Phytochemical Screening, in Vitro Antimycotic and Antioxidant Activities of Crude Extracts of Six Rwandan Medicinal Plants

Phytochemicals constitute reliable sources of antifungal and antioxidant compounds with low toxicity to mammals and safe to the environment which may serve as substitutes for synthetically produced chemicals. The present work aims at screening, and investigating the antimycotic and antioxidant activities of six medicinal plants, namely Croton macrostachyus (Euphorbiaceae), Clerodendrum (Rotheca) myricoides (Lamiaceae), Erucastrum arabicum (Brassicaceae), Melanthera scandens (Asteraceae), Senecio mannii (Asteraceae), and Senna didymobotrya (Fabaceae). A qualitative phytochemical screening was conducted by appropriate chemical methods. Antioxidant activity of 18 aqueous extracts has been tested by bleaching the solution of 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical. Antimycotic activity of all extracts was measured by the disc diffusion method on a potato dextrose agar plate against five fungal strains viz. Rhizopus stolonifer, Aspergillus niger, Aspergillus flavus, Aspergillus parasiticus and Fusarium oxysporum. The study revealed that the main phytochemical constituents of leaf, stem and root bark extracts were terpenoids, tannins, saponins, alkaloids. Furthermore, the results showed that leaf extracts were richer in antioxidant potentials than the rest of the plant parts tested. However, scavenging capacity of leaf extract was lower than that of the ascorbic acid for all extracts except C. macrostachyus and M. scandens which exhibited approximately the same values as ascorbic acid. The leaf extracts displayed the highest inhibition zone average for most of the fungi. The strong antimycotic activity ranged from 14 to 27 mm and was observed mainly in leaf extracts. The leaf extract of M. scandens was the strongest against F. oxysporum with 27.0 mm inhibition zone diameter. The root extract displayed the lowest average zones of inhibition such as 2.0 mm for E. arabicum extract against R. stolonifer and 3.0 mm for S. didymobotrya extract against A. niger. Antioxidant and antimycotic activities were probably attributed to the presence of different groups of phytochemicals as such as terpenoids, flavonoids, and tannins. From the results of this study, plant extracts with higher antioxidant and antimycotic activity could be further studied and eventually be used in the development of natural antioxidants and food preservatives that could replace the synthetic compounds.