Early-Exposure Legacy Effects of Polystyrene Nanoplastics on Lettuce: Trait-Specific Non-Monotonic Responses Mask Ionomic and Antioxidant Disruption

The prevalence of polystyrene nanoplastics (PS-NPs) in agricultural ecosystems necessitates a detailed understanding of their impacts on crop development, physiology, and nutritional quality. However, while toxicological effects of nanoplastics are increasingly documented, the integrated and dose-dependent consequences of short-term early exposure across later developmental stages remain poorly understood. In this study, the morphological, biochemical, and ionomic responses of hydroponically grown lettuce (Lactuca sativa L. cv. ‘Red Little Gem’) to 50 nm PS-NPs were investigated at concentrations of 0, 1, 10, and 100 mg/L. Importantly, plants were exposed to PS-NPs only during the initial 7-day germination phase and were then transferred to a clean hydroponic system for a further 28 days, allowing the assessment of early-exposure legacy effects or stress memory. A low dose of exposure (1 mg/L) improved early germination kinetics while increasing seedling vigor, shoot biomass, and root fresh weight. Exposure to high quantities (100 mg/L) caused significant physiological stress, including reduced antioxidant capacity, increased catalase activity, and proline buildup. Furthermore, mineral analysis revealed a severe disruption of ion homeostasis under high exposure, including a dose-dependent depletion of phosphate and a five-fold increase in calcium. At 100 mg/L, visible development, including sustained root biomass and maximum leaf area growth, disguised significant internal stress signs and nutritional deficits. The findings show that focusing solely on macroscopic morphological criteria is insufficient to establish crop safety and that multi-domain physiological and ionomic frameworks must be employed to estimate actual agricultural dangers of nanoplastic contamination.