A Survey of Chemical Compositions and Biological Activities of Yemeni Aromatic Medicinal Plants

Yemen is a small country located in the southwestern part of the Arabian Peninsula. Yemen’s coastal lowlands, eastern plateau, and deserts give it a diverse topography, which along with climatic factors make it opulent in flora. Despite the introduction of Western medicinal system during the middle of the twentieth century, herbal medicine still plays an important role in Yemen. In this review, we present a survey of several aromatic plants used in traditional medicine in Yemen, their traditional uses, their volatile chemical compositions, and their biological activities.

There are around 33 species of Schinus, all of which are native to the Neotropics [7]. There is only one species, Schinus molle, which has been introduced to Yemen. The plant, known as "filfil katheb", is used in Yemeni folk medicine as an expectorant, a diuretic and also for the treatment of stomach upsets [8]. Bioactivity studies of S. molle essential oil by various researchers have shown that it possesses antibacterial, antifungal [9], cytotoxic [10], insecticidal, and insect repellent [11] properties. The essential oil of S. molle from Yemen, obtained by hydrodistillation contained β-caryophyllene (13.5%), α-pinene (2.8%), carvacrol (2%), germacrene D (16.7%), δ-cadinene (3.2%), spathulenol (3.4%), caryophyllene oxide (6.7%), viridiflorol (3.3%), α-cadinol (2.5%), α-bisabolol (4%), and an unknown compound (18%). The same plant was also extracted by supercritical CO2 and analyzed by GC-MS. Here the major components were comparable with only with some minor differences in their concentrations. The notable compounds identified were α-pinene (1.6%), β-caryophyllene (9.1%), germacrene D (13.7%), bicyclogermacrene (2%), spathulenol (2.7%), shybunol (3.3%), abietol (5.0%) and an unknown compound (37%) [12]. The observed bioactivities of S. molle essential oil can be attributed to the relatively large concentrations of the sesquiterpenoids β-caryophyllene, germacrene D, and caryophyllene oxide, which have shown antimicrobial and cytotoxic activities [13].   There are around 400 species of Artemisia, distributed typically in Asia, Europe, and North America [7]. Artemisia abyssinica, known as "boitheran" in Yemen, is an aromatic, grey, silky-hairy plant with pale yellow flower-heads and is well known as a stimulant and an analgesic. It is short lived perennial plant, with sparingly branched stems that are grooved especially above. Leaves are alternate, grey-green, deeply bipinnatisect with linear segments, and 4-10 cm long. It is widely spread on the high plateau from 2200 to 3600 m and often abundant on roadsides, alluvial plains, and abandoned fields. It is used in Yemen for treating headache and as insect repellent. In Saudi Arabia, a decoction of fresh whole plant is traditionally used to treat diabetes mellitus [14]. The plant has also been used in folk medicine as an anthelmintic, antispasmodic, antirheumatic and antibacterial agent [15]. Antioxidant, antileishmanial and antitrypanosomal activities have also been recorded for Artemisia abyssinica essential oil [15]. Essential oil compositions have been determined for Artemisia abyssinica from three different regions of Yemen, namely Taiz (higher than 1500 m), Sana'a (higher than 3000 m) and Alhodiadah (coastal region). The major components of Artemisia abyssinica essential oils from Yemen are listed in Table 1 [16,17]. Artemisia abyssinica essential oil was rich in camphor and davanone with lesser amounts of (E)-nerolidol, cis-sabinene hydrate, terpinen-4-ol, linalool, and bornyl acetate. Artemisia abyssinica essential oil showed marginal cytotoxic activity against MCF-7 cells (30% cell viability reduction at a concentration of 100 μg/mL) [16]. Artemisia abyssinica oil from Yemen is remarkably different in composition from that reported from Ethiopia, which was rich in yomogi alcohol (38.5%), artemisyl acetate (24.9%), and artemisia alcohol (6.7%) [18].    Artemisia arborescens has been used traditionally as an anti-inflammatory remedy. The essential oil of Artemisia arborescens has been reported to have antibacterial [19] and antifungal activities [15] as well as antiviral activity against HSV-1 and HSV-2 [20]. The essential oil of Artemisia arborescens from Yemen has been reported to have α-terpinene (8.7%), artemisia ketone (51.5%), camphor (14.1%), α-bisabolol (12.6%) and palmitic acid (2.4%) as the major constituents [21]. The oil showed strong in vitro cytotoxic activity with an IC50 of 16.9 µg/mL against HT 29 human colorectal adenocarcinoma cells. Artemisia arborescens essential oil also exhibited antifungal activity against Cladosporium cucumerinum. The cytotoxic activity of Artemisia arborescens can be attributed to the presence of α-bisabolol [22] and palmitic acid [23]. The essential oil composition of Artemisia arborescens from Yemen [21] was very different from samples from Sicily [19] or Algeria [24], which were dominated by β-thujone and chamazulene.

