Allelopathy and Allelochemicals: Sources, Mechanisms of Action, and Their Role in Integrated and Sustainable Crop Protection

Abstract

Allelopathy is an ecological process in which released chemicals modify the performance of neighbouring organisms, whereas phytotoxicity describes an inhibitory response to a substance under a defined assay and, by itself, does not demonstrate field-level allelopathy. This critical narrative review combines a structured re-screening of the literature with targeted updating through 21 August 2026. Searches used combinations of allelopath*, allelochemical*, phytotox*, bioherbicid*, hormesis, biostimulant*, rhizosphere, microbiome, formulation, resistance, and regulation; peer-reviewed studies were complemented by authoritative regulatory and resistance databases. Evidence was appraised according to experimental context and mechanistic strength, explicitly distinguishing field or whole-plant validation from controlled bioassays, experimentally supported molecular targets from physiological or omics associations, and computational predictions. The synthesis covers chemical diversity, botanical and microbial sources, agro-industrial by-products, mechanisms of phytotoxicity and low-dose responses, rhizosphere interactions, and formulation strategies. Important corrections emerge from this evidence hierarchy: strigolactones are carotenoid-derived apocarotenoid hormones/signals rather than sesquiterpenes; multi-target activity does not preclude resistance evolution; cyanobacterial responses cannot be directly extrapolated to weeds or crops; and hormetic stimulation under controlled conditions is not equivalent to reliable field biostimulant performance. The principal translational gaps are insufficient dose–response standardisation, limited crop-selectivity and field validation, variable biomass chemistry, incomplete carrier and non-target safety data, and regulatory uncertainty for multifunctional products. Allelochemicals therefore represent promising components of integrated crop protection, but their agronomic value depends on rigorous evidence, formulation-specific validation, and context-dependent deployment rather than on laboratory phytotoxicity alone.