Essential oils, chemical composition, and biological activities of Eucalyptus oleosa F. Muell. : A review

Abstract


INTRODUCTION
Eucalyptus is a large genus of the Myrtaceae family (Grattapaglia et al., 2012) that includes 900 species and subspecies of which more than 300 species contain volatile essential oil in their leaves (Al-Snafi, 2017), and about twenty of these species are known for their richness in 1,8cineole, used in pharmaceutical and cosmetic industries (Pino et al., 2002;Dhakad et al., 2018).Although Eucalyptus is widely grown in many countries all over the world (Booth, 2013), Australia is probably the only one where such a single group of plants dominate most of the landscape (Crisp et al., 2011).Worldwide production of Eucalyptus essentials oils is around 3000 tons, and the major producers are China, Spain, Portugal, South Africa, and Chile (Ghaffar et al., 2015).It has a long history of use in traditional medicine in the treatment of cold, fever, flu, and general sickness (Salehi et al., 2019).In addition to its richness in essential oils, Eucalyptus is used for ornamentation, afforestation, and to obtain timber, gum, pulp and paper and it's known by its cosmetic and medicinal values (Saadaoui et al., 2017).Furthermore, Green synthesis of silver nanoparticles is possible to use the aqueous extract of E. oleosa as a green synthesis procedure without any catalyst, template, or surfactant.
Colloidal gold and silver nanoparticles are synthesized with Eucalyptus leaf extract at non-photomediated conditions (Pourmortazavi et al., 2015;Pourmortazavi et al., 2017).In addition to these biological activities, Eucalyptus oils have ecological roles; they act as herbivore deterrents, alleviate ozone toxicity, and mitigate the temperature throughout the time of fires (Sawalha et al., 2021).In food, the efficiency of Eucalyptus essential oils as a natural beverage preservative is verified; an effective and potent inhibitor of spoilage fungi and their in vitro antimicrobial effect is assessed against 17 food spoilage microorganisms (Boukhatem et al., 2020).This study focused on Eucalyptus oleosa, a xeric species, resistant to environmental stresses including salinity and drought; and grows in very low rainfall (Merchant et al., 2006;Hobbs et al., 2009).The choice was made essentially for its high content of 1,8-cineole more than 80% (Jaymand et al., 2009) and its diversified biological activities.In terms of chemical composition, essential oils of E. oleosa were complex mixtures of substances generally terpenes and terpenoids (Marzoug et al., 2011).Considering the versatility of Eucalyptus essential oils in terms of bioactivities as well as their industrial importance, the purpose of this study is to provide the readers with the latest information concerning the essential oils yields, the chemical composition, and biological activities of E. oleosa.

