Herbicide hormesis can act as a driver of resistance evolution in weeds - PSII-target site resistance in Chenopodium album L. as a case study.

Background: Herbicide hormesis may play a role in the evolution of weed resistance by increasing resistance selection. A standard herbicide rate may be subtoxic to resistant plants and make them more fit than untreated plants. If this increase in fitness is ultimately expressed in reproductive traits, resistance genes can accumulate more rapidly and exacerbate resistance evolution by magnifying the selection differential between resistant and sensitive plants. The hypothesis of hormetically enhanced reproductive fitness was studied for a photosystem II (PSII) target-site resistant (TSR) biotype of Chenopodium album exposed to the triazinone metamitron in comparison with its wild-type.

Results: Both biotypes showed an initial hormetic growth increase at different doses leading to fitness enhancements of between 19% and 61% above untreated plants. However, hormetic effects only resulted in higher fitness at maturity in resistant plants with a maximum stimulation in seed yield of 45% above untreated plants. Applying realistic metamitron rates, reproductive fitness of resistant plants was increased by 15-32%.

Conclusions: Agronomically relevant doses of metamitron induced considerable hormesis in a PSII-TSR C. album genotype leading to enhanced relative fitness through reproductive maturity. This increase in relative fitness suggests an impact on resistance selection and can compensate for the oft-reported fitness costs of the mutation studied. Field rates of herbicides can, thus, not only select for resistant plants, but also enhance their reproductive fitness. The finding that herbicide hormesis can be eco-evolutionary important may have important implications for understanding the evolution of herbicide resistance in weeds. © 2018 Society of Chemical Industry.