Metabolic response of lettuce (Lactuca sativa L.) to polystyrene nanoplastics and microplastics after foliar exposure†

Plastic particles deposited from the atmosphere can be absorbed by crops and have significant effects on crops. However, current knowledge regarding the particle size effects on the phytotoxicity of airborne plastic particles to leafy vegetables is limited. Therefore, in the present study, we examined the effects of foliar exposure to polystyrene nanoplastics (PS NPs, 100 nm) and polystyrene microplastics (PS MPs, 1 μm) of different concentrations on the biomass, physiological and biochemical indexes (i.e., chlorophyll, antioxidant enzyme and malonaldehyde (MDA), nutritional quality), and the metabolism of lettuce (Lactuca sativa L.). PS concentration, rather than particle size, exerted significant effects on these physiological and biochemical indexes. Both PS NPs and PS MPs at the high concentration of 35.0 mg L⁻¹ decreased the contents of chlorophyll a, chlorophyll b, and total chlorophyll in lettuce leaves. Further, the nutritional quality of lettuce leaves was generally improved as evidenced by the increased soluble protein and soluble sugar as well as the decreased nitrate. The decrease of superoxide dismutase activity and accumulation of MDA suggested oxidative stress induced by PS NPs and PS MPs. Metabolomics analysis showed that foliar exposure to PS NPs disturbed the energy metabolism, glutathione metabolism, and ABC transporter, whereas PS MPs perturbed the lipid metabolism and cutin, suberin and wax biosynthesis in lettuce leaves. The different metabolic responses between PS NP and PS MP treatments highlighted the importance of particle size in investigating the phytotoxicity of airborne plastic particles. These results provided effective information for the risk assessment of airborne plastic particles.