Conyza bonariensis (L.) Cronquist
Conyza bonariensis (L.) Cronquist is known as "sadaf" in Yemen. It is an annual or short lived perennial weed. A variety of phytochemicals have been reported from the genus that includes alkaloids, volatile oils, terpenoids, phenolic acids, flavonoids and hydrolyzable tannins. Conyza bonariensis is widely used in folk medicine for the treatment of rheumatism, cystitis, gout, nephritis, dysmenorrhea, tooth pain and headache. Other studied biological activities of Conyza bonariensis include molluscicidal activity against Biomphalaria snails, anti-inflammatory activity, antipyretic effects, and antimicrobial activity [25]. Essential oil from the aerial plant parts of Conyza bonariensis collected from Yemen was dominated by the sesquiterpenoids 8-cedren-13-ol (18.5%) and aromadendrene oxide (18.8%) [26].  There are 77 species of Pulicaria, distributed in temperate and warm Eurasia [7], and in folk medicine Pulicaria species have been used as insect repellents, galactagogues, antiepileptics, and as tonics [27]. Pulicaria inuloides is distributed from Morocco east throughout North Africa, south to Chad, Niger, and Somalia, and east to the Arabian Peninsula [28]. The plant, known as "sekba", is used traditionally in Yemen to treat wounds [29]. The essential oil of Pulicaria inuloides from Yemen was composed largely of carvotanacetone (47.3%) and palmitic acid (12.8%) [30]. The oil has demonstrated antimicrobial activity against Staphylococcus aureus, Streptococcus pneumoniae, Bacillus subtilis, Escherichia coli, and Candida albicans, and antioxidant activity in the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical-scavenging and β-carotene bleaching assays [31].

Pulicaria jaubertii Gamal-Eldin
Pulicaria jaubertii is a perennial fragrant herb with erect branches growing up to 50 cm [32]. The plant, known as "anssif" in Yemen, is distributed in the southern Arabian Peninsula, and is used traditionally as a diuretic and antipyretic. Leaves of Pulicaria jaubertii are also used flavoring agents for cooking. Similar to Pulicaria inuloides (above), the essential oil of Pulicaria jaubertii from Yemen was dominated by carvotanacetone (64.0%) [33]. The essential oil of Pulicaria jaubertii from southern Saudi Arabia had a higher proportion of carvotanacetone (98.6%), and this essential oil had excellent in vitro cytotoxic activity against MCF-7 and Hep-G2 cells (IC50 = 3.8 and 5.1 μg/mL, respectively), but was only marginally antibacterial [32].
An aromatic tea of Pulicaria undulata, known as "kho'ah", is used in the central Sahara to treat chills, diabetes, cardiac disorders, skin diseases, and abscesses [36], and in Egypt to treat inflammation, as an insect repellent, and an herbal tea [37]. Pulicaria undulata oil has shown antibacterial [38], sedative [39], and insecticidal activities [40]. Pulicaria undulata essential oil from Yemen has been reported to have an extremely high concentration of carvotanacetone (91.4%) along with 2,5-dimethoxy-p-cymene (2.6%) [41]. Although carvotanacetone has been found in Pulicaria undulata oil collected from different geographical regions, comparison shows that the highest concentration is found in Yemeni Pulicaria undulata. Thus, for example, Pulicaria undulata oil from Sudan was composed of 55.9% carvotanacetone [42].
The essential oil from Yemen did show moderate in vitro cytotoxic activity on MCF-7 breast tumor cells (IC50 = 64.6 μg/mL), which can be attributed to the high concentration of carvotanacetone (see above). The oil also exhibited antibacterial activity against Staphylococcus aureus with minimum bactericidal concentration (MBC) of 3.12 µL/mL for Staphylococcus aureus and methicillin-resistant Staphylococcus aureus. The MBC against Candida albicans and Bacillus subtilis was 6.25 µL/mL, whereas MBC against Escherichia coli was 12.5 µL/mL [41].

