A detailed study of the volatile components of Plectranthus asirensis of Saudi Arabian origin

Abstract Essential oil composition of Plectranthus asirensis grown in Saudi Arabia was chemically analysed for the first time by various 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). This analysis led to the characterisation of a total of 124 components representing 98.5% of the total oil composition. The results revealed that P. asirensis oil was mainly dominated by monoterpenoids (90.7%) in which most representative constituents were thymol (66.0 ± 0.36%), γ-terpinene (14.0 ± 0.18%), p-cymene (5.2 ± 0.06%) and β-caryophyllene (3.0 ± 0.03%). It is worth mentioning here that this is the first report on the phytochemical constituents of P. asirensis.


Introduction
Plectranthus asirensis J.R.I. Wood is an intensely aromatic stout branching shrub with large downy leaves and about 1.5-cm-long deep violet flowers in terminal spikes. It belongs to the important family Lamiaceae (Originally known as Labiateae), one of the most widely distributed plant groups, which contains about 236 genera and over 7000 species (Collenette 1985;Harley et al. 2004). Flower spikes of P. asirensis are sometimes very hairy (Collenette 1985). In Saudi Arabia, P. asirensis is locally famous with Arabic name 'Shar' and famous for its use in traditional medicine for diaper rash and itching and as an antiseptic for wound dressing (Abulfatih 1987a;Abulfatih 1987b;Thomas 2015). Antimicrobial activity of this plant against various pathogenic bacteria has also been reported (Marwah 2013). Essential oil composition ABSTRACT Essential oil composition of Plectranthus asirensis grown in Saudi Arabia was chemically analysed for the first time by various 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). This analysis led to the characterisation of a total of 124 components representing 98.5% of the total oil composition. The results revealed that P. asirensis oil was mainly dominated by monoterpenoids (90.7%) in which most representative constituents were thymol (66.0 ± 0.36%), γ-terpinene (14.0 ± 0.18%), p-cymene (5.2 ± 0.06%) and β-caryophyllene (3.0 ± 0.03%). It is worth mentioning here that this is the first report on the phytochemical constituents of P. asirensis. of various Plectranthus species has been studied earlier (Smith et al. 1996;Mallavarapu et al. 2005;Senthilkumar & Venkatesalu 2010;Padalia & Verma 2011;Ali et al. 2012;Balachandra et al. 2012;Gelmini et al. 2015). However, to the best of our knowledge, there is no previous report on the phytochemical constituents of P. asirensis which encouraged us to carry out this study.

Results and discussion
Perusal of literature on P. asirensis revealed that this plant has never been studied before for its phytochemical constituents. Hence, as part of our research interest on aromatic plants of Saudi Arabia (Khan et al. 2012;Khan et al. 2014;Khan et al. 2016), in the present study, we report a detailed chemical characterisation of volatile organic compounds of P. asirensis growing in Saudi Arabia. The hydro-distillation of fresh aerial parts of P. asirensis in a conventional Clevenger-type apparatus produced colourless oil in an excellent yield of 1.3%, v/w based on the fresh weight. The phytochemical constituents of P. asirensis aerial parts essential oil were analysed by GC-MS and GC-FID using two different types of stationary phase columns (polar and nonpolar columns) which led to the identification of 124 different compounds representing 98.5% of the total oil compositions. The identified compounds with their LRIs and their relative contents in the oil are listed in Table S1 according to their elution order on a nonpolar column (HP-5MS).
Since this is the first phytochemical study of P. asirensis, comparison of its oil composition with previous studies is not possible. However, it is significant to mention that thymol which was characterised in our present study as the main component of P. asirensis oil has previously been identified as principle component in many Plectranthus species, such as P. tenuiflorus growing in Saudi Arabia (Smith et al. 1996) and P. cylindraceus from Yemen (Ali et al. 2012). Thymol was also identified as the most leading component in P. marrubatus growing in Ethiopia (Asres et al. 2012) and as principle component in P. amboinicus from Brazil (Da Costa et al. 2010). Similarly, thymol was also characterised as one of the major components from various Plectranthus species growing in different countries (Mallavarapu et al. 2005;de Albuquerque et al. 2007;Senthilkumar & Venkatesalu 2010;Manjamalai et al. 2012;Khalid & El-Gohary 2014) (Table S2). Hence, P. asirensis growing in Saudi Arabia may be considered as a new thymol chemotype of Plectranthus species.

Conclusion
The data presented above are the first phytochemical study of P. asirensis. Analysis of its essential oil revealed the presence of monoterpenoids as the most dominating chemical group in the oil, in which thymol, γ-terpinene and p-cymene were identified as major constituents. These major compounds identified from the essential oil of P. asirensis are known to have widespread applications in perfumery, foods, cosmetics, pharmaceuticals, and flavour and fragrances industries. Thus, P. asirensis which grows wildly in Saudi Arabia can be used as an alternative cheap source for the isolation of these bioactive compounds. Moreover, identification of a large number of phytomolecules from the essential oil of P. asirensis might be exploited in the chemotaxonomic grouping of different Plectranthus species.

Disclosure statement
No potential conflict of interest was reported by the authors.