Ethnobotanical, phytochemical and pharmacological aspects of Bengal Pogostemon (Pogostemon benghalensis)

Introduction Plants produce a diverse assortment of secondary metabolites that do not participate directly in the growth and development of the plant (1). Due to their complex chemical structure and biosynthetic pathways, these myriad natural products have received little attention from the biological scientists. However, humans have been using the herbs and their products as medicines to cure various ailments and diseases like cough, cold, fever, digestive disorders, food poisoning, etc. since time immemorial (2,3). Owing to their various traditional uses, researchers are showing more and more interest in reevaluating and recognising various biological properties of natural plant products. Some of these natural plant products are essential oils, dyes, colouring agents, and pharmaceutical compounds. Now a day, essential oils are one of the most important secondary metabolites that are receiving considerable attention of the researchers. Because of various aromatic and therapeutic properties, these are widely used in food, flavour and fragrance, cosmetic, and other pharmaceutical industries (4). Pogostemon benghalensis (Burm.f.) Kuntze is an aromatic undershrub that grows in open riverine forest areas of tropical climates (5) and generally found in attitudes http://www.herbmedpharmacol.com doi: 10.34172/jhp.2020.40


Introduction
Plants produce a diverse assortment of secondary metabolites that do not participate directly in the growth and development of the plant (1). Due to their complex chemical structure and biosynthetic pathways, these myriad natural products have received little attention from the biological scientists. However, humans have been using the herbs and their products as medicines to cure various ailments and diseases like cough, cold, fever, digestive disorders, food poisoning, etc. since time immemorial (2,3). Owing to their various traditional uses, researchers are showing more and more interest in re-evaluating and recognising various biological properties of natural plant products. Some of these natural plant products are essential oils, dyes, colouring agents, and pharmaceutical compounds. Now a day, essential oils are one of the most important secondary metabolites that are receiving considerable attention of the researchers. Because of various aromatic and therapeutic properties, these are widely used in food, flavour and fragrance, cosmetic, and other pharmaceutical industries (4).
Pogostemon benghalensis (Burm.f.) Kuntze is an aromatic undershrub that grows in open riverine forest areas of tropical climates (5) and generally found in attitudes between 150-1300 m. It is native to South Asia and is widely distributed in India, Nepal, Bangladesh, Myanmar, Sri Lanka, China, Thailand, Vietnam and Bhutan (6,7). The floral buds and leaves are rich in essential oil, which in turn is rich in sesquiterpenes. Various researchers have studied the chemical profile of the essential oil of P. benghalensis. The essential oil and the leaf extracts of P. benghalensis have been studied for their antioxidant (10), antibacterial (11)(12)(13), antifungal (11,13), antiviral (14), larvicidal (9) and anticancerous activities (15). Traditionally, its leaves and roots have been used to cure cold, cough, pneumonia, diarrhoea, dysentery, skin diseases, and digestive problems (16,17). The present review compiles the incomplete information on the phytochemistry, traditional uses, and other pharmacological properties of P. benghalensis and highlights that the plants are a rich source of phytochemical worth exploiting for human benefits.

Botanical description
Pogostemon benghalensis is an undershrub to shrub with a strong, solid, angular and tomentose stem. The leaves are ovate with double dentate margin and acuminate apex, pubescent and bear epidermal hairs and secretary structures, arranged in opposite phyllotaxy (19). It bears purple or pinkish-white bilipped flowers with strong aroma in verticillaster inflorescence; stamens are exerted with long violet purple hairs on filaments; the ovary is glabrous with slenderical style and bilobed stigma (20,21). Fruits are trigonous, reddish brown and composed of four nutlets.

Phytochemistry
A little work has been done on the photochemistry of P. benghalensis. As per studies done using GC-MS (gas chromatography-mass spectrometer), the plant has a rich profile of phytochemicals like phenolics, steroids, tannins, flavonoids, terpenoids, and essential oils. The crude extracts and essential oils are mainly composed of sesquiterpenes, but also have terpene derivatives, aromatic alcohols and other organic compounds (5,6,8,9). In a study performed by Chanotiya et al, Elemol (11.6-20.5%), β-caryophyllene (8.1-12.5%), β-bisabolene (3.6%-18%), α-humulene (4.0-8.7%), β-ocimene (3.6-3.7%), germacrene B (2.5-4.8%), were reported as the major components in Pogostemon oil extracted from leaves and inflorescence of the plant (8). Moreover, the presence of these components was also confirmed by Bhuiyan et al (6). In a similar study of the phytochemistry of Pogostemon, Anjana and Thoppil (9) reported dehydranone (26.66%) as major component followed by δ-cadinol (23.06%) whereas, in contrast, Dhakal et al (5) reported 7-Isopropyl-1,4-dimethyl-azulen-2-ol (41.72%) as the major component followed by α-gurjunene isomer (9.23%). The variations observed in the chemical profile of P. benghalensis could have been possibly due to differences in geographical location, climatic conditions, harvesting methods, the growth stage of the target plant at the time of harvesting, plant parts used for oil extraction, soil profile etc. Some of the important components detected in the essential oil of P. benghalensis, along with their known pharmacological properties are given in Table 1  .

Nutritive value
Nutrient analysis of the leaves of P. benghalensis by Unni et al revealed the presence of approximately 0.25 ± 0.1% carbohydrates, 6.175 ± 0.2% fatty acids, 4.59 ± 0.1% proteins, 7.10 ± 0.1% fibre, 1.6 ± 0.2% tannin, 84.77 ± 2.4% moisture, and 75.01 ± 1.6% nutritive value. The investigators also determined the fatty acid composition through GC analysis after converting lipids into methyl esters which ensured the presence of methyl ester of palmitic acid (0.75%) and a mixture of oleic, linoleic and linolenic acids (3.75%), of which linoleic and linolenic are the essential fatty acids for human beings (64).

