Seasonal Variation in Essential Oil Compositions and Antioxidant Properties of Acorus calamus L. Accessions

Background: Acorus calamus (Sweet flag) is a known herbal drug commonly used in traditional medicine. Our aim was to perform seasonal and altitudinal phytochemical screening to assess the antioxidant activity of the essential oils in the rhizome and leaves of A. calamus from three different altitudes. Methods: Phytochemical screening was performed using GC/MS analysis and in vitro antioxidant assay was done by different methods. Results: The essential oils mainly contained α-asarone, β-asarone (35.3–90.6%), and Z-isoelemicin (1.7–7.3%) as the major constituents, besides linalool, Z-methyl isoeugenol, shyobunone, kessane, etc. All the oils exhibited vast molecular diversity in terms of quantitative ingredients. All essential oils were studied for their antioxidant activity by different methods, including their effect on the DPPH radical-scavenging activity, reducing power, and chelating properties of Fe2+. The oils isolated in all the different seasons exhibited antioxidant activity as a function of concentration, with IC50 values ranging from 475.48 ± 0.08 to 11.72 ± 0.03 compared to standards. Conclusion: From the results obtained it can be inferred that the herb may be a good source of bioactive compounds and can work as an antioxidant to prevent oxidative deterioration in food. The data provide a basis for its in-situ investigation for judicious exploitation.


Introduction
Acorus is a genus of wetland monocot flowering plants distributed in North America and northern and eastern Asia, and naturalised in southern Asia and Europe from ancient cultivation [1][2][3][4][5][6]; they grow as herbs with perennial tuberous thick rhizomes in wetlands, particularly marshes [7]. The species are used in traditional medicine for the treatment of epilepsy, mental ailments, chronic diarrhoea, dysentery, bronchial catarrh, intermittent fevers, and glandular and abdominal tumours [8]. Plant diversity has considerable importance as a source of pharmaceutically active substances [9]. The natural antioxidants from plants can protect the human body from the attack of free radicals and retard the progress of many chronic diseases [10,11]. Natural antioxidants are generally classified as phenols, including flavonoids, phenolic acids and volatile compounds [7]. Acorus calamus is a traditional indigenous herb generally used in the treatment of cough, bronchitis, gout, tumours, haemorrhoids, skin diseases, numbness, and general debility [12,13]. It possesses a wide range of pharmacological activities, such as anti-diabetic [14], central nervous system depressant [15], anti-inflammatory [16], antioxidant [17], antispasmodic [18], antibacterial [19], antifungal [20], and cardiovascular [21] and insecticidal agent [22]. It has been reported by different workers that medicinal plants show a remarkable variation of active ingredients during different

Extraction of Essential Oils
Fresh leaves/rhizomes of A. calamus were collected from their natural habitat and tested in the phytochemistry research lab. To extract essential oil, the plant material (500 g) was crushed and separately subjected to hydro-distillation in Clevenger apparatus for 3 h using the apparatus described in the European Pharmacopoeia [33]. using a GC MS-QP 2010, in the following conditions. Column DB-5 (30 m × 0.25 mm i.d.; 0.25 µm film thickness; J&W Scientific, Agilent, Santa Clara, CA, USA); carrier gas: helium, with a flow rate of 1 mL/min; injection temperature: 250 • C; oven temperature programme: initial temperature 80 • C, isothermal for 2 min, RAMP 7 • C/min, final temperature 280 • C, and isothermal for 10 • C/min. Ionization mode: EI (70 eV), mass range: 40-6500 amu. The compounds were identified with the help of NIST-MS, FFNSC Wiley Library, and comparing the data with literature reports and GC retention indices [34].

Antioxidant Activity
To analyse the in vitro antioxidant property, the essential oils of A. calamus were subjected to the following methods.

DPPH Radical Scavenging Activity
This is a quick method to study the scavenging ability of the antioxidants [35]. Briefly, the tested samples (5-25 µL/mL) were added to 5 mL of a 0.004% methanol solution of DPPH. Finally, the absorbance was read against a blank at 517 nm after 30 min of incubation at room temperature. Ascorbic acid was used as the standard antioxidant. Inhibition of free radical by DPPH in percent (IC %) was calculated using the equation. IC% = (A 0 − A t )/A 0 × 100, where A 0 = the absorbance value of the control sample, A t = the absorbance value of the test sample, and IC = inhibitory concentration. The radical scavenging activities of essential oils were discussed in terms of their IC 50 values.

