Cymbopogon nardus essential oil: a comprehensive review on its chemistry and bioactivity

Table of contents :
Abstract
1. Introduction
2. Botanical description
3. Traditional and modern utility
4. Physical properties
5. Chemical composition
6. Biological activities
6.1. Insecticidal
6.2. Antifungal activity
6.3. Antibacterial activity
6.4. Herbicidal activity
6.5. Antioxidant property
7. Safety and toxicity
8. Conclusion and future perspectives
Disclosure statement
References

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Journal of Essential Oil Research

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Cymbopogon nardus essential oil: a comprehensive review on its chemistry and bioactivity Harneet Kaur , Urvashi Bhardwaj & Ramandeep Kaur To cite this article: Harneet Kaur , Urvashi Bhardwaj & Ramandeep Kaur (2021): Cymbopogon nardus essential oil: a comprehensive review on its chemistry and bioactivity, Journal of Essential Oil Research, DOI: 10.1080/10412905.2021.1871976 To link to this article: https://doi.org/10.1080/10412905.2021.1871976

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JOURNAL OF ESSENTIAL OIL RESEARCH https://doi.org/10.1080/10412905.2021.1871976

Cymbopogon nardus essential oil: a comprehensive review on its chemistry and bioactivity Harneet Kaur, Urvashi Bhardwaj and Ramandeep Kaur Department of Chemistry, Punjab Agricultural University, Ludhiana, India ABSTRACT

ARTICLE HISTORY

Increasing knowledge about the various properties exhibited by essential oils led to a sharp expansion in their production and utilization. Citronella essential oil is one of the commercially important essential oil having multiple applications in the cosmetic and pharmaceutical industries. Its essential oil is highly valued in the National and International markets. It is extracted from perennial grass citronella (Cymbopogon nardus (L.) Rendle) belonging to the family Poaceae. This review aims to coherently discuss and compare various factors significantly affecting the chemical composition of essential oil. Citronellal, citronellol and geraniol are major constituents of C. nardus essential oil. The review also discusses the important biological properties exhibited by citronella essential oil along with the toxicological and safety aspects of citronella essential oil. This informa­ tion will offer a new perspective for the further utilization of essential oil in various fields.

Received 6 April 2020 Accepted 28 December 2020

1. Introduction Essential oils have been used for traditional medicinal purposes since ancient times (1). The recent interest in green consumerism has lead to the renewal of scientific interest in these substances classified as GRAS (Generally recognized as safe). These natural products are given preference to synthetic ones. In the last dec­ ade, there is continuous growth in the inclination of industry towards the utilization of essential oil due to their great application as flavour additives, fragrance in perfumes or for its pharmaceutical properties (2). India ranks 3rd position in essential oil production in the world. Among various crops grown for essential oil, lemon, roses, grapes, citrus, eucalyptus, citronella grass have attained commercial significance (3). Due to the success of natural herbs and sales of environment friendly products with encouraging returns, many entrepreneurs and herbal companies are currently developing plantations of aromatic plants such as citro­ nella grass (4). In 2011, production of citronella essen­ tial oil was 100 metric tonnes. Citronella essential oil is in particular highly valued due to its insecticidal activ­ ities (5). Citronella grass (Cymbopogon nardus L. Rendle) is perennial grass belonging to the family Poaceae. It includes 140 aromatic species (6). It is commonly known as ganjni (Hindi), kamakher (Bengali), usadhana (Marathi), kamachipillu (Tamil), kamkshi- kasuvu (Telgu), khavai (Punjabi). Citronella grass is native to CONTACT Urvashi Bhardwaj

[email protected]

© 2021 Informa UK Limited, trading as Taylor & Francis Group

KEYWORDS

Cymbopogon nardus; essential oil; chemical composition and biological activity

South India and Sri Lanka (2). It is cultivated mainly in tropical and subtropical regions of America, Africa and Asia including Brazil (7). It is commercially grown in the south-eastern parts of Asia (mainly in Burma, Sri Lanka, India, Ceylon, Taiwan, Indonesia) and West Indies. Citronella is usually confused with lemongrass but the lemongrass comes from the Cymbopogon species C. citratus, C. pendulus and C. flexuosus and consists of citral (cis and trans) as the active ingredient (8) whereas the two species of citronella i.e., C. winterianus and C. nardus contain citronellal (mono­ terpene aldehyde) as its major constituents and the other active compounds are citronellol and geraniol, respectively (9,10). The distinct chemical composition of lemon and citronella grass refers to different medic­ inal properties. Lemongrass is used as an analgesic, antipyretic, antiplasmodic, sedative, diuretic and in the treatment of gastrointestinal and nervous disturbances (11). On the other hand, citronella is used for rheuma­ tism, menstrual problems, fevers and intestinal para­ sites (12). Citronella refers to two species of perennial tropical, essential oil-bearing grasses of genus Cymbopogon viz. Java (C. winterianus) and Ceylon (C. nardus). These two species are distinct from each other both morphologi­ cally and biochemically (13). In the Java type, the root is shallow, culm is tall (2.5 m), the color of cross section of non-flowering branch is yellow, panicle is opened

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H. KAUR ET AL.

(50 cm long), three veins at lower glume, shorter and broader leaves, narrow-winged spikelets and the inflor­ escence is 60–100 cm tall while in case of Ceylon type, the root is deep, culm is short (2 m), cross section of non-flowering branch is red, panicle contracted (15–30 cm long), no vein at lower glume, long and narrow leaves, concave winged spikelets and the inflor­ escence is 15–60 cm long (14). The major constituents in C. winterianus essential oil includes citronellal, citro­ nellol, geraniol, nerol, elemol, methyl isoeugenol while citronellal, citronellol, geraniol, camphene, limonene, 1-borneol, methyl isoeugenol, geranyl formate are the major constituents of C. nardus essential oil. However, this article presents a literature review of C. nardus (Ceylon species) of citronella. The objective of this paper is to provide an overview of various factors affecting the chemical composition of citronella (C. nardus) essential oil. This review also compiled the published data on the various biological properties exhibited by essential oil to provide this essential oil as a potential source for the development of pharmacological agents. This study is based on a literature search from 1973 year to April 2020 to find all relevant articles published. The search was performed through specia­ lized databases (Lilacs, Scielo, Science Direct, and PubMed) using citronella, Cymbopogon nardus, Biological activity, chemical composition as keywords.

