E. globulus belongs to the family of Myrtaceae; an evergreen broadleaf tree, with a straight trunk, indigenous to Australia, the genus Eucalyptus comprises of more than 700 species. It is known as “the blue gum” or Tasmanian blue gum. E. globulus Labill. has precious bioactive constituents, antioxidants, antimicrobials, and phytoremediation, and herbicidal activities, which will pave the way to the development of new pharmaceuticals and agrochemicals, as well as food preservatives. They may also provide potential commercial applications to counteract the limitations of synthetic antioxidants.
E. globulus is extensively cultivated worldwide [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16][1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] because of its easy adaptability to environmental conditions, ease of cultivation, fast growth rate, and increase in the woody biomass. Furthermore, it can be planted in contaminated areas [17][16]. Obviously, Eucalyptus can adapt to biotic and abiotic stresses by selectively releasing defense compounds mainly of mono- and sesquiterpenes, as well as some hydrocarbons and quinones [18][17]. E. globulus has attracted the attention of researchers as well as environmentalists worldwide, due to it commonly being used in the pulp industry as a fast-growing source, and for the essential oil extracted from its leaves. Commercially, it is widely used for different purposes.
The essential oil of Eucalyptus spp. is among the 18 most frequently traded essential oils throughout the world. On account of this, there is an increasing amount of attention paid to their advantages as raw materials, which can be used in food, pharmaceuticals, and cosmetics, both in scientific and industrial research [1,19][18][19]. Among them E. globulus Labill. is the main source of the Eucalyptus leaf oil used globally [3][2]. Its leaves are alternate, with yellowish petioles, precisely lanceolate, 10–30 cm long, 2.5–5 cm wide, shiny dark green on both surfaces, and are a rich source of essential oil [7,10,20,21,22][6][9][20][21][22]. The oil is typically extracted by hydrodistillation, as the greatest amount of extracted chemical compounds can be obtained by utilizing this method [4,15,21][3][14][21]. The essential oil yield ranges from 1–3% [1,3,9,19,20,23][18][2][8][19][20][23]. The chemical composition of the Eucalyptus leaf essential oil is a complex mixture of substances, commonly containing 20 to 54 components with varying concentrations [4,8,13,15,24][3][7][12][14][24]. The oil comprised particularly of oxygenated monoterpenes (80.65–87.32%) as well as monoterpene hydrocarbons (18.33–12.45%) [9,25][8][25].
Eucalyptus leaf essential oil has drawn the attention of numerous researchers, as it represents a wide range of biological potentials, such as antibacterial efficacy toward both Gram-positive and Gram-negative bacteria [28,29][26][27] and considerable antibacterial potency against periodontal diseases [20,30][20][28]. Therefore, it can be incorporated into dental care products, as it is an anti-inflammatory agent [10,31][9][29]; has antifungal [16,28,32,33][15][26][30][31] and antidiabetic potential [34,35][32][33]; insecticidal [36,37,38,39][34][35][36][37], acaricidal, and repellent activities; is a biodegradable pesticide [40][38]; has bio-nematicide efficacy [7][6], phytopathogen control [8,23][7][23], and anthelmintic activity [41][39]; and it is a natural agent for food preservation [25]. Furthermore, this leaf essential oil has been previously applied in “folk medicine, in the treatment of respiratory problems, including, cold, cough, runny nose, sore throat, asthma, nasal congestion, bronchitis, and sinusitis” [42][40].
