Drought stress is one of the most important environmental constraints that negatively affect the growth and production of crops worldwide. Recently, nanotechnology has been increasingly used to improve the tolerance of plants exposed to abiotic stresses such as drought. The present study was designed to investigate the moderating effect of cerium oxide nanoparticles (CeO2 NPs) on alleviating drought stress for the apple cv. ‘Red Delicious’ on M9 rootstock. Drought stress caused a significant increase in CAT, GPX, APX, and SOD enzyme activities compared to control plants. Drought decreased the content of macro and microelements, and the application of CeO2 NPs led to significant changes in the content of these elements in plants under drought stress. CeO2 NPs significantly reduced chlorophyll damage under high drought levels. In addition, they alleviated the damage caused by drought, which was shown by lower levels of MDA and EL. When these nanoparticles were used during drought stress, they greatly increased the production of abscisic acid and indole-3-acetic acid hormone. In response to drought stress, the expression of DREB1A and DREB1E genes increased. The use of CeO2 NPs in stressful and non-stressful conditions had a positive effect on improving the studied traits of the apple plants and enhancing nutrient levels. Taken together, the findings suggest that CeO2 NPs can be used as promising drought stress-reducing agents in apples. Therefore, understanding the mechanisms of abiotic stress in global horticulture and the role of nanoparticles is essential for developing improved, drought-tolerant crops and the adoption of measures to deal with changing climatic conditions.
