Nanoparticles have unique physical and chemical properties, which can positively and negatively impact crop growth and tolerance to abiotic stresses. This study evaluated the potential of ZnO and SiO2 nanoparticles in alleviating salinity stress in hydroponically cultivated lettuce. Two-week-old lettuce seedlings (Lactuca sativa cv. Green Forest) were transplanted into a 5-L deep water culture system and grown for four weeks in a customized growth chamber set at 25°C with 230 µmol/m2/s PPFD. The nutrient solution was maintained at an electrical conductivity (EC) of 1.5 dS/m and pH 5.8, and replenished weekly. A factorial design was employed with four salinity stresses (non-saline, 50 mM NaCl, 33.3 mM CaCl2, 25 mM NaCl 16.6 mM CaCl2) and three nanoparticle treatments (no nanoparticle, 100 ppm ZnO, 100 ppm SiO2). Under non-saline conditions, both ZnO and SiO2 treatments showed no significant differences in shoot growth compared to the control plants. However, ZnO application reduced shoot biomass, leaf area, SPAD, chlorophyll fluorescence and net photosynthetic rate under CaCl2 and NaCl CaCl2 stress. SiO2-treated plants had higher SPAD than the control plants under CaCl2 stress but presented lower values under NaCl CaCl2 stress. Root growth also showed contrasting results based on the stress conditions. SiO2 application resulted in increased root dry weight, total root length and surface area under non-saline and CaCl2 stress, while they decreased under NaCl stress. Similarly, ZnO application enhanced root growth under non-saline conditions, but demonstrated negative effects under all salinity stress conditions. In conclusion, SiO2 nanoparticle application did not improve salinity tolerance in lettuce, except for root growth under CaCl2 stress, and ZnO nanoparticle treatments showed phytotoxicity in both shoots and roots under all salinity stress conditions.
