Size-, Shape-, and Number Concentration-Dependent Nanoplastics Accumulation and Growth Responses in Lettuce

Understanding the ecological impacts of nanoplastics requires evaluation metrics beyond conventional mass-based concentrations. In this study, we investigated the generation, characterization, and phytotoxic effects of environmentally relevant plastic particles in hydroponically grown lettuce (Lactuca sativa), focusing on particle size, shape, and number concentration. Low-density polyethylene (LDPE) was degraded using an advanced oxidation process, demonstrating that substantial oxidative degradation is required for the formation of nanoplastics; the resulting LDPE particles exhibited a broad size distribution ranging from the nanoscale to the micrometer scale, containing nanoscale domains (peak size ~20 nm, average size ~30 nm), and showed predominantly ellipsoidal morphologies derived from cross-linked polymer regions. In contrast, polystyrene (PS) particles of defined sizes (~600 nm and ~2000 nm) were prepared via mechanical fragmentation, exhibiting sharp-edged, flake-like morphologies. Laser scanning microscopy revealed uptake and translocation of LDPE particles from roots to aerial tissues, whereas larger PS particles showed limited transport. Growth inhibition analysis based on particle number concentration (1010–1017 particles/mL) showed a stepwise concentration–response relationship for LDPE particles, with inhibition levels increasing from approximately ~30% at low concentrations to high levels of inhibition at the highest concentrations In contrast, PS particles caused significant root damage (e.g., clear surface disruption observed in microscopy) and growth inhibition (~30–40%) even at relatively low number concentrations (~1010–1012 particles/mL), likely due to their sharp-edged morphology. Overall, plant responses to plastic particles were governed by an interplay of size, shape, and number concentration, highlighting the importance of particle morphology and concentration metrics in agroecosystem risk assessment.