The Essential Oil-Bearing Plants in the United Arab Emirates (UAE): An Overview

Essential Oils (EOs) are expensive hydrocarbons produced exclusively by specific species in the plant kingdom. Their applications have deep roots in traditional herbal medicine, which lacks scientific evidence. Nowadays, more than ever, there is a growing global interest in research-based discoveries that maintain and promote health conditions. Consequently, EOs became a much attractive topic for both research and industry, with revenues reaching billions of dollars annually. In this work, we provide key guidance to all essential oil-bearing plants growing in the United Arab Emirates (UAE). The comprehensive data were collected following an extensive, up-to-date literature review. The results identified 137 plant species, including indigenous and naturalized ones, in the UAE, citing over 180 published research articles. The general overview included plant botanical names, synonyms, common names (Arabic and English), families and taxonomic authority. The study acts as a baseline and accelerator for research, industry and discoveries in multiple disciplines relying on essential oil-bearing plants.


Introduction
Globally, the essential oils (EOs) industry creates billions of dollars of revenue annually. Therefore, more attention has been given recently to this sector, as a natural primary resource for phytochemicals. Indeed, the EOs industry has a wide range of enormous applications in various fields, such as pharmaceuticals, aromatherapy, healthcare, cosmetics, food flavoring, food preservation and the fragrance industry [1].
Briefly, EOs are concentrated phytochemicals comprised mostly of terpenes, oxygenated terpenes, sesquiterpenes and oxygenated sesquiterpenes [2,3]. EOs are volatiles associated with a characteristic aroma resulting from the complex interaction between hundreds of volatiles. These hydrophobic compounds are produced exclusively from certain plant species as secondary metabolites, acting as defense phytochemicals [4].
In fact, EOs applications have deep roots in old traditional practices, in which they were a natural resource to treat infections and sicknesses for hundreds of years [3,4]. However, such traditional practices lack scientific validation, and thus have to be remarkably studied and tested, looking for scientific justification. The studies about essential oils from UAE plants and their biological activities are scanty when compared to other research in plant-based fields. There are studies such as extraction of essential oil of Haplophyllum tuberculatum [5], Teucrium stocksianum [6], Pulicaria glutinosa [7], Cleome amblyocarpa [4] and Moringa peregrina [8] from the United Arab Emirates.
The main objective of this work is to provide key guidance to all EO-bearing plants indigenous and naturalized to the United Arab Emirates (UAE), which included highlights on all available EO-bearing plant species, their families, botanical names, common names

A Comprehensive Overview
All the indigenous and naturalized plants of the study location were evaluated and the result was establishing a full list of all Emirati EO-bearing plants (Table 1), including an overview of 137 Emirati EO-bearing plants belonging to 46 families, all cited based on up-to-date literature (over 180 references). Meaning that, EO-bearing plants comprise 17% of the estimated 800 indigenous and naturalized plants. 29 Pluchea dioscoridis (L.) DC.
(Syn. Sinapis arvensis L.) (Eng. Charlock, charlock mustard, wild mustard) [    [184] [15,21] According to Raut and Karuppayil [185] there are around 2000 identified EO-bearing plants globally. Therefore, it is of high value that a country (with estimated 83 km 2 land area and with limited freshwater resources) includes 137 EO-bearing plants. It is worth mentioning that although some of the UAE indigenous and naturalized plants have rich traditional therapeutic applications and belong to important medicinal families, there are no data yet available related to the potential of their EOs. Therefore, it is expected that many of the medicinal and aromatic plants that are available locally are not investigated yet, and the true estimation of the Emirati EO-bearing plants could be underestimated. Examples of these species include, Amaranthus graecizans and Amaranthus viridis from the Amaranthaceae family, which were used in the past by the Bedouin people of the UAE to treat scorpion stings, snake bites and itchy skin rashes as reported by Sakkir et al. [28].
Generally, the most important habitats are the plantations, farmlands and irrigated lands, which host 50% of the Emirati EO-bearing plants. Meanwhile, the most important natural habitats are mountains and rocky terrain, wadis, sandy dunes and coastal saline lines, hosting 36.3, 24.4, 15.5 and 14.8% of the total EO-bearing plants growing in the United Arab Emirates, respectively ( Figure 2).
Since plantations and farmlands are hosting 50% of the Emirati EO-bearing plants (which could be due to their ornamental or food production value, or just available naturally as weeds due to the accessibility of water), there should be educational campaigns to educate landlords about extra potentials and economic benefits related to EOs of their available indigenous plants. In addition, with the sharp population growth and the current expansion in the industrial and urbanization activities, strong efforts should be invested to conserve the natural habitats of the EO-bearing plants (e.g., mountains, wadis), and take the same into consideration in strategic planning and management.   Based on our results, shoots (particularly leaves and flowers) are the most important parts that have potential for EOs, in which, 56.3, 37.8 and 29.6% of the Emirati-EO-bearing plants have the potential to extract EOs from their shoots, leaves and flowers, respectively ( Figure 3).
Our results show that 82% of the status of the Emirati EO-bearing plants are reported as least concerned plants (low risks of becoming endangered), as shown in Figure 4. However, since recent references that report the status of indigenous plants are limited, and taking into consideration that status of 10% of the plants are not yet evaluated, their real status could be underestimated, especially with the current population growth and urbanization activities.

