From Membrane Destabilisation to Metabolic Collapse: Bioherbicidal Effects of Santolina chamaecyparissus L. Essential Oil on Bidens pilosa L.

The increasing incidence of herbicide-resistant weeds and concerns over synthetic herbicides are driving the search for natural alternatives. This study evaluated the post-emergence bioherbicidal activity and mode of action of Santolina chamaecyparissus L. essential oil against Bidens pilosa L. seedlings. Gas chromatography–mass spectrometry revealed a chemically complex oil dominated by oxygenated monoterpenes, including cuminic alcohol, cis-carveol, cis-pinocarveol, α-phellandrene, and limonene. A preliminary dose–response assay identified an ED50 of 0.82% (v/v). At this concentration, visible injury reached 75.9% at 72 h, while leaf relative water content declined to 40.0%. Electrolyte leakage reached 77.6% at 24 h, accompanied by a threefold increase in malondialdehyde, indicating severe membrane damage and lipid peroxidation. Chlorophyll fluorescence imaging showed impaired photosystem II function, reduced electron transport, altered energy dissipation, and loss of photosynthetic vitality. Photosynthetic pigments declined after 24 h, with chlorophyll a reduced by approximately 32.5%. Untargeted metabolomics revealed time-dependent depletion of soluble sugars, organic acids, ascorbate, and shikimate, together with accumulation of proline, γ-aminobutyric acid, trehalose, and branched-chain amino acids. Network analysis indicated loss of metabolic coordination. Overall, under the controlled experimental conditions used here, S. chamaecyparissus essential oil exhibited a rapid contact-type phytotoxic action characterised by membrane disruption, oxidative damage, dehydration, photosynthetic impairment, and extensive metabolic perturbation.