Objective To study the differences in salt tolerance among the Melia azedarach, Ulmus pumila and Robinia pseudoacacia seedlings and to select the evaluation indexes of salt tolerance.

Method Based on the one-year-old seedlings of M. azedarach, U. pumila and R. pseudoacacia, the effects of different NaCl concentrations (0, 100, 200 and 300 mmol·L⁻¹) on the seedling biomass, physiological and biochemical indexes were analyzed.

Result (1) NaCl stress inhibited the biomass of M. azedarach, U. pumila and R. pseudoacacia seedlings, decreased the relative water content of leaves and increased the electrolyte permeability. (2) The contents of chlorophyll a and b in R. pseudoacacia decreased significantly with the increase in NaCl concentration. But the chlorophyll content in M. azedarach and U. pumila seedlings only decreased significantly under 300 mmol·L⁻¹ NaCl treatment. The photosynthetic parameters Pn, Gs, Ci and E decreased under NaCl stress in the 3 tree seedlings, but the stomatal limitation and water use efficiency increased. Meanwhile, the concentrations of free proline and soluble protein, as well as the activities of CAT and APX in roots and leaves also increased. (3) NaCl stress changed the ion balance in the roots and leaves of the 3 tree seedlings. The Na⁺ concentration in roots and leaves and K⁺ concentration in leaves of the 3 tree seedlings significantly increased under NaCl stress, but the concentration of K⁺ in roots, as well as the concentrations of Mg²⁺ and Ca²⁺ in roots and leaves had differential changes.

Conclusion The seedlings of U. pumila have the strongest salt tolerance, followed by M. azedarach, R. pseudoacacia. The concentrations of free proline and soluble protein in roots, CAT and APX activities in roots and leaves, Ca²⁺ in roots and Na⁺ in leaves are highly correlated with salt tolerance of the 3 tree seedlings, which can be used as the evaluation indexes of salt tolerance.