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.
