The bioplastic paradox: Biodegradable microplastics trigger species-specific nitro-oxidative stress and root growth inhibition in Brassicaceae

• Biodegradable microplastics induce species-specific nitro-oxidative stress • Oilseed rape maintained redox stability through adaptive antioxidant defenses • Garden cress showed nitrosative imbalance and impaired detoxification capacity • Protein nitration verified irreversible stress in sensitive garden cress • Nitro-oxidative biomarkers offer sensitive tools for bioMP phytotoxicity screening Biodegradable plastics are increasingly being used in agriculture as alternatives to conventional polymers, yet their environmental safety remains poorly understood. Although marketed as environmentally friendly, many degrade incompletely and persist as microplastics, raising concerns about their biological effects. Here we assess how two biodegradable microplastic formulations influence early root development and nitro-oxidative stress in two Brassicaceae species, oilseed rape ( Brassica napus ) and garden cress ( Lepidium sativum ), under in vitro exposure at 0.5% and 1% (w/v). Oilseed rape maintained root growth and redox balance through coordinated activation of antioxidant enzymes and non-enzymatic defenses, with limited protein nitration. In contrast, garden cress exhibited strong growth inhibition, elevated reactive oxygen and nitrogen species, extensive protein nitration, and suppressed antioxidant responses. Increased metabolic activity under high-dose exposure was consistent with stress-related metabolic adjustment rather than recovery. These results reveal that biodegradable microplastics can induce species- and polymer-specific nitro-oxidative stress even at realistic exposure levels. The findings highlight nitrosative biomarkers as sensitive indicators of sub-lethal phytotoxicity and provide a mechanistic basis for improving ecological risk assessments of biodegradable plastics in agriculture. Biodegradable plastics are promoted as sustainable alternatives to conventional polymers, but their environmental safety is often assumed rather than demonstrated. This study shows that even short-term exposure to biodegradable microplastics can induce species- and polymer-specific nitro-oxidative stress in plants. Sensitive species such as garden cress exhibit clear biochemical and metabolic disruption, highlighting the need for caution in the adoption of such materials in agriculture. These findings underscore the importance of including nitro-oxidative biomarkers and species sensitivity screening in future risk assessments and regulatory frameworks addressing biodegradable plastic use in soil-based environments.