Integrative taxonomic revision of the Camellia rhytidocarpa complex (Theaceae) synonymous status of C. lipingensis and C. zengii supported by morphological, anatomical, palynological, and molecular evidence

Background The section Tuberculata ( Camellia L.), as a monophyletic group characterized by tuberculate fruits, exhibits persistent taxonomic ambiguities among its constituent species, exemplified by the unresolved delimitation of Camellia lipingensis , Camellia zengii , and Camellia rhytidocarpa . These three species are highly similar in terms of morphology, genetics, or ecology as a plant complex. Historical revisions have been hindered by the absence of key morphological characteristics in type specimens and the instability of morphological identification criteria, leading to unclear classification of species. This study, based on type locality specimens, morphology, and systematic molecular biology, systematically integrates macroscopic morphology, microscopic structure, and molecular systematics data for the first time, aiming to clarify the taxonomic relationships among the three species. Results Multidimensional evidence based on morphology, anatomy, palynology, and molecular systematics supports the merger of C. lipingensis and C. zengii into the synonym C. rhytidocarpa . Morphological analysis reveals continuous variation in key traits: leaves lanceolate (6.42–12.50 × 1.17–4.45 cm); floral parts with 6–9 rounded sepals, 3–5 hairy styles, and 2.2–4.1 cm long filaments; fruit subglobose (diameter 2.24–3.18 cm), ovary 3-4-loculed (1 seed per locule). Anatomical and pollen characteristics are conservative: leaf epidermal stomata are elliptical (39.9–41.2 × 31.4–36.7 µm), with a density of 62–86 per mm²; pollen is nearly spherical (polar axis 36.7–37.8 µm/equatorial axis 40.3–41.3 µm, P/E ratio 0.87–0.91). Molecular systematics confirmed that the three form a strongly supported monophyletic clade (ML/PP = 100/1.00), with consistent chloroplast genome structures (157,029, 157,029, 157,048 bp; GC 37.3%; containing 87 protein-coding genes, 37 tRNA genes, and 8 rRNA genes). Conclusions This integrative study consolidates C. lipingensis and C. zengii as conspecific synonyms of C. rhytidocarpa based on congruent morphological, anatomical, palynological, and molecular phylogenetic evidence. The taxonomic revision resolves persistent delimitation conflicts within sect. Tuberculata while establishing an empirical framework for: Phylogenetic reconstruction of Camellia lineages with overlapping morphological variation, Conservation prioritization of evolutionarily significant units in East Asian biodiversity hotspots, and Development of standardized species delimitation protocols for taxonomically complex plant groups.