The genus Fagopyrum, which includes more than 20 species, is centered in southwest China, with all species remaining wild except for the well-known buckwheat and tartary buckwheat. Traditional identification within this genus has relied heavily on visible traits such as leaf shape, inflorescence structure, and achene morphology. However, numerous previous studies have indicated that these morphological characteristics are often insufficient for reliably distinguishing between closely related species. Consequently, taxonomists have increasingly turned to micromorphological and molecular tools to clarify species boundaries. The current study follows this trend by combining pollen analysis, leaf anatomy, and DNA sequencing to provide a robust framework for classifying F. grandiflorum.

Morphological comparisons between F. caudatum and the newly elevated F. grandiflorum reveal several significant differences. While both taxa exhibit heterostyly, a floral polymorphism involving reciprocal style and stamen heights, their floral dimensions differ. The mean floral diameter of F. caudatum is approximately 5.23 millimeters, slightly smaller than the 5.43 millimeters observed in F. grandiflorum, though this difference was not statistically significant. More pronounced differences are seen in the achenes, the fruit of the plant. F. grandiflorum produces significantly larger achenes, with a mean length of 3.90 millimeters and width of 2.64 millimeters, compared to 3.51 millimeters and 2.08 millimeters in F. caudatum. Additionally, the leaf shape differs notably; F. caudatum has triangular-hastate leaves, whereas F. grandiflorum features narrowly hastate leaves. Petiole characteristics also diverge, as the petioles of F. grandiflorum gradually shorten until they are absent towards the branch apex, while those of F. caudatum remain present.

Pollen morphology provides further evidence for the separation of the two taxa. Both species possess spheroidal, 3-zonocolporate pollen grains with reticulate exine ornamentation, a characteristic typical of the genus. However, quantitative analysis shows that the pollen grains of F. grandiflorum are significantly smaller than those of F. caudatum. The polar axis of F. caudatum pollen measures 30.84 micrometers, compared to 29.71 micrometers in F. grandiflorum. The apertures, or colpi, also differ in dimensions; those in F. caudatum are longer but narrower than those in F. grandiflorum. Furthermore, the lumina, the spaces within the reticulate exine pattern, are significantly smaller in F. grandiflorum, with a higher density per square micrometer. These micromorphological differences, which are often more stable than external floral traits, support the view that the two plants represent distinct evolutionary lineages.

Leaf anatomical structures also exhibit significant divergence between the two taxa. Measurements of paraffin sections reveal that F. caudatum has a thicker upper and lower epidermis, as well as thicker palisade and spongy tissue, compared to F. grandiflorum. The overall leaf thickness is significantly greater in F. caudatum, measuring approximately 216.58 micrometers, versus 171.59 micrometers in F. grandiflorum. These anatomical differences suggest varying levels of environmental adaptability. Thicker leaf tissues are often associated with greater stress resistance, such as to drought or cold, which may partly explain the broader geographic distribution of F. caudatum in Yunnan, Sichuan, and Gansu, compared to the more restricted range of F. grandiflorum in Gansu and Sichuan. The consistency of these anatomical traits across multiple individuals indicates that they are genetically determined rather than plastic responses to local conditions.

Molecular phylogenetic analyses provide the strongest evidence for the interspecific relationship between the two taxa. The study utilized internal transcribed spacer (ITS) sequences from the nuclear genome and chloroplast DNA regions, specifically matK and psbA-trnH, to reconstruct evolutionary relationships. The nuclear ITS phylogeny recovered Fagopyrum as a monophyletic genus, with two strongly supported subclades. Within these subclades, F. caudatum accessions formed a monophyletic group, while F. grandiflorum clustered with F. lineare and F. callianthum, rather than with F. caudatum. This genetic separation indicates that F. grandiflorum is more closely related to other species in the genus than to F. caudatum, contradicting its previous classification as a variety.

The chloroplast DNA analysis yielded complementary results, further supporting the distinct status of F. grandiflorum. The plastid phylogeny also placed Fagopyrum species into two well-supported subclades. In this tree, F. caudatum clustered with F. macrocarpum, while F. grandiflorum formed a strongly supported subclade separate from F. caudatum. The discrepancy between the nuclear and plastid phylogenies, where F. grandiflorum groups with different relatives in each dataset, highlights the complex evolutionary history of the genus. However, both datasets clearly distinguish F. caudatum and F. grandiflorum, indicating substantial genetic divergence that is inconsistent with an intraspecific relationship. The combination of nuclear and plastid markers enhances the robustness of the phylogenetic inference, providing a comprehensive view of the genetic separation between the two taxa.

The geographic distribution and habitat of F. grandiflorum also support its recognition as a distinct species. The plant grows in valleys and mountain slopes at elevations between 1000 and 1500 meters, occurring in Wenxian County of Gansu Province and Jiuzhaigou County of Sichuan Province. It is morphologically and geographically distinct from F. lineare, which is restricted to Dali Bai Autonomous Prefecture in Yunnan, and F. callianthum, found in Aba Tibetan and Qiang Autonomous Prefecture in Sichuan. The limited range of F. grandiflorum, combined with its unique morphological and genetic profile, suggests that it has evolved independently from F. caudatum. This geographic isolation may have contributed to the accumulation of genetic differences, leading to the speciation event that the current study documents.

The reclassification of F. caudatum var. grandiflorum as Fagopyrum grandiflorum has implications for the conservation and management of wild buckwheat resources. As a distinct species, F. grandiflorum may possess unique genetic traits that are valuable for breeding programs or ecological studies. The study underscores the importance of integrating multiple lines of evidence in taxonomic revisions, particularly in genera where morphological variation is high and species boundaries are unclear. By combining morphological, palynological, anatomical, and molecular data, the researchers have provided a rigorous justification for the new species status. This approach serves as a model for future taxonomic studies in Fagopyrum and other plant groups where traditional classification methods have proven inadequate.

In conclusion, the comprehensive analysis of F. caudatum var. grandiflorum reveals significant differences from F. caudatum across multiple biological levels. The distinct pollen morphology, leaf anatomy, and genetic divergence support the conclusion that the two taxa are separate species. The elevation of F. caudatum var. grandiflorum to Fagopyrum grandiflorum reflects a more accurate understanding of the evolutionary relationships within the genus. This taxonomic revision not only clarifies the identity of the plant but also highlights the complexity of species formation in wild buckwheats. The study contributes to the broader knowledge of plant diversity in China and provides a foundation for future research on the ecology and genetics of Fagopyrum species.

Subject of Research: Revisiting the taxonomy of Fagopyrum caudatum var. grandiflorum (Polygonaceae) using morphological and molecular data

Article Title: Revisiting the taxonomy of Fagopyrum caudatum var. grandiflorum ( Polygonaceae ) using morphological and molecular data

Article References: Tan, L., Yan, R., Du, H.-M., Peng, Y., Fan, Z.-X., Wang, Q.-H., Valdés-Florido, A., Chen, H., Li, S.-C., & Wang, A.-H. (2026). Revisiting the taxonomy of Fagopyrum caudatum var. grandiflorum (Polygonaceae) using morphological and molecular data. PhytoKeys, 276, 261-274. https://doi.org/10.3897/phytokeys.276.196051

Image Credits: AI Generated

DOI: 10.3897/phytokeys.276.196051

Keywords: Revisiting, taxonomy, Fagopyrum, caudatum, grandiflorum, Polygonaceae, morphological, molecular, data, scientific research