The Himalayan region is known for its rich and great variety of medicinal plants. Western Himalaya refers to the western half of the Himalayan Mountain region, stretching from northeastern Afghanistan, southern Tajikistan, through Kashmir to central Nepal. The Eastern Himalayas extend from the Kaligandaki Valley in central Nepal to northwest Yunnan in China, including Bhutan, North-East (Sikkim) India, southeastern Tibet, and parts of northern Myanmar. This region is widely considered one of the most important biodiversity hotspots in the world [1,2,3,4,5]. These mountain regions are comprised of high altitude ecosystems (3,000 - 4,000 M), steep slopes into deep mountain valleys and thus create numerous river/stream systems (Yarlung-Tsangpo, Brahmaputra, and Mekong) with mild summer temperatures and high rainfall from monsoons .
Sikkim, for example, is the least populous and second smallest among the Indian states. As part of the Eastern Himalayans region, Sikkim is notable for its biodiversity, including alpine and subtropical climates. The Kingdom of Sikkim was founded on the Silk Road by the Namgyal dynasty in the 17th century. It is inhabited by native mountain tribes living across the Indian, Tibetan, Bhutan, and Chinese borders. The human inhabitants of these regions live in extremely isolated areas where trees, plants and herbs are an important source for drugs used against various diseases, insects, and aliments. Plants form the most basic components of their daily life from meals, medicines, mouth and teeth treatments, cosmetics and physical ornamentation to each tribe’s traditional ‘alcoholic drink’ which is used for pain relief .
While other various plant components form the basis for plant defense (mechanical, physical, chemical), the biological activities of a plant may often be attributed to compounds within the essential oil profile. Depending on the plant species, the plant parts (leaves, stems, flowers) of which the essential oil was extracted, and the equipment used for analyses, up to 400 chemicals or even more can be identified within individual oils . Terpenes, mainly mono- and sesquiterpenes, make up the largest group of chemicals from these multi-component mixtures. Natural products, especially essential oils, are important sources to discover potent new lead fungicides. A number of plant essential oils such as rosemary (Rosmarinus officinalis) oil (Sporan™) and thyme (Thymus vulgaris) oil (Promax™) are marketed as fungicides for organic farmers .
Due to global warming insects transporting infectious pathogens like Malaria or yellow fever are appreciably spreading to the northern hemisphere. Besides killing the adult mosquitoes using insecticides, larviciding is another approach to reduce mosquito densities by dispatching the larvae before they emerge into adult individuals. In the last decades customers generally tended to favor natural products over synthetic insecticides and repellents which might possess toxic side effects or trigger allergies when applied onto the skin. That is why the repellent and larvicidal activity of terpenes called attention to essential oils as potent pest management agents .
In our previous studies we screened essential oils of medicinal plants typical for the western Himalayas, either from wild growing sources [e.g. 11] or from cultures [e.g. 12,13], for their biological impact. In the present manuscript we focused on the essential oils of three herbs:
Zanthoxylum armatum DC. (Syn.: Zanthoxylum bungeanum DC., Rutaceae) is an erect, thorny shrub or small tree that grows in the hot valleys of subtropical Himalaya from Jammu to Butan ascending to 2,200 m of altitude. In the Khasi hills in the north-east it can be found at lower altitudes between 700 and 1000 m. The seeds, leaves and bark are highly aromatic. The natives use the soft twigs as toothbrush for the plant’s toothache relieving activity and against insects due to its repellent impact . The essential oil of the fruits further exhibits antiseptic and disinfectant as well as antimicrobial and anthelmintic properties .
