Although abundant chemical compounds have been isolated from oregano, the most important group from a commercial and application point of view, refers to its volatile oil. Oregano is the commercial name of those Origanum species that are rich in the phenolic monoterpenoids, mainly carvacrol and occasionally thymol (Azizi, 2010). EO is responsible for the most commercially important chemicals displaying its aroma, flavor, and pharmaceutical properties (Morshedloo et al.
, 2017). As presented in Table 2, a wide chemical diversity is found in the essential oil composition of O. vulgare in point of quantitative and qualitative, depending on geographical distribution or on the time of plant collection.
In general, the plant is rich in monocyclic monoterpenes (carvacrol, thymol, ?-terpinene and p-cymene), sesquiterpenoids (?-caryophyllene, ?-bisabolene, spathulenol, germacrene-D) and acyclic monoterpenes (linalool, linalyl acetate, ?-myrcene and geraniol). Bicyclic monoterpenes (sabinyl compounds) have also been found in some species depending on the chemotype. All subspecies of O. vulgare exhibited wide variations in oil yield and chemical composition of EO depending on the geographic origin, the mode of the extraction, plant’s developmental stage, harvest time and growing conditions (Shafiee-Hajiabad et al.
, 2014). One study with aimed to assess, the developmental stage of oregano, the quality and quantity of the herb yield, led to the following conclusion: The best term for herb harvesting, was the full flowering phase. The participation of predominant constituents in the oil was increasing, starting from the vegetative, until the generative phase but the profile of volatile terpenes remains more or less unchanged.
In particular, harvesting in October significantly affected the relative abundance of carvacrol (lower) and the abundance of its biosynthetic precursor’s p-cymene (higher) compared to harvesting at the flowering stage in July (Grevsen et al., 2009; Nurzynska-Wierdak, 2009). ?-Terpinene in the EO exhibited a high variation, due mainly to the reverse correlation with carvacrol in oregano species; ?-terpinene (outoxidation)? p-cymene (hydroxylation)? carvacrol (De Mastro et al.
, 2017). Extracts of 502 individual O. vulgare plants from 17 countries and 51 populations of European Origanum vulgare were analyzed by Lukas et al (2015); the cymyl- and/or the acyclic pathway were usually active in plants from the Mediterranean climate whereas an active sabinyl-pathway was a characteristic of plants from the Continental climate (Lukas et al., 2015).
In another study Giuliani et al. (2013) confirmed that ssp. vulgare from the Veneto region (NE Italy) growing in different ecological conditions within a restricted spatial area can synthesize different EO chemotypes; plants grown in the lowland are essentially richer in EOs and are characterized by numerous peltate glands, whereas those of midland and highland are oil-poorer and have fewer glands. Interestingly, thymol and carvacrol, previously considered absent or poorly represented in this subspecies, seem to be synthesized in populations growing at intermediate and high altitudes; moreover hedycaryol, detected in significant amounts in the population growing at low altitude, was detected for the first time in O. vulgare (Giuliani et al., 2013).
In 1998, Chalchat demonstrated that Origanum vulgare L. ssp. vulgare synthesize at least nine chemotypes of EO, including: ?-caryophyllene, thymol, spathulenol, sabinene, o-cymene, germacrene D, ?- ocimene, terpinen-4-ol, cis- sabinene hydrate (Chalchat and Pasquier, 1998).
The main constituents of the ssp. gracile from Iran were carvacrol (46.5-60.6%), ?-terpinene (13.91-16.64%) and p-cymene (7.
21-13.54%) (Moradi et al., 2014; Morshedloo et al.
, 2017); and of the Turkish oil ?-caryophyllene (17.54%), and germacrene D (12.75%) (Sezik et al.
, 1993), while oil from French characterized by greater amounts of the sabinene (26.0%), germacrene D (13.7%) and ?-caryophyllene (6.6%) (Chalchat and Pasquier, 1999). As shown in Figure 1, ssp.
virens has a high EO diversity. Thymol, carvacrol, linalol, germacrene D, ?-terpinene and ?-bisabolene are the main constituents of ssp. virens EOs from different regions. The thymol chemotype (58%) from Portugal (Castilho et al.
, 2012) and carvacrol chemotype (34-68%) from another region of Portugal (Camiletti et al., 2016; Vale-Silva et al., 2012) have been reported, while linalool (38.2-76.8%) (Figuérédo et al.
, 2006; García-Beltrán and Esteban, 2016) have reported in Mediterranean regions and Spain. Germacrene D chemotype (Chalchat and Pasquier, 1999) have reported in France. The oil from Iran characterized by ?-bisabolene and sabinene (Morshedloo et al., 2017).
?-Terpinene were reported from Corsica (20.1%) and Central Portugal (34.2%) (Lukas et al., 2008; Vale-Silva et al., 2012).
It was reported that Greek oregano (ssp. hirtum) is very rich in EO yield in comparison with other O. vulgare ssp. The thymol and carvacrol chemotypes were identified in O.
vulgare ssp. hirtum (Shafiee-Hajiabad et al., 2014). These chemotypes were most commonly carvacrol-rich and less commonly thymol-rich (Skoula et al.
, 1999). The thymol and carvacrol chemotypes were identified in O. vulgare ssp.
hirtum. It was reported that ssp. hirtum growing in Turkey contained carvacrol, p-cymene and ?-terpinene as major components, whereas one study showed that EO of ssp. hirtum from another region of Turkey contained mainly linalool (96.
31%) (Sarikurkcu et al., 2015). The main components of ssp. glandulosum EO were identified to be thymol, carvacrol and their methylethers (Houmani et al., 2002). The later subspecies from Tunisia (Béjaoui et al.
, 2013) was rich in carvacrol, whereas from another region of the same country (Mechergui et al., 2016) and also in Algeria (Semra et al., 2013) was rich in thymol and p- cymene.
Afsharypuor et al. (1997) reported the EO composition of ssp. viride, that grows wild in northern parts of Iran (with linalyl acetate, sabinene, ?-caryophyllene as main components) and differs from the composition of EO of the same species, growing wild in the Balkan area (Bulgaria, Albania, Turkey, Greece, Yugoslavia) (carvacrol chemotypes) or cultivated in Israel (thymol chemotype) (Afsharypour et al., 1997).
The other studies by researchers in Iran, revealed that both oils (Iran and Balkan) are rich in thymol as major constituent followed by 4- terpineol and ?- terpinene (Andi et al., 2011; Hashemi et al., 2017). Whereas, the EOs of ssp viride from Turkey were rich in caryophyllene oxide (25.01%) and linalool (8.32%) (Kolda? et al.
, 2015). Glycosidically bound volatiles can be considered as potential aroma precursors of plants, since the enzymatic or an acidic hydrolysis of these secondary metabolites releases free volatiles. Glycosidically bound volatiles were partially investigated in Origanum vulgare and identified that eugenol and an isomer of methoxy-vinylphenol as the main aglycones of relevant glycosides (Jerkovi? et al., 2001).