TiO2 nanoparticle exposure on lettuce (Lactuca sativa L.): dose-dependent deterioration of nutritional quality†
Titanium dioxide nanoparticles (TiO2 NPs) are among the most widely produced engineered nanoparticles (NPs) and are used in a number of fields. However, the impact of the released TiO2 NPs on nutritional quality of edible vegetable crops and the associated mechanisms responsible for those changes are poorly understood. In the present study, lettuce (Lactuca sativa L.) was exposed to 50% Hoagland's nutrient solution amended with 0, 50, 100, 200, or 400 mg L−1 TiO2 NPs for one week. The results showed that TiO2 NPs exposure at 50–400 mg L−1 negatively impacted the nutritional quality of lettuce. Specifically, 50–400 mg L−1 TiO2 NPs significantly decreased the content of Ca by 33.6–35.7%, Mg by 7.1–27.2%, Mn by 21.0–34.5%, Fe by 8.6–21.0%, Zn by 11.8–22.6%, and B by 27.3–44.1% in lettuce edible tissues, relative to the control. However, 50–200 mg L−1 TiO2 NPs exposure significantly increased the root and shoot dry biomass by 0.69–1.3-fold and 32.0–75.6%, respectively; the contents of chlorophyll a, total chlorophyll, and carotenoid were increased by 14.7–24.5%, 10.3–18.5%, and 12.6–20.5%, respectively, relative to the control. The reductions in nutrient content under 50–200 mg L−1 TiO2 NPs exposure were correlated with the significant increase in plant biomass, suggesting that the changes were a function of the biomass dilution effect. Alternatively, the nutrient content reduction at 400 mg L−1 TiO2 NPs did not result from this mechanism since plant growth (dry biomass) was unchanged. Here, cell structure damage including root cell membrane shrinkage and a disordering of the thylakoid arrangement resulting from strong oxidative stress occurred, which negatively affected nutrient uptake and led to nutrient content decreases. In addition, TiO2 NPs were adsorbed on root surfaces, which could also inhibit both water and nutrient uptake. Importantly, μ-XRF mapping showed that TiO2 NPs exposure did not change the nutrient distribution pattern in exposed leaf tissues. The findings from this work highlight that TiO2 NPs exposure negatively impacts the nutritional quality of lettuce, although the mechanisms of plant response vary with exposure level of TiO2 NPs.