The size of nCu particles (20-30 nm) was confirmed by transmission electron microscopy before usage, and nCu suspension was prepared as previously described (23, 24). Maize seeds were treated with or without nCu (20 mg/kg DW or 1000 mg/kg DW) for 24 hrs before sowing. After that, maize field trials were conducted on an experimental station of Maize Research Institute, Hanoi, Vietnam in randomized complete block design with three replications. In each replication, seedlings were grown in a plot (size 49 m2, where row to row and plant to plant spaces were 0.7 m and 0.25 m, respectively) having seven rows (each 10 m). The germination of maize seeds was evaluated after 7 days of growth.