Introduction

Maerua triphylla A. Rich. is a shrub or small tree belonging to the Capparaceae or caper family. The Capparaceae family is a tropical and subtropical taxon of trees, shrubs, herbs and lianas consisting of approximately 417 species (Iltis, Hall, Cochrane, & Sytsma, 2011; Kers, Kubitzki, & Bayer, 2003) The genus Maerua Forssk has been recorded in tropical and southern Africa, and tropical Asia with about 60 species recorded from the African continent (Elfers, Grahama, & Dewolf, 1964). The genus name "Maerua" means "drooping" about the drooping foliage (Venter & Venter, 2015). The specific epithet “triphylla” is a Greek word meaning “three leaflets" about the trifoliate leaves that characterize the species.

Maerua triphylla is variable and four varieties, namely var. calophylla (Gilg) DeWolf, var. Johannis (Volkens & Gilg) DeWolf, var. pubescens (Klotzsch) DeWolf and var. triphylla have been described (DeWolf, 1962). However, in Kenya, where all four varieties have been recorded, it was found to be impossible to eliminate the varieties as the morphological characteristics appeared to be continuous (Mollel, 2013). The English common names of M. triphylla are “small bead-bean” and “maerua” (Dharani, 2019). The synonyms associated with the name M. triphylla include Crateva avicularis Burch. ex DC., C. caffra Burch. ex DC., M. dupontii Hemsl., M. jasminifolia Gilg & Gilg-Ben., M. pubescens (Klotzsch) Gilg, M. trachycarpaGilg, M. trichocarpa Gilg & Gilg-Ben. and M. variifolia Gilg & Gilg-Ben (Elfers et al., 1964; Wild, 1960). Maerua triphylla is an evergreen, erect or scandent tree or shrub, growing to a height of nine meters (Palgrave, 2002)

The branches of M. triphylla are dense, long, pendulous, young branches brown, glabrous with several lenticels and forming a round crown. The leaves are alternate, simple, trifoliate, with entire margins, dull grey-green, narrowly oval with a rounded apex. The flowers are small, regular, bisexual, cream to white, borne singly in the upper leaf axils. The fruit is a cylindrical capsule and constricted between the seeds, bluish-green at first, turning creamy-brown when ripe. The seeds are irregularly kidney-shaped, pale brown and obscurely warted. Maerua triphylla has have been recorded in evergreen or deciduous bush land, wooded grassland, dry woodland, termite mounds, thickets and margins of the drier forests, along rivers and lakes. Maerua triphylla has have been recorded in the Democratic Republic of Congo, Ethiopia, Djibouti, Tanzania, Kenya, Uganda, Somalia, South Sudan, Sudan, Malawi, Mozambique, Zambia, Zimbabwe, Madagascar and Seychelles an altitude ranging from sea level to 2300 m above sea level (Elfers et al., 1964; Palgrave, 2002).

The leaves of M. triphylla are eaten as leafy vegetables in Ethiopia and Tanzania (Lulekal, Asfaw, Kelbessa, & Damme, 2011; Luoga, Witkowski, & Balkwill, 2000) and also consumed as famine food (Mollel, 2013). In Kenya and Tanzania, during periods of famine, the roots of M. triphylla are boiled for a long time to remove toxic compounds and prepared as porridge (Dharani, 2019; Hamilton & Hamilton, 2006). The species is also important for bee forage (Mollel, 2013). Wild and domesticated animals browse the leaves and shoots of M. triphylla in Kenya, Ethiopia and Uganda (Gemedo-Dalle, Isselstein, & Maass, 2006; Mbuvi, Kungu, Gachathi, Leley, & Muthini, 2019). In East Africa, M. triphyllais planted in rocky sites, and other degraded landscapes as a management strategy for soil reclamation, and the species is often spared when clearing land for agricultural purposes (Mollel, 2013). Maerua triphylla is one of the important medicinal plants in tropical Africa, and the species is included in the book “Plant resources of tropical Africa 11(2): Medicinal plants 2”, a photographic guide to the most commonly used medicinal plants in tropical Africa (Mollel, 2013). Therefore, M. triphylla is a valuable plant species with potential for improving food and nutritional security and primary healthcare of local communities in tropical Africa. Thus, this review aims to provide an integrated and detailed appraisal of the existing knowledge on the ethnomedicinal uses and ethnobotany of M. triphylla.

