Transgenerational responses of invasive cocklebur species to microplastic stress: Stage specificity and interspecific divergence

Microplastic pollution and plant invasions are converging threats to terrestrial ecosystems. Whether microplastics elicit adaptive transgenerational responses in invasive plants-rather than exerting uniform toxicity-remains untested. We hypothesize that parental microplastic exposure alters offspring performance, with the direction and magnitude of effects depending on polymer type, particle size, and abundance. Using a three-factor fully randomized design, we exposed three invasive cocklebur species (Cyclachaena xanthiifolia, Xanthium italicum, and Xanthium spinosum) to polyethylene terephthalate (PET), polyethylene (PE), and polypropylene (PP) at three sizes (5, 50, and 500 μm) and three abundances (519, 2074, and 4148 items kg^-1), and quantified responses across parental life-history stages and F₁ germination. Contemporary responses were stage-specific: germination in C. xanthiifolia and X. italicum was governed by three-way interactions, whereas later traits shifted toward polymer-type-dominated effects; X. spinosum showed size-dominated germination and type- or type × size-dependent later traits. Transgenerational effects diverged among species: C. xanthiifolia exhibited size-mediated F₁ facilitation concurrent with type-driven parental inhibition; X. italicum showed direct, type-dependent facilitation; and X. spinosum showed no systematic transgenerational transmission. PLS-SEM supported these species-specific pathways. Because F₁ performance was assayed only under control conditions, these patterns are consistent with-but do not demonstrate-a predictive adaptive response. Our results indicate that microplastics can act as species-specific modulators of invasive plant life histories across generations, highlighting the need to incorporate multi-factor, stage-specific, and transgenerational endpoints into microplastic risk assessment.