Discovery of new PSII inhibitors: systematic investigation for herbicidal activity screening, properties, and mode of action

BACKGROUND The research focused on developing effective photosystem II (PSII) inhibiting triazine‐derived herbicidal compounds from the internal chemical library. Initially, the compounds were screened for their herbicidal activity using Spirodela polyrhiza (SPIPO) assays. Subsequently, compounds with stronger activity were synthesized on a gram scale and further investigated for their phytotoxicity, mobility, and mode of action. RESULTS In the SPIPO screenings, two compounds, K‐31706 and K‐31487, showed superior efficacy at concentrations exceeding 0.4 μm, surpassing the commonly used PSII inhibition herbicide, atrazine. Both compounds showed promising performance, especially in pre‐emergence applications under greenhouse conditions, with the application rate causing 50% biomass reduction below 100 g a.i. ha−1. However, both compounds exhibited strong phytotoxicity on major monocot and dicot crops, particularly when applied pre‐emergence. Absorption studies revealed that K‐31706 and K‐31487 have strong activity in both soil‐only and overtop applications, with root‐based absorption occurring in monocots like Digitaria ciliaris. K‐31487 was effective across various application methods on Abutilon theophrasti, indicating dual root and leaf absorption in dicots. Nevertheless, leaf excision assays indicated that both compounds act as contact herbicides rather than systemic ones. Chlorophyll fluorescence induction (OJIP) curve and molecular docking analyses confirmed PSII inhibition, which is comparable with simazine and diuron, with suppressed electron transport via interaction with QB site amino acids of D1 protein. CONCLUSION Overall, K‐31706 and K‐31487 are nonselective PSII inhibitors that block the electron transport in PSII, positioning them as potentially useful active ingredients for further investigation.