Materials and Methods

Results of the current study are based on a literature search on the ethnomedicinal uses and ethnobotany of M. triphylla using information derived from several internet databases. The databases included Scopus, Google Scholar, PubMed and Science Direct. Other sources of information used included pre-electronic sources such as journal articles, theses, books, book chapters and other scientific articles obtained from the university library.

Results and Discussion

Medicinal uses of Maerua triphylla

The fruit, leaf, root, stem bark and twig infusion and decoction of M. triphylla are mainly used as ethnoveterinary medicine, aphrodisiac and antidote for poisoning, and traditional medicine for rheumatism, cancer, respiratory problems, snakebite, headache and gastrointestinal problems (Table 1 ;Figure 1). In Tanzania, the leaves of M. triphylla are mixed with those of Boscia salicifolia Oliv and taken orally as an antidote for poisoning and traditional medicine for tuberculosis (Hedberg et al., 1982; Maroyi, 2019). In Kenya, the roots of M. triphylla are mixed with those of Uvaria acuminata Oliv and taken orally as traditional medicine for breast cancer, dizziness and hallucinations (Waswala-Olewe et al., 2014).

Similarly, the roots of M. triphylla are mixed with those of Capparis cartilaginea Decne. As traditional medicine for headache, and the roots of M. triphylla are mixed with those of U. acuminata and bark of Boscia angustifolia A. Rich. and used against snakebites (Waswala-Olewe et al., 2014). In Ethiopia, leaves of M. triphylla are mixed with those of Parvonia spp., Phytolacca dodecandra L’Hér. and Tagetes minuta L. and used as ethnoveterinary medicine for babesiosis or red water disease (Katunguka-Rwakishaya et al., 2004).

Phytochemical and pharmacological properties of Maerua triphylla

The ethnobotanical data about M. triphylla is related to three primary use categories which are centered around herbal medicine, fodder and a source of food, especially during difficult times. However, there is very little information available concerning the phytochemistry and pharmacological properties of the crude extracts or compounds isolated from the species. Little work has been done to characterize the nutritional attributes of M. triphylla, but a report on its leaves showed high amounts of proteins averaging 21.4% (Lehouérou, 1980; Mollel, 2013). There is need for nutritional evaluation of both the leaves and roots of the species as M. triphylla is widely used as wild food, both as supplementary and famine food (Lulekal et al., 2011; Luoga et al., 2000). There is also need to evaluate the toxicological properties of the species as uncooked roots are perceived as toxic, and boiling and re-boiling them several times is suggested to render them non-toxic and edible (Hamilton et al., 2006; Tairo, 2011).

McLean, Blunden, and Jewers (1996) identified the betaines and quaternary ammonium compounds such as 3-hydroxyprolinebetaine, glycine betaine, proline betaine and 3-hydroxy-1,1-dimethyl pyrrolidinium from dried aerial parts and branches of M. triphylla. Loontjens (2013) argued that quaternary ammonium compounds are potent biocides widely used in medical applications, cosmetics, disinfectants, surfactants and solvents. Some of these phytochemical compounds may be responsible for the biological activities of the species. Zorloni (2008) evaluated the acaricidal activities of acetone and hexane extracts of M. triphylla leaves against adult Rhipicephalus pulchellus ticks. The extracts exhibited good repellent properties against the ticks at 10.0% concentration. These findings support the use of leaf infusion and decoction against ticks in Ethiopia (Zorloni, 2008). Matata et al. (2018) evaluated the cytotoxicity activities of methanol: dichloromethane (1:1) extracts of M. triphylla leaves using the brine shrimp toxicity assay with Catharanthus roseus (L.) G. Don. Anticancer compounds characterized it as a positive control. The extract exhibited moderate activities with median lethal concentration (LC₅₀)value of 57.5μg/ml in comparison to LC₅₀ value of 6.7 μg/ml exhibited by the positive control (Matata et al., 2018). Therefore, the brine shrimp assay results corroborate ethnomedicinal applications of the roots and stem bark of the species against breast cancer and leukaemia in Tanzania (Augustino et al., 2005; Matata et al., 2018).

Conclusions

Maerua triphylla is known to be poisonous, and there is a need for detailed clinical and toxicological evaluations of crude extracts and compounds isolated from the species. Therefore, the widespread use of M. triphylla as food plant and source of traditional medicines throughout its distributional range suggest that the species is not taken at toxic dosages. But the use of M. triphylla as food and for the treatment of human diseases and ailments should be treated with caution and rigorous toxicological and clinical studies of the bark, fruits, leaves, roots and tubers, and compounds isolated from the species are necessary.