A Detailed Overview
A detailed view on the results of the top three richest families (based on the number of EO-bearing species) is divided into three table groups (Tables 2-10). The first group represents the general botanical information related to the plant species, including form, life form, life cycle, economic value and folk medicine/applications, internationally and locally (Tables 2, 5 and 8). The second group illustrates the data related to the plants' natural habitats in the UAE, including important locations, soil, habitats, flowering period and wildlife status (Tables 3, 6 and 9). The third group shows detailed knowledge related to plants' EOs, including potential plant parts, yields, extraction methods, main chemical groups/constituents and biological activities (Tables 4, 7 and 10).     (+) (Pearl drivers used to apply an infusion of fruits to skin after dives) ("L": Poultice of "L" used to treat joint pains) (Resin mixed with egg-white applied to eyes to treat cararacts) ("L": Eaten to treat diarrhoea) ("Se": Soaked in water or milk drunk to treat diabetes) ("Pd": Smoke from burning "Pds" inhaled for colds) (UAE: Applied to soothe burns. "L": are pounded into a paste and used a poultice on boils and swellings or applied around boils to draw out the pus) [120] 10  (+) (Oman: "L": as deodorant + "L": Fragnance eases headaches and dizziness + "L": Crushed and placed in nose to treat colds and in ears to treat earaches + "L": Juice from young "L" as eye drops or to soothe insect bites) (UAE: "L": to treat vomiting, against itching)         [191,192] [7], who studied the EO of Pulicaria glutinosa grown in Jebal Al Faya, and Al Yousuf et al., [6] who studied Teucrium stocksianum grown in Khor Fakkan. Additional research studied EOs of Haplophyllum tuberculatum for plants also grown in Khor Fakkan [5]. Al-Marzouqi et al., [130] studied EO of Menthe spicata collected from different regions in the UAE.
Based on the above, there is scarcity in research performed on the EO-bearing plants of the UAE. This is the case while the country has rich biodiversity and has rich traditional medicine applications [16,17,28]. In addition, according to Sakkir et al., [28] 37% of the UAE medicinal plants are applied topically to treat skin problems. This is a direct/indirect indication that the UAE is a good niche for EO phytochemicals of healing benefits. Consequently, it is highly recommended to invest more efforts to study the local EO-bearing plants, seeking new natural resources of phytochemicals of proven biological activity to the country and the world.
Actually, our established databank of the UAE EO-bearing plants offers a solid background to take quick decisions in plant selection and to start up an innovative EOs-based research pathway, which can lead to new chemotypes and promising discoveries. Besides, the databank provides the interested parties (from academic and industrial fields) the opportunity to have an overview on all the Emirati EO-bearing plants, enabling them to highlight the most important indigenous species to supply their needs according to field of interest. At the same time, the databank lists all the UAE EO-bearing plants that need to be conserved from decision-makers to guarantee a sustainable future for the next generation.
It worth mentioning that rich traditional practices are linked (directly or indirectly) to the availability of EOs as active components that lead to particular biological activities of great healing benefits. Additionally, it was reported by a study conducted by Sakkir et al., [28] from the Environmental Agency of Abu Dhabi (EAD) that 37% of the indigenous plants have been used to treat skin problems in the traditional medicine of the UAE. Which can be linked, in one way or another, to the presence of therapeutic grade EO, and therefore could be a positive indicator that the flora of the UAE could pose an excellent resource for EO phytochemicals of various industrial applications.
Thus, it is fundamental to create a comprehensive reference that includes all the UAE indigenous and naturalized species capable of producing EOs. Focusing on the significant role of such a natural resource in a region where fresh water is expensive and where the country's leadership is working on diversifying the economic resources.
Indeed, it is expected that the essential oil of many indigenous and naturalized medicinal and aromatic plants has not been investigated yet, and the true estimation of the EO-bearing plants growing in the country could be higher than the current findings of this study. Examples of these species include, Amaranthus graecizans and Amaranthus viridis from the Amaranthaceae family, which were used in the past by Bedouin people of UAE to treat scorpion stings, snake bites and itchy skin rashes as reported by Sakkir et al., [28].
According to published research by Shahin et al., [4], the essential oil of the indigenous medicinal Cleome amblyocarpa was extracted and studied for the first time, declaring positive antioxidant activities. Recently, another study extracted essential oil from the seeds of Moringa peregrina and evaluated its chemical composition and antioxidant potentials [8]. Therefore, it is expected that similar results can be found while studying other medicinal and aromatic indigenous species.