YIELD OF E. OLEOSA ESSENTIAL OILS
The Eucalyptus essential oils were extracted by using different methods such as supercritical CO2, microwave and by solvents.Hydrodistillation is typically the most used method to obtain volatile compounds produced by plants (Richter and Schellenberg, 2007).A considerable variation in the yields of essential oils extracted from E. oleosa leaf has been detected and the values were rangingd from 0.06 to 7 % (Elaissi et al., 2007).Previous studies have reported that the essential oils yield from plants collected in Iran was 6.7% (Ebadollahi et al., 2013;Rahimi-Nasrabadi et al., 2013), whereas others reported that the yield of E. oleosa essential oils varied from 2.31% to 3.2% collected from the same country (Jaymand et al., 2009;Ebadollahi et al., 2017).Relatively essential oils yield extracted from E. oleosa harvested in Tunisia was 4.90% (Marzoug et al., 2011) and similar results was detected with E. oleosa volatile oils from Australia that contained 4.60% of essential oils (Bignell et al., 1995).The detected values were more important than that reported in other species in Tunisia (E.maideni, E. astrengens, E. cinerea, E. leucoxylon, E. lehmani, E. sideroxylon and E. bicostata) in which the yields were ranging from 1.2% to 3% (Sebei et al., 2015).According to the literature, the yields of essential oils varies significantly between species of the genus Eucalyptus such as 0.29% in E. microtheca (Hashemi-Moghaddam et al., 2013), 0.5% in E. camaldulensis (Ndiaye et al., 2018), 3.9% in E. sargentii (Fathi and Sefidkon, 2012), and 1.8% in E. globulus (Damjanović-Vratnica et al., 2011).Moreover, other investigation reported variable oil content (0.45%-1.12%) from different aerial parts of E. oleosa originating from Tunisia (Marzoug et al., 2011).Other Eucalyptus species analyzed from Marocco (E.cinerea, E. baueriana, E. smithii, E. bridgesiana, E. microtheca, E. foecunda, E. propinqua and E. erythrocorys) have similar oil yields ranging from 0.2% to 1.15% (Zrira et al., 2004).As previously stated, the observed variability not only might have been derived from harvest time, local, climatic, and seasonal factors but also it could be greatly depending upon the different parts of the plants extracted.
Other study reported the effect of the extraction methods on the yield (Chamali et al., 2019).
All these variations might be due to the influence of geographical differences, environmental and growing conditions, physiological and biochemical states of plants, different extraction and analytical procedures, and genetic factors (Kokkini et al., 2004;Hassanpouraghdam et al., 2011).Furthermore, such variation can be attributed to several factors including plant age, climate, vegetative cycle stage, harvest time, geographical location, part plant used and genetic variation (Barra, 2009;Ben Hassine et al., 2012;Barbosa et al., 2016;Dorsaf et al., 2016;Almas et al., 2018).Several clinical studies indicate that Eucalyptol, the monoterpene, due to different medicinal properties including antioxidant (Ciftci et al., 2011), antimicrobial (Schürmann et al., 2019), anti-inflammatory (Zhao et al., 2014) and respiratory disorder treatments (Sudhoff et al., 2015).On other hand, the richness in 1,8-cineole revealed several potential applications; as an insect repellent (Aldoghaim et al., 2018).Furthermore, it is often used as a flavoring agent for food products (Santos and Rao, 2001).

Antioxidant activities
Antioxidants play an important role in food preservation by inhibiting oxidation processes and contributing to the health promotion provided by many dietary supplements, nutraceuticals, and functional food ingredients (Shahidi and Zhong, 2015).Furthermore, natural antioxidants are in demand for pharmaceuticals products (Brewer, 2011).Therefore, in recent years, considerable attention has been detected towards the identification of plants with antioxidant activity (Moon and Shibamoto, 2009).Previous studies revealed that E. oleosa leaf essential oils from Tunisia exhibited high antioxidant potential in both assays (DPPH and ABTS), the IC50 were 52.8 ± 0.7 mg/mL and 176.5 ± 3.1mg/mL respectively (Ben Marzoug et al., 2010).In addition, the antioxidant capacity of four parts of E. oleosa essential oils (stems, adult leaves, fruits, and immature flowers) showed moderate antioxidant activities in which the best IC50 is found for the adult leaves essential oil (0.013 ± 0.0006 mg/mL) in the ABTS assay (Marzoug et al., 2011).Whereas another study showed that the leaf essential oils of E. oleosa from Iran did not show any antioxidant activity (Rahimi-Nasrabadi et al., 2013).This moderate antioxidant activity is probably due to the low content of phenolic compounds presents that is related to the extraction technical used and origin of plant.

Antimicrobial activities
The antimicrobial activity of the E. oleosa essential oils has been studied by several researchers and discussed in the text; the essential oils exhibit toxicity against a wide range of microbes, including bacteria, fungi, yeast but the bioactivity against virus has not investigated (Table 2).

Antibacterial activities
It has been reported that E. oleosa is active against Gram+ strains Enterococcus feacalis and Staphylococcus aureus with minimal inhibitory concentration which was situated between 0.028-0.056mg/ml.In addition, the bactericidal dose against all organisms tested was ranged between 28-56 µg/ml (Ben Hassine et al., 2012).These results agreed with the study showed that the E. oleosa essential oils of different parts (stems, adult leaves, fruits, and immature flowers) appeared more active against the tested Gram+ such as Staphylococcus aureus and Listeria monocytogenes than Gram negative bacteria, although the immature flowers presented a larger prevalence of activity 0.39-3.72 mg/mL (Srinivasan et al., 2001).But another study revealed that the essential oil of E. oleosa exhibited high antibacterial activity against Gram negative ones, with highest inhibition zone 19.0 mm diameter and lowest MIC value 0.062 mg/ml against E. coli which shows that this microorganism is sensitive to E. oleosa essential oil (Rahimi-Nasrabadi et al., 2013).Hence, the activity against both types of bacteria Gram+ and Gram-, may be indicative of presence of broad spectrum antibiotic compounds or simply general metabolic toxins (Srinivasan et al., 2001).

