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Allelopathy Journal 25 (2): 313-330 (2010) International Allelopathy Foundation 2010
Tables: 5, Fig : -
Chemistry and bioactivity of Eucalyptus essential oils
JINBIAO ZHANG*1, MIN AN1, HANWEN WU1,2, REX STANTON1
and DEIRDRE LEMERLE1
Test Centre, Fujian Agriculture and Forestry University, Fuzhou, Fujian 350002, China.
E. Mail: jbzhangfj@yahoo.com
(Received in revised form: November 23, 2009)
CONTENTS
1. INTRODUCTION
2. ESSENTIAL OIL CHEMISTRY
2.1. Extraction methods
2.2. Yields of essential oils
2.3. Chemical analysis
2.4. Chemical composition
3. ESSENTIAL OIL BIOACTIVITY
3.1. Bioactivity on organisms
3.2. Bioactivity on weeds and crops
3.3. Mechanism of bioactivity
4. RELATIONSHIP OF BIOACTIVITY AND CHEMICAL
COMPONENTS
5. APPLICATIONS OF EUCALYPTUS ESSENTIAL OILS
6. CONCLUSIONS
7. REFERENCES
ABSTRACT
The essential oils obtained from the eucalyptus have many medicinal and
commercial uses. The oils possess many bioactivities (antimicrobial, antiviral,
fungicidal, insecticidal and herbicidal activities). The commercial uses of essential oils
have increased the research on their extraction, chemical composition, bioactivity and
mode of actions. Eucalyptus species differs in their chemical composition. The
bioactivity of essential oils is highly associated with their unique chemical
composition. The novel biological functions of eucalyptus essential oils suggest
research on all eucalyptus species to fully exploit their commercial benefits. This
review, discusses the recent progresses in above research areas and future research
prospects.
Keyword: Allelopathy, bioactivity, chemical composition, essential oils, eucalyptus.
*
Correspondence
author,
1
E.H. Graham Centre for Agricultural Innovation (Industry & Investment NSW and
Charles Sturt University), Wagga Wagga Agricultural Institute, Wagga Wagga, NSW 2678, Australia,
2Industry & Investment NSW, Primary Industry, Wagga Wagga Agricultural Institute, Wagga Wagga, NSW
2650, Australia
Zhang et al
314
1. INTRODUCTION
Eucalyptus (family Myrtaceae) is native to Australia, however, its few species are
indigenous to neighbouring countries (30). It grows over wide range of soils and climates,
hence, has been introduced into North and South Africa, Asia and Southern Europe (3).
The interspecific hybrids are grown in Brazil, Congo, China, Indonesia and South Africa
and small plantations in Phillippines, Vietnam, Thailand and Malaysia and South
Americas (Argentina, Chile, Paraguay and Uruguay) (35). There are approximately 700
species worldwide and only about 1% are used for industrial purposes (76). Its timber is
now used for fuel, shelterbelts, windbreaks and for hardwood fibre (86).
Its essential oils have many medicinal and commercial uses (47). They can be
used as folk medicine and are anesthetic, anodyne, antiperiodic, antiphlogistic, antiseptic,
astringent, deodorant, diaphoretic, disinfectant, expectorant, febrifuge, fumigant, hemostat,
inhalant, insect repellant, preventitive, rubefacient, sedative yet stimulant, suppurative,
tonic, and vermifuge (34). Its 9-species are used for medicinal and commercial purposes
(Table 1). Besides, E. cinerea and E. cneorifolia also possess medicinal value and E.
macarthurii in perfumery.
Table 1. Cultivated Eucalyptus species
Uses Species Country
Eucalyptus globulus Labill.
China, Portugal, Spain, India, Brazil,
Chile,Bolivia, Uruguay, Paraguay
E. smithii R. Baker South Africa, Swaziland,
Zimbabwe
E. polybractea R. Baker (syn. E.
fruticetorum F. Muell. ex Miq.)
