Due to increased food demand, the use of herbicides is both necessary and on the rise. Several herbicide classes target photosynthetic electron transport: Herbicide Resistance Action Committee (HRAC) Groups 5, 6, and 22. These herbicides are used in large amounts in many different cropping systems to control several species of broadleaf and grass weeds. This article provides a comprehensive review of what these photosynthesis inhibitors are, how they are used, and their modes of action. Presently, commercial herbicides only inhibit electron flow at two different sites: photosystem II (PSII) and photosystem I (PSI). Herbicides that inhibit electron flow at PSII block the movement of electrons down the electron transport chain, while those that inhibit electron flow at PSI accept electrons. Necrosis developing on the leaves of plants treated with PSII and PSI inhibitors is due to the accumulation of reactive oxygen species. Evolution of resistance, toxicity concerns, and other limitations of these herbicides call for the exploration of new chemistries that can be used to target this pathway.

A goosegrass [Eleusine indica (L.) Gaertn.] population uncontrolled by paraquat (R) in a vegetable production field in St. Clair County, AL, was collected in summer 2019. Research was conducted to assess the level of resistance of the suspected resistant population compared with three populations with no suspected paraquat resistance (S1, S2, and S3). Visual injury at all rating dates and biomass reduction at 28 d after treatment (DAT) of S populations occurred exponentially to increasing paraquat rates. S biotypes were injured more than R at 3 DAT, with biomass recovery at 28 DAT only occurring at rates <0.28 kg ha−1. Plant death or biomass reduction did not occur for any rate at any date for R. Paraquat rates that induced 50% or 90% injury or reduced biomass 50% or 90% compared with the non-treated (I50 or I90, respectively) ranged from 10 to 124 times higher I50 for R compared with S and 54 to 116 times higher I90 for R compared with S biotypes. These data confirm a paraquat-resistant E. indica biotype in Alabama, providing additional germplasm for study of resistance to photosystem I electron-diverting (PSI-ED) resistance mechanisms.

Herbicides inhibit biochemical and physiological processes or both with lethal consequences. The target sites of these small molecules are usually enzymes involved in primary metabolic pathways or proteins carrying out essential physiological functions. Herbicides tend to be highly specific for their respective target sites and have served as tools to study these physiological and biochemical processes in plants (Dayan et al. 2010b).