CuO NPs application has either a neutral or inhibitory effect on seed germination , with few showing positive effects . However, in all cases, the application method was adding the CuO NPs to the substrate (usually filter paper). In this work, seeds were immersed in CuO NP solutions for 5h only and then transferred to Germitest papers moistened with distilled water. We speculate that this treatment was sufficient to raise the ROS levels to the "oxidative window for germination" while avoiding the detrimental effects of excessive ROS. Excessive amounts of Cu can interfere with photosynthetic and respiratory processes, enzyme synthesis and plant ultrastructural development . This may also explain why the lowest CuO NPs concentrations improved seed vigor while higher concentrations presented no such effects.