ReviewPlectranthus: A review of ethnobotanical uses
Introduction
The family Lamiaceae contains several genera, such as sage (Salvia), basil (Ocimum) and mint (Mentha), with a rich diversity of ethnobotanical uses. Another important genus is Plectranthus, a large genus containing about 300 species found in Tropical Africa, Asia and Australia. Some species of Plectranthus are difficult to identify because of a lack of clear-cut morphological criteria to discriminate not only among species within the genus but also among the closely related genera. This has resulted in numerous taxonomic problems in the naming of species with the result that species have often been placed in several closely related genera like Coleus, Solenostemon and Englerastrum. In addition, some species formally placed in Plectranthus, are now recognized as the more distantly related Isodon (Paton et al., 2004).
Because of these taxonomic issues, different names have been used for the same species of Plectranthus and thus it has been difficult to collate information about the ethnobotanical uses of this genus. Furthermore, the most commonly used medicinal species of Plectranthus have a high degree of synonymy. This review aims to collate data about the different uses of species of Plectranthus, held in disparate sources and frequently under several synonymous Latin names. Information about the uses of the different species was gathered from searching the published literature using the databases NAPRALERT (Pharmacological Sciences (PCRPS), College of Pharmacy, University of Illinois), Web of Science (http://isiknowledge.com), CAB-direct (http://www.cabdirect.org), SEPASAL (http://www.kew.org/ceb/sepasal), Ingenta connect (http://www.ingentaconnect.com/;jsessionid=fq5k5kae0rgi9.victoria), Medline (http://medline.cos.com), Kew Library Catalogue, Elsevier ScienceDirect (www.sciencedirect.com), Dr. Duke's database (http://www.ars-grin.gov/duke/ethnobot.html) as well as looking through the Floras of countries in Africa, South America, Asia and Australia. Terms used in the searches included ‘ethnobotany, traditional uses, chemicals, phytochemistry, of Plectranthus, Coleus, Solenostemon, Anisochilus and Tetradenia’. All voucher specimens of Plectranthus and Coleus as well as relevant ones of in Solenostemon in herbaria at Kew (K), British Museum (BR), East Africa (EA) and Nairobi (NAI) were also examined to verify the different synonyms used to describe the species and to examine them for notes about their uses. Collectors often made notes either on their vouchers or in their field notebooks about the local use made of species they collect. (Herbarium vouchers cited in the text and/or tables are given at the end of the reference section.) Finally, the ethnobotanical data were mapped onto the most recent phylogeny of the genera (Paton et al., 2004) to see if there is any relationship among species of Plectranthus with similar ethnobotanical uses.
Section snippets
Synonymy
This review covers the ethnobotanical uses of 62 species of Plectranthus and about 30% of the literature citations covered by this review use synonyms to describe these species. The 10 species with the most uses also have the greatest number of synonyms. For example, Plectranthus barbatus Andr. has commonly been referred to as Plectranthus forskohlii Briq, Plectranthus forskalaei Willd., Plectranthus kilimandschari (Gürke) H.L. Maass., Plectranthus grandis (Cramer) R.H. Willemse, Coleus
Ethnobotanical uses
The ethnobotanical data collated for this review have been grouped using the categories in Economic Botany Data Collection Standard (Cook, 1995). A summary of the different ethnobotanical uses of the 62 best documented species of Plectranthus is presented in Table 1, Table 2.
Geographical distribution
A survey of the ethnobotanical information by geographical areas shows that 45 species are used in the African continent (Watt and Breyer-Brandwijk, 1962, Roberts, 1990, Kokwaro, 1993, Githinji and Kokwaro, 1993, Morris, 1996, Neuwinger, 2000), 14 species in Asia (Morton, 1992, Yoganarasimhan, 2000), 10 species in America (Prudent et al., 1995, Ruiz et al., 1996), five species in Australasia (Morton, 1992) and one species in the pacific (Mayenda, 1991) (Table 1, Table 2). In Africa, the most
Chemistry of Plectranthus species
To date, the majority of phytochemical studies on species of Plectranthus have focused on the isolation of a range of diterpenoids. Although, some of these studies report the biological activity of the diterpenoids isolated (Abdel-Mogib et al., 2002), very few have used activity-guided fractionation to isolate the compounds associated with a specific ethnobotanical use. Of the different types of diterpenoids found in the genus, the abietane diterpenoids are the most diverse group. Some of the
Ethnobotanical uses and phylogeny
In order to see if there are any relationships among the species of Plectranthus with ethnobotanical uses, the ethnobotanical data were mapped onto a phylogeny of the genus. The most recent traditional classification of the genus proposed by Codd (1975) is problematic because it does not include all species encountered in the literature and it groups species by superficial morphological similarity based on one or two characters leading to groupings which are incongruent with the phylogeny
Conclusion
Of the 300 species of Plectranthus, 62 species are reported to be used as medicines, ornamentals, foods, flavours, fodder and/or material. It could be that the number of species used is greater as there were citations to unconfirmed species that were only cited at the level of genus, but because it was unclear as to the species used very few of these references have been cited in this review. There are some interesting trends that emerge from this review. Species with ethnobotanical uses are
Acknowledgements
The authors are grateful to the BAT Biodiversity Partnership with RBG Kew for the grant that enabled this work to be carried out. We are also grateful to Dr. T. Kokubun, Royal Botanic Gardens, Kew, for designing the database used in this project. We thank the following herbaria for use of their libraries and access to their collections: Royal Botanic Gardens, Kew (K), British Museum (BM), East Africa Herbarium (EA) and University of Nairobi (NAI).
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