Effects of foliar application of zinc oxide nanoparticles in lettuce (Lactuca sativa L.) plants

Metallic nanoparticles (NPs) are increasingly incorporated into agricultural products. Foliar NPs sprays are being increasingly employed in agriculture, but the data regarding foliar uptake of NPs, associated biophysiochemical changes inside plants, and possible health hazards are limited. Zinc (Zn) is among the micronutrients with poor bioavailability, though this element is essential for the vital functions of plants. This study tested the effect of nano-enabled Zn (ZnO-NP) delivery via foliar application on Zn accumulation in addition to physiological and biochemical changes in lettuce (Lactuca sativa L. ‘Little Gem’). Plants were grown hydroponically in controlled environment growth chambers (PFD ‒ 220 µmol m^‑2 s^‑1, 18 h photoperiod, 21/17±2°C temperature, 60±5% relative humidity). Foliar sprays were applied using varying particle sizes of ZnO-NPs (18, 40 and 140 nm) at a concentration of 200 ppm. Foliar sprays of ZnO-NPs caused no visual phytotoxicity to lettuce leaves, resulted in improved fresh and dry biomass, leaf area, affected leaf nitrogen balance index (NBI), and macro- and microelement uptake. ZnO-NPs-treated lettuce leaves contained higher amounts of Zn. After foliar ZnO-NP sprays, phenolic compounds, antioxidant capacity (ABTS, DPPH, FRAP assay), and antioxidant enzyme (CAT, SOD, APX, GR) activity were significantly increased. These results indicate that the application of ZnO-NPs could be applied in lettuce production to improve yield, quality, and nutraceutical properties.