Damage and recovery from drift of synthetic-auxin herbicide dicamba depends on concentration and varies among floral, vegetative, and lifetime traits in rapid cycling Brassica rapa

Herbicides can drift from intended plants onto non-target species. It remains unclear
how drift impacts plant functional traits that are important for fitness. To address this
gap, we conducted an experiment where fast cycling Brassica rapa plants were
exposed to one of three drift concentrations (0.5%, 1%, 10%) of synthetic-auxin
dicamba. We evaluated damage to and capacity of floral and vegetative traits to
recover as well as lifetime fitness by comparing treated plants to controls. Response to
dicamba exposure was concentration-dependent across all traits but varied with trait
type. At 0.5% dicamba, three out of five floral traits were affected, while at 1%
dicamba, four floral traits and one out of two vegetative traits were negatively impacted.
At 10% dicamba all floral and vegetative traits were stunted. Overall, floral traits were
more responsive to all dicamba drift concentrations than vegetative traits and displayed
a wide range of variation ranging from no response (e.g., pistil length) to up to 84%
reduction (ovule number). However, despite floral traits were more affected across the
dicamba drift concentrations they were also more likely to recover than the vegetative
traits. There was also variation among lifetime traits; the onset of flowering was
delayed, and reproductive fitness was negatively affected in a concentrationdependent
manner, but the final biomass and total flower production were not affected.
Altogether, we show substantial variation across plant traits in their response to
dicamba and conclude that accounting for this variation is essential to understand the
full impact of herbicide drift on plants and the ecological interactions these traits
mediate.