1. Introduction
The genus
Murraya (Rutaceae) is made up of about 14 species.
Murraya paniculata (L.) Jack is a small tropical evergreen shrub, native to the tropical and subtropical parts of the world, including southern China, Taiwan, India, Nepal, Northeastern Pakistan, Sri Lanka, Southeastern Asia (
i.e., Cambodia, Laos, Myanmar, Thailand, Vietnam, Indonesia, Malaysia, and the Philippines), and Northern Australia. It is widely naturalized in the southern part of Australia, Southeastern USA and Central America.
M. paniculata is also known as
Chalcas exotica, Chalcas paniculata, and
Camunium exoticum [
1].
M. paniculata is commonly known as orange jasmine or mock orange. In Nepal, it is known as
bajardante [
2]. The average shrub can grow up to 7 m high. Morphologically, the plant can be distinguished by its alternate, glabrous, and glossy leaves that are once-compound, occurring in 3–7 oddly pinnate leaflets. Leaflets are elliptic to cuneate-obovate, 2–9 cm long × 1.5–6 cm wide [
3,
4].
M. paniculata blooms throughout the year. Inflorescences are terminal, corymbose, few-flowered, and dense. Flowers are pentamerous, bisexual, and sweetly fragrant. Petals are 12–18 mm long, narrowly elliptic to oblanceolate, curved backwards, and white to fading cream in color. The fruit is a fleshy berry, oblong-ovoid, red to orange, and grows up to 2.5 cm in length [
3,
4].
For many years,
M. paniculata has been used as an ornamental and a medicinal plant [
5]. Due to its hardiness and wide range of soil tolerance, orange jasmine is commonly used as a hedge. The leaves have been used as a food additive in many Indian and Malay dishes due to their strong fragrance [
6].
M. paniculata is commonly used in traditional medicine for treatment of diarrhea, abdominal pain, stomach ache, dysentery, headache, edema, thrombosis, and stasis of blood. Moreover, it was used as a detoxication agent, anticonvulsant, local anesthetic, and expectorant. Previous reports have shown that the extracts from bark and leaf are stimulant and astringent, and had antinociceptive [
7], anti-inflammatory, antidiarrheal [
8], antitrypanocidal, antidiabetic, antimalarial, antibacterial, antifungal, and antioxidant activities [
9,
10]. The essential oil was reported to possess anti-amebic activity [
11]. Pangnakorn and Poonpaiboonpipattana reported that the aqueous extract of
M. paniculata leaves possesses phytotoxic effects on seed germination and seedling growth of
Bidens pilosa,
Amarathus spinosus,
Echinochloa crusgalli, and
Chloris barbata [
12].
M. paniculata has been the subject of several phytochemical studies. The leaf extract was reported to contain coumarins [
13,
14] and flavonoids [
15,
16,
17]. The components of leaf essential oils of
M. paniculata from Bangladesh [
18], China [
19], Cuba [
20], and Nigeria [
5] have been previously reported. However, many factors, including provenance, weather, soil conditions, time of harvest, and the drying technique, can change the chemical composition and yield of essential oils [
21].
M. paniculata has been described as synonymous with
M. exotica [
22], but this has been controversial and has recently been challenged [
23]. The current study was conducted to investigate the composition of the leaf essential oil of
M. paniculata from Nepal as well as its biological activities.
3. Results and Discussion
The
M. paniculata leaf essential oil composition is shown in
Table 1. The leaf oil was mainly composed of methyl palmitate (11.05%), isospathulenol (9.44%), (
E,
E)-geranyl linalool (5.29%), benzyl benzoate (4.20%), selin-6-en-4-ol (4.01%), β-caryophyllene (3.97%), germacrene B (3.62%), germacrene D (3.39%), and γ-elemene (3.19%) ,with other minor constituents (<3%). The current study revealed that the essential oil composition and percentages are significantly different from the previously published reports from Bangladesh and China. The major constituents of the leaf oil of
M. paniculata of Bangladeshi origin were caryophyllene oxide (16.6%), β-caryophyllene (11.8%), spathulenol (10.2%), β-elemene (8.9%), germacrene D (6.9%), and methylene-6-4-(1-propenylidene)cyclooctene (6.4%) [
18], while the main components of Chinese
M. paniculata essential oil were β-caryophyllene (23.3%), spathulenol (16.1%), (
E)-α-bergamotene (9.3%), (
E)-nerolidol (4.6%), and δ-elemene (3.3%) [
19]. The leaf oil of
M. paniculata from Cuba was also rich in β-caryophyllene (29.8%) and spathulenol (5.1%), but had significant quantities of caryophyllene oxide (6.3%), viridiflorol (5.7%), δ-cadinene (5.6%), bicyclogermacrene (5.6%), α-humulene (5.3%), and β-cubebene (5.3%) [
20]. The leaf oil of Nigerian
M. paniculata was mainly composed of β-cyclocitral (22.9%), methyl salicylate (22.4%), (
E)-nerolidol (11.7%), α-cubebene (7.9%), cubenol (6.8%), β-cubebene (5.8%) and isogermacrene (5.7%) [
5].
