Hormesis without evolution: plastic compensatory responses to herbicide drift in Oxalis stricta, a common weed of agriculture

O_LIHerbicide drift exposes non-target plants to sublethal doses of agrochemicals, yet its ecological and evolutionary consequences remain poorly understood. Although hormesis--defined as stimulatory responses to low doses of otherwise toxic compounds--has been documented following herbicide exposure, it has rarely been evaluated within an evolutionary ecology framework. Here, we integrate concepts of tolerance and overcompensation to examine herbicide-induced hormesis in the common agricultural weed Oxalis stricta across two field experiments. C_LIO_LIWe exposed replicated maternal lines to a drift-relevant dose of dicamba and quantified growth, reproductive traits, floral allocation, and pollinator visitation. Dicamba drift consistently increased flower production in both years, revealing a robust plastic shift toward reproductive allocation. However, the fitness consequences of increased flowering differed between years: in 2022, increased flowering was associated with higher seed production through indirect, trait-mediated pathways, whereas in 2023 increased flowering did not translate into detectable differences in reproductive output. Structural equation modelling indicated that dicamba effects on reproduction were largely indirect, mediated through correlated trait responses rather than direct stimulation of fitness. C_LIO_LIDespite consistent plastic responses, we detected little genetic variation in the magnitude of hormesis, suggesting limited potential for adaptive evolution of overcompensation. Dicamba drift also reduced individual flower size, indicating a shift toward larger floral displays composed of smaller flowers. These allocation shifts altered pollinator visitation patterns, primarily through changes in flower number, linking herbicide exposure to trait-mediated changes in plant-pollinator interactions. C_LIO_LITogether, our results demonstrate that low-dose herbicide exposure can generate repeatable compensatory responses that reshape ecological interactions, even when evolutionary responses are constrained. C_LI