Plant Parts | Major Constituents | References |
---|---|---|
Leaves | 1,8-cineole (31.42%) and trans-3-carven-2-ol (10.10%), 2-Octen-1-ol, 3,7-dimethyl (9.37), and Cis-p-Menth-2,8-dienol (6.33) | [34] |
Leaves | 1,8-cineole (86.51%), α-pinene (4.74%), γ-terpinene (2.57%) and α-phellandrene (1.40%) |
[2] |
Leaves | 1,8-cineole (95.61%) and alpha-pinene (1.5%) | [63] |
Aerial parts | 1,8-cineole (79.85%), Limonene (6.72%), p-cymene (5.14%), and γ-terpinene (3.93%) | [25] |
Leaves | 1,8-cineole (85.8%), α-pinene (7.2%), and β-myrcene (1.5%) | [8] |
Leaves | γ-terpinene (94.48%) and 1,8-cineole (3.20%) | [56] |
Leaves | 1,8-cineole (33.6%), α-pinene (14.2%), and d-limonene (10.1%) | [36] |
Leaves | Terpinen-4-ol (23.46%), γ-terpinene (17.01%), pathulenol (8.94%), ρ-ymene (8.10%) and ρ-cymen-7-ol (6.39 %), globulol (2.52%), and α-phellandrene (2.20%) | [55] |
Leaves | 1,8-cineole (22.35%), limonene (7.01%), solanol (6.05%), β-pinene (5.20%), trans-verbenol (4.02%), and terpinen-4-ol (3.10%) | [7] |
Leaves | 1,8 cineole (51.08%), α-pinene (24.60%), L-pinocarveol (9.98%), and globulol (2.81) | [4] |
Leaves | 1,8-Cineole (71.05%) and α-pinene (8.30%) | [18] |
Leaves | Phenolics (quercetin and luteolin) | [1] |
Leaves | 1,8-cineole (76.65%), α-pinene (5.65%), α-terpineol acetate (4.85%), and alloaromadendrene (3.98%) | [10] |
Leaves | 1,8-cineole (62.38%), α-pinene (23.79%), α-terpinyl acetate (5.41%), globulol (1.68%), and β-pinene (1.1%) | [14] |
Leaves | 1,8-cineole (55.29%), spathulenol (7.44%), and α-terpineol (5.46%) | [20] |
Leaves | 1,8-cineole (36.68%), β-pinene (9.25%), aromedendrene (6.33%), and globulol (5.11%) | [5] |
Leaves | Chlorogenic acid, rutin, and quercetin 3-glucuronide and ellagic acid derivatives | [41] |
Leaves | 1,8-cineole (54.79%), β-pinene (18.54%), α-pinene (11.46%), β-eudesmol (4.68%), α-phellandrene (2.06%), para cymene (1.60%), and gamma-eudesmol (1.20%) | [3] |
Leaves | p-Cymene (18.18%), methyl eugenol (8.83%), 4-Terpinenol (8.45%), s-methyl 3-methylbutanethioate (7.26%), g-terpinene (5.12%), and 1,8-cineole (3.16%). | [57] |
leaves and small branches | 1,8-cineole (63.81%), α-pinene (16.06%), aromadendrene (3.68%), and o-cymene (2.35%) | [12] |
Leaves | 1,8-cineole (63.00%), α-pinene (16.10%), and camphor (3.42%) | [13] |
Leaves | 1,8-cineole (48.2%), α-pinene (16.1%), γ-terpinene (8.9%) and p-cymene (8.8%) | [33] |
Leaves | 1,8-cineole (75.8%), p-cymene (7.5%), α-pinene (7.4%), and limonene (6.4%) | [11] |
Leaves | 1,8-cineole (69.32%), camphene (9.41%), α-pinene (7.48%), and α-terpineol (5.08%) | [23] |
Leaves | 1,8-cineole (46.76%), D-limonene (9.61%), and o-cymene (6.49%) | [35] |
Leaves | Phenolic compounds (quercetin, luteolin, kaempferol, iso-rhamnetin, phloretin, chlorogenic acid) | [52] |
Leaves | 1,8-cineole, phenolic acids (Gallic acid, ellagic acid, vanillic acid, p-hydroxybenzoic acid, p-coumaric acid, and quercetin), phenolics (catechin, rutin, and luteolin) | [42] |
Leaves | 1,8-cineole (70.94%), 3-cyclohexene-1-ol (3.13%), beta. fenchyl (5.38%), 1,2-benzenedicarboxylic acid (6.08%), dodecane (1.50%) | [64] |
Leaves | Eucalyptol (59.63%), p-cymene (15.55%), and DL-limonene (14.90%) | [15] |
Leaves | Eucalyptol (55.43%), α-pinene (25.55%), and D-limonene (5.687%) | [24] |
Leaves | 1,8-cineol (56.83%), L-pinocarveol (10.42%), α-pinene (9.47%), globulol (7.68%), and carvacrol (1.59%) | [31] |
Leaves | p-cymene (20.24%), spathulenol (14.10%), and eucalyptol (11.30%) | [54] |
Leaves | 1,8-cineole (23.3%), citronellal (18.1%), geranial (17.6%), isopulegol (10.4%), myrcene (13.0%), cuminaldehyde (9.1%), and 2-pinene (8.5%) | [6] |
Leaves | 1,8-cineole (80.2%), p-cymene (6.6%), and limonene (5%) | [9] |
Leaves | D-limonene (23.5%), m-cymene (24.8%), o- cymene (9.9 and 5.4%), 6-camphenol (7.2 and 10.7%), terpinen-4-ol (5.2 and 4.5%), and globulol (4.0 and 12.9%) | [32] |