2. Botanical description Citronella is evergreen and aromatic grass. It grows 2.5 m tall from rootstock and is clamp-forming grass. It is a long-lived grass having narrow leaf-blades. The panicles are 15–30 cm long and are narrow with unbranched inflorescence racemes, 8–10 mm long with long and soft hairs present around it. The spike­ lets are without stalks, flat and concave winged keels on the back (15). It is branched above and a large decom­ pound nodding panicle is formed (16). At the base, culms are up to 10 mm in diameter and are solid, polished, with black finely glabrescent nodes. The leaves are long and narrow 0.5 to 1.6 cm wide, 1 m long with ribbon-like blade at the greenish green end, with a white-coloured midrib and upper cauline over 9 mm wide which is narrowed to the base. The apex of the leaf is filiform and is glaucous beneath with scab­ rous margins and is glabrous except the top of the sheath. It can tolerate the temperatures and annual rainfall of 16–36°C and 750–4100 mm but grows best in the areas where temperature and annual rainfall ranges between 20 and 30°C and 1300–2000 mm. The sandy loamy soil with high moisture content (without

water logging) is best for the proper growth of the citronella plant. The most ideal season for the planting of the plant is rainy season while summers and early winters are favourable during harvest. The first harvest takes place 6 months after the crop plantation and can be harvested three to four times in the year with the 2.5–3 months interval. The oil yield gets reduced if the harvest is done in the late winters (17). The postharvest drying of the plant material can increase the shelf life as it leads to the reduction of the moisture content and prevention of the enzymatic and microbial activity of the plant (18). The Scientific classification of C. nardus is as follows: Kingdom: Clade: Clade: Clade: Order: Family: Genus: Species:

Plantae Angiosperms Monocots Commelinids Poales Poaceae Cymbopogon Nardus

3. Traditional and modern utility In Ayurveda, citronella is referred as Pangiri and is used in the treatment of redness, irritation, toothaches and inflammation of the skin, rheumatism, digestive problems, headache, infectious diseases, fatigue and childbirth wash. In Thailand and China, citronella essential oil was used traditionally for the treatment of irritable bowel, stomach ache, intestinal cramps, gastritis, indigestion, flatulence and also as a blood tonic (19,20). Since Vedic times, it has been used in aromatic tea, diuretic, antipyretic, vermifuge and also in the treatment of mental illness (21). It is also used for the treatment of intestinal problems, fever, and in aromatherapy for treatment of cold, headaches and flu (20). Nowadays, the essential oil obtained is found to have values in soaps, toiletry, perfumery, tobacco, cos­ metics, insect repellent packaging, body care products and also in the pharmaceutical industries (2,21,22). The essential oil was also used to repel cats (23). C. nardus is found to contain cellulosic fibres and is used in the paper industry for preparation of pulp for paper industry with less environmental threats (24). The properties of the pulp, paper and raw material of C. nardus is same as that of non-wood materials. C. nardus contained high α-cellulose (35%), holocellu­ lose, low alcohol-benzene extractive (5.14%), low lignin content low ash content (3.66%) and lowest sodium hydroxide extractives (25.99%). C. nardus pulp required less energy for pulping as compared to wood pulp and the properties can be improved by beating pulp up to

JOURNAL OF ESSENTIAL OIL RESEARCH

500 PFI revolutions. C. nardus grass has a potential to replace hardwood pulp in news, toilet, writing papers. Hence, C. nardus grass is recommended for paper pro­ duction (24). The fumigation effect of citronella essential oil on the quality of potato tubers and sprout suppression during storage was investigated. They involve the treatment of potato tubers with citronella essential oil fumigation. It was reported that the citronella essential oil fumigation results in reducing sugar content, suppressing the gib­ berellin production, decreasing the level of α-solanine, and inhibits the degradation of starch. Hence, citronella oil fumigation leads to the control of sprouting and helps in improving the quality of potato tubers while storing (25).

4. Physical properties The yellowish coloured citronella essential oil (b.p. 70° C) has a refractive index and the specific gravity of 1.47 and 0.89 g/cm3 respectively obtained by different meth­ ods at 20°C. The red and yellow colour of citronella oil has also been reported. The reddish colour of citronella oil might be due to the presence of trans-βcaryophyllene and γ-cadinene. The oxidation of lipids in the extraction systems might also be reason behind the appearance of the red colour. No red colour appeared during the extraction using hydrodistillation process (26). The variation of physical properties of citronella essential oil with the age of leaves is given in Table 1. The essential oil is slightly soluble in water and solu­ ble in hexane, ether. It can be stored under normal conditions and is noncorrosive. It can persist for about 425 h with a flash point of 170°C.

5. Chemical composition The chemical composition of citronella essential oil varies with the geographical origin, environmental fac­ tors, ecological and climatic conditions, developmental stages, harvest time, genetic factors. In addition to these, Table 1. The comparison of physical parameters of the citronella essential oil at 25°C (41). Density Refractive index Main composition (%) gmL−1 0.854 1.466 Citronellal (39.66), Citronellol (12.98), Geraniol (18.83) 6–10 months Pungent 0.873 1.475 Citronellal (4.8), lemon Citronellol (6.69) Geraniol (46.10), Citral (cis, trans)(26.61), Nerol(4.23).

