Thymus × citriodorus (Pers.) Schreb., Lamiaceae is a perennial, evergreen, pubescent, erect shrub up to 30 cm tall with a characteristic lemon scent. The leaves are narrow, oval to lanceolate, and typically green, although some patterns of yellow and white can appear in some cultivars. The flowers are white or slightly pink and the flowering period occurs during summer months. Thymus × citriodorus grows in well drained soils, preferably calcareous and with plenty of sun. Propagation by herbaceous stem cuttings is preferred to propagation by seed, minimizing heterogeneous characteristics associated with its hybrid status (Tavares et al. 2010; Mourão 2012).

Thymus × citriodorus is considered a hybrid between T. pulegioides L., geraniol chemotype, and T. vulgaris L., although some controversy is present regarding its characterization (Stahl-Biskup and Holthuijzen 1995). Margaret Easter (2021), in her article in Plant Heritage, referred to T. citriodorus as a species, not as a hybrid. Thymus × citriodorus was first described as a species in 1811, but later in the 1970s it was regarded as a hybrid of European garden origin, between T. pulegioides and T. vulgaris, and consequently perpetuated by the nursery trade and became gradually accepted as such (original reference not available—accessed in (Easter), on March 2023). In the same article, the author discussed about a previous study conducted by herself and colleagues, where Random Amplified Polymorphic DNA analysis was carried out to identify genetic relationships between Thymus plants. The study showed that T. citriodorus was not included in the cluster formed by the species T. vulgaris and T. pulegioides, its supposed hybrid parents. Based on these findings, the author proposed that it would be acceptable to revert T. citriodorus to its original species status. Still, in a different study conducted by Kerem et al. (2023), the authors analyzed different Thymus species using molecular characterization with inter-simple sequence repeat markers and clustered together T. vulgaris and T. citriodorus. The hybrid status is also repeatedly presented by the scientific community, accepted by the nursery trade, and by the Royal Botanic Kew Gardens, London UK, that accepts and adopts the hybrid status (POWO 2023).

There is also no unanimity regarding its origin, as some authors refer that T. citriodorus has no natural distribution, while others, along with the Royal Botanic Kew Gardens, refer to it as native to Southern Europe and Vaičiulytė widely cultivated in the Mediterranean region (Omidbaigi et al. 2005, 2009; Bagdat et al. 2011; Duman and Özcan 2017; Toncer et al. 2017; POWO 2023). Several varieties (or cultivars) of this hybrid are also mentioned in the literature, although most of the conducted scientific studies refers only to T. citriodorus (Trindade et al. 2018). These different cultivars are distinguishable by the appearance and color of the leaves and include varieties such as T. citriodorus “aureus,” T. citriodorus “archer’s gold,” T. citriodorus “golden dwarf,” and T. citriodorus “silver queen” (Horváth et al. 2006; Aprotosoaie et al. 2019; Steshenko et al. 2021; Steshenko and Mazulin 2022).

Overall, several species of the genus Thymus are associated with a vast list of pharmacological effects, and thyme-derived products (herbal substances and preparations) are applied in traditional medicine for several conditions (Horváth et al. 2004, 2006; Pina-Vaz et al. 2004; Hajimehdipoor et al. 2009; Park 2011).

In the specific case of T. citriodorus, its major application is for culinary purposes due to its pleasant lemon flavor (Garibaldi et al. 2007; Ntalli et al. 2020a). Thymus × citriodorus is used as an ingredient for confection of several recipes of starters, snacks, sauces, and different meat and vegetarian dishes and soups and is also consumed in salads as well as in marinades. It is also used in jellies and desserts (Li et al. 2019). Besides its direct raw culinary use, T. citriodorus is also vastly consumed in the form of infusion or tisane (Omidbaigi et al. 2009; Pereira et al. 2013b). In fact, due to its high consumption and interesting profile as a source of bioactive compounds with potential health effects, it has been recently studied as a potential functional food, specifically as a nutraceutical (Taghouti et al. 2020). The by-products that remain after harvesting for culinary purposes, freely available as waste, are also used for animal feed, contributing to a circular economy (Ntalli et al. 2020a).

Apart from its value in the culinary field, some medicinal uses are also described for this plant, despite there are no ethnopharmacological studies available. Most of its described medicinal/pharmacological uses are for respiratory tract conditions. Its VO is used in aromatherapy for symptomatic relief of asthma and of other respiratory disorders, especially in children, as it is considered better tolerated than other Thymus VOs (Horváth et al. 2006; Pereira et al. 2013a). Tinctures prepared from this species are used for alleviating respiratory symptoms in cases of pneumonia and bronchitis (Steshenko and Mazulin 2022). The dried leaves of the plant prepared in potpourri and herbal-filled pillows are also used for symptomatic relief of respiratory disorders, as bronchitis (Horváth et al. 2006; Omidbaigi et al. 2009; Rita et al. 2018). In addition to the traditional uses for respiratory tract affections, compresses and baths prepared from the stems and leaves of the plant are used as analgesic, to reduce pain in arthritis (Steshenko and Mazulin 2022). Its VO is also used due to its antiseptic and deodorant properties (Pereira et al. 2013a). Other preparations of this plant, such as hydrolates (by-products of VO production), have also been valorized, and their properties as regulators of oily skin and hair, anti-dandruff agents, and adjuvants in the treatment of pediculosis and as hair tonics are often advertised by sellers of these products (Oliveira et al. 2022).