Australia
E. exserta F. Muell. China
E. radiata Sieber ex DC. (syn. E.
australiana, E. radiata var. australiana)
South Africa, Australia
E. dives Schauer Australia
Medicinal
E. camaldulensis Dehnh. (syn. E. rostrata
Schldl.)
Nepal
E. citriodora Hook. China, Brazil, India Perfumery
E. staigeriana F. Muell. ex Bailey Brazil
Source : 38
Growth inhibition of plants by other plants in their vicinity has been known for a
long time. The chemical interaction between plants is referred to as allelopathy (80).
Intensive allelopathy research has been conducted in recent decades and progress has made
in analysis and identification of allelochemicals (10,37,42,51,84,116). The rapid advances
in the identification of bioactive allelochemicals in eucalyptus oils have prompted research
into potential natural herbicidal compounds for weed management in agriculture
(16,17,81).
Owing to the wide range of traditional uses and potential commercial prospects of
eucalyptus oils, extensive research on yield and chemical composition of oils has been
undertaken, as well as development of extraction and analytical techniques. This paper
Chemistry and bioactivity of Eucalyptus essential oils
315
reviews the progress on isolation, identification, bioactivity and mode of actions of
essential eucalyptus oils.
2. ESSENTIAL OIL CHEMISTRY
2.1. Extraction methods
Essential oils are secondary metabolites produced by plants in response to stress,
infection or parasitic attack. Eucalyptus oils are volatile organic compounds found in
fruits, flowers, bark, seeds, wood and roots. However, they are mainly extracted from
leaves (3,20). There are 5-extraction methods (11,19,43,45,82,90) :
(i). Hydrodistillation: It involves placing plant material and water in a container and
heating from below. The heat releases the aromatic vapour, which is condensed into liquid
containing little hydrosol (water). Total oil yield increases with increase in distillation
time, but the principle component (%) reduces accordingly (11).
(ii). Steam distillation: It involves placing the plant material in a flask and forcing steam
through the plant material to vaporize the volatile oils. It is similar to hydrodistillation, but
higher oil yield is obtained (90).
(iii). Vacuum distillation: To extract essential oils from many eucalyptus species, vacuum
distillation is done at lower temperatures, avoiding the change in essential oil chemical
composition during distillation(19).
(iv). Supercritical fluid extraction (SFE): It uses the ability of certain gases (generally
CO2 is used) to act as non-polar solvents at certain temperature and pressure. It is more
efficient and faster than hydrodistillation (82).
(v). Subcritical-water extraction (SWE): It uses hot water (100-374oC) as extractant and
high pressure. It is faster and efficient than hydrodistillation (45). The advantages are
shorter extraction time, high quality of extract, lower costs of extracting agent and
environmental friendly (43).
2.2. Essential oils Yields
The yields of eucalyptus oils from leaves depends on the species studied and
varies from 0.10 to 9.0 % on dry weight basis (Table 2). Several factors affect the
eucalyptus oils yields: tree age (5), leaf age (39,96), altitude (62), season (77), harvest time
(32) and fertilizer application (61). Young leaves contains more oil than old leaves (39),
while leaves from older trees gave slightly higher yield (5). However, oil production is
more influenced by altitude than age (62). Oil content also fluctuates with season. E.
citriodora leaves collected in Pakistan in April-May contained 0.9% oil than 1.3% in
December (77). Good eucalyptus oil yields in central Thailand were obtained between
January to July (32). Application of nitrogen, phosphorus and potassium fertilizers
increased the production of essential oils (61).
