Comprehensive Analytical Studies on the Efficiency of Foliar Application of Nanofertilizers

The study investigated the effects of foliar-applied nano fertilizers containing nanoparticles (NPs) of Cu, Zn, and Fe, in both non-complexed form and as urea-aqua nanometal complexes (UAM), on physiological and biochemical parameters of Champion apple trees planted in 2016. Metal sulfates served as controls. Foliar applications were performed four times during the growing season, with leaves sampled in mid-July, shortly after full development. Photosynthetic activity, fruit quality, and heavy metal content were assessed. Metal concentrations in nanomaterials were quantified by iodometric and complexometric methods. Chlorophylls a, b, and carotenoids were measured colorimetrically, soluble solids content (SSC) refractometrically, and sugars and titratable acids titrimetrically. The sugar-acid index (SAI) was calculated accordingly. Cu²+, Zn²+, Cd²+, and Pb²+ in apples were determined by inversion chronopotentiometry. UAM applications (200 and 400 ml/ha) enhanced chlorophyll a and b concentrations, increased SSC by 1.45–3.57%, and improved fruit quality (SAI increased by 1.48–7.85 units). Conversely, treatments with Cu²+, Zn²+, and Fe²+ sulfates negatively affected photosynthetic pigments and apple quality. Importantly, UAM treatments reduced Cd²+ and Pb²+ accumulation in apples, whereas sulfate treatments increased these toxic elements to or above maximum permissible concentrations (MPC). Thus, foliar application of Cu, Zn, and Fe UAM (200 and 400 ml/ha) optimizes mineral nutrition, enhances photosynthesis, improves apple quality, and reduces contamination by toxic heavy metals compared to sulfate fertilizers.