Abstract: Taking the pot-cultured seedlings of four leguminous tree species (Albizia julibrissin, Robinia pseudoacacia, Sophora japonica, and Gleditsia sinensis) as test materials, this paper studied their growth indices, critical salt concentration (C₅₀), and K⁺ and Na⁺ allocation under different levels of NaCl stress, aimed to understand the difference of test tree species in salt tolerance. NaCl stress inhibited the seedling growth of the tree species. Under NaCl stress, the  dry matter accumulation decreased, while the root/shoot ratio increased, especially for A. julibrissin and G. sinensis. Quadratic regression analysis showed that the C₅₀ of A. julibrissin, R. pseudoacacia, S. japonica, and G. sinensis was 3.0‰, 5.0‰, 4.5‰, and 3.9‰, respectively,i.e., the salt tolerance of the four tree species was in the order of R. pseudoacacia > S. japonica > G. sinensis > A. julibrissin. In the root, stem, and leaf of the four tree species seedlings, the Na⁺ content increased with the increase of NaCl stress, while the K⁺ content (except in the root of A. julibrissin) decreased after an initial increase, resulting in a larger difference in the K⁺/Na⁺ ratio in the organs. Under the same NaCl stress, the allocation of Na⁺ in different organs of the four tree species seedlings decreased in the order of root>stem>leaf, while that of K⁺ differed with tree species and NaCl stress, and leaf was the main storage organ for K⁺. The K⁺/Na⁺ ratio in different organs decreased in the sequence of leaf>stem>root. R. pseudoacacia under NaCl stress accumulated more K⁺ and less Na⁺ in stem and leaf, and had higher K⁺/Na⁺ ratio in all organs and higher dry mass, being assessed to be more salt-tolerant. In contrast, A. julibrissin under high NaCl stress accumulated more Na⁺ in stem and leaf, and had a lower K⁺/Na⁺ ratio in all organs and lower dry mass, being evaluated to be lesser salt-tolerant. The K⁺ accumulation in seedling stem and leaf and the Na⁺ retention in seedling root could be the main reasons for the salt tolerance of leguminous tree species under NaCl stress.

Key words: leguminous tree species, NaCl stress, critical salt concentration, ion distribution, salt tolerance assessment