Lawsonia inermis L. (Henna): A Comprehensive Review of Its Phytochemistry, Pharmacological Potential, Traditional Uses, and Commercial Applications
1. Introduction Lawsonia inermis L., commonly referred to as henna, is a perennial shrub belonging to the family Lythraceae, which has a long history of use in traditional medicine and cultural rituals. The plant is particularly famous for yielding a reddish-orange colour from the compound lawsone (2-hydroxy-1,4-naphthoquinone), which is found primarily in its leaves. Lawsone, as reported by Oda et al. (2018), is the major active compound that gives henna its colouring properties, which have contributed to its being a standard ingredient in cosmetic and ceremonial applications for centuries. [49] highlighted the broad geographical and cultural significance of L. inermis in Asia, Africa, and the Middle East, where it finds application in rituals, healing, and as a plant dye. Apart from its colouring role, the plant contains a wide variety of phytochemicals—e.g., triterpenoids, flavonoids, and phenolic acids—which serve to promote a wide variety of biological activities. [9] has named these compounds as being responsible for their drug efficacy. Traditionally, L. inermis‘ therapeutic uses have been recognised by traditional medical systems worldwide. It was reported used by [35] in Ayurvedic medicine, and [20] used it in wound care, infection management, and metabolism disorder treatment via generation-to-generation transmission of ethnomedicinal information. These conventional claims have been provided with scientific justification by recent experimental research. Its antioxidant, antidiabetic, antimicrobial, and anti-inflammatory activities were documented by [42], [50], and [2] further investigated its immunomodulatory and wound-healing activities, further establishing its significance in current phytotherapeutic research. Notwithstanding the growing amount of evidence, hurdles like extract standardisation, toxicity testing, and strict clinical trials are still present. The review herein covers an overall discussion on L. inermis, including its taxonomy, phytochemistry, ethnopharmacological history, therapeutic attributes, and prospects in commercialisation. This is aimed at bridging traditional knowledge with contemporary science to aid future innovation and sustainable use of this multipurpose plant. 2. Taxonomy and Botanical Description L. inermis L. is the single known species in the genus Lawsonia and is found in the family Lythraceae, which is placed in the order Myrtales. Its taxonomic status as a dicotyledonous angiosperm has been repeatedly upheld in every classification scheme that has been put forth by Bentham and Hooker, Engler and Prantl, Cronquist, and the Angiosperm Phylogeny Group (APG). [49] had characterised L. inermis as a much-branched, smooth shrub or small tree, usually 2 to 6 metres tall. The plant is readily identified by its opposite, simple leaves that range from elliptic to lance-shaped shapes, usually 1.5 to 5 centimetres long [4,6]. These leaves have entire margins and acute tips and are either sessile or attached by very short petioles. The plant bears fragrant flowers arranged in terminal panicles. Each flower consists of four petals, from white to pale pink, and has prominent stamens. [9] reports that the fruits are tiny, spherical capsules measuring 4–8 millimetres in diameter, containing numerous angular seeds. The bark is typically thin and greyish-brown, and it peels in fine layers as the plant grows [7,10]. One of the major botanical features of L. inermis is that it produces the orange-red pigment lawsone, which is deposited in the leaves [49]. observed that the pigment is significant from a historical point of view for its applications in traditional body adornment, natural hair dyeing, and colouring of fabrics. Table 1. Comparative taxonomic classification of L. inermis L. according to different botanical systems (Bentham & Hooker, Engler & Prantl, Cronquist, and the Angiosperm Phylogeny Group). Figure I. This figure details the morphological features of L. inermis (henna). (a) presents the mature plant naturally occurring in its environment, with its bushy appearance- (b) provides a flowered branch, which emphasises the opposite phyllotaxy also seen here clearly in the inset. (c) offers a single flowered twig with minute, opposite leaves. (d) gives a close-up of the leaves, which are elliptic, entire, and oppositely arranged on the stem. (e) gives the inflorescence with many minute flowers aggregated in terminal cymes. (t) gives a close-up of one flower, displaying whitish-yellowish petals and bold stamens. (g) gives a single stamen with a bilobed anther, and (h) gives a mature green fruit, which is a tiny capsule. 3. Synonyms and Vernacular Names The botanical name Lawsonia was given in commemoration of Dr. Isaac Lawson, an 18th-century Scottish doctor. The species name inermis refers to the usually spineless twigs of the plant, which separate it from other thorny shrubs. Because of its wide geographic range and morphological variability, L. inermis has been placed under different synonyms in the past. These synonyms, documented in ancient botanical works, are listed in Table 2. L. inermis has a large number of vernacular names, which highlight its extensive cultural and medicinal use. It is “henna” or “Egyptian privet” in the English language [1]. It is named as “mehndi” in Urdu and Hindi, “mendi” in Gujarati and Marathi, and “maruthani” in Tamil. In the Arabic-speaking world, it is also known as “hina” or “henné.” There are similar names given to it in Persian and Swahili cultures. As identified by [1], an array of vernacular names is employed throughout Asia and Africa, each signifying the plant’s function in religious, cosmetic, and medicinal practices. Some examples include “inai” in Indonesian and Malay, “dan” in Burmese, “kaaw” in Lao, and “thian daeng” in Thai. Table 3 presents a list of these names to show the local vocabulary and cultural extent of L. inermis. 4. Cultivation and Distribution L. inermis, or henna, is a hardy plant species that withstands dryness and semi-desert conditions. The plant tends to grow up to a size of about six metres but can be kept in check at a shorter height by way of pruning, thereby maximising production of leaves as a harvest commodity, as has been observed by [50]. Ideal growth is achieved in sandy, drained soils under full sun and is particularly well-suited for tropical and subtropical environments. It is restricted in the colder areas by its frost sensitivity [50]. In the conventional farm environments, L. inermis is commonly planted on field borders or around residences and serves … Read more