Nanoparticles properties such as solubility, tunable surface charges, and singular reactivity might be explored to improve the performance of fertilizers. Nevertheless, these unique properties may also bring risks to the environment since the fate of nanoparticles is poorly understood. This study investigated the impact of a range of CuO nanoparticles sizes and concentrations on the germination and seedling development of Phaseolus vulgaris L. Nanoparticles did not affect seed germination, but seedling weight gain was promoted by 100 mg Cu L-1 and inhibited by 1 000 mg Cu L-1 of 25 nm CuO and CuSO4. Most of the Cu taken up remained in the seed coat with Cu hotspots in the hilum. X-ray absorption spectroscopy unraveled that most of the Cu remained in its pristine form. The higher surface reactivity of the 25 nm CuO nanoparticles might be responsible for its deleterious effects. The present study therefore highlights the importance of the nanoparticle structure for its physiological impacts.