Juniperus communis L. (Cupressaceae) is a large shrub or small tree native to Europe, South Asia and North America. In India it grows in temperate or subarctic zones in western Himalaya at altitudes between 2,600 to 4,500 m. Juniperus communis is a well-known and well described medicinal plant that possesses a wide range of pharmacological activities and thus has been used in traditional as well as academic medicine for centuries. The native population in India considers the plant as sacred and uses its needles for incense. The essential oil of the fruits exhibits carminative, stimulant and diuretic properties . Furthermore its antimicrobial and antioxidant impact have been studied. The chemical composition of the essential oils from leaves and berries derived from plants from various countries all over Europe and Asia has been characterized. However, depending on the plants’ origin their composition varied distinctively: Although α-pinene was the main component in most European leave oils, sabinene was dominated in several samples from India, Iran and Morocco .
Dysphania ambrosioides (L.) Mosyakin & Clemants (Syn.: Cheopodium ambrosioides L., Amaranthaceae) is an annual herb native to Mexico but it also grows in tropical and subtropical parts in India. Because of its anthelmintic activity it is known as “wormseed”. Cattle generally do not graze this plant due to its strong pungent aroma. The Indian native population uses it for its anthelmintic activity but also the activities against influenza and pneumonia. The essential oil is often rich in ascaridole, a bicyclic monoterpene-peroxide, but several chemotypes containing other monoterpenes as main components have been described in the literature. Further, the essential oil exhibits antifungal, antioxidant, antiviral, antimicrobial and insecticidal activities .
In continuation of our investigations on plants typical for the Himalayan region, the main purpose of the present study was to describe the chemical composition of the essential oils of Z. armatum, J. communis, and D. ambrosioides collected from wild and evaluate these samples for their antifungal, larvicidal as well as repellent activity.
2 Experimental Procedures
2.1 Plant material and isolation
Leaves and inflorescence of Z. armatum as well as D. ambrosioides were collected from wild at the area of Palampur in the Kangra district at an altitude of 1,300 m during July and August 2010. Leaves of J. communis were collected from Lahaul valley of Himalchal Pradesh at an altitude of 2,800 m during the same time period.
The voucher specimens (Z. armatum PLP 1224, J. communis PLP 4820, D. ambrosioides PLP 1216) were deposited in the herbarium-PLP of the institute of Himalayan Bioresource Technology, Palampur. All samples were identified by Brij Lal, taxonomist of the Biodiversity division of IHBT, Palampur.
Plant materials of Z. armatum (2.021 kg) and D. ambrosioides (1.350 kg) as well as the fresh green needle-like leaves of J. communis (1.562 kg) were air dried separately in the shade at 25°C. In a glass Clevenger type apparatus of 5 kg capacity plant material was hydrodistilled until total recovery of essential oils. Juniperus leaves were chopped before hydrodistillation. Z. armatum yielded in 0.4% (fresh wt. basis) of yellow oil. For D. ambrosioides 0.1% (fresh wt. basis) of light yellow oil was obtained, and 1.2% (fresh wt. basis) of colorless oil from J. communis was collected. All oil samples were dried by anhydrous sodium sulphate, filtered, and stored at 4°C until analyses.
2.2 Essential oil analysis
GC-FID and GC-MS analyses were performed in one run and one GC with the help of a MS-FID-splitter consisting of a quartz Y-splitter and a short (ca. 20 cm) 0.1 mm ID fused silica restrictor column as an inlet to the GC-MS interface and a ca. 1 m X 0.25 mm deactivated fused silica column serving as a transfer line to the FID detector. The restriction column limits the flow into the MS vacuum and prevents entering combustion gases from the FID which is operated at atmospheric pressure. The flow in the analytical column must be greater than the inflow to the MS detector which is limited to about 1 mL/min by means of the restriction line. The GC column flow must be held constant otherwise the FID-MS split ratio changes with temperature. The following instrumentation was used:
A Thermo Fisher Scientific Trace GC with a split/splitless injector heated at 250°C and connected to a 50 m X 0.25 mm X 1.0 μm SE-54 (95% Polydimethyl-, 1% Polydivinyl-, 4% Polydiphenylsiloxan) capillary column (made and tested for deactivation and separation efficency in our lab ), a FID detector operated at 250°C. 0.1 μL essential oil samples were injected neat with a 0.5 μL plunger-in-needle syringe at a split ratio of 1:100.
