Abstract
The genus Premna, one of the largest woody genera of Lamiaceae, comprises approximately 130 species distributed across the tropics and subtropics of the Old World. Previous classification and identification of the genus has mainly relied on morphological studies, resulting in significant taxonomic ambiguity and misidentification. In Thailand, over 20 Premna species have been recorded but their phylogenetic relationships have never been well investigated, particularly within the P. serratifolia complex which is the most taxonomically complicated species with the most extensive synonymy within the genus, leading to uncertainty regarding species recognition. This study presents the first comprehensive molecular phylogeny of Premna in Thailand by utilizing four chloroplast regions (ndhF, rbcL, rps16, and trnL-F), and focuses on resolving the intricate relationships within the P. serratifolia complex while providing taxonomic notes for Thai species. The results confirm the monophyly of Premna and reveal two primary clades which are discussed in the context of both morphological and molecular evidence. We successfully disentangled the taxonomic confusion surrounding the P. serratifolia complex with reinstatement of P. paniculata, and P. punctulata while P. cordifolia was confirmed as a distinct species and occurs in Thailand. We also treated P. octonervia under P. serratifolia. Additionally, P. interrupta var. smitinandii was reduced to a synonym of P. interrupta, P. coriacea was excluded from the Thai flora, and P. coriacea var. villosa was lectotypified. As a result, 21 species are now recognized in Thailand. To facilitate future identification efforts, we provide an updated diagnostic key for all species, detailed morphological descriptions, a comprehensive list of examined specimens within the P. serratifolia complex, and field photographs of several species. These findings enhance our understanding of the taxonomy and phylogeny of Premna in Thailand, offering a robust framework for further research on this taxonomically challenging genus.