Age of leaves Smell 1–5 months Fragrant lemon

3

the chemical composition also depends upon the method of extraction and drying techniques. Initially, steam distillation and hydrodistillation were considered most convenient methods for the extraction of citronella essential oil (10,27). Though these processes are very simple but these are time and energy-intensive and thereby resulting in water solubility, thermal degradation and hydrolysis of fra­ grance constituents (28). Further Micro hydrodistilla­ tion (MHD) was introduced for extraction of citronella essential oil (29). In the last decade, supercritical car­ bon dioxide extraction has been introduced involving the use of carbon dioxide as a supercritical fluid for the extraction of essential oil (30–32). The extraction with supercritical carbon dioxide was generally performed at low temperature. It prevents thermal degradation and loss of thermolabile compounds. Moreover, low critical pressure and temperature, inertness, low toxi­ city, selectivity, non-carcinogenic nature are some of the properties of supercritical carbon dioxide (28). For small-scale extractions, an alternative eco-friendly method, i.e. ohmic-heated hydrodistillation has been introduced (33). This method does not involve use of any organic solvent during extraction and saved 46% of total process time (34). Moreover, this process leads to reduction in energy consumption along with the enhancement of the product quality. The ohmicheated hydrodistillation results in the formation of the transient pores in cell membranes and results in the release of the citronella oil by opening the pores of schizogenous cavities and hence is a better method for the extraction of the citronella oil (35). It was found that due to the localized heating and rapid increase in temperature during ohmic heating, it results in the cell and structural eruptions along with the increase in membrane permeability (36). It was reported that on hydrodistillation, the thyme glands were shrunk while upon ohmic-heated hydro-distillation these glands were erupted (34). However, metal ion migration and residue pollution take place because of the electroche­ mical reaction between electrodes and the extraction media (37). Therefore, selection of the proper material as the electrodes, careful handling of the waste of production and the proper process parameters are necessary for the better utilization of this method. Another method, namely, Accelerated solvent extrac­ tion (ASE) has been introduced for extraction of essen­ tial oil (29). This method utilizes the organic solvents at high pressure and temperature and favoured because of its speed and requirement of a small amount of organic solvent. The essential oils sensitive to oxidative degradation or volatilization through action of air or light may be extracted using this method in absence of

4

H. KAUR ET AL.

light and presence of inert atmosphere (38). To increase the efficiency of extraction another technique i.e, sono hydrodistillation was introduced (39). Various mechanisms involved in sonication include fragmenta­ tion, sonoporation, sonocapillary formation, erosion, detexuration, which results in the faster recovery of the oil (33). Due to the better heat and mass transfer, ultrasonic waves results in faster extraction process (40). Moreover, less energy is required for completion of the extraction process. The yield of essential oil extracted is largely dependent upon the method used for extraction (Table 2). In terms of energy consump­ tion, ohmic-heated hydrodistillation is superior to steam distillation and hydrodistillation as it requires less energy to operate. In a particular method employed, plant part used, harvesting time, moisture content and packing of mate­ rial also significantly affects the yield of essential oil. (i) Packing of leaves: It was found that the yield of the essential oil was 0.64% in the sleeping position of the leaves while it was 0.43% in the standing position of the leaves. Also, the yield was 0.70% in the loose packing while it was 0.40% in the close packing of the leaves (41). The old leaves of C. nardus consist of high content of geraniol while the younger leaves have high content of citronellal and citronellol. (ii) Moisture content: It was found that the drying affects greatly the composition and content of the oil. Based on dried weight, the yield of the oil from the dried sample after 3 h distillation was 2.12% while the yield from the fresh sample was 2.43% (42). It was hence supposed that some of the volatile compounds are lost during drying and hence result in the decreased oil content. The oil obtained after 4 h distillation of dried sample was found to have yield of 0.78% (42). Hence, sufficient amount of water must be maintained through­ out the process otherwise it results either in the burning of the plant material or insufficient evaporation of the essential oil (43). However, the presence of excess water, it results in more heat and time consumption during the extraction. (iii) Parts of the plant: On comparison of the yield of the oil extracted from different parts of the plants, the essential oil content in the leaves was 40 and 17%

Table 2. The yield of essential oil obtained using various methods. Sr. No. 1.

Method used Hydrodistillation

2. 3. 4

Steam distillation Ohmic heated hydro-distillation Supercritical carbon dioxide extraction

Yield (%) 9.4 1 0.7 0.37 2.06

References (43) (51) (41) (42) (42)

higher than the stem and whole of the aerial parts. Also, the whole aerial part was found to contain 21% of the more essential oil as compared to the stem. The essential oil yield obtained from the different parts of the plant such as stems, leaves and whole aerial plant was found to be 1.58, 2.38 and 1.92%, respectively. The essential oil highest and lowest yield from leaves was 2.38 and 1.75%, respectively, while highest and lowest yield from whole aerial parts was 2 and 1.75%, respec­ tively (44). (iv)Harvesting Season: The spring-harvested leaves yielded 3.55% of the essential oil while the winter season leaves yielded 2.33–2.67% of the oil upon hydrodistilla­ tion (17). (v) Extraction time: It was further observed that with the increase in the extraction time (up to 2 h), the oil yield increases up to 3 h but after 3 h the oil quality decreases (45). Hence, the yield of citronella essential oil was higher from the fresh leaves present in the sleeping position with loose packing. Presence of sufficient amount of water is necessary for better yield of essential oil. The yield of essential oil extracted from leaves was highest followed by the essential oil extracted from whole aerial part and the yield of essential oil extracted from stem was the lowest. The spring-harvested leaves yielded greater amount of essential oil with the extraction time of 3 h. The variations in the method used for extraction of essential oil markedly affect the percentage composi­ tions of components of citronella essential oil (2). However, major constituents were estimated in all extraction methods (Table 3). In all of the extraction methods, citronellal (16–36%), citronellol (4–13 %) and geraniol (7–22 %) were found to be the major constitu­ ents and Eugenol (0.78–2.5%), δ-cadinene (0.36–­ 1.09%), β-myrcene (0.09–2.9%) were the minor constituents of citronella essential oil. The chemical composition of C. nardus essential oil was studied and its major constituents included cam­ phene, β-caryophyllene, limonene, myrcene, terpino­ lene, borneol, citronellol, geraniol, linalool, piperitol, citral (cis and trans), citronellal, methyl heptenone, citronellic acid, piperitone, citronellyl acetate, caryo­ phyllene oxide, geranyl acetate, geranyl butyrate, methyl eugenol, chavicol, eugenol, methyl isoeugenol, nerol, ocimene, elemol, η-propyl alcohol, 4-terpineol, menthane, α-terpinene, α-thujiene, α-terpineol, αpinene and β-pinene (46). The citronella essential oil mainly consists of mono­ terpenes. It contained 16 monoterpenes (73.3%), two aldehydes (30%), five hydrocarbons (8.9%), eight alco­ hols (40.4%), one phenolic hydrocarbon (0.5%). The

