Synthesis and Characterization of Nano Zinc Oxide Foliar Fertilizer and Its Influence on Yield and Postharvest Quality of Tomato

Nanoparticles of ZnO were synthesized via an alkaline precipitation technique that is ideal for large-scale and cost-effective production, using ZnSO4 and ZnCl2 as starting materials. Scanning electron microscopy (SEM) analysis revealed that rod-shaped ZnO crystals with nanoscale dimensions were produced from both precursors, but ZnCl2 produced smaller and less aggregated crystals compared to ZnSO4. Using ZnCl2 as starting material, we refined the synthesis parameters by varying the solution pH and temperature, drying and calcination temperature, and adding cationic surfactant. Scanning electron microscopy - energy dispersive x-ray spectroscopy (SEM-EDS) analysis verified the nano size of ZnO (average of 45 nm via SEM), with low degree of aggregation, and high chemical purity (59.96% Zn via EDS). X-ray diffraction (XRD) patterns identified hexagonal wurtzite structure with high crystallinity. Mineralogical analysis by X-ray fluorescence (XRF) showed that the nano ZnO were free from impurities (90.39% ZnO) while FTIR analysis authenticated the presence of Zn and O bonds. There was sufficient evidence to conclude that nano ZnO with high chemical purity, and suitable characteristics as Zn foliar fertilizer source, was successfully synthesized using ZnCl2 as starting material. The synthesized nano ZnO (SNZ) was tested as foliar fertilizer on pot-grown 'Marimar' F1 tomato, and compared with bulk ZnO (BZ), commercial nano ZnO (CNZ), and granular zinc sulfate (ZS). Boric acid was incorporated to a separate set of the Zn foliar treatments (SNZ+B, BZ+B, CNZ+B, ZS+B) to determine the synergistic effects of Zn and B on the growth and yield of tomato. SNZ+B resulted in positive responses in Zn and B uptake and dry matter yield. Marketable yield was found to be significantly highest in SNZ+B. Statistically, SNZ+B was either better than or equal to the SNZ and CNZ+B treatments in terms of the growth and yield parameters. SNZ+B also led to significant improvements in fruit quality, including higher TSS, %TA, and ascorbic acid content. SNZ enhanced the agronomic effectiveness of Zn foliar fertilizers as evident in the improvements in yield components of tomato. Nanoscaling made ZnO more available to stomates, making nutrient use more efficient. It also resulted in increased number of particles per unit weight of applied Zn, while increasing the specific surface area and solubility of ZnO in water, thus enhancing plant uptake which ultimately led to improvements on yield as well as postharvest quality.