Zhang et al
316
Table 2. Yields of eucalyptus essential oils
Species Yield a (%) Reference
E. bakeri 1.8-3 22
E. brachycorys, E. crucis 0.75-3.75 19
E. caesia 0.97 7
E. camaldulensis 0.39 b 113
E. camaldulensis 0.25-0.46 39
E. cinerea 1.2-2.3 39
E. citriodora 0.6 16
E. citriodora 0.9-1.3 77
E. crebra 0.29-1.33 3
E. dunnii 3.67 112
E. globulus 1.3-2.7 b 96
E. grandis 0.36% b 113
E. microtheca, E. spahulata 0.38-1.88 88
E. nitens, E. denticulata 0.7-0.8 54
E. porosa 0.57 6
E. propinqua, E. pulverulenta 0.22-3.56 120
E. radiata ssp. robertsonii 6.7-8.4 97
E. radiate ssp. radiata, E. smithii 6.0-9.0 28
E. remophil, E. oleosa, E. patellaris, E. ranscontienalis, E. salubris 0.1-5.3 36
E. tereticornis 1.57-2.09 94
E. viminalis 0.5-1.2 39
aDry weight basis; bFresh weight basis.
2.3. Chemical composition
The oil composition of different eucalyptus species has been reviewed till 1998
(30). This current review collates the research advances from 1999 (Table 3). The number
of components identified and main components in essential oils from different eucalyptus
species are presented in increasing order of 1,8-cineole content. The main components are
monoterpenes (1,8-cineole, p-cymene, citronellal, citronellol, limonene, α-phellandrene, β-
phellandrene, α-pinene, β-pinene, trans-pinocarveol, terpinolene, α-terpineol, α-thujene)
and sesquiterpenes (β-caryophyllene, β-eudesmol, globulol, spathulenol and virdiflorol).
The chemical profile and main components of oils from eucalyptus leaves varied
significantly between species (88,120). For example, the main components of E.
largiflorens oil were 1,8-cineole (37.5%), p-cymene (17.4%), neo-isoverbenol (9.1%),
limonene (6.5%) and spathulenol (6.7%), while those of E. spathulata oil were 1,8-cineole
(72.5%), α-pinene (12.7%) and trans-pinocarveol (3.3%), with the absence of neo-
isoverbenol, limonene and spathulenol or at lower amounts (88). The monoterpenes, 1,8-
cineole and α-pinene, are the main components in most species, while, E. citriodora is rich
in citronellal (49.5-87%) and citronellol (8-20%). The content of 1,8-cineole in eucalyptus
oils ranges from 10-90%. Generally, the content of α-pinene is below 20%. The
composition of eucalyptus oils differs even between the trees of same species in different
periods, sites (25) and extraction method/time (11,90).
Chemistry and bioactivity of Eucalyptus essential oils
317
Zhang et al
318
Chemistry and bioactivity of Eucalyptus essential oils
319
3. ESSENTIAL OIL BIOACTIVITY
Eucalyptus essential oil has range of bioactivity [antimicrobial (53,56,107),
antiviral (26), fungicidal (92,93), insecticidal (57,63), anti-inflammatory (95),
antinociceptive activity (55), antioxidant capacity (104,119) and phytotoxic (91) activity]
(Tables 4,5).
3.1. Bioactivity on organisms
3.1.1. Antimicrobial activity: Sumitra and Sharma (107) reported that E. teriticornis
essential oils had antibacterial activity against six bacteria (Staphylococcus aureus,
Bacillus cereus, Escherichia coli, Micrococcus luteus, Proteus mirabilis and Alcaligenes
faecalis). Cermelli et al. (26) showed that Haemophilus influenzae, Haemophilus
parainfluenzae and Stenotrophomonas maltophilia were the most susceptible to E.
globulus essential oils, followed by Streptococcus pneumoniae and Streptococcus
agalactiae. The highest activity was found at 1.25µ l ml-1 for H. influenzae, H.
parainfluenzae and S. maltophilia. Antiviral activity assays using virus yield experiments
indicated mild activity on the mumps virus (26).
3.1.2. Antifungal activity: E. citriodora essential oil is effective against Microsporum
nanum, Trichophyton mentagrophytes and Trichophyton rubrum (93). Pure oil controlled
M. nanum in 20 s and T. mentagrophytes and T. rubrum in only 15 s. It was more effective
than prevalent synthetic antifungal drugs (Dactrine, Nizral and Tenaderm) without any
adverse side effects on mammalian skin up to 5% concentration. E. citriodora oil has also
clinical value in dermatophytoses, when used as a broad spectrum antimycotic ointment.
