**2. Bioactive compounds of** *T. capitata* **EO**

According to several studies, the EO of *T. capitata* is characterized by its high chemical homogeneity. Russo et al. [23], in an experiment carried out with wild populations of *T. capitata* in Calabria (Italy), observed that all the collected specimens, despite having grown under different environmental conditions, had a very similar chemical composition and all the specimens were of chemotype carvacrol (81.5–78.4%). A relatively high percentage of *p*-cymene, γ-terpinene, and β-caryophyllene were also found in these specimens. In addition, it has been observed that the percentages of *p*-cymene and γ-terpinene decreased when the percentage of carvacrol increased, which indicated that both compounds were its precursors [24]. Saija et al. [25], studying the chemical composition of this EO found that, all the wild specimens of *T. capitata* analyzed were of chemotype carvacrol. These results agree with studies conducted by Miguel et al. [9, 26, 27], where the major component was carvacrol, regardless of both the part of the plant used and the state of development. Likewise, Tuttolomondo et al. [28], in Sicily (Italy), found 38 compounds, being the most representative α-pinene, myrcene, α-terpinene, *p*-cymene, γ-terpinene, borneol, β-caryophyllene and carvacrol (67.4–79.5%), being the 13 biotypes studied of carvacrol chemotype. These results suggest that there is no polymorphism in the EO of *T. capitata*. However, other studies are contradictory to the results mentioned above, showing the existence

*Thymus*

and Canada [16].

interior, as Teruel [20].

Interest in EOs has skyrocketed in recent times. The demand for "natural" products increases year after year, and aromatic plants and EOs are becoming part of daily life. Likewise, more and more people are investigating the use of compounds obtained from plant extracts in medicine, such is the case of the EOs of many aromatic plants such as lavender (*Lavandula angustifolia*), eucalyptus (*Eucalyptus globulus*), or mint (*Mentha piperita*), which are being investigated for their neuroprotective effects [14]. Others such as EOs from oregano (*Origanum spp*.) are

It is estimated that more than 250,000 hectares are currently used to produce about 250 different plant extracts and, so on, different EOs, so they have a high socioeconomic importance in the places where they are produced, being generally rural areas in developing countries. These EOs are often used in the food industry as well as in other products of daily use such as bath gels, soaps, detergents, oral care products and body lotions. They are also widely used in aromatherapy (International Trade Center 2014). This justifies that, on a global level, 45,000 tons of EOs are produced annually, which implies an investment of more than 600 million euros, according to a study carried out by the Ministry of Agriculture of France. The main exporters are China, the USA, Brazil, EU countries, India, and Indonesia, and the largest imports are Switzerland, the USA, EU countries, Japan,

In the industry of the EOs, one of the aromatic plants with greater use is thyme. Thyme is a small shrub and perennial aromatic plant, belonging taxonomically to the genera *Thymus* and *Thymbra*, of the family *Lamiaceae*, which includes 220 genera with plants such as mint, peppermint, basil, oregano, or pennyroyal, known throughout the world [17]. Thyme is very abundant in the Mediterranean Region. In the Iberian Peninsula, there is a high number of endemism, and it is common to find them in groups of thickets commonly known as "tomillares" [18]. Spain is one of the main suppliers of thyme worldwide [19], being the provinces with higher production Almeria, Murcia, and Granada, although it is also important in other areas of Andalusia, Castilla-La Mancha, and other provinces of

Within the Region of Murcia, we found several species of thyme, two of them being of special relevance, both for their properties and for their environmental situation [18]: (1) *Thymbra capitata* (L.) Cav., commonly known as the Andalusian thyme, has a compact and stiff, fairly branched shrubby appearance, with pink flowers arranged in pineapple-shaped heads and leaves that are linear, glandular, and fleshy-looking, with a flat margin (**Figure 3**) [21]. (2) *Thymus hyemalis* Lange, commonly known as purple thyme or winter thyme, since the flowering stage

studied for their antioxidant and antibacterial activities [15].

**116**

**Figure 3.**

T. capitata *inflorescence, characteristical disposal, and leaf morphology.*

of three different chemotypes for *T. capitata*. In this sense, Miceli et al. [29] found 75 components and the majority being carvacrol and thymol, which, in all cases, constituted more than 50% of the composition of EO, followed by γ-terpinene, borneol, and *p*-cymene, when the chemical composition of the EO of *T. capitata* specimens were analyzed in flowering stage. The analysis of the compounds found in this EO revealed that there was a direct correlation between myrcene, α-terpinene, and γ-terpinene, whose concentrations decreased as the thymol concentration increased. An inverse relationship between linalool and myrcene was also observed. Thus, the analysis of the compounds presents in the EO of the specimens studied revealed that there were three distinct chemotypes: thymol, carvacrol, and thymol/ carvacrol, the most common being those of chemotype thymol. For the first two chemotypes, a negative correlation was observed between thymol and carvacrol, so when one of the components was majority, the other was at low concentration. The thymol /carvacrol chemotype resulted from the crossing between the specimens with the two previous chemotypes. In short, independently of the chemotype, it was observed that the content of monoterpenes reached 78% of the total of compounds present in the EO of *T. capitata* [29].

In this sense, the experiments carried out by [10] confirmed the existence of these three chemotypes, which supports the hypothesis that *T. capitata* has a high polymorphism in the EO composition. To carry out these experiments, specimens grown in areas at different temperatures and degrees of humidity were used. As a result of this experiment, it was observed that those of carvacrol chemotype only appeared under conditions of high temperatures and low humidity. On the other hand, an experiment was carried out in which nine specimens were used, collected from three different areas, to later be cultivated under the same controlled climatic conditions. The results showed that the specimens maintained the chemotype that they originally presented, which is determined genetically, and did not change in the absence of climatic variations. These data suggest that the chemical composition of the EO is determined by the genetic endowment of the specimen and the different chemotypes are distributed according to the environmental conditions of the area in which they are cultivated [30].

Finally, in relation to other components found in smaller proportion (such as geraniol, camphor, or β-caryophyllene, among others), there is a high variability between populations and even within the same population [24, 31]. This variability can influence the bioactivity of *T. capitata* EO, which does not only depend on the majority component but also depends on the synergistic and antagonistic interactions that occur among all the phenolic and non-phenolic components [9, 25–27].