JOURNAL OF ESSENTIAL OIL RESEARCH

5

Table 3. Variation of chemical composition with methods of extraction. Hydro Distillation (%) Chemical Constituents Citronellal Citronellol Geraniol Neral Eugenol Geranial α-Copaene β-caryophyllene Humulene α-cubebene α-pinene δ- cardinol Limonene oxide Germacrene-D γ- terpeniol Cis-sabinene hydrate β-myrcene Borneol Nerol β-pinene Nerolidol Germacren-4-ol Isoplugeol α-cadinol Elemol δ-cadinene β-Elemene Citral linalool Limonene Geranyl acetate Citronellyl acetate β-cubinene γ-cadinene

2 22.09 12.26 41.48 7.97 0.91 8.39 4.73 1.37 -

7 26.23 12.96 19.75 1.45 0.99 2.75 5.07 4.21 2.80 0.67 0.71 2.96 3.58 -

48 27.87 11.85 22.77 14.54 11.21 -

49 29.2 12.7 29.3 5 4.1 -

50 0.63 4.33 55.57 8.34 2.51 10.18 6.61 2.32 1.63 1.23 1.14 0.58 0.47 0.2 -

Steam distillation (%) 51 16.9 10.4 9.1 8 7 7.6 7.2 -

major component identified was citronellal (29.7%) fol­ lowed by geraniol (24.2%), γ-terpineol (9.2%), along with cis-sabinene hydrate, β-myrcene, borneol, nerol with concentrations 3.8, 2.9, 2.5, 1.5%, respectively. Nine sesquiterpenes (11.5%) including three alcohols (6.5%) and six hydrocarbons (5%) were also identified. Predominant sesquiterpene was (E)-nerolidol (4.8%) followed by caryophyllene and germacren-4-ol, 2.2 and 1.5%, respectively. Four non-terpenic components which comprised 1.4% of the total sample were also detected (10). The citronella essential oil obtained from different geographical sources has different distri­ bution of oxygenated monoterpenes (Table 4). In all the geographical regions, alcohol content was highest in citronella essential oil followed by aldehyde/ketone and least was in hydrocarbons. Among all the regions, alcohol content (80.1%) was highest in essential oil obtained from Bangladesh while carbonyl content (47.2%) was highest in the essential oil obtained from Brazil. Citronella essential oil was characterized by the pre­ sence of two major chemical groups, monoterpenes and

52 47.2 11.1 18.6 -

53 75.1 0.4 0.5 0.1 0.1 0.2 0.2 1 0.4 7.6 -

2 20.99 13.64 42.43 4.94 0.78 4.39 7.74 2.13 1.35

10 29.7 24.2 2.2 9.2 3.8 2.9 2.5 1.5 1.5 4.8 1.5 -

54 29.6 4.8 1.5 6.5 2.7 -

Ohmic heated hydro distillation 71 36.53 13.10 25.56 0.69 1.4 8.24 1.09 1.1 1.51 2.22 -

2 19.61 12.97 41.15 6.6 1.06 6.42 1.63 5.66 1.75 1.26

Table 4. Percentage contribution of oxygenated monoterpenes in C. nardus essential oil. Countries Malaysia (2,54) China (7) India (10,50) Morocco (51) Brazil (48,52,71) Africa (Ivory coast) (49) Bangladesh (53)

Hydrocarbons 6.1 9.2 17.86 17.86 7.2 5.51 4.1 9

Aldehyde/ketone 39.25 6.3 26.23 29.7 19.15 16.9 42.41 47.2 36.53 29.2 0.9

Alcohol 54.35 29.6 40.53 33.4 62.99 20.3 45.83 29.7 51.09 47 80.1

sesquiterpenes, along with great amount of their oxyge­ nated derivatives. C. nardus essential oil was charac­ terised by large amount of β- citronellal, β-citronellol, nerol, limonene, elemol, β-elemene, α-cadinol, germa­ crene-D, cubenol, δ-cadinene, geranyl acetate and τcadinol (47). Gas Chromatography-Mass Spectrometry (GC/MS) analysis of essential oil obtained from C. nardus con­ tained oxygen-containing monoterpenes (90.61%) which comprised of citronellal (27.87%), geraniol

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(22.77%), trans- citral (14.54%), citronellol (11.85%) and nerol (11.21%) as the major compounds (48) while the similar study reported that the percentage composition of major constituents such as citronellal, citronellol, geraniol, elemol and limonene was 29.2%, 12.7%, 29.3, 5 and 4.1%, respectively (49). Further, it was reported that 96.68% of essential oil isolated from citronella grass consists of 75 and 21% of monoterpenes and sesquiterpenes, respectively. It was a complex mixture of 23 compounds. The percentage of major constituents citronellal, geraniol and citronel­ lol was 26.23, 19.75 and 12.96 %, respectively, while the other minor constituents such as α- cadinol, elemol, δcadinene, β- elemene, citral, linalool, limonene and geranyl acetate were present in 2.75, 5.07, 4.21, 2.8, 0.67, 0.71, 2.96 and 3.58% (7) while in the similar study it was reported that C. nardus essential oil included geraniol (55.77%), trans-citral (10.18%) and cis- citral (8.34%) as major constituents whereas αcopaene (6.61%), citronellol (4.33%), humulene (1.63%), eugenol (2.51%), α-cubebene (1.23%), βcaryophyllene (2.32%), α-pinene (1.14%), δ-cardinol (0.58%), citronellal (0.63%), germacrene-D (0.20%)

and limonene oxide (0.47%) were the minor constitu­ ents (50). Geographical sources were responsible for the che­ mical composition differences of C. nardus essential oil (Table 5). In Morocco, the yield of C. nardus essential oil obtained from its leaves was 1% and was colourless. The essential oil obtained was then analyzed by GC/MS and 13 chemical compounds were identified. The major constituents obtained were citronellal (16.9%), followed by citronellol (10.4%), elemol (9.1%), nerol (8%), citro­ nellyl acetate (7.6%), β -cubinene (7.2%), and geranyl acetate (7%) in the essential oil obtained from Morocco (51) while the essential oil obtained from C. nardus collected from China was found to contain citronellal (26.23%), citronellol (12.96%), Elemol (5.07%), geranyl acetate (3.58%), geraniol (19.75%), δ- cadinene (4.21%), Limonene (2.96%), β-elemene (2.8%) and α-cadinol (2.75%) (7). The chemical composition of hydro-distilled citro­ nella leaves essential oil obtained from Congo and Benin region were compared and found that it contains 91 and 86% of monoterpenes, respectively. The citronellal and geraniol were 41.3% and 23% respectively in essential oil