Since some of the beneficial activities associated with T. citriodorus are presumably related to its volatile metabolites, the composition of the VO obtained from this plant has been the focus of many investigations (Toncer et al. 2017; Aprotosoaie et al. 2019; Malankina et al. 2019; Jurevičiūtė et al. 2019; Pasławska et al. 2020; Ložienė et al. 2021; Oliveira et al. 2022). Table S1 abridges the source, processing features, and chemical characterization of the T. citriodorus VOs analyzed by gas chromatography–mass spectrometry (GC–MS), along with information regarding the major compounds in each VO and their relative abundance. In most studies, the VO was obtained from hydrodistillation of air-dried plant material using a Clevenger-type apparatus. Hydrodistillation duration varied across studies but was majorly between 2 and 4 h. The use of steam distillation was also described in some studies (Horváth et al. 2004, 2006; Sacchetti et al. 2005; Wu et al. 2013; Checcucci et al. 2017; Oliveira et al. 2022).

The identification and quantitative analysis of VO components was typically performed by GC–MS. Most of the studies reported compositions characterized by high percentages of oxygen-containing monoterpenes followed by moderate percentages of monoterpene hydrocarbons. The major compound usually reported in T. citriodorus VOs was geraniol (1), an open-chain monoterpenic alcohol derived from geranyl pyrophosphate (2) (Stahl-Biskup and Holthuijzen 1995; Horváth et al. 2004, 2006; Omidbaigi et al. 2005, 2009; Bertoli et al. 2010; Keyhani and Keyhani 2010; De Lisi et al. 2011; Teixeira and Rodrigues 2014; Toncer et al. 2017; Malankina et al. 2019; Jurevičiūtė et al. 2019; Ntalli et al. 2020a; Ložienė et al. 2021; Vaičiulytė et al. 2022; Oliveira et al. 2022). Relative amounts of this component ranged widely across different studies varying from 4.3 to 83.3% (Tátrai et al. 2016; Malankina et al. 2019). Details of how intrinsic or extrinsic factors influenced the major components of T. citriodorus will be addressed in detail in the section “External Influences on the Chemical Variation of Thymus × citriodorus VO and Extracts.” Other relevant components described in T. citriodorus VO plants were nerol (3)—the cis-isomer of geraniol—and the structurally and biogenetically related aldehydes, geranial (4) and neral (5), together known as citral (Chen and Viljoen 2010). These two aldehydes are responsible for the lemon-like odor and flavor as studies show that citral flavor is much more associated to lemon by consumers than even d-limonene, despite the last being the primary component of lemon (Hirai et al. 2022). Altogether, these oxygen-containing monoterpenes assign to T. citriodorus its characteristic citrus-sweet rose flavor and aroma (Pasławska et al. 2020).

In additon to the most common geraniol chemotype, until 2018, other three were proposed for this hybrid, specifically, the carvacrol, borneol, and terpinolene chemotypes (Trindade et al. 2018). Horváth et al. (2006) reported carvacrol (6) and p-cymene (7) in considerable relative amounts (43.5% for carvacrol; 21.1% for p-cymene) in a T. citriodorus VO obtained from plants cultivated in Hungary, being carvacrol (6) the major component. This VO was obtained from the “archer’s gold” cultivar. Terpinolene (8) and α-terpineol (9) were reported as the major compounds in a VO from T. citriodorus cultivated in south-eastern Anatolia, Turkey (Toncer et al. 2017). Also, Wu et al. (2013) reported borneol (10) and thymol (11) (28.82 and 14.43%, respectively) as the major compounds in a T. citriodorus VO. There was also a report of citronellol (40.1%) (12), followed by geranyl acetate (27.5%) (13) as the major compounds of an VO distilled from fresh plants grown in Italy (Checcucci et al. 2017).

Recently, other major compounds were reported in relative amounts that could account for new pure chemotypes, where the dominant component represented over 50% of the total composition. Malankina et al. (2019) reported thymol (11) (53.46%), followed by isobornyl propionate (14) (12.92%), as the main components of VO from T. citriodorus var. variegata, a hybrid with different parental forms. Also in 2019, Aprotosoaie et al. (2019) reported (R)-(-)-lavandulol (15) (54.27%) as the major constituent in a Moldavian T. citriodorus VO.

Other extracts have also been studied for their chemical composition, specifically regarding phenolic compounds (Park 2011; Pereira et al. 2013a, b; Villanueva Bermejo et al. 2015; Rita et al. 2018; Taghouti et al. 2020). Table S2 compiles information of the source, processing, and extraction features (when disclosed in the original paper) and chemical characterization of T. citriodorus extracts. From these, the majority consisted of aqueous extracts, prepared either as an infusion or as a decoction (Rita et al. 2018; Ntalli et al. 2020a, b; Taghouti et al. 2020). Ethanol extracts and methanol extracts were also prepared and studied by several authors (Pereira et al.