Antifungal activities
An antifungal activity of the different part (stems, adult leaves, fruits, and immature flowers) of E. oleosa essential oils were tested against three pathogenic fungi Aspergillus ochraceus, Mucor ramamnianus and Fusarium culmorum and demonstrated that immature flowers and stems had strongest antifungal activity with minimal inhibition concentration value between 2.79-3.88mg/ml (Srinivasan et al., 2001).Additionally, these findings were consistent with another study performed by Kouki et al. (2023) who reported that E. oleosa EOs exhibited a significant antifungal activity against five Fusarium ssp.
Generally, E. oleosa essential oils showed variable antimicrobial activity against the different test organisms.This variability could be related to several factors such as chemical composition of essential oils and geographic location of the plant material, also the sensitivity of the bacterial strains and its nature (Sabo and Knezevic, 2019).The intense application of insecticides leads to the development of insecticide resistance in insect pest populations worldwide (Pittendrigh et al., 2008) and resulted in an increased risk of pesticides resistance, toxicological implications for human health and environmental pollution (Batish et al., 2006;Mahmood et al., 2016;Alengebawy et al., 2021).Thus, there has been a growing interest in research concerning the possible use of plant extracts as alternatives to synthetic pesticides (Ghosh et al., 2012).
Recently, the insecticidal activity elicited by certain essential oils indicates that these botanical compounds could be used as alternative tools (Isman, 2000).Furthermore, essential oils are applied similarly to other pesticides and their biological activity is manifested both by exposure to their vapors and by topical application (Isman, 2000;Tarelli et al., 2009).Further, it has been reported that Eucalyptus species essential oils have significant pesticidal potentials (Maciel et al., 2010;Alzogaray et al., 2011;Pant et al., 2014;Filomeno et al., 2017;Ainane et al., 2019).Insecticidal activity of E. oleosa oils from Iran are assessed against American white moth, Hyphantria cunea Drury 1773 (Lepidoptera: Arctiidae) at different concentrations (0.1, 0.21, 0.45, 0.95 and 2%) used at three times (24h, 48h 72h) in which found the LC50 (Lethal Concentration to kill 50% of insects) values were estimated as 0.36% at a shorter duration (24h) (Ebadollahi et al., 2013).In addition, the acaricidal effects of E. oleosa essential oils against Tetranychus urticae Koch (Acarina: Tetranychidae) have been reported (Ebadollahi et al., 2017).Hence, volatile oils from E. oleosa show strong phytotoxicity towards H. cunea and T. urticae.Indeed, it could provide opportunities for new biodegradable products for pest control considering their noticeable effects at low applied concentrations and short times of exposure.Indeed, the toxicity of the essential oils tested varies widely depending on the nature of the essential oil, the concentration used and the duration of the treatment.Studies have revealed that monoterpenes have insecticidal activities against the stored-product insects (Rajendran and Sriranjini, 2008;Papachristos et al., 2004).

CONCLUSION
Eucalyptus oleosa shows high subspecific variability and resistance to arid conditions.It showed a great richness in essential oils and their chemical composition indicated that the most abundant component is 1,8-cineole, followed by α-pinene.These characteristics represent advantages for the use of the species in afforestation and the valorization of these oils.Furthermore, E. oleosa essential oils possess a broad spectrum of biological effects, such as antioxidant, antibacterial, anti-fungal and antiinsecticidal activities.E. oleosa deserve to be deepened by studies on the relation subspecies, essential oils, and activities for a selection of promising subspecies for their aridity tolerance and their oils quantity, quality, and activities.Zhao, C., Sun, J., Fang, C., Tang, F. (2014).1, 8cineol attenuates LPS-induced acute pulmonary inflammation in mice.Inflammation 37, 566-572. Zrira, S., Bessiere, J.M., Menut, C., Elamrani, A., Benjilali, B. (2004).Chemical composition of the essential oil of nine Eucalyptus species growing in Morocco.Flavour and fragrance journal 19, 172-175.
Tableau 1. Major components present in essential oils of Eucalyptus oleosa samples.

Table 2 .
Antimicrobial effects of Eucalyptus oleosa EOs investigated on the pathogenic microorganisms.