List of Abbreviations
This section provided the meaning and description of all the abbreviations that were used to construct the tables of the UAE (native and naturalized) EO-bearing plants (Tables 2-10).

•
Main Chemical Groups/Components: (Main EOs Chemical Groups) and/or (Main/Potential Chemical Constituents) Plant part + extraction method.

•
General Notes: The use of "!" means information uncertainty.

Phytochemicals and Biological Activities from UAE Based Plants
Reviewing the literature, essential oils of the following six native/naturalized UAE plants were investigated under UAE climatic conditions, including Pulicaria glutinosa (Asteraceae) [7], Cleome amblyocarpa (Cleomaceae) [4], Mentha spicata (Lamiaceae) [130], Teucrium stocksianum (Lamiaceae) [6], Haplophyllum tuberculatum (Rutaceae) [5] and Moringa peregrine (Moringaceae) [8], with rich therapeutic applications for the last five species in folk medicine generally and the UAE traditional practices specifically. For example, infusion of C. amblyocarpa leaves was used to treat abdominal and rheumatic pain. M. spicata was used to promote general health-care benefits. Meanwhile, T. stocksianum has various applications related to kidney, stomach pains, thyroid problems and the common cold. The leaves of H. tuberculatum were used to treat scorpion stings, eaten as sedative and crushed in water and drunk to treat painful joints. In the UAE folk practices, the seeds' oil of M. peregrine has been taken orally for constipation and stomach cramp, and the seeds' oil mixture with clove oil and cardamom oil has been taken as a drink during labor. Besides, the seeds' oily extract is used to treat headaches, fever, muscle pains, burns, abdominal pain and constipation. M. peregrine leaves' extract can be rubbed on skin to treat a skin rash [22].
The rocky soils of the Hajar mountain are among the most famous places for P. glutinosa, T. stocksianum, H. tuberculatum and M. peregrine, including Khor Fakkan and Ru'us Al-Jibal for T. stocksianum and M. peregrine. While the sandy soils of the North Emirates of Dubai, Sharjah, Ajman and Umm Al Quwain are rich in C. amblyocarpa, the mountains, hillsides and wadis of Fujairah, Ras Al Khaimah, Sharjah and Abu Dhabi are rich places for H. tuberculatum [21]. The mint herb M. spicata is widely cultivated in farms for food production purposes, and contributes to the richest essential oils yield which is 10.90%, extracted from shoots using Supercritical carbon dioxide (SCCO 2 ) (Press: 350 bar, Temp: 50 • C) [130]. While the essential oil average yields of the aerial parts of each of P. glutinosa, T. stocksianum, H. tuberculatum and C. amblyocarpa, were (according to the highest yield reported in the literature based on UAE) 0.5 [7], 0.34 [6], 0.04 [5] and 0.0266% [4], respectively. The essential oil seeds' oil of M. peregrine extracted by hydrodistillation reported to be 0.22% [8].
A study of the phytochemicals showed that the major constituents of P. glutinosa essential oils extracted from aerial parts (including flowers) by steam distillation were p-elemene, 7-cadinol and a-cadinol (Sesquiterpenes) [7]. No studies were found to test the biological activities of the essential oil extracts for this shrub.
As reported by Al-Marzouqi et al., [130], the main chemical composition of M. spicata leaves' essential oil (extracted by SCCO 2 ) included carvone, a-pinene, limonene and linalool, which were significantly higher in the locally cultivated M. spicata in comparison to herbs imported from France, Syria and India. Although many studies in the literature reported the various biological activities of M. spicata (e.g., antibacterial, antifungal, antimicrobial, antioxidant, insecticidal and pesticidal, larvicidal activity, mosquito attractant/repellent and antimutagenic properties), however, no studies thus far have tested the biological activity of the oil for the herb cultivated under UAE climatic conditions. According to Al Yousuf et al. [6], the oil of the aerial parts of T. stocksianum, collected from the UAE, was characterized by a-cadinol and 6-cadinene. Studies based on other countries reported the antinociceptive activity of the oil, with no studies found related to the biological activity of the oil based on the UAE.
Based on the research findings of Al Yousuf et al., [5], the oil extracted from the aerial parts of H. tuberculatum α-phellandrene (10.7-32.9%) being the major component and with significant amounts of other phytochemicals varied in existence and percentages according to the harvesting season. Such phytochemicals include β-caryophyllene, β-pinene, limonene, δ-3-carene, linalool, linalyl acetate, β-caryophyllene and α-terpineol. The biological studies related to H. tuberculatum carried out based on other countries reported that the oil exhibits various biological activities including antifungal, antibacterial, antimicrobial, mosquito attractant/repellent, insecticidal and pesticidal activity and larvicidal activity. On the other hand, no studies have been conducted yet to test these activities and others for the essential oil of this perennial herb grown under the UAE climate.
Studies of phytochemicals and biological studies provide scientific justification for the rich therapeutic applications of the previously mentioned native/naturalized plants in the UAE traditional practices. At the same time, it is obvious that there is a lack of essential oil studies based on the UAE, and more efforts are needed to investigate the phytochemicals and biological activities of oils extracted from locally grown and harvested native plants. Besides, comparative studies to compare the essential oil yield (quantitatively and qualitatively) for plants grown in the UAE and other countries are required. This is needed to highlight the native/naturalized plants of superior quality and biological activity, and utilize the same (after standardization) for commercial purposes in various industries (e.g., pharmaceuticals, cosmetics, food preservatives, fragrance and flavor industries).

