Impacts of Olive Pomace Stress on Vicia faba L.’s Growth, Secondary Metabolism, and Nutrient Uptake

Olive pomace (OP), an olive mill byproduct, poses environmental risks if mismanaged due to its high phenolic content, acidic pH, organic load, and electrical conductivity. This study evaluated the impact of olive pomace filtrate (OPF) at varying doses (OP-5, OP-10, OP-15) on broad bean (Vicia faba L.) growth, secondary metabolites, and nutrient accumulation. The highest OPF dose (OP-15) exhibited a clear negative, dose-dependent phytotoxic effect, causing stem discoloration, reduced root growth, necrosis, and chlorosis, while untreated controls showed vigorous growth. This significantly (p < 0.05) reduced leaf development, average number of leaves, and total leaf area, even at the lowest concentration (5%). Consequently, OP-15 reduced dry and fresh biomass by over 50% and shoot/root lengths by up to 61.55% compared to the control. Liquid chromatography mass spectrometry (LC-MS/MS) analysis revealed a positive dose-dependent effect of OPF on beneficial phenol and flavonoid accumulation, with significantly higher amounts of ferulic, isoferulic, caffeic, chlorogenic, and 4-hydroxybenzoic acids, as well as luteolin-4′-rutinoside and 4,7-dihydroxyflavone. OP application significantly (p < 0.05) decreased relative water content and increased electrolyte leakage and malondialdehyde, indicating stress. Furthermore, OP decreased the uptake of K, P, Fe, S, Zn, and Cu. Therefore, the intrinsic phytotoxicity of OPF suggests that mitigation measures are essential before considering environmental application to prevent potential adverse effects on sensitive crops and the wider ecosystem.