Characterization of Mexican mint (Plectranthus amboinicus) using morphological, molecular, and GC-MS analyses

Abstract

Plectranthus amboinicus (Lour.) Spreng., commonly known as Mexican mint, is an aromatic and medicinal herb extensively used in traditional medicine across tropical regions. Despite its pharmacological and economic importance, information on its morphological, genetic, and chemical diversity in Sri Lanka is scarce. This study comprehensively characterized P. amboinicus accessions collected from different regions of the Northern Province of Sri Lanka through integrated morphological, sensory, molecular, phylogenetic, and chemical analyses. Morphological assessment revealed significant variation among five identified morphotypes based on plant stature, leaf size, branching pattern, and trichome density. Among them, morphotype M004 exhibited the strongest aroma intensity, despite its shorter stature, likely due to its dense glandular trichomes and compact leaf structure. Molecular confirmation using partial Internal Transcribed Spacer (ITS) sequence identified all morphotypes as P. amboinicus. Phylogenetic analysis clustered the Sri Lankan accessions (PQ387080, PQ390379, PQ390380) within the P. amboinicus clade but as a distinct subcluster separated from Indian and Indonesian sequences, indicating a possible unique lineage shaped by local adaptation, although this has to be tested further with more samples and by analyzing more DNA markers. GC–MS analysis of hexane extract from morphotype M004 revealed 28 volatile compounds dominated by monoterpenes and sesquiterpenes; p-Cymene (12.91%), Thymol (12.01%), cyclohexanol derivative (10.84%), Germacrene D (9.10%), γ-Terpinene (7.44%), Humulene (6.00%), Caryophyllene (5.54%), and α-Copaene (4.13%). Compared to earlier reports, Thymol was lower while sesquiterpenes were higher, with a rare cyclohexanol derivative underscoring a unique chemical profile of the accession as well. The distinct genetic and chemical divergence of the Sri Lankan population highlights the emergence of a region-specific variant with potential applications in pharmacology, aromatics, and genetic improvement programs.