| Summary: | The chemical composition of the essential oils of Plectranthus cylindraceus and Plectranthus arabicus grown in Saudi Arabia were analyzed using gas chromatography techniques (GC–MS, GC–FID, Co-GC, LRI determination, and database and literature searches) using two different stationary phase columns (polar and nonpolar). The analysis led to the characterization of a total of 157 different compounds from both oils. In the oil derived from P. cylindraceus, 79 compounds were identified, whereas 132 compounds were identified in the oil derived from P. arabicus; these compounds account for 95.2% and 98.4% of the total oil compositions, respectively. The major constituents of P. cylindraceus oil were patchouli alcohol (55.5 ± 0.01%), 1,8-cineole (6.0 ± 0.01%) and valerianol (3.8 ± 0.18%), whereas, the main compounds of the P. arabicus oil were 1,8-cineole (50.5 ± 1.37%), β-pinene (7.0 ± 0.08%), camphor (6.3 ± 0.19%) and β-myrcene (4.1 ± 0.10%). To the best of our knowledge, patchouli alcohol found in high concentration in the P. cylindraceus oil has never been reported from the genus Plectranthus. Moreover, this is the first phytochemical study of P. arabicus.
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