Objectives We investigated the differences in the effects of salt stress and saline-alkali stress on the activities of nitrogen metabolism enzymes, and the expression of genes related to nitrogen absorption and assimilation in grape root stock seedlings.

Methods Sand culture experiment was conducted using ‘5BB’ rootstock seedlings as materials, which were under three treatments for 14 days, CK (normal nutrient solution), salt stress (nutrient solution containing 0.20 mol/L NaCl), and saline-alkali stress (nutrient solution containing 0.10 mol/L NaCl & 0.10 mol/L NaHCO₃). Plants were sampled for the measurement of dry and fresh weight, root morphology, SPAD value, Na⁺ and K⁺ content, activities of five nitrogen metabolism enzymes, ¹⁵N content, and the expression levels of nine genes.

Results Compared with CK, saline-alkali stress significantly reduced the fresh weight of whole plants, and markedly decreased the SPAD values. Salt and saline-alkali stress increased Na⁺ content to 1.50 and 2.50 fold in leaves, and 1.68 and 1.79 fold in roots, compared to CK, and were recorded higher Na⁺ content in roots than in leaves. Saline-alkali stress was tested the highest leaf K⁺ content, while CK was tested the highest root K⁺ content. Compare with CK, salt and saline-alkali stress decreased the activities of enzymes in leaves and roots, and decreased NO₃⁻, NH₄⁺ and total nitrogen content in different extent. Compare with CK, salt and saline-alkali stress decreased the NO₃⁻ and total N contents in leaves and roots, saline-alkali decreased the NH₄⁺ contents in leaves, salt and saline-alkali stress decreased the NH₄⁺ contents in roots. Salt and saline-alkali stress down-regulated the expression levels of nine nitrogen metabolism-related genes in leaves and roots. Compared with CK, the expression levels of leaf VvNIA decreased by 68.61%, 92.55%, and VvNIR decreased by 66.26%, 81.61%, respectively; both salt and saline-alkali stress decreased the expression levels of VvNRT1.1, VvNRT1.2, VvNRT2.1 and VvNRT2.2 in leaves, and the difference in the expression levels of the four genes was significant between salt and saline-alkali stress. Saline-alkali stress down-regulated the expression levels of VvNRT1.1 and VvNRT1.2 in roots, and salt and saline-alkali stress down-regulated the expression levels of VvNRT2.1 and VvNRT2.2 in roots. The result of K¹⁵NO₃ isotope labeling showed that ¹⁵N use efficiency of grape rootstock seedlings under salt and saline-alkali stress were decreased by 63.83% and 75.95%, respectively.

Conclusions Under salt and saline-alkali stress, grape seedlings would reduce the activities of key nitrogen metabolism enzymes in leaves and roots, down-regulate the expression levels of genes related to nitrogen absorption and assimilation to some extent, thus reduce the NO₃⁻, NH₄⁺ and total N content in leaves and roots, and result in low ¹⁵N use efficiency and growth of grape root stock seedlings. The adverse effect of saline-alkali stress is worse than salt stress on grape root stock seedlings.