For substance identification a Thermo Scientific Automass Solo Mass Spectrometer was used with the GC-MS interface heating at 250°C, ion source 230°C, EI mode at 70 eV, filament 500 μA, scan range 40 - 500 amu. The following oven temperature program was used: 60°C for 1 min. then heated to 230°C at a rate of 3°C/Min, 230°C isotherm for 12.3 min. The carrier gas was helium 5.0, with a constant flow rate of 1.5 mL/min.
Thermo Xcalibur 2.2 software was used for identifying the compounds by correlating mass spectra to databases of NIST 08 , Wiley 8th ed. , Adams library , MassFinder terpenoids library  and our own library. Retention indices are determined with the use of the measured retention times of a series of n-alkanes that elute over the whole span of the chromatogram and calculated according to the method of van den Dool and Kratz .
Quantification was performed using normalized peak area calculations of the FID chromatogram without (by first approximation) relative FID-response factors.
2.3 Olfactory evaluation
For olfactory evaluation, one droplet of each essential oil was applied onto commercially available paper blotters. The samples were examined by a panel consisting of two aroma-chemists and a professional perfumer over 90 min to control for odor progression. Odor descriptions were compared to our own database of referenced aroma compounds.
2.4 Fungal pathogen production and inoculum
Isolates of Colletotrichum acutatum Simmonds, C. fragariae Brooks, and C. gloeosporioides (Penz.) Penz. and Sacc. were obtained from B. J. Smith, USDA, ARS, Poplarville, MS and isolated from strawberry (Fragaria x ananassa Duchesne). Conidia of each fungal species are harvested from 7-10 day-old cultures by flooding plates with 5 mL of sterile distilled water and dislodging conidia by softly brushing the colonies. Conidial suspensions are then filtered to remove mycelia then conidia concentrations are determined photometrically from a standard curve based on the percent of transmittance (%T) at 625 nm and manual hemocytometer counts. Conidial stock suspensions are adjusted with sterile distilled water to a concentration of 1.0 X 106 conidia/mL .
2.5 Direct Bioautography
Matrix bioautography was used to screen for Z. armatum, J. communis and D. ambrosioides essential oils. All essential oils were diluted in acetone and spots at 80 and 160 μg/spot were applied onto thin-layer chromatography (TLC) plates. Essential oils were diluted in acetone. Each TLC plate was subsequently sprayed with a spore suspension (105 spores/mL) of these three Colletotrichum and incubated in a moisture chamber for four days at 26°C with a 12 h photoperiod. Bioautography experiments were performed twice and technical grade commercial fungicides benomyl, cyprodinil, azoxystrobin, and captan (Chem Service, Inc., West Chester, PA) were used as fungicide standards at 2 mM in 2 μL of 95% ethanol.
2.6 Mosquito Bioassays
Aedes aegypti larvae and adults used in these studies were from the laboratory colonies maintained at the Mosquito and Fly Research Unit at the Center for Medical, Agricultural and Veterinary Entomology, USDA-ARS, Gainesville, Florida. For biting deterrence bioassays, eggs were hatched and the insects were reared to the adult stage in the laboratory and maintained at 27 ± 2°C and 60 ± 10% RH with a photoperiod regimen of 12:12 h (L:D). 8-13 day-old adult females were used. For larval bioassays, the eggs were hatched and the larvae were maintained at the above temperature.
Mosquito Biting Bioassays
Experiments were conducted by using a six-celled in vitro Klun and Debboun (K&D) module bioassay system developed by Klun et al.  for quantitative evaluation of biting deterrence of candidate compounds. The assay system consisted of a six well reservoir with each of the 3 X 4 cm wells containing 6 mL of blood. Further procedures were performed as described by Ali et al. .
