Fulvic Acid Enhances Zinc Uptake and Grain Biofortification Efficiency of Low-Dose ZnO Nanoparticles in Wheat

Abstract

Zinc (Zn) deficiency is a widespread nutritional problem, and increasing Zn concentration in wheat grain through agronomic biofortification represents a practical strategy to improve dietary Zn supply. However, improving Zn-use efficiency while reducing nanoparticle inputs remains an important challenge. A factorial field experiment arranged in a randomized complete block design with three replications evaluated four wheat cultivars (‘Anaj-17’, ‘Akbar-19’, ‘Nawab-21’, and ‘Zincol-16’) under six ZnO nanoparticle (ZnO NP) and fulvic acid (FA) treatments. The treatments included a control, low-dose ZnO NPs (50 mg L⁻¹), high-dose ZnO NPs (100 mg L⁻¹), low-dose ZnO NPs combined with FA as a foliar application, FA alone, and ZnO NPs plus FA applied through irrigation. Cultivar, treatment, and cultivar × treatment interactions significantly influenced several physiological, Zn-related, and yield traits. Across cultivars, the foliar application of low-dose ZnO NPs combined with FA produced the highest mean grain Zn concentration (39.65 mg kg⁻¹), representing a 2.30% increase compared with the high-dose ZnO NP treatment (38.76 mg kg⁻¹), despite using a 50% lower ZnO concentration in the spray solution. The same treatment also increased leaf Zn concentration by 6.46%, total chlorophyll by 11.95%, carotenoid concentration by 25.00%, and grain yield by 6.71% relative to the high-dose ZnO NP treatment. Responses, however, differed among cultivars, with the magnitude and direction of the T3 response varying for grain Zn accumulation and yield-related traits. Overall, the results indicate that combining FA with a lower concentration of foliar-applied ZnO NPs can achieve Zn biofortification and productivity responses comparable to or greater than those obtained with the higher ZnO NP concentration, depending on cultivar.