The effect of water deficits on the growth of indigenous salvia species and the anti-fungal activity of their leaf extracts against Fusarium Oxysporum Schitdl. (Hypocreales), a causal agent of seedling blight in maize

Abstract

Synthetic chemicals come with negative implications on human health and also cause environmental degradation. There is a call for natural, environmentally friendly products to be used in the cultivation, storage, and processing of food crops. The laws and restrictions on some of the harmful synthetic chemicals are becoming stricter. The Republic of South Africa is rich in medicinal plants, of which its traditional healers use 3000 species to treat a plethora of illnesses and ailments. Most plants' healing properties are due or at least partly due to their phenolic compounds. One of the uses of secondary metabolites by the plants is for protection against pests and pathogens. Thus, humans can use these phytochemicals during crop production to manage diseases caused by phytopathogenic fungi and bacteria. Products utilising these natural compounds are the alternative to synthetic agricultural products. By producing these green products, South Africa can contribute to the global organic market worth multi-billions of US dollars, increases its GDP, and creates employment opportunities for citizens. South Africa is also a water-scarce country, which can be a limiting factor in crop production. Studies have shown that cultivating medicinal plants under water deficits enhances the quality and quantity of the same compound intended to be extracted to produce these green products. It is, therefore, why this study was initiated. Six species of indigenous Salvia were screened for their anti-fungal activity against Fusarium oxyproum, a phytopathogenic pathogen that causes seedling blight in Zea mays (Maize). Maize is one of the most important food crops in many countries across the globe. Poor stands due to seedling blight are financially devastating for commercial farmers, but it could mean starvation for subsistence farmers and developing countries. Fungicides are recommended for the management of seedling blight. However, the indiscriminate use of these chemicals causes the degradation of human health and the world's environment. In the first part of this study (chapter 2), the aim was to evaluate the anti-fungal activity of extracts against the phytopathogenic fungus F. oxysporum. The plant extracts were screened in vitro for their anti-fungal activity against F. oxysporum in a Minimal Inhibitory Concentration (MIC) assay. Extracts of all the six species inhibited the F. oxysporum at the 6, 12, 18, and 24 h post-treatment. The MIC values of the extracts ranged from 0.75 mg/ml and 12 mg/ml. However, S. dolomitica, S. namaensis, and S. repens with MIC values of 3.0, 4.2, and 9.6 mg/ml, respectively, continued to inhibit fungal growth beyond the 18th hour time period suggesting extracts from these species have fungistatic activities. S. chameleagnea showed the weakest activity compared to the other species in all four of the tested time slots. In contrast, S. dolomitica and S. namaensis showed the best activity was subsequently selected for further study. In conclusion, the six Salvia spp. studied are potential sources of anti-fungal extracts and compounds. In the second part of this study (Chapter 4), effects of different levels of water deficit treatments on the growth, concentration of polyphenols, and anti-Fusarium oxysporum activity of S. dolomitica and S. namaensis were assessed. Four-weeks old seedlings of the two species were subjected to 3-day, 6-day, 9-day, or 12-day water deficit regimes. Secondary metabolites such as polyphenols, alkaloids, and flavonols were assessed using spectroscopic methods, and the anti-fungal activities of crude extracts obtained from plants exposed to the various treatments were evaluated in a micro-dilution bioassay. In all treatments, the plant height, crown width, the number of stems and leaves, and fresh and dry weights reduced at increased intervals between irrigation. Acetone extracts from all treatments showed anti-fungal activity in MIC bioassay — the MIC values ranged from 0.75–6 mg/ml and 1.5–6 mg/ml for S. dolomitica and S. namaensis, respectively. However, extracts from the treatment with moderate water deficit (6-day watering interval) recorded significantly (P<0.01) better inhibition of F. oxysporum at the 18 h post-incubation than Mancozeb. This research has revealed that moderate water deficit level favours the accumulation of polyphenols, flavonols, and alkaloids in S. dolomitica. In contrast, severe water deficits treatment was associated with higher alkaloid content in S. namaensis extracts. Generally, total alkaloid content increased as the level of water deficits increased in both species. There is a possible correlation between this increase in plant secondary metabolites and the enhanced anti-fungal activity compared to the extracts of plants that received no water deficits. The final part of the study (Chapter 5) aimed to observe the effects of the acetone extracts of S. dolomitica plants on the germination of maize seeds when used as a seed treatment in the presence and absence of thy phyto-pathogenic fungus, F. oxysporum. The extracts were derived from plants subjected to four different water treatments (none, mild, moderate, severe), and each extracts having three concentrations (10, 20, 30 mg/ml). The germination bio-assay was conducted in sterilized closed systems in a research laboratory maintained at 25 °C for 14 days. Most of the extracts tested had no significant effects (P > 0.05) on the germination of the maize seeds; however, the extracts were associated with shorter shoot lengths and shoot lengths of the maize seedlings. The bioassays results indicated that the extracts of S. dolomitica might contain allopathic compounds that may be used to develop green alternatives to synthetic herbicides. The present study's findings pave the way for developing bioactive natural products as a green alternative to synthetic fungicides and using less water to cultivate these medicinal plants, which benefit a water-scarce country like South Africa.