Identifying foliar volatile organic compounds of Plectranthus and Coleus (Lamiaceae) as predictive markers of genus using GC×GC-TOFMS and machine learning

Abstract

Two prominent plant genera, Plectranthus, and Coleus, many species of which are indigenous to southern Africa, have been previously classified as a single genus of the name Plectranthus. However, phylogenetic analysis of markers of the plastid genome of subtribe Plectranthinae (family: Lamiaceae) has led to the recognition of Coleus as a sister taxon to Plectranthus. The purpose of this study is to analyse the profiles of foliar volatile organic compounds (VOCs), from the leaves of southern African species of Plectranthus and Coleus, as predictive chemotaxonomic markers of genus, using two-dimensional gas chromatography-time-of-flight mass spectrometry (GC×GC-TOFMS) and machine learning. Foliar VOCs from fresh crushed leaves of representative species of each genera (nine Plectranthus, six Coleus) were extracted in triplicate using static headspace solid-phase microextraction (HS-SPME), and analysed using GC×GC-TOFMS. Profiles of the foliar VOCs for each sample were constructed from their total ion chromatograms, and machine learning algorithms were used to model the data, to make predictions on the genus of new samples, and to tentatively identify putative markers of genus for Plectranthus and Coleus. A high predictive accuracy (up to 90%) was obtained, with a sensitivity (for genus Coleus) of up to 100%. Top ranking variables included C6C15 compounds of various chemical classes, but most notably of the sesquiterpene isomers, which were found to occur more prevalently in genus Coleus.