Acclimation to nitrogen × salt stress in Populus bolleana mediated by potassium/sodium balance

•Effective distribution of nutrition (potassium and nitrate) to young leaves when Populus bolleana suffer salt stress.•Nitrogen deficiency exacerbated salt-stressed effects in poplar.•High level of nitrogen nutrition modestly alleviated salt toxicity in both leaves.•Potassium/sodium ratio plays a vital role against salt stress across nitrogen levels in both young and old leaves. Nitrogen plays an important role in plant tolerance to salt stress, but it remains largely unknown how the young and old leaves in woody plants differently respond to salinity under different nitrogen supplies. In this study, the young saplings of Populus bolleana Lauche were exposed to moderate salt condition and different nitrogen levels, and the young and old leaves were separately analysed to compare their responses in ion balance, carbon/nitrogen balance, and redox balance. NaCl stress alone profoundly affected the old leaves but only exerted mild effects on young leaves, showing in the former lower K+/Na+ ratio, ion imbalance, lipid peroxidation, chlorophyll degradation, and reduction of carbon and nitrogen concentration. There is an important strategy for the effective distribution of nutrition (K+ and NO3−) to young leaves when P. bolleana suffer salt stress, displayed by the differential regulation of K+ / Na+ and NO3- transporters across leaf ages. When salt stress was accompanied by nitrogen deficiency, the young leaves displayed similar damaged effects with old leaves, with a comparable K+/Na+ ratio in all the damaged leaves. Conversely, high nitrogen supply modestly alleviated salt toxicity in both leaf type, accompanying with modest change of K+/Na+ concentrations. The present study demonstrated that K+/Na+ ratio plays a vital role against salt stress across nitrogen levels in both young and old leaves, and ion imbalance in young leaves can serve as an indicator related to the survival of plants under salt stress. Nitrogen deficiency significantly endanger the survival of salt-stressed poplar for its protection strategy on the young leaves has been lost in this condition.