Antifungal activities of Bulbine frutescens extracts against Fusarium oxysporum

Abstract

Natural products from medicinal plants, either as pure compounds or as standardized extracts, provide unlimited opportunities for production of new and safer plant fungicides. Due to implications caused by synthetic chemical fungicides there has been an increasing interest and demand for searching for a natural way that will be less toxic to human beings and environmental health. Medicinal plants are considered to be a natural source of antifungal agents. Thus leading to an increasing demand for medicinal plants and its cultivation strategies that will yield high amount of secondary metabolites and favours growth and development of these plants. The proper cultivation strategy including growing media can play a major role in increasing plant metabolites, growth and development. Therefore, Bulbine frutescens was cultivated under varied growth media combinations with the aim of evaluating the plants growth and physiological response as well as to determine the optimum growing media for increasing B. frutescens secondary metabolites production. In chapters 1 and 2, the introduction and conceptual background and scientific realizations of the study are presented. In chapter 3, the research objective was to assess the effect of vermicompost, biochar and sand on plant growth and development in B. frutescens. Plant growth parameters such as number of leaves, leaf length, plant fresh weight, dry weight, root length, root weight and number of roots were recorded. Leaf samples were analysed for micro and macro nutrients. Results obtained showed a significant difference (P < 0.05) on the number of leaves, leaf length, plant fresh weight, dry weight, and number of roots among the treatments. When all the treatments were compared, T1 in the above mentioned parameters except for plant fresh and dry weight produced the best results among the treatments followed by T3, and T4, whilst T2 performed the least. Nutrient analysis results indicated that the treatments were significantly different (P < 0.05) on the uptake of N, P, Ca and Zn where T1 in N and Zn showed better uptake of nutrients in all the treatments. These results indicate that the different growing media have a better influence and also negative influence on growth of the plant depending on the ratio of the substrates incorporated. The nutrient uptake of B. frutescens was not influenced by the growth media. However, the treatment with high proportion of vermicompost indicated the best results among all other treatments. In chapter 4, the study was conducted to (i) To evaluate the in vitro antifungal activity of extracts from B. frutescens cultivated on vermicompost, biochar and sand. (ii)To identify bioactive compounds in the extract which have shown promising antifungal activity. At 10 weeks post treatment, plants were harvested, washed and air dried before preparing them for bisassays. The acetone extracts of B. frutescens were then screened for antifungal activity against F. oxysporum using minimum inhibitory concentration (MIC) method. A preliminary study for anti F. oxysporum of normal soil grown B. frutescens leaves was also piloted. The MIC values of extracts of B. frutescens leaves following exposure to different media combinations were not statistically different (P > 0.05) among treatments. However the MIC values at 6 hr of plants grown in T2 (0.62±0.12 mg/ml) showed high antifungal activity against F. oxysporum although it was not significantly different to other treatments and T1 (1.25±0.25 mg/ml) values were lower compared to other treatments (T2, T3, T4) respectively. There was no significant difference (P > 0.05) in the MIC values among the treatments at 12 hr. Nonetheless, with T2 (0.50±0.12 mg/ml) and T3 (0.50±0.12 mg/ml) exhibited high anti F. oxysporum activity. The positive control at 6 and 12 hr showed activity while the negative control did not show any activity. Additionally, there was no significant difference (P > 0.05) between the extracts of B. frutescens grown in normal soil and the one exposed to different growing media. The leaf samples obtained from different treatments showed that B. frutescens have the following secondary metabolites: polyphenols, flavonols and alkaloids. The polyphenols (mg GAE/g) were significantly different (P < 0.05) and high in T1 (9.316 ± 0.17) while there was no significant difference (P > 0.05) in flavonol (mg QE/g) content among treatments. However, T1 produced higher flavonol content compared to other treatments. There was a significant difference (P > 0.05) in alkaloids content with T1 (6.29mg/g) recording the highest content of alkaloids. These results indicate that B. frutescens has antifungal activities against F. oxysporum. This study also demonstrated that vermicompost, biochar and sand have a positive effect on antifungal activity of B. frutescens against F. oxysporum and the combination ration of 2:1:1 significantly favoured the accumulation of secondary metabolites.