Characterisation of Essential Oil Compounds and Optimisation of Water and Potassium for Production of Lantana camara (L.) for Tuta absoluta Management

Abstract

Tuta absoluta (Meyrick) is a serious threat to the sustainable production of tomatoes (Solanum lycopersicum L.) in Kenya. Tomato is one of the most important vegetables grown in Kenya, playing a critical role in meeting food nutritional requirements and creating employment. The pest causes up to 100% loss of production leading to reduced income and loss of livelihood. In the effort to control the pest, synthetic insecticides have been rampant, posing serious environmental and health risks. Lantana camara on the other hand is an invasive weed commonly found in tropical and sub-tropical ecosystems, including Kenya. It is a multipurpose plant with copious secondary metabolites and numerous biological activities that can be exploited in the control of T. absoluta. However, the synthesis of the secondary metabolites is significantly influenced by the ecological aspects and agronomic practices during growth and development, thereby inducing variations in quantity, quality, and distribution of the active compounds. The objectives of this study were to; characterise the secondary metabolite profiles of the leaf essential oils of Lantana camara from six different agro-ecological zones (AEZ) and soils in Kenya, evaluate the efficacy of the leaf essential oils of L. camara against T. absoluta, and assess the influence of potassium nutrients and water application rates on the oil yield and secondary metabolite profiles of L. camara. The leaf and soil samples and corresponding monthly climatic data were sampled at 8-9 a.m. from six representative AEZs namely; Lower Highland-Njoro (LH-NJ), Upper Midland 1-Kakamega (UM1-KK), Upper Midland 2-Kandara (UM2-KA), Upper Midland 3-Embu (UM3-EM), Lower Midland-Kiboko (LM-KI) and Coastal Lowland-Mtwapa (CL-MT) located in six counties of Kenya during the wet (May) and dry (September) seasons (2018, 2019 and 2020). The essential oils were extracted from the plant leaf samples by steam distillation and analysed through GC-MS (Gas Chromatography-mass Spectrometer). The GC-MS data were analysed with environmental variables (soil and climate data) using unimodally constrained and unconstrained ordination methods for untargeted metabolomics analysis. Tomato (Rio Grande VF) plants (L. esculentum) were grown in the greenhouse conditions in 2-litre pots, inside large insect‐proof cages and used for rearing Tuta absoluta and for performing the bioassays experiments. The larvicidal and repellent activity of essential oils against T. absoluta was performed using six different oil treatments from the six AEZs collected in May 2018. The larvicidal activity was performed using the leaf-dip bioassay protocol, while the repellency activity was performed using the repellent response method for phytophagous pests, and data were analysed using the ANOVA test and Probit analysis. Lastly, a greenhouse experiment studied the effects of potassium supplementation and water regimen on essential oil content and Lantana camara composition. Cuttings were collected from Lower Midland-Kiboko (LM-KI) and propagated under the greenhouse. Potassium was supplied at five rates as muriate of potash (MOP) (0.00g (F1), 0.25g (F2), 0.51g (F3), 0.76g (F4) and 0.81g (F5)) and three watering level (10% (W1), 40% (W2) and 80% (W3)) field capacity. The results showed that regional and seasonal variability was observed for secondary metabolites (SMs) in the leaf essential oil, which correlated to soil attributes and climatic factors. The study highlights the seasonal-geographic metabolism relationship for L. camara and the combined analytical method to obtain data that contributes to understanding the environmental factors' influence on the secondary metabolites' accumulation and synthesis. The bioassay test showed that L. camara essential oil has good larvicidal activity with higher mortality (89%) on the 2nd instar larvae with a higher concentration (0.01µl/µl oil/o.1% Tween 20). The repellence test also showed a higher average repellence (93.44%) effect with a higher concentration (0.01µl/µl oil/0.1% Tween 20) of the essential oils. The potassium nutrition supplementation and water regime treatments affected the essential oil yield from the leaves. The highest essential oil content (0.76%) was obtained in the lowest moisture (10% VMC (W1)) and potassium supplement levels (0.00g (F1)). Whereas the highest moisture (80% VMC (W3)) and potassium supplement levels (0.81g (F5)) produced the lowest essential oil content (0.34%). Sixty-eight major compounds were identified in the essential oil from the grown plants. The metabolites synthesised with low moisture content comprised most of the metabolites detected at five different potassium treatments (79.36-94.46%) compared to those treated with optimum moisture level and potassium treatment (69.77-63.24%). The potassium and watering treatments affected the production of the secondary metabolites. The results showed that increasing the water and potassium levels showed negative and positive correlations on different metabolites, indicating the effect is metabolite-dependent. The research has shown that the essential oil of Lantana camara may be a sustainable, eco-friendly alternative for synthetic insecticide in the T. absoluta management program. Due to the variability in active compounds, one could choose the location of plant growth for a particular compound or add/omit nutrients to modify the secondary metabolites. Therefore, this exploratory analysis of L. camara was able to deduce that the environmental and soil variables and water and fertilizer can modulate metabolite accumulation. Lastly, developing agro techniques for local production is essential for assuring the production of desired metabolites in L. camara plant leaf essential oil for exploitation in commercial production for pest management.