E. robusta, E. rostrata, E. camaldulensis, E. tereticornis, E. globulus and E. citriodora oils
are antibacterial (66,83) and antifungal (53,56,103).
3.1.3. Insecticidal activity: E. globulus essential oil has high insecticidal activity against
Aphis gossypii (63). E. grandis and E. globulus oils are resistant to termite followed by
E. citriodora and E. urophylla oils (57).
3.1.4. Antioxidant: Eucalyptus oils also antioxidant, due to the presence of phenolic
compounds (104,119) and their radical scavenging properties (119). They can also trigger
a series of induced chemical defence responses (21,110).
3.2. Bioactivity on weeds and crops
Volatile allelochemicals derived from eucalyptus oils also have herbicidal activity
against many weed species (13,16,17,79,91). Volatile essential oils from E. citriodora was
phytotoxic to Bidens pilosa, Amaranthus viridis, Rumex nepalensis and Leucaena
leucocephala, with no germination occurring at 0.06% eucalypt oil concentration under
laboratory conditions (91). Essential oils from E. nicholii strongly inhibited the
germination of Amaranthus retroflexus, Portulaca oleracea and Acroptilon repens (79).
Zhang et al
320
Table 4. Bioactivity of eucalyptus essential oils on organisms
Species Bioactivity Reference
A. Antimicrobial activity
E. camaldulensis,
E. tereticornis
Antimicrobial activity 27
E. citriodora
Antibacterial activity against Escherichia coli,
Staphylococcus aureus, Proteus mirabilis, Pseudomonas
aeruginosa, Proteus vulgaris.
66
E. citriodora
Antimicrobial activity against Trichophyton rubrum,
Histoplasma capsulatum, Candida albicans, E.coli and
Mycobacterium smegmatis
60
E. globulus Antimicrobial activity against S. aureus, E. coli 4
E. maidenii Antimicrobial activity against 12 bacteria 41
E. robusta,
E. saligna
Antimicrobial activity against S. aureus, E. coli and C.
albicans
83
E. teriticornis
Antibacterial activity against S. aureus, Bacillus cereus, E.
coli, Micrococcus luteus, P. mirabilis and Alcaligenes
faecalis
107
E. globulus Antiviral activity 26
B. Antifungal activity
E. camaldulensis,
E. tereticornis
Antifungal activity 27
E. citriodora Antifungal effects against mildew and wood decay fungi 106
E. dalrympleana Antifungal agent against Epidermophyton floccosum,
Microsporum gypseum and T. rubrum
92
E. globulus Antifungal effects against three Candida species
56
E. rostrata,
E. camaldulensis
Fungitoxic properties against four human pathogens:
Trichophyton mentagrophytes, Epidermophyton floccosum and
Microsporum canis
103
C. Insecticidal activity
E. globulus Insecticidal activity against Aphis gossypii adults 63
E. globulus Fumigant toxicities 53
E. globulus,
E. citriodora
Repellent effects against adults Apriona germarii, Psacothea
hilaris and Monochamus alternatus
117
E. grandis Larvicidal activity 59
E. saligna Repellent effects against Sitophilus zeamais and Tribolium
confusum
111
E. urophylla,
E. citriodora
Antitermite activity 57
D. Antioxidant
E. camaldulensis Antioxidative activities 104
E. staigeriana Antioxidant capacity 119
D. Others
E. camaldulensis Antinociceptive properties 55
E. citriodora,
E. tereticornis,
E. globulus
Anti-inflammatory and analgesic effects 95
E. tereticornis Relaxant effects on guinea-pig tracheal smooth muscle 29
Chemistry and bioactivity of Eucalyptus essential oils
321
Table 5. Biological activity of eucalyptus essential oils on plants
Eucalyptus species Bioactivity Reference
Crops
E. camaldulensis Growth inhibition against Allium cepa, Spinacia oleracea,
Lepidium sativa, Zea mays and Lycopersicon esculentum
64
E. citriodora Phytotoxicity against crops (Triticum aestivum and Oryza sativa) 17
E. exserta,
E. urophylla
Growth inhibition against Raphanus sativus and Lactuca sativa 118
Weeds
E. camaldulensis Growth inhibition against Echinochloa crus-galli, Avena fatua
and Rumex acetosella.