Obstacles and Difficulties
The greatest obstacles and difficulties that were faced are related to the scarcity of the references to UAE wildflowers (Shahin, 2018c). There is so much confusion in the literature between the botanical names and the synonyms, including spelling mistakes that make the task of data collection to list all the Emirati plants (followed by screening and listing the Emirati EO-bearing plants) a difficult and complicated mission.
For example, Cornulaca arabica Botsch and Cornulaca monacantha Delile were mentioned as two different species in the reference of Brown and Sakkir [22], while according to published study [21] Cornulaca arabica Botsch is a synonym of Cornulaca monacantha Delile. Besides, the plants Actiniopteris semiflabellata, Commicarpus boissieri and Cymbopogon jwarancusa were mentioned with minor spelling mistakes as Actioniopteris semiflabellata, Commicarpus boisieri and Cymbopogon jwarancuse, (respectively) in the textbook of Jongbloed et al., [21] which is one of the most important references of the UAE indigenous and naturalized plants.
Moreover, some publications use either the synonyms or the common names instead of using the botanical names. Therefore, while reviewing the literature using the formal botanical names (to screen the EO potential) no results will appear, although, in many cases the plant would be a rich resource of EO phytochemicals. For example, some publications will use Dipcadi serotinum, Cymbopogon parkeri Stapf. Heliotropium europaeum and Calligonum polygonoides instead of using the botanical names, which are Dipcadi erythraeum Webb and Berth., Cymbopogon commutatus (Steud.) Stapf., Heliotropium lasiocarpum and Calligonum comosum, respectively.

Conclusions and Future Perspectives
Based on our comprehensive and detailed screening of all the families of the UAE wildflowers, we concluded that there are at least 137 EO-bearing plants in the UAE (17% of the UAE wildflowers) belonging to 46 families. The top three richest families, based on the number of their species, are Asteraceae, Fabaceae and Lamiaceae.
Most of the UAE EO-bearing plants have rich traditional medicinal applications and other economic values, such as pharmaceuticals, nutrition, aromatherapy, fragrance and flavoring. Generally, the shoots (especially leaves and flowers) are the most important parts to extract EO phytochemicals (e.g., terpenoids) of valuable biological activities, such as antioxidant, antimicrobial and antitumor properties.
The UAE EO-bearing plants are widespread in the areas of plantations, mountains and wadis of the country. Serious efforts to educate landlords about the great value of the UAE EO-bearing plants are needed, to make sure that these expensive species are well-cultivated in a sustainable manner. Besides, strong efforts related to management and strategic planning should be employed to conserve the natural habitats of the EO-bearing plants.
All our obtained results support that the UAE is a rich natural resource for the native and naturalized EO-bearing plants that have rich ethnobotanical applications of multiple economic potential.
Therefore, serious efforts are needed to standardize the oil yield (quantitatively and qualitatively) for all listed essential oil-bearing plants of the UAE, and to focus sustainability on native essential oil-bearing plants of industrial applications at research and commercialization level. Taking into consideration that this field is promising for multiple research disciplines and discoveries.

Conflicts of Interest:
The authors declare no conflict of interest.