Microplastics are gaining increasing recognition as co-occurring xenobiotics in agricultural systems, yet their functional role in shaping plants' exposure to other contaminants remains insufficiently resolved. This study investigates whether aged polyethylene microplastics (PE MPs) behave as inert background particles or as active components of plant exposure scenarios.

Using Brassica napus L. cultivated under controlled hydroponic conditions, we examined the independent and combined effects of aged PE MPs (~500 µm) with two chemically distinct agrochemicals: tetracycline and a glyphosate-based ionic liquid. Despite the absence of particle internalization, PE MPs adhered to root surfaces and induced significant reductions in chlorophyll and carotenoid levels, accompanied by pronounced alterations in antioxidant enzyme activities (CAT, APx, POx), demonstrating that physical root–particle contact alone is sufficient to disrupt redox balance.

Sorption experiments revealed compound-specific partitioning behavior. Tetracycline and the surfactant cation showed measurable affinity for aged PE, resulting in reduced freely dissolved concentrations and partial attenuation of selected biochemical stress markers. In contrast, the glyphosate anion displayed limited sorption and retained strong oxidative impact in co-exposure treatments.

The results demonstrate that microplastics simultaneously function as stress inducers and as modulators of xenobiotic availability. Importantly, reductions in dissolved concentrations did not consistently translate into proportional mitigation of plant stress responses. These findings emphasize that microplastics should be considered active participants in contaminant exposure pathways rather than passive environmental debris, particularly in agroecosystems where multiple xenobiotics co-occur.