64
E. citriodora Inhibition on germination and growth against Cassia occidentalis
and E. crus-galli
16
E. citriodora Phytotoxicity against weeds (Amaranthus viridis and E. crus-
galli)
17
E. citriodora Herbicidal activity against Anagallis arvensis, Chenopodium
album and Spergula arvensis
65
E. citriodora Herbicidal activity against Bidens pilosa, A. viridis, Rumex
nepalensis and Leucaena leucocephala
91
E. citriodora Germination inhibition in Parthenium hysterophorus 100
E. globulus Germination inhibition in Amaranthus retroflexus and Portulaca
oleraceae
9
E. globulus Germination inhibition in P. hysterophorus 49
E. nicholii Herbicidal activity against A. retroflexus, P. oleracea and
Acroptilon repens
79
The eucalyptus oils are also herbicidal against Parthenium hysterophorus, Cassia
occidentalis, Echinochloa crus-galli and A. viridis (16,17,100). Thus compounds in
eucalyptus oils have potential for further use as natural herbicides.
However, the phytotoxic activity of eucalyptus oils can also cause damage to
some crops (64,118). Batish et al. (13,17) found that eucalyptus oils from E. citriodora not
only inhibited weed (A. viridis and E. crus-galli) growth, but also caused injuries to wheat
(Triticum aestivum), maize (Zea mays), radish (Raphanus sativus) and rice (Oryza sativa).
It is therefore critical to maximise the herbicidal activity of eucalyptus oils against weeds
but at the same time to minimise the negative impact of crop growth.
3.3. Mechanism of oil bioactivity
Antibacterial and antifungal compounds may target various cell structures or
chemical pathways, such as cell wall degradation, membrane damage, dissipation of the
proton motive force, decrease in extracellular protease activity, o-lipopolysaccharide
rhamnose content, ergosterol content or unsaturated fatty acids (23,44,72,73). However,
research data are very limited on the mechanism of antimicrobial and antifungal activity of
eucalyptus oils. The mode of actions of essential oils and chemical compounds from other
plant species may assist the understanding of the possible mechanism involved in
eucalyptus oil bioactivity.
Cox et al. (31) reported that tea tree (Melaleuca alternifolia) essential oils
inhibited the growth of E. coli through reduction in glucose-dependent respiration and
Zhang et al
322
enhanced leakage of intracellular K+. Cytoplasmic membrane damage was reported in S.
aureus after treatment with tea tree oil or its main monoterpene components, leading to
lysis, the loss of 260-nm-absorbing material and the loss of tolerance to NaCl (24). It was
shown that the oils of E. citriodora exerted its antimicrobial activity through the
synergistic action of citronellal and citronellol (58).
The antifungal mechanism of Thymus spp. essential oils against yeasts and
filamentous fungi was identified due to the inhibition of germ tube formation (74), with
additional impairment of ergosterol biosynthesis in some strains (75). Rapid propidium
iodide (PI) penetration into cells detected by flow cytometry assays indicates that the
fungicidal effect is through primary cell membrane damage rather than secondary
membrane damage caused by metabolic impairment. The biochemical nature of the
monoterpenes suggests they probably act as a solvent of the cell membrane. The major
components of Thymus spp. essential oils such as carvacrol, thymol and p-cymene
exhibited similar fungicidal activity against Candida species with synergistic effects noted
for thymol/p-cymene and thymol/1,8-cineole mixtures (74). The synergism helps explain
the differences in bioactivity between essential oils and their pure major compounds.
