Title : Anatomical markers in plant systematics: Discriminating Nigerian Synsepalum (A.DC.) Daniell species via vegetative microanatomy
Abstract:
Background: The genus Synsepalum (A.DC.) Daniell has three reported species in Nigeria. However, the lack of recent formal revisions has made it difficult to determine the exact number of species within the genus. This study investigated the leaf micromorphology and the anatomical structures of the epidermis, midrib, petiole, stem transverse section (TS) and wood maceration of S. stipulatum and S. dulcificum using light microscopy. The objective was to identify additional characters to support the taxonomic circumscription of these taxa. Methods: Plant samples collected during field trips were subjected to comprehensive macromorphological and micromorphological assessments of the leaf and stem characters. Results: Macromorphological analysis reveals strong similarities across the taxa in leaf surface, venation, and margins. However, distinct variations were noted: S. stipulatum (Afi) grew as a tree with an aristate-cuspidate leaf apex, whereas S. dulcificum exhibited an acute leaf base. Micromorphologically, S. stipulatum (Akpap) displayed a larger epidermis cell length (27.08 μm), epidermal cell width (16.28 μm), and stomatal index (25.53 %). Conversely, epidermal cell number (29-40) and stomatal number (8-13) were higher in S. dulcificum, while pore/diameter was higher in the S. stipulatum (Afi) (11.4 μm). Epidermal cells were predominantly polygonal or irregular, except on the adaxial surface of S. dulcificum, which featured circular, cylindrical and polygonal cells. All studied species lacked trichomes, and stomata on the (paracytic, hemiparacytic, anisocytic, tetracytic and anomocytic) were restricted to the abaxial surfaces. Notably, both collections of S. stipulatum possesses crystals on the cells of both surfaces. Midrib outlines were generally arc-shaped, except in the S. stipulatum (Afi), which presented an almost circular-irregular shape. Highly discriminative variations were observed in petiole characters: S. stipulatum (Afi) exhibited an arc-shaped outline with a groove and 10 free vascular bundle (VB), S. stipulatum (Akpap) showed an arc-shaped outline without a groove and 3 free VB, and S. dulcificum presented a circular-oval outline with a single bean-shaped VB. Despite these differences, the two S. stipulatum collections shared similarities in cuticle thickness, shape, surface, epidermis, and vascular bundle type. Stem characters indicated that pore chain/multiple, pore cluster and solitary vessels were higher in S. stipulatum than in S. dulcificum. Furthermore, the bark was hardly distinguishable into outer and inner layers in the S. stipulatum (Akpap) and S. dulcificum, whereas it was completely indistinguishable in the S. stipulatum (Afi).
Conclusion: These findings underscore the strong taxonomic relevance of midrib, petiole, and stem characters in discriminating between these taxa, particularly in separating the two distinct collections of S. stipulatum. To further clarify their taxonomic status, subsequent investigations utilizing cytological, chemical, and molecular markers are highly recommended.
Keywords: Taxonomy, Morphology, Anatomy, Synsepalum, Nigeria.
