The effects of water deficiency and the fungus Beauveria bassiana on the bioactivity of Allium cepa (onion) extracts against grapevine mealybug (Planococcus ficus)

Abstract

For many years chemical insecticides have been utilized extensively in the agricultural sector to reduce the harm that insect pests inflict on plants. However, these substances are hazardous to humans and the environment. Studies that search for “green” alternatives that could replace synthetic insecticides are gaining significant attention. Biological control agents like fungi hold great promise as worthy alternative insecticides and plant growth stimulants. They are environmentally friendly and have proven efficacy against insect pest populations in greenhouse and field environments as well as their ability to enhance secondary metabolite production in plants. Furthermore, plants can develop symbiotic associations with some endophytic entomopathogenic fungus species, increasing the plants' resilience to abiotic stressors, such as water stress. Water deficiency is one of the most important abiotic factors limiting plant growth. However, despite the negative effects of water stress on plants, it can induce plants to produce more secondary metabolites. This study focuses on the singular and interactive effects of Beauveria bassiana inoculum and water stress on plant growth, secondary metabolite contents, antioxidant and repellent activities of Allium cepa extracts. The specific objectives were: (i) to determine the effects of water deficiency and the inoculation of a growth medium with B. bassiana conidia on the growth parameters (number of leaves, plant height, fresh and dry weights), (ii) to determine whether water deficiency and the fungus B. bassiana influence the nutrient contents of A. cepa tissues, (iii) to determine the variations in the profile of bioactive chemical constituents of A. cepa in response to water deficiency and the inoculation of growth medium with B. bassiana spores and (iv) to determine the anti-mealybug activities of A. cepa in response to water deficiency and inoculation of growth medium with B. bassiana spores. The first experimental part of the study, presented in chapter three, evaluated the interactions between water deficiency and the inoculation of a growth medium with B. bassiana on plant growth, tissue nutrient contents, secondary metabolites, and antioxidant capacity of Allium cepa. Allium cepa seedlings were also exposed to B. bassiana or a no-fungus treatment and to one of three watering regime treatments (3-day, 5-day, or 7-day watering intervals). Plants treated with B. bassiana performed better than those in the no-fungus treatment, despite the longest watering interval causing reduced plant growth. The interaction between the watering schedule and the fungus had a significant impact on the contents of P, K, and Fe. Interestingly, the B. bassiana-treated plants had a considerably higher polyphenol (64.0 mg GAE/L) at the 7-day watering interval than the plants that had not been exposed to fungus. Total flavonol is among the fungus-treated plants was significantly impacted by the watering interval. The quantity of flavonoids present in the onion bulbs and the antioxidant capacity in the FRAP bioassay were both substantially affected by the interaction between the watering interval and B. bassiana inoculation. In the second part of the study, presented in chapter four, the effects of water stress and B. bassiana on the volatile organic compounds and the repellent activity of extracts of Allium cepa (onion) against grapevine mealybug were evaluated. Simultaneously, fungus and no-fungus treatment A. cepa seedlings were applied to one of three watering regimes (3-day, 5-day, or 7-day watering intervals). After ten weeks, A. cepa bulbs were collected, crushed, and then extracted with dichloromethane (DCM) for 24 hours at room temperature (25 °C). The DCM extract was filtered and dried, and then reformulated in DCM to obtain the following concentrations: 20% w/v, 10% w/v, and 5% w/v. These concentrations were then tested in a disc-repellent bioassay. The disc-repellent bioassay showed that repellencies differed significantly across the treatments (fungus, no-fungus, positive and negative control), regardless of the watering interval. When extracts were evaluated at 20% w/v, B. bassiana inoculation of A. cepa plants considerably increased the repellent ability against grapevine mealybugs. Plant extracts from the fungal treatments demonstrated the strongest insect repellent efficacy. The Gas chromatography–mass spectrometry analysis of the DCM extracts revealed that the B. bassiana-inoculated plants' bulbs produced considerably more volatile organic chemicals than the plants without the fungus. The current study advances existing knowledge on the relationship of plants, fungi, and water deficiency in the contexts of plant response to stress and pest control.