Context dependence shapes the evolution of glyphosate and herbivory resistance in Ipomoea purpurea

Plant defense evolution has often been examined under single-stressor scenarios, overlooking the complex, interacting stressors of natural environments. Recent research incorporating multiple stressors has largely emphasized physiological and molecular responses rather than evolutionary dynamics, leaving such dynamics less explored. To address this gap, we examined the potential for fitness costs and benefits to the herbicide glyphosate and insect herbivory in Ipomoea purpurea (common morning glory). Through a factorial field experiment, we manipulated glyphosate and the insecticide spinosad to estimate selection on glyphosate and herbivory resistance. We found that glyphosate resistance was under positive selection with no detectable fitness costs, whereas herbivory resistance showed neither costs nor benefits. Notably, positive correlational selection favored higher glyphosate and herbivory resistance in the combined-stressor environment, suggesting these traits may be genetically linked. Further, the strength of positive selection on glyphosate resistance depended on the environmental context of herbivory. Together, these results indicate that selection for glyphosate resistance predominantly drives evolution in this system and may promote increased herbivory resistance. By applying a fitness cost-benefit framework to multiple interacting stressors, our study underscores the context-dependent nature of selection on plant defense traits and highlights how combined stressors can shape their evolutionary trajectories.