Application of insecticides on peppermint (Mentha × piperita L.) induces lignin accumulation in leaves by consuming phenolic acids and thus potentially deteriorates quality.

Irrational use of pesticides may lead to physiological and metabolic disorders in different crops. However, there are limited investigations on impacts of insecticides on physiology and biochemistry, secondary metabolic pathways, and associated quality of medicinal plants such as peppermint (Mentha × piperita L.). In this study, target metabolites in peppermint were monitored following foliar spraying of five insecticides: imidacloprid, pyriproxyfen, acetamiprid, chlorantraniliprole, and chlorfenapyr. Compared with the control, all insecticide treatments caused a significant loss of soluble protein (decreased by 22.3-38.7%) in peppermint leaves. Insecticides induced an increase in the levels of phytohormones jasmonic acid and abscisic acid in response to these chemical stresses. Among them, imidacloprid increased jasmonic acid by 388.3%, and pyriproxyfen increased abscisic acid by 98.8%. The contents of phenylpropanoid metabolites, including rutin, quercetin, apigenin, caffeic acid, 4-hydroxybenzoic acid, ferulic acid, syringic acid, and sinapic acid showed a decreasing trend, with pyriproxyfen decreasing the levels of quercetin and 4-hydroxybenzoic acid by 78.8% and 72.6%, respectively. Combined with correlation analysis, the content of lignin in leaves shows different degrees of negative correlations with several phenolic acids. It could be inferred that insecticides may trigger plant defense mechanisms that accumulate lignin (increased by 24.6-49.1%) in leaves by consuming phenolic acids to barricade absorption of insecticides. Through constructing networks between phytohormones and secondary metabolites, peppermint may regulate the contents of caffeic acid, 4-hydroxybenzoic acid, and sinapic acid by the antagonistic effect between salicylic acid and abscisic acid in response to insecticidal stresses. Principal component analysis and systemic cluster analysis revealed that the most pronounced changes in physiological indexes and metabolites were caused by the pyriproxyfen treatment. In conclusion, this study improves our understanding of the mechanism by which insecticides affect plant physiological and metabolic processes, thus potentially altering the quality and therapeutic value of peppermint as an example.