The bioactivity mechanism of eucalyptus oils on plants reduces cell survival,
chlorophyll content, RNA contents, acid soluble carbohydrate and water soluble
carbohydrate (50). Moradshahi et al. (64) found that the presence of different
concentrations of crude volatile oils from E. camaldulensis decreased the mitotic index in
the root apical meristem of Allium cepa, affected the Hill reaction in isolated spinach
(Spinacia oleracea) chloroplast and reduced peroxidase activity in Lepidium sativa, E.
crus-galli, Avena fatua, Rumex acetosella, Z. mays and Lycopersicon esculentum. Under
laboratory conditions, volatile oils from lemon-scented eucalypt (E. citriodora) decreased
weed emergence and earlier seedling growth by severely inhibiting photosynthetesis and
respiration at 0.06% (93) and 0.07% (15) concentration levels. Rapid electrolyte leakage in
the leaf tissue suggested that the mode of actions of the eucalyptus oils is a result of the
disruption of membrane integrity (15). α-Pinene, one of the common monoterpenoids from
eucalyptus oils, has been regarded as a key compound contributing to the disruption effect
(102). From these studies, it can be concluded that eucalyptus species suppress the growth
of other plant species by affecting several biochemical and physiological processes.
4. RELATIONSHIP BETWEEN BIOACTIVITY AND
CHEMICAL COMPONENTS
Progress has been made toward better understanding of relationship between the
observed biological activity and the key bioactive chemical components of eucalyptus oils.
Major compounds, such as 1,8-cineole, α-pinene, β-pinene, p-cymene, limonene,
citronellal and citronellol are principle bioactive compounds in essential oils.
Sartorelli et al. (83) reported that E. robusta oil is more active than E. saligna oil
against both E. coli and S. aureus due to the higher concentrations of the monoterpenes α-
pinene, β-pinene and limonene. Pure solutions of 1,8-cineole and α-pinene are bioactive
against Pseudomonas tolaasii, exhibiting bacteriostatic activity at 7.0 µl ml-1 and
bactericidal activity at 10.0 µ l ml-1 (105). Antibacterial activity of citronellal and
citronellol suggest that most monoterpenes in eucalyptus oils are antibacterial (85,87). The
Chemistry and bioactivity of Eucalyptus essential oils
323
relationship between the chemical composition and antifungal activity has also been
documented. Su et al. (106) reported that the chemical components and antifungal
activities varied significantly among E. urophylla, E. grandis, E. camaldulensis, and E.
citriodora, with E. citriodora the most effective for controlling mildrew. E. urophylla, on
the other hand, was least effective, suggesting that antifungal activity is associated with
citronellal and citronellol, two major compounds in the essential oils of E. citriodora. This
conclusion was supported by antifungal tests using the pure citronellal and citronellol
compounds (46).
Antitermite properties of eucalyptus oils are conferred by monoterpene α-pinene,
followed by monoterpenes p-cymene and 1,8-cineole, while the other components in the
oils were less effective (57). α-Pinene has stronger larvicidal activity than pure 1,8-cineole,
suggesting that α-pinene is a principal larvicidal component of E. grandis essential oils
(59).
The phytotoxic and herbicidal activities of eucalyptus oils are highly related to
chemical composition of essential oils, with E. citriodora essential oils being strongly
phytotoxic (13,65,100). The monoterpenes (citronellal, citronellol, 1,8-cineole and α-
pinene) inhibited the germination and initial seedling growth of C. occidentalis, A.
conyzoides, E. crus-galli, Cassia obtusifolia and Z. mays (1,81,98,99,102). Singh et al.
(101) assessed the phytotoxicity of citronellal against Ageratum conyzoides, Chenopodium
album, P. hysterophorus, Malvastrum coromandelianum, C. occidentalis and Phalaris
minor. Emergence of A. conyzoides and P. hysterophorus was inhibited by citronellal at a
concentration of 50 µ g g-1 (sand) and the weed emergence and early seedling growth of
other species were completely inhibited at 100 µ g g-1.