The temperature of the solution in the reservoirs was maintained at 37°C by continuously passing the warm water through the reservoir using a circulatory bath. The reservoirs were covered with a layer of collagen membrane (Devro, Sandy Run, SC). The test compounds were randomly applied to six 4 X 5 cm areas of organdy cloth and positioned over the membrane-covered CPDA-1+ATP solution with a Teflon separator placed between the treated cloth and the six-celled module to prevent the contamination of the module. A six celled K&D module containing five female mosquitoes per cell was positioned over cloth treatments covering the six CPDA-1 + ATP solution membrane wells, and trap doors were opened to expose the treatments to these females. The number of mosquitoes biting through cloth treatments in each cell was recorded after a 3 min exposure and mosquitoes were prodded back into the cells to check the actual feeding. Mosquitoes were squashed and the presence of green fluorescent tracer dye (or not) in the gut was used as an indicator of feeding. A replicate consisted of six treatments: four test compounds, DEET (a standard biting deterrent) and ethanol treated organdy as solvent control applied randomly. Five replications with five females per treatment were conducted using newly treated organdy and a new batch of females in each replication.
Bioassays were conducted to test the essential oils of D. ambrosioides, Z. armatum and J. communis for their larvicidal activity against Ae. aegypti by using the bioassay system described by Ali and coworkers . Eggs were hatched and larvae were held overnight in the hatching cup in a temperature-controlled room. Five one day-old Ae. aegypti larvae were added in a droplet of water to each well of 24-well plates (BD Labware, Franklin Lakes, NJ) by use of a disposable 22.5 cm Pasteur pipette. Fifty microliters of larval diet (2% slurry of 3:2 Beef Liver powder (Now Foods, Bloomingdale, Illinois) and Brewer’s yeast (Lewis Laboratories Ltd., Westport, CT) was added to each well by using a Finnpipette stepper (Thermo Fisher, Vantaa, Finland). All chemicals tested were diluted in dimethyl sulfoxide (DMSO). Eleven microliters of the test chemical was added to the labeled wells, while 11 μL of DMSO was added to control treatments. After the treatment application, the plates were swirled in clock-wise and counter clockwise motions, front and back, and side to side five times to ensure even mixing of the chemicals. Larval mortality was recorded 24 h post treatment. Larvae that showed no movement in the well after manual disturbance of water were recorded as dead. A series of concentrations ranging between 125 and 31.25 ppm were used in each treatment to obtain a range of mortality.
For statistical analyses, proportion not biting (PNB) was calculated using the following formula:
Data on the PNB values were then analyzed using the ANOVA procedure of SAS (version 9.2; SAS Institute Cary, NC).
The conducted research is not related to either human or animals use.
3 Results and Discussion
The essential oils were analyzed using simultaneous GC-FID and GC-MS. A total of 59 compounds, accounting for 96.8%, were identified for Z. armatum oil. It possessed a high amount of 2-undecanone (47.7%) and 1,8-cineol (20.5%) followed by α-pinene (6.0%) and 2-tridecanone (4.7%) (Table 1). This is in accordance with the literature on Z. armatum leaf essential oil from India . In general very little on leaf oil has been published in the literature, most of the research has been done on seed oil with linalool and limonene as major components from India [10,29] and borneol and isobornyl acetate form samples from Pakistan . The odor of Z. armatum oil can be described as spicy refreshing odor with a touch of citrus and some terpeny notes. Its heart note is soft spicy with cypress and woody connotations, the fond note herbal-balsamic. In an older investigation Z. armatum leaf oil collected at 1,200 m altitude from an area near Palampur in 1992, close to the region our plants were collected, was described by Weyerstahl and coworkers. The authors found linalool (18.8%), 2-undecanone (17.0%), 1,8-cineole (15.7%) and β-phellandrene (12.7%) as main constituents. 2-Tridecanone, another characteristic component of Z. armatum leaf oil, appeared in a ratio of 2.0%. The odor was described as green and fruity with a dominant tropical note as well as sweaty-animalic undertones . This odor-description as well as the high amount of linalool is in contrast to our findings. Since this discrepancy can rather not be due to altitudinal or regional growth differences of the plants, one could assume that a climatic influence might be the cause [32,33]. Even more that almost 20 years lie between these two investigations.