Tarchonanthus camphoratus L.
Tarchonanthus camphoratus, known locally as "bayad" or "mukar" in Yemen, is a strongly aromatic shrub found growing in the hillsides between 1200-2500 m [53]. Traditionally, its aromatic leaves are used for wounds and urinary tract infections [29]. An infusion of the leaves is used to treat stomach ailments and bronchitis. Burning of the aerial parts of the plant liberates smoke that is inhaled for the treatment of sinus-related complaints and for headaches. The burned seeds and leaves are used traditionally as fumigating agents in funeral rituals. Leaves can also be used as hot poultices for treating chest complaints whereas chewing the leaves is believed to alleviate toothache. There have been numerous studies on the essential oil of Tarchonanthus camphoratus and a wide variety of biological properties (e.g., antimicrobial, antifungal, antioxidant, antidiabetic, insecticidal, insect repellant, analgesic, and antipyretic) have been reported [53]. Tarchonanthus camphoratus leaf essential oil from Yemen is composed largely of oxygenated monoterpenoids (48.3%) and oxygenated sesquiterpenoids (32.7%). The principle compounds were endo-fenchol (21.2%), trans-pinene hydrate (8.8%), caryophyllene oxide (7.5%), α-terpineol (6.4%), τ-cadinol (6.4%) and α-cadinol (5.2%). The essential oil was also reported to have moderate antimicrobial activity in disc diffusion assay against a number of pathogenic bacteria including methicillin-resistant Staphylococcus aureus (MRSA) and Candida albicans. The oil also showed moderate in vitro cytotoxic activity against HT29 (human colonic adenocarcinoma) tumor cells with an IC50 of 84.7 µg/mL [53].
Catha edulis, a member of the Celastraceae, is a dicotyledonous evergreen shrub known locally known as "khat". The major cultivation areas of khat is Ethiopia, particularly the Harar district, and Yemen. Chewing khat is a primary recreation in Yemen [4,64]. Catha edulis leaves contain several chemical groups of compounds including alkaloids, tannins, flavonoids, terpenes and sterols, and essential oils. Many studies have confirmed the presence of a complex set of alkaloids called kathdulinat, which are responsible for its stimulant effects especially phenylalkylamine compounds (cathinone and cathine) [65,66]. The leaves of khat are chewed to increase activity and elevate the mood. The roots and leaves are used traditionally for the treatment of influenza, cough, gonorrhea, chest and stomach problems [67]. The essential oil of C. edulis leaves from Yemen has been reported to have a high concentration of carvotanacetone (84.4%) in addition to trans-pulegol (2.2%) and 2,5-dimethoxy-p-cymene (1.9%) [67].

Leucas virgata Balf. f.
There are around 100 species of Leucas distributed in Africa, Arabia, and Indomalaysia [7]. The Soqotra island of Yemen has 10 different species of Leucas of which Leucas virgata is endemic and known locally as "sa-lel hon". It is an abundant aromatic shrub and grows up to 1 m in height [75]. The leaves and sprigs are used for tea and it has been traditionally used for the treatment of several stomach problems. The essential oil of Leucas virgata is rich in oxygenated monoterpenoids (50.8%), the major ones being camphor (20.5%), exo-fenchol (3.4%), fenchone (5.4%), and borneol (3.1%). It also has a high percentage of oxygenated sesquiterpenoids like β-eudesmol (6.1%) and caryophyllene oxide (5.1%). The essential oil exhibits very good antibacterial activity against Staphylococcus aureus, Bacillus subtilis and Esherichia coli. A methanolic extract of Leucas virgata has shown good activity against Trypanosoma brucei with an IC50 of 8.1 µg/mL [76]. Leucas virgata extracts have been reported to have moderate antimicrobial activity against various bacterial strains with MIC values ≤ to 250 µg/mL [62].

