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

Herbicide drift exposes nontarget plants to sublethal agrochemical doses, yet its ecological and evolutionary consequences remain poorly understood. Although hormesis - stimulatory responses to low doses of otherwise toxic compounds - has been documented following herbicide exposure, it has rarely been examined within an evolutionary ecology framework. Here, we examine herbicide-induced hormesis in the agricultural weed Oxalis stricta across two field experiments. We exposed replicated maternal lines to dicamba drift and quantified growth, reproductive traits, floral allocation, and pollinator visitation. Dicamba drift consistently increased flower production in both years, revealing a plastic shift toward reproductive allocation. Fitness consequences were context-dependent: 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. We detected little genetic variation in hormetic responses, suggesting limited potential for adaptive evolution. Dicamba drift also reduced individual flower size, indicating a shift toward larger floral displays composed of smaller flowers. These allocation shifts reduced pollinator visitation primarily through changes in flower number, linking herbicide exposure to trait-mediated changes in plant-pollinator interactions. Together, our results show that low-dose herbicide exposure can generate compensatory responses that reshape ecological interactions, even when evolutionary responses are constrained.