Zinc-Oxide-Nanoparticles in Conjugation with Zn-Solubilizing Bacteria Improve Zn Biofortification and Nitrogen Use Efficiency in Wheat

Soil zinc (Zn) deficiency is a major cause of Zn-malnutrition, low yields, and low nitrogen use efficiency (NUE) in wheat. Improving grain Zn concentration and NUE in wheat without compromising yield has become a global concern. A study was therefore conducted to explore the potential of Zn-solubilizing bacteria (ZnSB) and Zn oxide nanoparticles (ZnONPs) for improving Zn biofortification and nitrogen use efficiency (NUE) in wheat. Two strains of ZnSB (Pseudomonas aeruginosa (YZn1) and Stenotrophomonas maltophilia (WZn1)) were isolated from field soil and selected for study based on Zn solubilization efficiency, IAA production, and Zn release efficiency. The potential of soil and foliar applications of ZnONPs separately, or in combination with consortia of ZnSB, to enhance wheat Zn concentrations, productivity and NUE was evaluated. The treatments tested were: Control (T₁), ZnSB (T₂), ZnSO₄ (soil application; T₃), ZnONPs (foliar application; T₄), ZnSB + ZnONPs (soil and foliar applications respectively; T₅), and ZnONPs + ZnSB (soil applications of both) + ZnONPs (foliar application) (T₆). Soil application of ZnONPs when combined with ZnSB and a foliar application of ZnONPs (T₆) significantly (P ≤ 0.05) improved yield and yield traits compared to the control (T₁) and ZnSO₄ (T₃) treatments. Notably, T₆ increased chlorophyll SPAD value, 1000-grain weight, grain yield, harvest index (HI), and grain Zn concentration by 27.61%, 29.63%, 53.54%, 23.07%, and 89.06% respectively, over control (T₁). The T₆ treatment also increased grain zinc concentration and yield relative to T₃ by 20.95% and 6.12% respectively. The NUE was also increased in response to T₆, with significantly higher nitrogen physiological efficiency (48.79 g g^− 1), agronomic use efficiency (27.08 g g^− 1), nitrogen harvest index (80.84%), and partial factor productivity (61.34 g g^− 1). However, the maximum Zn apparent recovery (71.02%) was observed in plants subjected to T₅. Combined application of ZnSB and ZnONPs in the soil along with foliar application of ZnONPs can replace conventional application of ZnSO₄ for maximum yield, Zn-enriched grain, and improved NUE in wheat when grown in Zn-deficient soils.