Mentha spicata L.
Mentha is a comparatively small genus having 18-19 species [7]. It is a well-known genus for its medicinal properties and is found in the temperate regions of Eurasia, Australia, and South Africa, and is cultivated from tropical to temperate climates of America, Europe, China, Brazil, and India [77]. Mentha spicata (spearmint) is a perennial, rhizomatous and glabrous herb that has a strong aromatic odor. The spearmint odor of Mentha spicata comes from the high concentration of (R)-carvone present in it. There are various ethno-medicinal uses of Mentha spicata essential oil and the genus is one of the most researched for its components as well as for the biological activities. It has been extensively used for flatulence, acidity neutralization, gastro stimulation and digestive problems. It has also been used for cough and cold, as a diuretic and spasmolytic [78]. It is considered to have analgesic, antipyretic and anti-inflammatory effects as well [79]. The essential oil of Mentha spicata from the Sana'a region of Yemen was composed largely of carvone (63.0%), with lesser amounts of limonene (7.9%), 1,8-cineole (4.8%), terpinen-4-ol (2.9%), cis-dihydrocarvone (6.1%), and β-caryophyllene (2.4%) [16]. Carvone is the characteristic volatile component of Mentha spicata from which the plant species gets its distinct smell. There are various reports on the presence of carvone as the major component from Mentha spicata of various geographical origins.
The Yemeni oil sample was assayed for antimicrobial, antifungal and cytotoxicity against MCF-7 and MDA-MB-231 cells. The oil showed considerable activity against Bacillus cereus, Esherichia coli, and Botrytis cinerea with MIC of 312.5 µg/mL, 156 µg/mL and 78 µg/mL, respectively. However, the oil did not show any cytotoxic activity against either MCF-7 cells or MDA-MB-231 cells. Apart from these, Mentha spicata oil has been investigated for other biological activities like antioxidant activity, analgesic, anti-inflammatory and antipyretic effects. In a recent study, Liu and co-workers reported that Mentha spicata oil showed considerably strong cytotoxicity on HeLa cells with an IC50 of 2.08 µg/mL [80]. The oil was also reported to have high antibacterial effect against Escherichia coli, Saccharomyces cerevisiae and Penicillium citrinum. The major compounds in the essential oil were carvone (65.3%), limonene (18.2%), dihydrocarvone (3.0%) and camphene (2.3%). Agrawal and co-workers have examined the antimicrobial activities of both enantiomers of limonene and carvone [81]. These investigators found that both optical isomers of carvone showed activity against a wide spectrum of human pathogenic fungi and bacteria. Carvone has been found to inhibit the transformation of Candida albicans from a coccus to the filamentous form, so making them a potentially good therapeutic agent against infections caused by fungus. Other properties of carvone include its strong insect repellent activity and a promising sprouting inhibitory action on potatoes. (S)-carvone is a good potato sprout inhibitor, and is commercialized in the Netherlands under the name "Talent" [82].
Meriandra bengalensis, known as "dharo", is an aromatic shrub that grows to a height of about 2 m and is highly branched. It has been used traditionally as an antiseptic, astringent, antirheumatic, and carminative [83]. Meriandra bengalensis from Yemen showed camphor (43.6%), 1,8-cineole (10.7%), borneol (3.4%), caryophyllene oxide (5.8%) and α-eudesmol (5.8%) as the major constituents [83]. Although the essential oil sample did not show significant antibacterial or antifungal activity, some of its major constituents do have biological activity. Camphor is considered to have good activity against human-and soil-born fungi. At a concentration of 500 µL/L, camphor has been shown to reduce the radial growth of Sclerotinia sclerotiorum and Rhizoctonia solani by 51.7% and 64.9%, respectively [84]. Camphor has been found to have good fungicidal activity against Botrytis cinerea where it showed complete inhibition of mycelia growth at 1.75 g/L [85].

Ocimum basilicum L.
The genus Ocimum is made up of around 65 species found throughout tropical and subtropical regions [7]. The genus is represented by seven species in Yemen, namely Ocimum basilicum, Ocimum tenuiflorum, Ocimum suave, Ocimum spicatum, Ocimum gratissimum, and Ocimum forskolei. Ocimum basilicum is used in Yemeni traditional medicine to treat various ailments, including abdominal cramps, gastroenteritis, dysentery, and diarrhea. In northern Oman and Saudi Arabia, juice of leaves or crushed leaves is used in the treatment of wounds, acne, and vitiligo. It is used also as a deodorant and is considered to be an aphrodisiac [27,88]. Ocimum basilicum from Yemen was dominated by linalool (74.5%) with lower concentrations of 1,8-cineole (7.4%) and estragole (7.2%) [16]. The Yemeni basil oil was screened for in vitro cytotoxic activity against MCF-7 (human mammary ductal carcinoma) cells, but was inactive; only 18.5% reduction of cell viability at a concentration of 100 μg/mL.

