Soil-applied polystyrene nanoplastics (PSNPs) remain cortically confined but trigger systemic oxidative and metabolic disruption in Zea mays L. seedlings.

Nanoplastics (NPs) are emerging soil contaminants, yet their phytotoxic effects under realistic exposure conditions remain poorly understood. This study evaluated maize (Zea mays L.) responses to soil-applied polystyrene nanoplastics (PSNPs; ~47 nm, zeta potential ζ = -65 mV) across 0.1-50 mg kg-1, spanning environmentally relevant to high-end concentrations. Growth responses were minimal at ≤0.1 mg kg-1 but became strongly inhibitory from 1 mg kg-1 onward. PSNP exposure reduced biomass and chlorophyll content, depleted soluble proteins and carbohydrates, and sharply increased proline and antioxidant enzymes-catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD). A severe decline in the reduced-to-oxidized glutathione ratio (GSH:GSSG) indicated a marked impairment of glutathione redox buffering. Confocal imaging showed that PSNP-associated fluorescence was restricted to the epidermal and cortical tissues, with negligible signal in the stele, suggesting that systemic effects may arise from root-localized stress rather than particle movement. Untargeted liquid chromatography-high-resolution mass spectrometry (LC-HRMS) metabolomics of shoots revealed coordinated downregulation of amino-acid-related metabolites and suppression of porphyrin/chlorophyll-pathway intermediates, alongside selective engagement of phenylpropanoid and flavonoid pathways-consistent with oxidative stress-driven metabolic reprogramming. Together, these findings suggest that soil-applied PSNPs disrupt maize growth and redox homeostasis via oxidative and signaling-associated processes, even in the absence of detectable vascular translocation. This study provides ecologically realistic evidence of PSNP-induced metabolic and physiological impairment in a major food crop and underscores the need to incorporate nanoplastic monitoring into soil health and agricultural sustainability frameworks.