Table 5. Comparison of C. nardus essential oil content from different geographical sources. Geographical Distribution Malaysia (2,54) Component Citronellal Citronellol Geraniol Neral Eugenol Geranial α-Copaene β-caryophyllene Humulene α-cubebene α-pinene δ- cardinol Limonene oxide Germacrene-D γ- terpeniol Cis-sabinene hydrate β-myrcene Borneol Nerol β-pinene Nerolidol Germacren-4-ol Isoplugeol α-cadinol Elemol δ-cadinene β-Elemene Citral linalool Limonene Geranyl acetate Citronellyl acetate β-cubinene γ-cadinene

22.09 12.26 41.48 7.97 0.91 8.39 4.73 1.37 -

29.6 4.8 1.5 6.5 2.7 -

China (7) 26.23 12.96 19.75 1.45 0.99 2.75 5.07 4.21 2.80 0.67 0.71 2.96 3.58 -

India (10,50) 29.7 24.2 2.2 9.2 3.8 2.9 2.5 1.5 1.5 4.8 1.5 -

0.63 4.33 55.57 8.34 2.51 10.18 6.61 2.32 1.63 1.23 1.14 0.58 0.47 0.2 -

Morocco (51)

Brazil (48,52,71)

Composition (%) 16.9 27.87 47.2 10.4 11.85 11.1 9.1 22.77 18.6 14.54 8 11.21 7 7.6 7.2 -

36.53 13.10 25.56 0.69 1.4 8.24 1.09 1.1 1.51 2.22 -

Africa (Ivory coast) (49)

Bangladesh (53)

29.2 12.7 29.3 5 4.1 -

75.1 0.4 0.5 0.1 0.1 0.2 0.2 1 0.4 7.6 -

JOURNAL OF ESSENTIAL OIL RESEARCH

of Benin while it was 37.5 and 29.4%, respectively, in that of Congo. The oxygen-containing compounds were also detected and the aldehydes comprised 40% in aver­ age. The sesquiterpenes such as β-elemene, trans-βcaryophyllene and δ-cadinene were detected from congo essential oil while α-farnesene and elemol from Benin essential oil (12). The citronella essential oil obtained from Ooty (India) contained citronellal, geraniol and γ–terpineol present 29.7, 24.2 and 9.2%, respectively (10) whereas the specimen collected in Gerais (Brazil) contain citro­ nellal (47.2%), geraniol (18.6%) and citronellol (11.1%) as the major constituents (52). Citronella essential oil obtained from Bangladesh majorly consists of 9-% monoterpene hydrocarbons. Sesquiterpene hydrocar­ bon includes p-Elemene, p-caryophyllene and αhumulene. Alcohols were the major components of citronella essential oil and constitute 75% of the total oil, Geraniol (75.1%) and geranyl acetate (7.6%) were the major components of citronella essential oil (53). In the similar study, the essential oil obtained from Malaysia consists of alcohol, aldehyde/ketone and hydrocarbons present 29.6, 6.3 and 9.2% respec­ tively (54). Through all the studies conducted to determine the chemical composition, the chemical structures and the molecular formulas of the major components of C. nardus essential oil are given in Figure 1.

6. Biological activities The biological properties exhibited by citronella essen­ tial oil are categorised into various types depending upon the target organism as follows. 6.1. Insecticidal Mosquito repellent activity is the prime activity of citro­ nella essential oil (10,55,56). Since thousands of years, citronella and its oil is used as an insect repellent even before the manufacture of synthetic insect repellents like DEET. Citronella is largely used as a disinfectant in treating snake bites and stings of venomous insects as well as an air purifier which aids in warding off insects. Moreover, mosquito repellent cream containing citro­ nella essential oil has been developed (57). It was reported that at the concentrations of 12.5% citronella oil was effective in killing tropical horse tick larvae (Anocentor nitens) while at the concentration of 0.006% citronella is one of the active ingredients of the pesticide product, Bug Assassin (58). This pesticide product along with peppermint, eugenol, Sodium lauryl sulfate provided 90% control of spotted spider mites

7

(Tetranychus urticae). Hence it showed good insectici­ dal activity (59). However, citronella essential oil was not found effective against Homopteran pests and was unable to repel three species of kissing bugs,i.e. Triatoma protracta, T. recurva and T. Rubida (58). Contact toxicity and repellency of essential oil of C. nardus against red imported fire ant (Solenopus invicta) and argentine ant (Linepithema humile) were evaluated (60). It was found that 50% of argentine ants were killed in 34.3 min by citronella oil and 100% after twenty-four hours. On the other hand, 50.6% of red imported fire ants were killed on 24 h exposure to citronella oil and hence caused significant mortality. The essential oil obtained from C. nardus essential oil repelled both body (Pediculus humanus var. humanus) and head lice (P. varcapitata) (61). Citronella essential oil along with neem and coconut oil was effective in preventing head lice transmission. The mode of action of citronella against insects was reported to be non-toxic to humans and plants (60). It was reported that the terpenes in citronella essential oil blocks the neural pathways of the insects and hence disrupt movements and the metabolism of insects. It was reported that by adding 5% vanillin to the 10–25% citronella oil extracted from citronella doubled its pro­ tection against Anopheles mosquitoes (55). At 10% con­ centration and with 24-h exposure, citronella oil killed 100% of Anopheles dirusa and Culexquinque fasciatus and 97% of Aedes aegypti. It was found that the micro­ encapsulated citronella oil-treated fabrics provide greater and longer protection against A. aegypti than that of the alcohol-treated citronella oil. The microen­ capsulated citronella oil-treated fabrics repelled mosqui­ toes for almost 21 days while alcohol-treated citronella oil repelled mosquitoes for 5 days (62). Citronella essen­ tial oil has been cited in one-third of all patents as biological insect repellent (63). The insect repellent activity of citronella essential oil was tested against Bemisia tabaci (Sweet potato whitefly) at various concentrations, i.e., 0, 0.416, 1.66, 3.33 and 6.66 μL/L. Citronella essential oil showed high repel­ lency against B. tabaci and deterred females from laying eggs on leaves treated with citronella essential oil. The mortality of B. tabaci increases with increase in concen­ tration of citronella essential oil and the effective con­ centration was 6.66 μL/L (64). A similar study was undertaken to demonstrate the insecticidal activity of Citronella essential oil against Aedes aegypti. At 400 μL, Citronella essential oil was effective in inhibiting A. aegypti (65). The insect repellent activity of nine potential plants namely Finger root (Boesenbergia pandurata), Greater galangale (Alpiniaga langa), Cardamom (Elettaria

