Chemical Ecology of Allelopathic Interactions in Nigerian Savanna Agroecosystems: Isolation, Identification, and Bioactivity Mechanisms of Phytotoxic Compounds

Allelopathy, the chemical inhibition of plant growth by secondary metabolites released from neighbouring plants, is a significant but poorly characterised phenomenon in Nigerian savanna agroecosystems, where weed competition causes substantial yield losses. This dissertation presents an original chemical ecology investigation of allelopathic interactions in guinea and Sudan savanna agroecosystems of North Central and North West Nigeria, focusing on the isolation, structural identification, and mechanistic bioactivity characterisation of phytotoxic compounds from three dominant savanna species: Striga hermonthica, Sorghum bicolor, and Pennisetum pedicellatum. Phytotoxic metabolites were isolated by bioassay-guided fractionation using successive solvent extraction, column chromatography, and preparative HPLC. Structural elucidation employed 1D and 2D NMR spectroscopy, high-resolution mass spectrometry, and single-crystal X-ray diffraction for novel compounds. Bioactivity was evaluated in seed germination and root elongation assays against staple crops including cowpea, groundnut, and millet. Mechanisms of phytotoxicity were investigated using plant physiological assays including photosystem II efficiency measurement, reactive oxygen species quantification, antioxidant enzyme activity, and cell membrane integrity. Molecular docking studies against target plant enzymes were conducted for the most potent compounds. Eleven phytotoxic compounds were isolated, including three novel natural products contributing new structures to the chemical literature. Strigolactone analogues from Striga hermonthica showed mechanism-based inhibition of cowpea germination enzymes at nanomolar concentrations. The dissertation provides an original chemical ecological framework for understanding and potentially exploiting allelopathic chemistry to develop botanical herbicides for sustainable weed management in Nigerian savanna agriculture. Keywords: allelopathy, phytotoxic compounds, savanna agroecosystem, chemical ecology, Nigeria