Alleviation of nitrogen stress in rice (Oryza sativa) by ceria nanoparticles

This study explored the potential of ceria nanoparticles (CeO2 NPs) to alleviate stress in hydroponic rice caused by low N (LN) and high N (HN) stresses. The N content in plants was measured after 3 weeks of treatment with CeO2 NPs. The impact of CeO2 NPs on plants under medium N (MN, a normal condition) was studied as a comparison. LN resulted in N deficiency while HN led to N excess in plants while impairing plant growth. CeO2 NPs (100 and 500 mg L-1) increased the N levels in roots and shoots under LN by 6-12% and 22-30%, respectively. However, under HN stress, 500 mg L-1 CeO2 NPs reduced the N levels in roots and shoots by 9% and 6%, respectively. CeO2 NP treatment enhanced the activities of key enzymes involved in N assimilation, glutamine synthetase (GS), glutamine oxoglutarate aminotransferase (GOGAT) and glutamate dehydrogenase (GDH), accounting for the increased N content in plants under LN. Under HN, 500 mg L-1 CeO2 NPs downregulated the GS and GDH activity by 42% and 36%, respectively. CeO2 NPs reduced oxidative membrane and DNA damage and enhanced plant tolerance to N-stress by regulating the antioxidant enzyme system and the levels of proline and phytohormones including gibberellin 3, abscisic acid, zeatin riboside and indole-3-acetic acid. However, when N is supplied normally, CeO2 NPs caused oxidative stress in plants thereby impairing plant growth. A change of N conditions altered the root exudate composition and led to different extents of transformation of CeO2 NPs at the root interface and different Ce uptake and translocation in plants. This study for the first time reported that CeO2 NPs could act as an alleviator of N stress in rice, while it might be a risk when the N supply is normal.