8

H. KAUR ET AL.

Monoterpenes

Camphene (C10H16)

Limonene (C10H16)

Sesquiterpenes

Myrcene (C10H16)

β-Elemene (C15H24)

trans β-caryophyllene δ-cadinene (C15H24) (C15H24)

α-Farnesene (C15H24) Hydrocarbons

Hydrocarbons

CH2OH

CH2OH H3CO

OH

HO

Citronellol

Geraniol

Eugenol

(C10H20O)

(C10H18O)

(C10H12O2)

OH

Elemol (C15H26O)

CH2OH

CH2

Linalool (C10H18O)

HO

HO

Nerol (C10H18O) Alcohols

Nerolidol (C15H26O)

Alcohols

γ-terpineol (C10H18O)

CHO

CHO

CHO O

Citronellal (C10H18O)

cis-citral trans-citral (C10H16O) (C10H16O) Aldehydes

Piperitone (C10H16O) Ketones

Figure 1. Chemical structures of important constituents of C. nardus essential oil.

cardamomum), Turmeric (Curcuma longa), Siamese cassia (Cassia siamea), Neem (Azadirachta indica), Eucalyptus (Eucalyptus citriodora), Citronella grass (Cymbopogon nardus), and Siam weed (Eupatorium odoratum) was checked and it was found that the

citronella grass was most effective against mosquito with 2 h of repellency (66). The insect repellant activity of citronella oil might be due to the presence of geraniol, limonene, eugenol and citronellal (67). Citronella essen­ tial oil in the concentration range of 0.05–15% alone or

JOURNAL OF ESSENTIAL OIL RESEARCH

in combination with natural products shows good insect repellent property against 24 mosquitoes of Aedes, Anopheles, Armigeres, Culex and Monia genus (68). Moreover, evaluation of the insect repellant activity of citronella in combination with other binders such as potato starch, neem powder, wood powder, cow dung, corn starch and coconut shell powder revealed that the best result was given by citronella leaf cake with neem powder with burning time of 21 min and the residual percentage is 24.1%. Moreover, this combination also showed antibacterial and medicinal properties. Hence, Neem powder cake impregnated with 10% citronella oil was recommended as good insect repellent. It can be interpreted from these reports that crude citronella essential is was effective against various insects (ticks, mosquitoes and mites) at concentration of 0.05–15%. The effectiveness of the essential oil can be increased with use of different binders. Microencapsulation further increased the repellent action due to the slow release of essential oil. 6.2. Antifungal activity The antifungal activity of C. nardus essential oil was investigated against Aspergillus niger and Penicillium putida. The essential oil inhibited the growth of A. niger at higher concentration (400 mg/L) (69) whereas it showed inhibition at much lower concentra­ tion (4 mg/ml) against P. putida (70). The antifungal potential of C. nardus essential oil was investigated against P. grisea, Aspergillus species and Colletotrichum musae. It was found that C. nardus essential oil inhibited the growth of P. grisea, Aspergillus species and C. musae through inhibition of their spore production and this antifungal property was concentration-dependent (71). Citronella essential oil was evaluated for its antifun­ gal activity against Alternaria alternata (fungus that spoils fruits and vegetables especially cherry tomato) and found that the citronella essential oil at the dose of 1.5 μl/ml was very effective against A. alternata without any negative effect on the quality of the fruit. The mini­ mum inhibitory concentration in PDA (Potato dextrose agar) medium was found to be 1.5 μl/ml (72). The antifungal activity of C. nardus essential oil was investigated against cereals and pulses contaminating fungi belonging to Penicillium and Mucor genera (A. flavus, A. niger and A. fumigates) (73). At 50% concentration of citronella essential oil, 90 mm inhibi­ tion zone was observed against all the test strains. With decrease in concentration to 25 and 12.5%, the antifun­ gal activity was observed to be maximum against