Reducing Power
The reducing power of essential oils was determined by the method developed earlier [36]. In brief, varying concentrations of tested samples (5-25 µL/mL) were mixed with 2.5 mL of phosphate buffer (200 mM, pH = 6.6) and 2.5 mL of 1% potassium ferricyanide, K 3 [FeCN 6 ]. The mixtures were incubated for 20 min at 50 • C. After incubation, 2.5 mL of trichloroacetic acid was added to the mixtures, followed by centrifugation at 650 rpm for 10 min. The upper layer (1 mL) was mixed with 5 mL distilled water and 1 mL of 0.1% ferric chloride and the absorbance of the resultant solution was measured at 700 nm. Reducing power % = (A 0 − A t )/A 0 × 100, where A 0 = the absorbance value of the control sample and A t = the absorbance value of the test sample. The percent of chelating ability was plotted against concentrations and using a standard (gallic acid). The reducing potential of essential oils was discussed in terms of their RP 50 values.

Metal Chelating Activity
The chelation of Fe 2+ by essential oils was evaluated using the method developed earlier [37]. In brief, 0.1 mL of 2 mM FeCl 2 ·4H 2 O, 0.2 mL of 5 mM ferrozine, and 4.7 mL of methanol were added to different concentrations of a test sample (5-25 µL/mL). The solutions were mixed and allowed to react for 10 min. The absorbance was determined at 562 nm; IC% = (A 0 − A t )/A 0 × 100, where A 0 = the absorbance value of the control sample and A t = the absorbance value of the test sample. The percent of chelating ability was plotted against the concentration, and the standard curve was drawn using a standard antioxidant (EDTA). The metal chelating ability of essential oils was discussed in terms of their IC 50 values.

Results and Discussion
A noticeable variation was observed in the percentage yield of the hydro distilled essential oils (EOs) samples, taken at four seasons (winter, spring, summer, and autumn), The essential oil was pale yellow with a characteristic odour, and produced an irritating sensation in the eyes. The yields of essential oils in different seasons were 0.02-1.3% for leaves and 1.2-4.8% w/v for rhizomes. However, in previous reports the yields of essential oils in A. gramineus and A. calamus from different regions have been reported to range from 1.0 to 3.5% [31,38,39]. In the present study, the yield showed the highest percentage (4.8%) during the summer in all accessions, whereas during autumn and winter the yields obtained were only up to 0.02%. GC/MS showed marked variation in the major ingredients of oils prepared in spring, summer, autumn, and winter, respectively. The composition of the essential oils differed quantitatively and qualitatively according to the time of collection. A detailed comparative analysis of all the oils with different seasons has been recorded in Table 1, with phenyl propanoids in Table 2 and Figure 1, while the class composition is in Tables 3 and 4.
The oils were dominated by phenylpropanoids, and were detected in all the accessions in all seasons but in different quantities. Interestingly, the β-asarone content was highest in winter (57.0-90.6%) and lowest in summer, except Bhimtal, in which spring has the lowest content; whereas the α-asarone content was highest in summer and lowest in winter in all the accessions. On the basis of major components (phenyl propanoids) analysis by cluster analysis, as in Figure 2, it was observed that there were little difference in principal components like α-asarone, Z-isoelemicin, β-asarone, etc., both altitude-wise and season-wise.
Over 78-96% of constituents were identified in both ACLEO and ACREO. The major constituents identified in both the oils in all four seasons were trans-methyl isoeugenol, Z-isoelimicin, α-asarone, β-asarone, etc., but with different respective yield. Both ACLEO and ACREO exhibited similar qualitative diversity in terms of terpenoid composition, but their quantity varied in different seasons. However, the constituents, viz., limonene, α-humulene, 2,4,5-trimethoxy benzoic acid, and heptadecanol, could be detected only in ACLEO, whereas elimicin and β-calacorene were detected only in ACREO.
The seasonal variation in the chemical composition of essential oils might be due to different altitudes, environmental conditions, and the developmental stage/season of the plant materials, which is in agreement with results reported earlier [40][41][42]. In terms of class composition, the monoterpenoids in ACLEO from all three accessions in four different seasons ranged from 0.2 to 15.7%, mainly represented by limonene, β oscimene, linalool, bornyl acetate, etc. The sesquiterpenoids ranged from 2.6 to 16.6% with E caryophyllene, β elemene, α copaene, calarene, Z methyl isoeugenol, α humulene, spathulenol, germacrene D, α cadenene, α calacorene, etc. The marker class of, phenyl propanoids, ranged from 36.9 to 90.8%, mainly represented by α asarone, β asarone, Z isoelimicin, elimicin, etc. (Tables 2 and 3). Similarly, in ACREO the monoterpenoids ranged from 0.1 to 9.5%, and sesquiterpenoids from 0.6 to 9.4%, while phenyl propanoids ranged from 60.8 to 82.4% in three accessions for the four seasons. The essential oil composition of rhizomes from our laboratory has already been reported. The essential oil was found to possess significant antibacterial and antihelmintic activity [31,32]. It has also been reported that asarone and sesquiterpenoids were the major constituents in two phylogenetically different accessions [38]. Patra and Mitra [43] and Tamas et al. [44] have also reported acoramone and phenylpropane derivatives like α-asarone, β-asarone, γ-asarone, isoeugenol, and methyl ether in essential oils. The components reported earlier were also identified, besides some other constituents, but in different quantities. Seasonal along with altitudinal variation in both rhizomes (ACREO) and the aerial part (ACLEO) are reported here for the first time, demonstrating the qualitative and quantitative diversity of constituents in the essential oils of A. calamus.                   All the essential oils isolated in different seasons exhibited DPPH radical scavenging activity in a dose-dependent manner (5 µL/mL-25 µL/mL) ( Table 5). The radical scavenging potential of ACREO and ACLEO from three altitudes in the form of their IC 50 (Table 5).