The bioassay results suggest that the bioactivity of eucalyptus oils may be due to
their monoterpene components. However, essential oils are a complex chemical mixture
and they possess higher activities than their individual components (52). Therefore, total
bioactivity of essential oils may be due to the combined effects of several minor
components or the synergistic effect of essential oil components (12,52).
5. APPLICATIONS OF EUCALYPTUS ESSENTIAL OILS
Eucalyptus essential oils are widely used in many industries and mainly in
therapeutic natural medicine (108).
Eucalyptus is a traditional medicinal plant to treat cold, flue, fever, diabetics and
bronchial infections (70,92,95) or to prevent and control other pathogenic diseases (4,109).
Eucalyptus oil based products are used as topically applied medication to relieve muscular
pain and as solvent/sealer in dentistry (3) and as disinfectant (38). The bioactive
components identified from eucalyptus essential oils may develop new classes of analgesic
and anti-inflammatory drugs (95).
Similarly, the insecticidal activity of eucalyptus oils could provide opportunities
for new biodegradable products for pest control. The oil is used as an insect repellent since
1948 (48) and now many commercial repellents are available in US and China (14). A
commercial mosquito repellent has also been developed from p-manthane-3,8-diol isolated
from E. citriodora (14). Kegley et al. (48) has reported that eucalyptus oil based products
ranked 4th amongst the insecticides used for repelling insects from beehives.
Zhang et al
324
The widespread use of synthetic herbicides has resulted in the rapid evolution of
herbicides-resistant weeds and increasing public concern over the impact of synthetic
herbicides on human health and the environment (114). Alternative weed management
options based on natural products are being sought (33), with essential oils from plants
receiving much attention due to their ready availability and low cost (8,33). A commercial
herbicide with the active ingredient cinmethylin, has been developed based on the natural
chemistry of 1,8-cineole, the major monoterpene in eucalyptus essential oils (40). A
myriad of compounds are present in eucalyptus essential oils and determination of their
chemical structure and bioactivity may provide novel leads for development of herbicides
with new modes of action.
Eucalyptus oils are widely used in fragrance industry (soaps, detergents, creams,
lotion, deodorizer, perfumes) and as flavouring agents in food industry (38,69,78).
Eucalyptus oils are also used as a flotation agent in the mining industry (3) and as a source
of citronellal for the chemical industry (38).
6. CONCLUSIONS
Eucalyptus essential oils are gaining increasing interest due to their varied
commercial applications particularly as insect repellents, fragrants and traditional
medicines. Despite the commercial prospects, till now only limited eucalyptus species
have been studied. Considering that the chemical composition of eucalyptus oils and its
associated biological activities varies significantly between species, other species deserves
further attention. It should be emphasized that the research into eucalyptus oil chemistry
may lead to the development of natural pesticides with new mode of actions.
The relationship between the bioactivity and chemical composition has been
reported, yet information on the mode of actions of eucalyptus oils is limited. Modern
molecular and biochemical approaches have provided excellent research tools to facilitate
the study of mechanisms of action and the biosynthesis of essential oils.
Progress has been made in identification of genes responsible for essential oil
biosynthesis in peppermint (Mentha piperita), with subsequent metabolic engineering
significantly altering essential oil composition and yield. This success in genetic
manipulation of essential oil biosynthesis in peppermint suggests similar improvements in
eucalyptus essential oil production may be possible. An extensive review on the advances
in genetic transformation of eucalypts indicates that significant progress has already been
made into understanding eucalyptus genetics. Quantitative trait loci associated with
essential oil biosynthesis in E. grandis × E. urophylla hybrids have already been identified.
Future modification of eucalyptus essential oil biosynthesis will require close collaboration
between plant biologists, biochemists and molecular biologists as well as plant breeders to
develop products for new applications.
ACKNOWLEDGEMENTS
Authors acknowledge the funding support from Meat and Livestock of Australia.
Chemistry and bioactivity of Eucalyptus essential oils
325
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