9

A. niger (Inhibition zone 50 and 40 mm, respectively) while it was minimum against P. italicum (Inhibition zone 27 and 16 mm, respectively). Citronella essential oil was comparatively more effective against A. niger. Citronella essential oil inhibited the formation of hyphae of Candida albicans and also inhibited the growth of yeast at the concentration of 15.8–1000 g/ ml. This antifungal property of C. nardus essential oil was attributed to the presence of terpenes (52). Similarly; Zhu et al (2005) reported that the antifungal potential of C. nardus essential oil might be due to the presence of phenolic compounds, oxygenated monoter­ penes and sesquiterpene hydrocarbons (74). The emulsification of essential oil into nano emulsion is a cost effective method to increase its effectiveness (75). The essential oil which is volatile and sensitive to degradation is best for nano emulsion formation using cavitation assisted techniques (76,77). The nano emul­ sions of neem oil and citronella oil mixture possessed better antifungal activity against phytopathogenic fungi (Sclerotium rolfsii and Rhizoctonia solani). This obser­ vation made a step towards the utilization of citronella oil in combination with the neem oil for control of diseases caused by S. rolfsii and R. solani (77). A study was undertaken for evaluation of the citro­ nella oil in treatment of major coffee diseases, (Rust and brown eye spot). Twelve months old coffee plant was sprayed with 1000 μl/L citronella oil, 200 mg/L aciben­ zolar-S-methyl and 200 mg/L tebuconazole fungicide separately. Seven days later, the plants were inoculated with Cercospora coffeicola and Hemileia vastatrix and application was repeated after every 30 days for 5 months. With the efficiencies of 47.2 and 29.7%, respec­ tively, citronella oil controlled the rust and brown eye spot while tebuconazole fungicide-controlled rust and brown eye spot with the efficiencies of 96.5 and 90.5%, respectively. Acibenzolar-S-methyl showed no control of rust but with efficiencies of 55.9% reduced brown eye spot. Hence, it was found that the citronella oil not only controls the rust and brown eye spot in coffee plant but also activates their defense system (78). A study was undertaken for demonstration of anti­ fungal potential of citronella essential oil against Collectotrichum acutatum (causing chilli anthracnose) at various concentrations, i.e., 0.25, 1.25 and 2.5 μl/ml using poisoned food technique. It was found that citro­ nella essential oil inhibited the mycelial growth at all the three concentrations. At concentration 1.25 μl/ml, it inhibited germ tube elongation and reduced conidial germination. Although the complete mycelial growth inhibition was observed at 2.5 μl/ml but it was toxic to the fruit at this concentration. Hence, 1.25 μl/ml was the

10

H. KAUR ET AL.

selected as most effective concentration of citronella essential oil in controlling C. acutatum (79). The antifungal potential of C. nardus essential oil was attributed to cell wall changes, interference in wall synthesizing enzymatic reactions which affect morpho­ genesis and growth of fungus. It caused intracellular constituent leakage, increased ion permeability or impairment of fungal enzymatic systems. The antifungal potential of citronella essential oil was due to its inter­ ference in integrity of cell membrane functioning through the changes in membrane potential, respiratory chain inhibition and loss of cytoplasmic material (52). Thus, C. nardus essential oil can be used for develop­ ment of natural and safer fungicide. The constituents of C. nardus essential oil (citronellal and linalool) were also tested for their antifungal poten­ tial against fungal strains- A. candidus, A. versicolor, A. flavus, Eurotium amstelodami, Penicillium adametzii, E. chevalieri, P. griseofulvum, P. citrinum, P. griseofulvum, and P. islandicum and it was found that the citronellal and linalool obtained from C. nardus essential oil were responsible for activity against all these fungal strains (16). It was reported that the synergism between the citronellal and linalool (components of C. nardus essential oil) was responsible for its fungicidal activity (16). Coming to conclusion of all these studies reported on the antifungal potential of citronella essential oil, it has been found that citronella essential oil was effective against fungi belonging to variety of genus. The essential oil inhibited the fungal growth at very low concentra­ tion range. Further, nano emulsion formation can increase its effectiveness as antifungal agent. The anti­ fungal activity was primarily due to presence of mono­ terpenoids such as citronellal. In order to prevent or alleviate fungal infections, C. nardus essential oil can be an important ‘natural’ and safer fungicide. 6.3. Antibacterial activity A few studies conducted on the bactericidal action of C. nardus essential oil can be efficiently used as anti­ bacterial agents against human pathogens such as Acinetobacter baumanii, Escherichia coli, Enterococcus faecalis, Pseudomonas aeruginosa, Klebsiella pneumo­ niae, Serratia marcescens, Salmonella typhimurium,

and Staphylococcus aureus at concentration range of 1200–20,000 μg/ml. The higher MIC value indicated that it was more susceptible to human pathogens. Both Gram positive and Gram negative bacterial species were inhibited by citronella essential oil (80). The strong antibacterial activity of oil was due to the presence of the components elemol (9.1%), citronellol (10.4%), citronellal (16.9%) and nerol (8%) (81). 6.4. Herbicidal activity Germination of seeds of six weed species such as billy goat weed (Ageratum conyzoides), parthenium ragweed (Parthenium hysterophorus), common lambsquarters (Chenopodium album), coffee weed (Cassia occidenta­ lis), prickly malvastrum (Malvastrum coromandelia­ num) and little seed canary grass (Phalaris minor) were assayed against citronella essential oil. Citronellal (major compound of citronella essential oil) was applied to seeds at concentrations of 5, 10, 25, 50 and 100 μg citronellal/g of sand, in which they were planted. It was found that at the concentration of 100 μg/g, neither of the seeds of weed species emerged. The most susceptible species of weed was A. conyzoides and P. hysterophorus and did not emerge even at 50 μg/g seed treatment. The mode of action essential oil might be necrosis followed by the death of plant tissue (82). Aqueous methanol extracts of C. nardus leaves, roots and stalks inhibited the growth of certain common weeds of agricultural fields such as cress (Lepidum sati­ vum L.), Alfalfa (Medicago sativa L.), lettuce (Lactuca sativa L.), Italian ryegrass (Lolium moltiflorum Lam.), barnyard grass (Echinochloa crus-galli L.) and jungle rice (Echinochloa colonum L.). Strong growth inhibition was shown particularly by leaf and root extracts. It had been demonstrated that this growth inhibitory activity was due to presence of allelopathic substances in C. nardus (83). The allelopathic activity of leaf, root and shoot methanol extracts of C. nardus is given in Table 6. A study was undertaken to demonstrate the herbici­ dal activity of citronella essential oil against Crebgrass (Digitaria horizontalis) and Burrgrass (Cenchrus echina­ tus). The phytotoxic effects of citronella essential oil were demonstrated at various concentrations, i.e., 1, 10 and 20%. A drastic reduction in the seed germination

Table 6. Allelopathic activity of leaf, root and shoot methanol extract of C. nardus (83). Plant part Leaf extract Root extract Stalk Extract