The complex mixture of J. communis essential oil consisted mainly of monoterpene hydrocarbons with α-pinene (43.2%) and limonene (34.7%) as dominant constituents out of a total of 79 components (97.9%) (Table 2). Although Dahmane and coworkers  as well as Kumar and colleagues  also detected mainly monoterpenoids in their oils from Algeria and India, respectively, these oils differed in the ratio of the main compounds. Here again, most of available publication described the oil of Juniper berries, just little research was done on the needles/leaves. Olfactory evaluation of J. communis essential oil resulted in a fresh and terpeny odor with a touch of gin as a top note, a soft spicy and herbaceous heart note and a balsamic fond odor with a tender woody touch.
For D. ambrosioides essential oil, 16 compounds, comprising 82.9% of the oil, were identified. p-Cymene (36.3%) and ascaridole (31.2%) were most prominent (Table 3). The oil exhibited a terpeny, slightly metallic and harsh top note, reminding of myrcene, and a herbal camphorous, rather disgusting heart note. Its fond note was unpleasant, somewhat dusty-minty. A study on D. ambrosioides (Chenopodium ambrosioides) essential leaf oil derived from the same area in Palampur in 1993 showed high concentration of a-terpinene (65%) and p-cymene (29.4%). Interestingly the ratio of ascaridole was very low (0.7%) which is rather unusual since this monoterpenoid is the lead-component of this oil and responsible for its anthelmintic activity . The reason for this could again be climatic issues. Despite local variation our findings are in accordance with literature on oil samples from Brasil derived from cultivated plants in Lavaras . However, they differ significantly from samples collected from wild in Sao Paolo which, besides p-cymene (14.5%), included cis-piperitone oxide (35.2%) and trans-isoascaridole (14.1%) as main components .
In bioautography assays, D. ambrosioides essential oil showed the best antifungal activity with fungal growth inhibition zones from 6.5 to 8.0 mm and 11.0 to 14.5 mm using two concentrations of 80 and 160 μg/spot against Colletoirichum species. Antifungal activity was evident by the presence of clear zones with a dark background where fungal mycelia or reproductive stroma were not present on the TLC plate. At the highest tested concentration (160 μg/spot), the zone inhibitions were almost close to commercial fungicide captan (1.2 μg/spot) against all three Colletoirichum species (Table 4). Data published by a French research group confirmed this high antifungal activity using different screening methods on several essential oils against cellulolytic mold strains: D. ambrosioides showed the best results . The other two essential oils, Z. armatum and J. communis, showed moderate antifungal activity. In our previous antifungal studies, essential oils rich in non-oxygenated monoterpenes such as α-/β-pinenes, p-cymene, limonene, myrcene and the oxygenated monoterpene 1,8-cineole were not active in bioautography bioassays against Colletotrichum species [39,40,41,42,43]. The low antifungal results of Z. armatum and J. communis oils in the present investigation are another prove of the rather inefficient antifungal impact of oils comprising these terpenoids as main components. However, the essential oil of D. ambrosioides was the most antifungal active oil tested. This high antifungal impact might be due to the oil’s high amount of ascaridole: It is a bicyclic monoterpene with a rather unusual bridging peroxide functional group, and has long been the only known natural peroxide, first described in 1908. Salimena and coworkers noted high antifungal activity of D. ambrosioides oil against spore germination of Botrytis cinerea (Syn. Botrytinia fuckeliana), a plant pathogen mainly affecting roses. The authors also pointed out that, besides thymol and carvacrol, also ascaridole has a high antifungal impact which has repeatedly been reported in the literature . Other authors also demonstrated its antifungal activity against fungus Sclerotium rolfsii  as well as its potent allelopathic effect on Amaranthus hypochondriacus .
