Responses of soybean (Glycine max [L.] Merr.) to zinc oxide nanoparticles: Understanding changes in root system architecture, zinc tissue partitioning and soil characteristics

Addressing global Zinc (Zn) deficiency in food and feed requires innovation in Zn fertilizer. Recently, Zn oxide nanoparticles (ZnONPs) have piqued interest for potential use as a novel nano-Zn fertilizer. However, little is known about potential factors influencing ZnONPs partitioning in different plant tissues, and changes in root system architecture (RSA) and soil characteristics. Herein, we tested the effects of particle size (38, 59, and > 500 nm) and concentration (0-500 mg/kg) of ZnONPs on Zn bioaccumulation in multiple tissues in soil-grown soybean (Glycine max) grown for 120 days, including changes in RSA (root biomass, length, area, volume, and density) and soil characteristics (pH and electrical conductance [EC]). Our results showed significant effects of Zn compound types, Zn concentrations and their interaction on RSA, and Zn uptake by root, stem, leaf, and seed, in soybean. Concentration-response curves for root structures with varied sized ZnONPs and Zn2+ ions were deemed nonlinear, whereas for Zn distribution between different tissues the concentration-response curves were linear. Interestingly, ZnONPs and Zn2+ ions up to 200 mg/kg showed beneficial effects on root growth and development, but toxic response was observed at higher concentrations for both compounds. Root dry weight, length, volume, and area with 200 mg/kg ZnONPs-38 nm were higher by 48%, 56%, 33% and 44%, respectively, compared to control, and were higher by 15%, 23%, 15% and 19%, respectively, compared to 200 mg/kg ZnCl2. In general, soybean responses to the smallest size ZnONPs-38 nm were higher for all parameters evaluated compared to the larger-sized ZnONPs (59 and > 500 nm) and Zn2+ ions. Zn bioaccumulation varied among tissues in the order: root > seed > leaf > stem. A minor but steady decrease in soil pH and EC occurred among different concentrations for both ZnONPs and Zn2+ ions. Improved RSA can facilitate water and nutrient uptake in soybean, promoting growth and yield, especially considering arid and semi-arid climates where water is a limiting factor. Further, improving seed and shoot Zn levels, as demonstrated herein using ZnONPs, is paramount to addressing Zn deficiency in food and feed. Future studies assessing potential impacts on soil microbes, soil health and food safety upon ZnONPs application is critical for risk assessment of the novel nanofertilizer.