Concentration (g/mL) 0.03 0.03 0.1

Percentage inhibition of shoot growth Lettuce 100 4.21 100

Alfalfa 23.61 23.91 22.38

Cress 28.74 25.29 26.92

Jungle rice 51.28 28.49 33.95

Barnyard grass 39.30 30.39 36.02

Italian ryegrass 29.39 31.91 20.76

JOURNAL OF ESSENTIAL OIL RESEARCH

was reported (97–99%). Citronella essential oil showed a negative effect on the plant height and dry mass of shoots and roots after 12 h of treatment. At the concen­ tration of 20%, it showed a reduction in the dry mass of shoots. Finally, reduction in the chlorophyll and protein content by 80–90% was demonstrated on application of Citronella essential oil (84). The mode of action of C. nardus essential oil was reported that the photosynthetic and respiratory meta­ bolism was impaired by the citronellal component of the C. nardus grass which disrupts the cuticular wax, shrink epidermal cells, clog stomata and causes rapid leakage of electrolyte. Along with this citronella essential oil also causes necrosis, chlorosis, wilting followed by death of weeds (85). Hence, Citronella essential oil has the potential to be used as a natural herbicide. 6.5. Antioxidant property Citronella oil was found to have moderate antioxidant property with IC50 value of 206 μg/ml (86). The absence of phenolic compounds might be responsible for its low antioxidant property (87). The antioxidant potential varies with the methods of extraction of the C. nardus essential oil. The essential oil obtained by ohmic-heated hydrodistillation contained high concentration of anti­ oxidant compounds such as citronellal and limonene. It was found that the pure citronellal possessed good anti­ oxidant property with IC50 value of 49 μg/ml (88). The antioxidant property of essential oil was temperature-

11

dependent and was largely affected by subjecting the plant material to high temperature which deteriorates the extract (89). Hence, Citronella grass possesses various biological properties. All of these properties are compiled and given in Table 7.

7. Safety and toxicity It was reported that citronella oil has been used exten­ sively since ancient times and considered safe without any side effects (5). The estimated acute toxicity of citronella essential oil was >5000 mg/kg (90). The subchronic toxicity of citronella essential oil has been waived by EPA (91). Few skin dermatitis and eczema have been reported by the use of citronella oil in few sensitive species (92). Citronella oil is included in the list of non-carcinogenic compounds (93). However, it was said that at high doses citronella can be toxic to bene­ ficial insects such as pollinators but it is not con­ firmed yet.

8. Conclusion and future perspectives In this article, we reviewed studies of citronella essential oil, its traditional importance, chemical composition and some biological activities reported so far. We have provided a detailed botanical description to distinguish two varieties of citronella grass. Since ancient times, Citronella is used in Ayurvedic medicine for treatment

Table 7. Biological properties of citronella grass essential oil. Biological Property Insecticidal (55,60,64–66)

Antifungal (16,47,69,70,72,73,75,78,79)

Antibacterial (80) Acaricidal (58,59) Herbicidal (26,83,84)

Target organism Anopheles dirus, Culex quinquefasciatus, Aedes aegypti Solenopsis invicta, Linepithema humile Bemisia tabaci Aedes aegypti Aedes, Anopheles, Armigeres, Culex, Monsonia A. candidus, A. versicolor, A. flavus, Eurotiumams telodami, Penicillium adametzii, E. chevalieri, P. griseofulvum, P. citrinum, P. griseofulvum, P. islandicum. Candida albicans Aspergillus niger P. putida A. alternate A. flavus, A. niger, A. fumigates S. rolfsii, R. solani Cercospora coffeicola, Hemileta vastatrix Collectotrichum acutatum Acinetobacter baumanii, Escherichia coli, Enterococcus faecalis, Pseudomonas aeruginosa, Klebsiella pneumoniae, Serratiam arcescens, Salmonella typhimurium, and Staphylococcus aureus Tetranychus urticae Triatoma protracta, T. recurva, T. rubida Anocentor nitens Ageratum conyzoides, Parthenium hysterophorus, Chenopodium album, Cassia occidentalis, Malvastrum coromandelianum, Phalaris minor Lepidum sativum L. Crebgrass (Digitaria horizontalis), Burrgrass (Cenchrus echinatus)

Positive/ negative Concentration Positive 10% Positive Positive 6.66 μL/L Positive 400 μL Positive 0.05–15% Positive Positive Positive Positive Positive Positive Positive Positive Positive Positive

15.8–1000 g/ ml 400 mg/L 4 mg/ml 1.5 μl/ml 50% 1000 μl/L 1.25 μl/mL 1200–20,000 μg/ml

Positive Negative Positive Positive

0.006% 12.5% 100 μg/g

Positive Positive

0.03 g/ml 20%

12

H. KAUR ET AL.

of redness, irritation, toothaches, rheumatism, digestive problems, headaches, infectious diseases, fatigue and childbirth washes. Recently, it found its use in the paper industry for paper production due to its high cellulose content. It is majorly exploited for its essential oil content. The yield and the chemical composition of C. nardus essential oil also varied with the methods of extraction. Geographical distribution results in variable chemical composition of essential oil. Researches car­ ried out till date on C. nardus essential oil lead to the identification of 100 compounds. The majority of sam­ ples contained citronellal, citronellol and geraniol as major components. Variable composition accounts for the promising insecticidal, fungicidal, herbicidal and antioxidant activ­ ities shown by essential oil. It is a cheap, environmental friendly and effective substitute for synthetic chemicals. Apart from its biological potential, it can be used as a bioadditive (94) and its waste can be used to produce biogas, especially methane (95). It could be used as an alternative for future mouthwash’s formulations (96). In recent times, the preference of the consumers for nat­ ural ingredient containing products have ushered in a ‘green wave’ and this results in the increased demand for natural essential oil. Some major constraints in sus­ tainable industrial exploitation of medicinal and aro­ matic plants (MAPs) such as citronella are due to geographical variations and non-standardized proce­ dure of extraction of essential oil. Efforts should be to get quality and quantity as consistent as possible. In recent times, techniques like nano emulsion and micro­ encapsulation have been used to eliminate the problems (liphophilic nature, high dose and less activity than isolated compounds) related to the use of essential oils. Hence, there is a need of awareness of the benefits of citronella essential oil in pharmacological and agricul­ tural fields.

Disclosure statement No potential conflict of interest was reported by the author(s).

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