Influence of the Application of Different Zinc Oxide Nanoparticles on a Lettuce Crop Grown in an Acidic Mediterranean Soil

Abstract

Zinc (Zn) is a crucial micronutrient essential for the growth and development of crops. Recently, there has been growing interest in harnessing its benefits through the application of zinc oxide (ZnO) nanoparticles (NPs) as an alternative to conventional fertilizers. Different types of ZnO NPs were synthesized in the laboratory by a co-precipitation method using different precursor metal (Zn(NO3)2 for ZnO-1 and ZnSO4 for ZnO-2) and sources of hydroxyl groups (NaOH for ZnO-1 and NH4OH for ZnO-2) or by a sol–gel method, using ZnC4H6O4 (ZnO-3) or ZnSO4 (ZnO-4) as precursor metal. This study focused on the effect of these Zn sources on the impact on lettuce (Lactuca sativa L.) cultivation under acidic and Zn-deficient soil conditions. The efficacy of these Zn sources was evaluated by measuring the lettuce fresh weight, the stem diameter, the Zn concentrations in young leaves and mature leaves, the photosynthetic pigment content (chlorophyll and carotenoid), and the overall Zn status in the soil and soil pore water. The ZnO NPs with particle sizes of 76–104 nm positively affected the stem thickness (with an increase of up to 1.4 times that of the control) and crop biofortification with Zn (up to 3.2 and 12.6 times the Zn in young leaf and mature leaf, compared to the control). The smaller ZnO NPs (ZnO-1 and ZnO-3) showed the highest concentrations of bioavailable Zn and Zn in pore water at the end of the cultivation period (with an average increase of 41% compared to larger sources), resulting in high biofortification levels in both mature and young leaves. Peak concentrations of dissolved Zn in soil pore water were observed at 18 days after planting, followed by a decline attributed to the retention of Zn in more insoluble forms in the soil. The difference in Zn concentration between mature leaves and young leaves indicated its limited mobility in the plant, with more Zn accumulating in mature leaves.