Leaves | Eucalyptol (51.62%), α-pinene (23.62%), p-cymene (10%), β-myrcene (8.74%), terpinen-4-ol (2.74%), and γ-terpinene (2.59%) | [22] |
Leaves | 1,8-cineol (67.4 and 67.6%) and α-pinene (12.8 and 13.1%) | [21] |
Fruits | Aromadendrene (31.17%), 1,8-cineole (14.55%), globulol (10.69%), and ledene (7.13%) | [65] |
Fruits | Globulol (23.6%), aromadendrene (19.7%), 1,8-cineole (19.8%), and α-pinene (3.8%) | [66] |
Bark | Polyphenol and tannin | [59] |
Deciduous bark | Fatty acids, aliphatic alcohols, sterols, and triterpenoids | [61] |
Bark | Polygalloyl glucoses (gallotannins), catechin, epicatechin, ellagic acid, quercetin-3-o-rhamnoside, and isorhamnetin (phenolic compounds) | [67] |
Stump | Phenolic compounds and flavonoids | [53] |
The leaf essential oil of Nigerian-grown E. globulus exhibited a low antioxidant capacity via its potential to scavenge DPPH radicals, with elevated IC50 values (136.87 µL/mL) as compared to the standard antioxidant ascorbic acid. This may be attributed to the absence of some components such as 1,8-cineole in the leaf oil as well as the potential antagonistic impact between other components in Eucalyptus oil [27][55]. Conversely, Luis et al. [13][12] reported that the essential oil of E. globulus exerted a remarkable antioxidant efficacy through its ability to scavenge DPPH radicals with an IC50 value (2.90 ± 0.35 v/v), with respect to the IC50 value (4.56 ± 0.70 v/v) for Eucalyptus radiata. This may be related to the existence of 1,8-cineole as the main constituent only in E. globulus essential oil, as well as the synergistic effect between other oil components. Moreover, the essential oil of E. globulus showed great activity to inhibit the lipid peroxidation with an IC50 value (2.72 ± 0.01 v/v) inferior to the activity of the synthetic antioxidant BHT, with an IC50 value of 3.58 ± 0.02 w/v, in the β-carotene bleaching test. This was considered a promising result that supported the E. globulus essential oil as a potential natural substitute, to overcome the adverse side effects of synthetic antioxidants, especially for food preservation.
The extracted essential oil from fruits exerted pronounced antibacterial potency against tested multidrug-resistant bacteria. Furthermore, the combination of 1,8-cineole and aromadendrene from fruit oils produced a higher inhibition through an additive and synergistic effect against methicillin-resistant Staphylococcus aureus, Streptococcus pyogenes, and Bacillus subtilis, as compared to using a single compound [43][65]. The antibacterial efficacy was ascribed to the highest percentage of oxygenated monoterpenes (87.32%) in Eucalyptus leaf oil, and the synergism also resulted from other minor components [9][8]. The antimicrobial effects of the methanolic extract from leaves, against S. aureus and B. subtilis, could be attributed to the existence of tannins and saponins [60][58]. Similarly, leaf extracts proved the anticariogenic activity, due to the existence of sesquiterpene alpha-farnesene that would lead to an advancement of effective drugs for the treatment of dental caries [19].
The greatest antibacterial activity was obtained from the synergism between E. globulus essential oil or leaf extracts and antibiotics toward P. aeruginosa [2][1]. Additionally, Goldbeck et al. [1][18] observed a synergism effect as a result of the combination between E. globulus and E. urograndis essential oils against Streptococcus mutans. Furthermore, the highest antibacterial activity was correlated with the elevated concentration of 1,8-cineole (71%) in E. globulus, as compared to E. urograndis (36%), which supported the potential usage of E. globulus essential oil, through its incorporation into biodegradable films, as an environmentally benign strategy to control S. mutans, as an important oral pathogen.