Ethnobotanical uses
Different parts of P. benghalensis have been traditionally used by different ethnic groups in India and other countries for domestic and therapeutic purposes. Generally, genus Pogostemon was used by tribal people for its roots and leaves (65). Different plant parts were used in formulations like decoction, fresh extract, poultice, and infusion etc. to get rid of health ailments. The plant is used as an antidepressant, antiseptic, aphrodisiac and to cure skin problems in aromatherapy (64). It has been widely used in folk medicine for the treatment of intestinal disorder and intermittent fever (66). Tribal people also used the plant as an antidote to snakebite. The paste of soft leaves and fresh roots was applied to the snake bites. Boiled root extract was orally given to the patient (67). Traditionally, leaf and root juice have been given to cure cough and cold (11,68), haemorrhage (69), malaria, Dahiya et al  (70), fever (71,72), vomiting, food poisoning, stomach problems (17) and respiratory tract infections (68). Leaves are used to cure scabies and ringworms (73) and burning (74). Its leaves are also used as vegetable (75). A decoction of fresh leaves is given orally to cure dyspepsia (76). Bhattarai et al reported that the decoction of roots of Pogostemon plant along with the root of the plant of Ageratum conyzoides was used to cure typhoid by the local people of Nawalparasi district, Nepal (77). Fresh leaves of P. benghalensis are used in Southern Assam for the herbal preparation known as "Shuktani", which is used for the treatment of diarrhoea, dysentery, and indigestion, and also used by women for lactation and body strength after parturition (16). The young leaves are also used as a vegetable, and used to prepare pancake with powdered rice (78). The essential oil of P. benghalensis is used in the perfumery industry, and its dried leaves are used to scent cloth (79). Leaves and shoots of this plant are also used to extract natural colour and dyes (80), which are further used to decorate the wall of the houses (81). Various traditional uses of P. benghalensis are summarized in Table 2.

Pharmacological activities
Various pharmacological properties have been studied in plant P. benghalensis. Some of such activities like antibacterial, antiviral, antioxidant, antifungal, antiinflammatory of the plant are given in Table 3 and discussed below.

Antibacterial activity
Taylor et al reported that methanolic extracts of the aerial parts of P. benghalensis possessed antibacterial activity and inhibited the growth of bacteria Bacillus subtilis and Staphylococcus aureus at 2 g/mL (11 (14).

Antifungal activity
Traditionally, leaves of P. benghalensis have been used by local people of Panchthar district (Nepal) to cure scabies, ringworms, thus, implicating the antifungal property of the plant. Various extracts of P. benghalensis were reported to possess antifungal activity. The methanolic extracts of the aerial parts of this aromatic plant showed antifungal activity against Microsporum gypseum and Trichophyton mentagrophytes (11). Similarly, Thoppil et al (13) revealed the antifungal potential of crude essential oil of three species of genus Pogostemon including P. benghalensis against eight fungal strains, and reported that P. benghalensis inhibited the growth of Fusarium solani and Candida albicans with maximum inhibition zone of 32.33±2.08 mm and 32.33±2.52 mm, respectively over the standard antibiotic nystatin (inhibition zone; 30.33±1.53).

Antioxidant activity
In a study conducted by Singh et al, the antioxidant activity of crude essential oil of ten species of family Lamiaceae including P. benghalensis was evaluated through 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) scavenging activity and total antioxidant activity (TAA). The essential oil of P. benghalensis exhibited DPPH scavenging activity and TAA with an EC50 (Halfmaximal Effective Concentration) value of 171.3±5.74 µg/mL, 89.5±2.33 µg/mL, respectively whereas, the EC50 value of standard butylated hydroxytoluene (BHT) for DPPH scavenging activity and standard ascorbic acid for TAA were 156.4 and 165.7 µg/mL. The EC50 value of Pogostemon oil was nearly close to the standard value for DPPH scavenging activity, and the EC50 for TAA was lower than the standard value (10).

Larvicidal activity
Anjana and Thoppil evaluated the larvicidal potential of essential oil of four species of genus Pogostemon including P. benghalensis against the fourth instar larvae of Aedes albopictus Skuse. P. benghalensis essential oil showed 100% larval mortality at 100 and 200 ppm concentrations and the activity was attributed to sesquiterpene rich essential oil (9). Several reports have confirmed the larvicidal  (15).

Conclusions
The present review congregated information about the botanical aspects, ethnobotanical uses and recent studies on phytochemistry, and biological activities of different extracts of P. benghalensis. Essential oil is highly rich in sesquiterpenes. Various studies have evaluated the anticancerous, antibacterial, antifungal, antiviral, antioxidant and larvicidal properties of the essential oil and different extracts of P. benghalensis. It has been found that essential oil and various extracts of P. benghalensis possess good pharmacological potential and have broad spectrum activities on various ailments as evident from the traditional knowledge and reported bioassays. Despite ample traditional uses of P. benghalensis, only limited in vivo model studies have been conducted to evaluate its pharmacological properties. Therefore, there is an utmost need for in vivo clinical trials to confirm these pharmacological activities. To further strengthen the pharmacological profile of P. benghalensis for drug development, more rigorous research should be conducted on the extraction, identification and the mode of action of the bioactive components at the molecular level. Further, more investigations are required to elucidate the correlation between traditional uses and its pharmacological activities.