Sites of Collection Sites of Collection
Similarly, the dose-dependent response for chelating activity for all the Eos (Table 5) Table 5).
The EOs from A. calamus exhibited good in vitro antioxidant activity, which might be because of the mixture of essential oils containing mono and sesquiterpenoids and the synergetic effects of the constituents. This is proven by a report that says that antioxidant capacity is affected by other bioactive compounds and could involve synergistic effects [45]. Several reports have shown in vitro antioxidant properties of many natural products, including essential oils. Antioxidants are believed to be directly anti mutagenic. In vitro physicochemical assays characterize most of them as antioxidants. However, the published report shows that in eukaryotic cells, essential oils can act as pro-oxidants, affecting inner cell membranes and organelles like mitochondria, and are usually non-genotoxic, and hence the beneficial effects of essential oils are due to pro-oxidants' effects on the cellular level [46]. The anticancer and antioxidant properties of certain medicinal herbs are used to treat trauma over a longer period of time, which is promising. Acorus calamus extracts and essential oils have been reported to possess anticancer and anti-angiogenic effects on cancer cells [47][48][49], which might be due to its antioxidant activity. In folk medicine the herb A. calamus has been used as a wound healing agent for many years, which has been proven scientifically by reporting the significant wound-healing activity of aqueous extracts in the animal model of excise wound healing, and anti-inflammatory activity in vitro [50]. It has been reported that essential oils containing linalool and the corresponding acetate play a major role in terms of anti-inflammatory activity [51]. The compounds shyobunone and isoshyobunone, isolated from essential oils, have been reported to possess insecticidal and repellant activity against Lasioderma serricorne (LS) and Tribolium castaneum (TC) [52]. Z methyl isoeugenol is used in perfumes as a flavouring agent [53]. Elimicin and caryophyllene have been reported to possess anti-inflammatory, antimicrobial, and analgesic activity [54][55][56]. Palmitic acid, regardless of obesity, impairs leptin and insulin's ability to regulate food intake and body weight. In addition, it has been reported that fatty acids containing palmitic acid and its ester possess significant antifungal and antibacterial activity [57,58]. Various essential oil components present in essential oils like linalool, 1,8-cineol, caryophyllene, α humulene, and asarone have been reported to possess antioxidant activity [59][60][61]. Components like linalool, asarone, α humulene, and caryophyllene oxide are also present in essential oils. Based on the reported data, it can be inferred that the antioxidant activity of A. calamus essential oil is because of these compounds, besides the synergetic effects of other constituents in the oil.

Conclusions
On the basis of our results, we can conclude that seasonal fluctuations periodically impact on the production of the constituents in medicinal plants, and also likely influence their therapeutic efficiency. Our study reveals that A. calamus plants show a rhythmic increase in oil production throughout the growing season and decline towards the winter. Hence, late summer can be the best time for collecting A. calamus plants. The present study reveals the presence of bioactive compounds, the antioxidant activity, and the free radical scavenging activity of A. calamus. Thus, this study supports the use of A. calamus against various ailments. More bioactive compounds present in EOs of A. calamus may warrant further characterisation.