Plectranthus barbatus Andrews
Plectranthus is a large genus that belongs to the Lamiaceae and contains around 200 species of herbs and shrubs, most of which are aromatic plants tropical and sub-tropical areas of the Old World [7]. There are 12 species of Plectranthus found growing in Yemen, and this genus has been widely used for its medicinal properties. They have been used for treating skin, digestive, and respiratory complications [27,91]. Plectranthus barbatus is a perennial shrub that grows over the southern and subtropical regions of India, Pakistan, Sri Lanka, tropical east Africa, Asia (South of Arabian Peninsula), China, and Brazil [92]. The main constituents isolated from Plectranthus barbatus have been diterpenoids and essential oils. The essential oil composition varies according to the location and the date of harvest and contains mainly mono-and sesquiterpenes [92]. A wide range of biological activity has been reported for Plectranthus species. They are very important in ethno-medicinal uses to treat a range of ailments, particularly digestive, skin, infective, and respiratory problems. The plant has also been used widely for food, flavor and fodder.
Plectranthus cylindraceus is a strong aromatic plant found growing in different parts of United Arab Emirates, Saudi Arabia, Oman, Yemen, and East African countries. GC-MS analysis of the essential oil Plectranthus cylindraceus growing in Yemen showed high concentrations of thymol (68.5%), terpinolene (5.3%), β-selinene (4.7%), β-caryophyllene (4.0%), δ-cadinol (2.1%) and ar-curcumene (1.7%). The oil showed very good antibacterial and antifungal activity against Staphylococcus aureus, Bacillus subtilis and Candida albicans with MIC of 390, 180 and 180 µL/mL, respectively. It also showed considerable 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical-scavenging activity as an antioxidant with IC50 of 34.5 µg/mL [83]. A similar study on Plectranthus cylindraceus oil from Oman has shown carvacrol (46.8%) and terpinolene (18.2%) to be the major constituents [96]. Note that thymol and carvacrol have similar retention indices on DB-1 or DB-5 columns and virtually identical mass spectra; these may be the same compound or overlapping mixtures of thymol and carvacrol in the Plectranthus cylindraceus samples. The oil also showed antimicrobial [96] and nematicidal activity [97]. The antibacterial and antifungal activity of Plectranthus cylindraceus is probably due to the presence of high concentration of thymol/carvacrol (see above).

Thymus laevigatus Vahl
Thymus laevigatus is an endemic species to Yemen and is the only species that represents this genus in Yemen [103]. The genus Thymus has about 250 species that are mostly perennial, aromatic and evergreen plants [7]. Thymus laevigatus, known locally as "zatar", is mostly found in the higher mountains in North Yemen in Haggah (2500 m) and in Dhamar (2200 m). In Yemeni folk medicine, Thymus laevigatus dried leaves are used as powder in warm milk, sesame oil, or olive oil for the treatment of different stomach diseases, cough, tonsillitis, pharyngitis and renal colic [103]. The essential oils from many species of Thymus have been reported to have very good antibacterial and antifungal activities, which are attributed to the high concentrations of thymol and carvacrol is those oils [104,105]. An essential oil study on Thymus laevigatus from Yemen has shown a very high concentration of carvacrol (84.3%), along with p-cymene (4.1%), γ-terpinene (4.0%) and trans-anethole (3.6%) as the major constituents [103]. Thymus laevigatus oil was found to have a wide range of antimicrobial activity and a potent fungicidal effect against Candida albicans with a MIC of 0.0313% (v/v) [104].

Conclusions
Yemen is a land of diverse landscapes including mountains, desert, gorge-like wadis, and coastal escarpments. The variety of habitats of mainland Yemen, coupled with the endemism of the Soqotra archipelago, give Yemeni flora a great deal of diversity. There are currently around 2930 species of higher plants in Yemen [106,107] of which 699 species are endemic [108]. Despite the introduction of Western medicinal system during the middle of the twentieth century, traditional herbal medicine continues to play an important role in many parts of Yemen. Many of these medicinal plants and their essential oils have shown notable biological activities. In addition to research activities on these traditional medicinal plants, it is hoped that future studies will provide new insights into pharmacological activities of understudied Yemeni flora. Unfortunately, increasing environmental degradation due to human activity, invasive plant species, and climate change threaten the native flora of Yemen. It is hoped that steps be undertaken to safeguard the fragile ecology of Yemen, protect the native flora and fauna, as well as preserve the traditional knowledge of the people.

Author Contributions
N.A.A.A. and W.N.S. conceived and organized the review; B.K.C., N.A.A.A. and W.N.S. contributed to the writing and editing of the manuscript. Some of this work was taken from the M.S. Thesis of B.K.C. [16].