Abstract: [Objective] Thisresearch was aimed to study responses of Siberia elm (Ulmus pumila) clonestodifferent concentrations of NaCl treatment, find out the salt stress response mechanism and provide a theoretical basis for the salt-tolerant mechanism of elm clones. [Method] Based on the previousstudy of the elm clones salt-tolerantselection, three salt-tolerantelm clones and three salt-sensitive elm clones were used as the materials in thisstudy. The seedlings were subjected to different salt stressesof (0%, 0.3%, 0.5%, 0.7%, and 0.9% concentration) with adequate soil moisture. The growth index (height growth rate, ground diameter growth rate and biomass), antioxidant enzyme activates (SOD and POD), osmoregulation substances (free proline, soluble protein and sugar), tissue Na⁺ and Cl^- content, chlorophyll and gas exchange (photosynthetic rate, transpiration rate, intercellular carbon dioxide concentration, stomatal conductance, water use efficiency, apparent mesophyll conductance and stomatal limitationpercentage) were measured andanalyzed. [Result] The results showed(1) that the growth index reduced with the increase of salt concentrationin both salt-tolerantand salt-sensitive clones.The soluble protein content, free proline content and soluble sugar content increased with the increase of NaCl concentration. The antioxidant enzyme activates (SOD and POD) in cells increased under the lower salt concentrations andthen decreased at the high salt concentration.Photosynthetic characteristics, such as photosynthetic rate, transpiration rate, intercellular carbon dioxide concentration and stomatal conductance, were all decreased to different degrees with the increase of NaCl concentration.Chlorophyll concentration showed a decrease tendency withincreasing the salt concentration. The transportation of Na⁺ and Cl^- from the root to the shootwas reduced, andthus a large amount of Na⁺ and Cl^- was accumulated in the root section. (2): Incomparisonwith the salt-sensitive clones, the salt-tolerantelm clones accumulated large amount of soluble sugar and maintaineda high activity of SOD enzyme in the tissue to cope with the increasingsalt stress.[Conclusion] The resultsindicatedthat the soluble sugar content and SOD activity playedakey role in the salt-tolerance of elm clones, and theycould be used as important indicators for the salt tolerance ofelm clones.

Key words: in vitro culture, elm clones, salt-tolerance, salt response