono...