Phytochemical and pharmacological investigations of invasive Chromolaena odorata (L.) R.M. King & H. Rob. (Asteraceae).
Omokhua, Aitebiremen Gift.
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Chromolaena odorata (L.) R.M. King & H. Rob. (Asteraceae) is an invasive weedy scrambling perennial shrub native to the Americas that has proven to be a significant threat to both natural and semi-natural ecosystems as well as to livelihoods in the tropics and sub-tropics (including sub-Saharan Africa). Two biotypes of C. odorata are invasive in sub-Saharan Africa. The Asian/West African biotype (AWAB) is the more widespread form on the continent (being present in West, Central and East Africa), while the southern African biotype (SAB) is restricted to south-eastern Africa. Although the negative impact of the plant has received considerable attention in Africa, its medicinal and pharmacological significance is only beginning to be explored. The AWAB plant is exploited as a source of medicine in West and Central Africa for the treatment of malaria, wounds, diarrhoea, skin infections, toothache, dysentery, stomach ache, sore throat, convulsions, piles, coughs and colds, possibly because of the presence of flavonoids, essential oils, phenolics, tannins and saponins. The plant is reported to have antibacterial, anti-inflammatory, antioxidant, anthelminthic, antifungal, cytotoxic, anti-convulsant, anti-protozoal, antispasmodic, anti-pyretic and analgesic properties. Though the above usefulness has been reported with reference to the AWAB plant, the SAB plant has not been investigated. Hence this thesis attempts to comparatively document the phytochemistry and ethnopharmacological importance of both C. odorata biotypes. The antibacterial and antifungal activities of the leaf extract of the AWAB (mature non-flowering plants, AMNF) and the SAB (mature non-flowering plants, SMNF) were evaluated. Both biotypes exhibited good activity against E. faecalis (AWAB, 0.78; SAB, 0.78), but only the AWAB exhibited good activity against K. pneumoniae (AWAB, 0.78; SAB, 1.56) and S. aureus (AWAB, 0.39, 3.12), showing that the AWAB extracts were more effective than those of the SAB. For the antifungal activity against C. albicans only the SAB exhibited good fungicidal (SAB, 0.78; AWAB, 1.56) and fungistatic (SAB, 0.78; AWAB, 1.56) activity. The results of a further investigation of the antimicrobial activities of the different growth stages of the SAB plant showed that all growth stages exhibited some level of activity against the tested bacterial and fungal strains, although young and mature non-flowering plants displayed the better activities. Phytochemical analysis revealed the presence of saponins, phenolics, flavonoids and condensed tannins in varying amounts in the leaf extracts of the AWAB and SAB plants but alkaloids were only present in the AWAB plant. Quantitative determination showed that the AWAB contained higher amounts of phenolics and flavonoids than the SAB, but condensed tannins were higher in the SAB than the AWAB. In comparing the levels of phytochemicals between the three growth stages of the SAB, mature non-flowering plants contained the highest amount of phenolics, flavonoids and tannins compared to the young and flowering plants. This showed that the antimicrobial activity displayed by the extracts of the various growth stages of the SAB could not be correlated with the levels of various phytochemicals present. The tetrazolium-based colorimetric (MTT) assay for cytotoxicity and Ames test for mutagenicity were used to evaluate the safety of the plant extracts prepared from the three growth stages of the SAB. The results from the cytotoxicity assay showed that the young plant extract which showed the best antimicrobial activity was more cytotoxic than the mature flowering and mature non-flowering plants. The Ames test using Salmonella typhimurium tester strains TA98 and TA102 without S9 metabolic activation revealed that all plant extracts of the three growth stages of the SAB were non-mutagenic towards the S. typhimurium strains tested. The screening of the plant extracts for pharmacological activity and phytochemical composition provided valuable preliminary information in that both the AWAB and SAB may be good sources of antimicrobial agents. This study further demonstrated that the leaf extract of the young and mature non-flowering plants of the SAB plants may be exploited for medicinal purposes. While the medicinal potential of the AWAB sub-type has been demonstrated in this thesis and by other workers, this is the first study that simultaneously examined the phytochemistry and pharmacological potential of the SAB plant. The results suggest that the SAB plant can be exploited in southern Africa as a source of traditional medicine. This serves the dual purpose of exploring a use for this burgeoning weed problem as well as finding a possible alternative to highly exploited plant species with the same medicinal potential.
- Masters Degrees (Botany)