In plants, ammonium (NH 4 +) is the main nitrogen source and acts as a physiological and morphological response signaling molecule. Melatonin and dopamine are associated with plant responses to abiotic stress. However, previous studies have rarely focused on nutrient stress, and the roles of melatonin and dopamine in the uptake and metabolism of nitrogen in plants remain unclear. In this study, we investigated the regulatory effects of melatonin and dopamine on nitrogen utilization efficiency in apple seedlings under two NH 4 + concentrations (2 and 0.1 mM) by measuring plant growth, root system architecture, 15NH 4 + content, and related enzyme activity and gene expression. Under low nitrogen supply, apple seedling growth slowed and showed marked reductions in biomass accumulation, chlorophyll content, and nutrient uptake. However, both melatonin and dopamine significantly improved plant growth, chlorophyll content, and root development and enhanced antioxidant enzyme activity. Exogenous application of melatonin or dopamine also promoted the absorption and accumulation of 15NH 4 + and enhanced nitrogen metabolism-related enzyme activity. At the molecular level, melatonin and dopamine significantly increased the expression levels of nitrogen metabolism genes and transporter genes. Overall, these results suggest that melatonin and dopamine can relieve nutrient stress caused by low concentrations of NH 4 + through regulating the absorption and metabolism of nitrogen. • The effect of low ammonium-induced stress can be alleviated by melatonin and dopamine. • Melatonin and dopamine increased 15 NH 4 + uptake, transport and accumulation. • Activity of enzymes involved in N-metabolism can be increased by melatonin and dopamine. • Melatonin and dopamine increased nutrient uptake by increasing related genes transcription. [ABSTRACT FROM AUTHOR]