Long-term exogenous application of melatonin improves nutrient uptake fluxes in apple plants under moderate drought stress

•The effect of drought-induced stress can be alleviated by melatonin.•Melatonin positively influenced the growth and physiological parameter of Malus.•Melatonin increased 15N uptake, utilization and accumulation.•Activity of enzymes involved in N-metabolism can be increased by melatonin.•Melatonin increased nutrient uptake by increasing related genes transcription.