Biodegradable nanoplastics pose a greater risk: Polylactic acid exceeds polystyrene in phytotoxicity and bioaccumulation in lettuce

Nanoplastics (NPs) pose a global threat to terrestrial ecosystems through plant uptake and phytotoxicity. While previous studies have focused on the size and charge of NPs, the differential effects of biodegradable versus conventional NPs remain unclear. This study investigated the impact of biodegradable (polylactic acid, PLA) and conventional (polystyrene, PS) NPs (both with a nominal diameter of 100 nm) on lettuce at concentrations up to 20 mg·L. Through integrated phenotypic, physiological, biochemical, and microanatomical analyses, we assessed the lettuce plant's response to these NPs. Lettuce roots absorbed both PS and PLA particles, transporting them to the stems and leaves via the vascular system. This process disrupted cellular and organellar structures. The accumulation followed a root > stem > leaf pattern. PLA particle concentrations were 2291, 286.9, and 9.18 mg·L in roots, stems, and leaves, respectively, while the corresponding concentrations of PS particles were 994, 47.9, and 9.12 mg·L. PLA particles triggered oxidative stress, resulting in cellular damage and subsequent growth inhibition. In contrast, PS particles showed lower uptake due to aggregation and exhibited minimal toxicity, with a slight stimulatory effect on biomass observed at 5-20 mg·L, potentially indicative of a hormetic response. In response to NPs stress, lettuce roots enhanced the key physiological processes of lignification and suberization. At 20 mg·L, root lignin content increased by 31.3% (PS) and 49.1% (PLA) compared to the control. These results contribute to a growing awareness of the potential risks associated with PLA-based biodegradable NPs.