Effect of zinc oxide nanoparticles (nZnO) on antioxidant defense, lignin metabolism and cadmium subcellular distribution in lettuce (Lactuca sativa L) under low-dose cadmium stress (hormesis)

Most studies on cadmium (Cd) have focused on its toxicity at high concentrations, while its hormetic effects at low doses remain underexplored. Lettuce (Lactuca sativa L) is cultivated on a wide scale around the world, and its leaves continue to exhibit a high capacity for Cd accumulation at trace concentrations, thereby posing a significant threat to human health. This study investigated the role of foliar-applied zinc oxide nanoparticles (nZnO, 50 μmol l-1 and 100 μmol l-1) in mitigating low-dose cadmium (Cd) stress in lettuce. Exposure to 2.5 μmol L-1 Cd significantly enhanced lettuce growth, demonstrating a classic hormetic response. However, this growth stimulation was accompanied by Cd accumulation in leaves (4.45 mg kg-1 DW, 0.22 mg kg-1 FW), exceeding the FAO/WHO safety limit for edible vegetables (0.2 mg kg-1 FW). Foliar application of nZnO significantly reduced the levels of reactive oxygen species (ROS) and malondialdehyde (MDA) in lettuce. This was accompanied by increased activities of antioxidant enzymes and elevated endogenous hormone levels, which collectively contributed to enhanced lettuce growth. Furthermore, it was demonstrated that nZnO led to a substantial reduction in Cd accumulation in lettuce tissues. This reduction was attributed to an increase in total phenolic content and changes in the subcellular distribution and chemical forms of Cd. Our results indicate that foliar application of zinc oxide nanoparticles is a viable strategy for reducing Cd accumulation in lettuce and other leafy vegetables under Cd-induced hormesis, thereby ensuring food safety.