Comparative Analysis of Leaf Micromorphology, Phytochemical Profiles, and Antioxidant Activity in Two Vallaris Species (Apocynaceae)

In Thailand, only two species of Vallaris are commonly recognized: Vallaris glabra (L.) Kuntze and Vallaris solanacea (Roth) Kuntze. Both species also have demonstrated promising anticancer and other biological activities. This study aims to investigate the leaf micromorphology, chemical profiles, and antioxidant properties of these two species to support their differentiation and potential for development into medicinal extracts. Leaves were examined using a light microscope and a scanning electron microscope. Chemical profiles of the leaf extracts were analyzed using Thin Layer Chromatography (TLC) to produce species-specific fingerprints. Antioxidant activities were evaluated through DPPH, ABTS, and FRAP assays. The leaf micromorphology revealed that both species possess polygonal epidermal cells and predominantly paracytic stomata, with a few anisocytic types, mainly located on the abaxial surface. Although the number and size of epidermal cells and stomata varied slightly between species, both exhibited unicellular trichomes on the leaf surface. Idioblasts containing druse crystals were observed under a polarized light microscope, with V. solanacea showing a greater abundance of druse crystals compared to V. glabra. TLC profiles of the leaf extracts showed distinct differences of compounds based on retention factor (Rf) values, particularly in ethanolic and aqueous extracts. Notably, the ethanolic extracts of V. glabra exhibited strongest DPPH radical scavenging activity (IC50 67.55 mg/L). Overall, micromorphological traits, particularly the abundance of druse crystals and quantitative differences in epidermal cell and stomatal characteristics, and TLC profile of ethanolic leaf extract, provide reliable taxonomic markers for distinguishing between Vallaris species. Meanwhile, antioxidant activities highlight the potential of V. glabra as a promising natural source of bioactive compounds for pharmaceutical development.