Optimized zinc oxide and copper oxide nanoparticle applications increase soybean productivity and drought resiliency

Climate-induced water stress is projected to hamper soybean yield and seed/grain quality globally, unless climate adaptive management practices are adopted. Soybean ( Glycine max L.) was cultivated under field conditions using pots following randomized complete block design and subjected to well-watered or drought stress (40–50% water capacity). Plants were foliar-sprayed with 0–200 mg L −1 copper oxide (CuONPs) or zinc oxide (ZnONPs) nanoparticles, or with 400 mg L −1 of ionic Cu/Zn form. Under well-watered conditions, 50 mg L −1 CuONPs increased seed yield and protein content by 25% and 11%, respectively; whereas increasing concentrations of CuONPs to 100 or 200 mg L −1 reduced yield by 8–24%. On the other hand, ZnONPs at 100 and 200 mg L −1 increased average seed yield by 59% and 80%, and seed number by 39% and 48%, respectively. Drought stress reduced seed yield, seed weight, and micronutrient content by approximately 15–25%, 15%, and 34–46%, respectively, compared to well-watered conditions. Under drought stress, ZnONPs at 100–200 mg L −1 improved seed yield by 25–39%, protein content by 20–31%, and oil content by 24–29%; while CuONPs above 50 mg L −1 reduced seed number and yield by 24–54% and 37–47%, respectively. At 400 mg L −1 , ionic Cu or Zn had no adverse effects on seed quality, except for ionic Zn that increased average protein under drought stress. Overall, CuONPs at 50 mg L −1 and ZnONPs at 100–200 mg L −1 increased soybean productivity and seed quality under both well-watered and drought conditions, likely due to improved micronutrient (Cu & Zn) and nitrogen assimilations. Low dose CuONP or moderate dose ZnONP foliar applications represent promising and sustainable approaches for enhancing soybean productivity while maintaining seed nutritional quality under varying water availability.