Toxicity of S-metolachlor to non-target crop Vigna angularis: Influences from soil properties to molecular mechanism

S-metolachlor (ME) is a widely used pre-emergence herbicide, and yet there are frequent incidents of non-target crop damage caused by ME application in production practice. Understanding the factors and mechanisms influencing the toxicity of ME on non-target crops is crucial for ensuring the rational use of pesticides in agriculture. This study shows that while the permissible concentration of ME does not significantly affect the germination rate of Vigna angularis (V. angularis), it exerts an obvious inhibitory effect on the post-germination growth of V. angularis, such as fresh weight, total root length, and root tip number (p < 0.05), with the highest inhibition rate for above-ground fresh weight reaching up to 88 %. Among the three soil types (T1, T2, T3), T2 soil exhibited the most significant damage to V. angularis. Mantel correlation analysis reveals that this may be attributed to the lower cation exchange capacity, organic matter, and microbial biomass carbon levels in T2 soil, which impeded ME degradation. Consequently, higher residual ME concentrations contribute to more significant damage to V. angularis. Furthermore, combined metabolomic and transcriptomic analyses revealed that the damage to V. angularis caused by ME was primarily associated with pathways such as "tyrosine metabolism," "glycine, serine, and threonine metabolism," "phenylalanine metabolism," and "phenylpropanoid biosynthesis." The gene regulatory network and metabolic pathways involving 28 differentially abundant metabolites and 119 differentially expressed genes were associated with the metabolic and physiological processes related to ME damage in V. angularis. Based on the soil types examined in this study, ME appears unsuitable for use in fields cultivated with V. angularis. This study provides insights into the ME toxicity on non-target crops and offers strategic considerations for the application of herbicides in agriculture.