Biting deterrent activity of the essential oils of D. ambrosioides, Z. armatum, and J. communis is given in Fig. 1. Dysphania ambrosioides essential oil showed highest biting deterrence with PNB value of 0.8 which was similar to DEET with PNB value of 0.88 followed by Z. armatum and J. communis demonstrating PNB values of 0.48 and 0.52, respectively. Bigoga et al.  reported 100% repellency of C. ambrosioides essential oil against Anopheles gambiae females at a dose of 200 ppm for four hours for both seed and leaf essential oils. These findings indicated that C. ambrosioides possesses mosquito larvicidal and repellent potential that might be further exploited in combating mosquito vectored diseases through anti-vector interventions.
Data on the toxicity of Z. armatum, J. communis and D. ambrosioide essential oils against one day-old larvae of Ae. aegypti at 24 h post treatment is given in Figure 2. Mortality in DMSO, which was used as solvent control, was 0%. All the three essential oils gave 100% mortality at the dose of 125 ppm. Mortality at 62.5 ppm was 100, 90 and 80%, in the essential oils of Z. armatum, J. communis and D. ambrosioides, respectively, whereas only Z. armatum and J. communis essential oils gave 100 and 40% mortality, respectively, at the dose of 31.25 ppm. Massebo and colleagues reported LD50 values of 17.5 and 9.1 ppm of C. ambrosioides essential oil against 4th instar larvae of An. arabiensis and Ae. aegypti, respectively . Bigoga et al. reported 100% larval mortality at 200 ppm and 300 ppm for the essential oils from seeds and leaves, respectively . Rajkumar and Jebanesan reported 50% mortality of 3rd instar Culex quinquefasciatus larvae at a dose of 7.12 ppm whereas mortality was 100% at 25 ppm. The extract also affected the egg laying of the mosquitoes . Tiwary and coworkers reported larvicidal activity of Z. armatum essential oil against 3rd instar larvae of Cx. quinquefasciatus (LC50 = 49 ppm), Ae. aegypti (LC50 = 54 ppm) and An. stephensi (LC50 = 58 ppm) . These data corroborate the above findings against Ae. Aegypti.
From all three essential oils being discussed in this study, only Z. armatum gave 100% mortality against Ae. Aegypti larvae at all concentrations tested, and therefore can be considered as potent natural insecticide. Ascaridole rich C. ambrosioides demonstrated the highest antifungal activity against three Colletotrichum species. Additionally, C. ambrosioides essential oil showed potential repellent activity against Ae. aegypti and ascaridole, again, might be the responsible repellent compound against these mosquitoes . There is no study that reports antifungal activity of this monoterpenoid against Colletotrichum species. Therefore, D. ambrosioides essential oil should further be studied using 1D-TLC bioautography and the active antifungal compounds should be easily detectable directly on the TLC surface and further structurally identified. Although it has been stated that the whole essential oil usually is more active than its single components , isolated ascaridole should be tested on its biological activity in a follow up investigation.
This study was supported in part by USDA/ARS grant No. 58-6066-6-043. We thank Ms. J.L. Robertson and Ms. R. Pace for their help in antifungal bioassays and Dr. James J. Becnel, Mosquito and Fly Research Unit, Center for Medical, Agricultural and Veterinary Entomology (CMAVE), USDA-ARS, Gainesville, Florida, USA for supplying Ae. aegypti eggs. Authors Vijay K Kaul, Brij Lal and Vikas Jaitak are thankful to the Director CSIR-IHBT Palampur for providing necessary facilities to carry out plant survey/collection of plant materials as well as for hydrodistillation and analytical work.
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About the article
Published Online: 2018-04-23
Conflict of interest: The authors declare no conflict of interest.
Citation Information: Open Chemistry, Volume 16, Issue 1, Pages 306–316, ISSN (Online) 2391-5420, DOI: https://doi.org/10.1515/chem-2018-0028.
© 2018 Iris Stappen et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. BY-NC-ND 4.0