Abstract: The biomass allocation, salt sensitive index and salt tolerance index were compared between Robinia pseudoacacia and Fraxinus velutina under the mixed salt stress with different concentrations of NaCl and Na₂CO₃. The paraffin sections were made to observe the effect of salt stress on the root structure. The results showed that the growth of both R. pseudoacacia and F. velutina were suppressed with the increase of salt stress. The biomass of R. pseudoacacia increased at the low concentration of salt stress（0.2%）, meanwhile R. pseudoacacia performed the highest salt sensitive index and tolerance index. F. velutina performed the highest tolerance index in the moderate salt concentration（0.4%）and stronger sensitivity under high salt concentration（0.8%）. The root periderm proportion of F. velutina was higher than that of R. pseudoacacia’s in the mixed salt stress, while the cortex ratio increased less than R. pseudoacacia. The vascular bundle proportion of F. velutina root was reduced, while the opposition trend was observed in R. pseudoacacia. Under salt stress, R. pseudoacacia and F. velutina have different salt tolerance strategy.

Key words: Robinia pseudoacacia, Fraxinus velutina, root system, vascular bundle, cortex