Since
M. paniculata has often been classified as a species, if it is synonymous with
M. exotica [
5], it is unclear about which essential oil composition may belong to which, or if they are, indeed, separate species. Lv and co-workers [
19] have treated
M. paniculata and
M. exotica as separate species and have examined the essential oil compositions of both. These workers found
M. exotica from Guangxi Province, China, to be qualitatively similar to
M. paniculata (see above), and was dominated by spathulenol (25.6%),
trans-β-guaiene (13.7%), β-caryophyllene (11.7%), and bicyclogermacrene (4.1%) [
19]. There is much variation in the compositions of
M. exotica essential oils, however.
M. exotica oil from Hainan, China, was rich in β-caryophyllene (45.5%) and cedrene (15.1%) [
30], while a sample from Guangdong, China, had spathulenol (17.7%), α-pinene (13.2%), caryophyllene oxide (8.6%), and bicyclogermacrene (7.1%) as major components [
31]. In order to attempt to sort out the volatile phytochemistry of
Murraya paniculata/exotica, a hierarchical cluster analysis was carried out on the essential oil compositions of
M. paniculata and
M. exotica reported in the literature (
Figure 1) [
5,
18,
19,
20,
30,
31,
32,
33,
34,
35,
36]. The components used in the cluster analysis are summarized in
Table 2 and illustrate the chemical differences between these essential oil samples. Although there are only 13 essential oil samples, too few to provide a comprehensive chemotaxonomic representation of this species, this analysis does serve to place
M. paniculata leaf oil from Nepal into context with previously-reported essential oils of
M. paniculata and
M. exotica.
The cluster analysis reveals at least eight chemotypes for the
Murraya paniculata/exotica complex based on volatiles: (1) a methyl salicylate/β-cyclocitral chemotype represented by the
M. paniculata sample from Nigeria [
5]; (2) a β-humulene chemotype represented by the
M. exotica sample from India [
36]; (3) a chemotype dominated by α-pinene represented by the
M. exotica sample from Egypt [
32]; (4) a cluster rich in β-caryophyllene with
M. paniculata samples from China [
19] and Cuba [
20] and
M. exotica samples from China [
30,
35] and Cuba [
34]; (
5) a caryophyllene oxide/β-caryophyllene/spathulenol chemotype represented by the
M. paniculata sample from Bangladesh [
18]; (6) a spathulenol-rich cluster with
M. exotica samples from China [
19,
31]; (7) a β-caryophyllene/α-zingiberene chemotype represented by the
M. exotica sample from India [
33]; and (8) the sample from Nepal (this work), rich in methyl palmitate. In addition to genetic variation [
23], age [
37], vegetative cycle stage [
38], climate [
39], season [
40], soil composition [
41], and edaphic factors [
42] are among several factors responsible for the considerable variation in essential oil compositions [
43,
44]. Based on the observed composition, the Nepalese
M. paniculata leaf oil is chemically distinct from previously-reported analyses and may represent a distinct chemotype.
The essential oil of
M. paniculata was screened for potential antimicrobial activity. Based on our experience [
26,
45,
46], we consider samples to have good antimicrobial activity with MIC < 156 μg/mL, moderate activity with MIC between 156 and 313 μg/mL, and weak activity between 313 and 625 μg/mL. Samples with MIC > 625 we consider to be inactive. Based on these criteria,
M. paniculata essential oil was inactive against
Bacillus cereus and
Candida albicans (MIC = 2500 μg/mL) and marginally antifungal against
Aspergillus niger (MIC = 313 μg/mL). Nevertheless, the antifungal activity of
M. paniculata leaf oil was better than many of the essential oils we have tested [
47,
48], comparable to
Mitracarpus scaber leaf essential oil (MIC = 313 μg/mL) [
49] and
Betula nigra buds essential oil (MIC = 313 μg/mL) [
50], but not as effective as
Pinus roxburghii cone essential oil (MIC = 39 μg/mL) [
51],
Cinnamomum camphora leaf essential oil (MIC = 19.5 μg/mL) [
52],
Curcuma longa leaf essential oil (MIC = 19.5 μg/mL) [
53], or
Canthium subcordatum fruit essential oil (MIC = 39 μg/mL) [
54]. Although in low concentrations, α-humulene (1.1%) and germacrene D (3.4%) may contribute to the antifungal activity of
M. paniculata leaf oil, both have shown activity against
A. niger (MIC = 78 and 39 μg/mL, respectively, for α-humulene and germacrene D [
47]). Methyl palmitate (11.1% in
M. paniculata oil) has shown antifungal activity (MIC = 333 μg/mL) against
Blumeria graminis [
55].
M. paniculata oil showed moderate activity in the brine shrimp (
Artemia salina) lethality test with LC
50 value of 41 μg/mL. Essential oils showing
A. salina toxicity with LC
50 < 10 μg/mL are considered very active [
56], 10 μg/mL < LC
50 < 50 μg/mL are moderately active [
51,
57], and 50 μg/mL < LC
50 < 100 μg/mL, weakly active. In our nematicidal activity screening against
C. elegans, we have found LC
50 values ranging from 18 to 1100 μg/mL (unpublished), and we consider nematicidal activities LC
50 < 100 μg/mL to be very active, LC
50 values between 100 and 200 μg/mL to be moderately active [
52,
58], between 200 and 300 μg/mL to be weakly active, and > 300 μg/mL to be inactive. Thus,
M. paniculata oil was highly nematicidal to
Caenorhabditis elegans (LC
50 = 37 μg/mL). It is difficult to speculate as to which compound(s) in the essential oil may be responsible for the brine shrimp lethality or nematicidal activity; there are many components in the leaf oil and none are especially dominant.