NaCl-induced changes of ion homeostasis and nitrogen metabolism in two sweet potato (Ipomoea batatas L.) cultivars exhibit different salt tolerance at adventitious root stage

• We studied ion fluxes and N metabolism in NaCl-stressed sweet potato adventitious roots.
• NaCl up-regulation of PM H+-ATPase contributes to K+ retention in salt-tolerant cultivar.
• NaCl triggers NH4+ efflux and NO3− uptake in salt-tolerant cultivar.
• Such transition of NH4+/NO3− fluxes contributes to ion homeostasis and N metabolism.
• A model showing salt-adaptive mechanisms in salt-tolerant cultivar is proposed.

Efficient control of ion homeostasis and nitrogen (N) metabolism contributes to the salt tolerance in crops. However, limited information is available on the contribution of ion homeostasis and N metabolism regulation towards salt tolerance in sweet potato (Ipomoea batatas L.) cultivars at adventitious root stage. In this study, NaCl-induced ion homeostasis and N metabolism changes in adventitious roots of two different sweet potato cultivars were investigated. Salt tolerant cultivar (Xushu 22, Xu 22) exhibited better capacity controlling Na+, Cl−, K+ and Mg2+ homeostasis than the salt sensitive cultivar (Xushu Shi 5, Shi 5) under prolonged salinity condition. Net H+ efflux and plasma membrane H+-ATPase gene expression were significantly enhanced in NaCl-stressed Xu 22 roots but not in Shi 5 roots. Salt-upregulated PM H+-ATPase contributed to less K+ efflux in Xu 22 roots. NaCl stimulated Mg2+ uptake in Xu 22 roots under prolonged salinity, but promoted Mg2+ efflux in Shi 5 roots. In addition, NaCl significantly enhanced transcript abundance of genes related to vacuolar Na+ and Cl− sequestration in Xu 22 roots. In Xu 22 roots, NaCl triggered an obvious net NH4+ efflux, which caused a rapid increase of NH4+ concentration around the rhizosphere. Exogenous supply of NH4+ markedly inhibited Na+ influx and K+ efflux under salt shock condition. Moreover, a salt-enhanced NO3− net influx was also detected in mature Xu 22 root region and this process may contributes to the normal N assimilation in this cultivar under prolonged saline condition. Taken together, our results suggest that the better capacities of root K+ and Mg2+ retention, vacuolar Na+ and Cl− sequestration, transition of NH4+/NO3− transport and N assimilation maintenance contribute to the salt tolerance of Xu 22 at adventitious root stage. A model showing salt-adaptive mechanisms in salt-tolerant Xu 22 is proposed.