Abstract: Populus euphratica, as the dominant species of the riparian forest along the Tarim River, northwest China, plays an important role in maintaining functions of the riparian ecosystem. Recent embankments along the river have altered the soil salinity condition due to water use by human or climate change, which is expected to influence regeneration of the riparian forest. To assess the potential effects of these alterations and elucidate the adaptation and endurance of P. euphratica seedlings to salinity stress, the paper examined growth performance, ionic distribution and transportation of P. euphratica seedlings across experimental soil salinity gradients.Results indicated that seedlings were limited in their growth performance, as evidenced by decreases height growth rates, leaf area and biomass across the salinity gradients. The height growth rate values in moderate (100 mol·L^-1) and severe (200 mol·L^-1) salinity stress achieved of approximately 50% and 20% respectively, in relation to control. In the same level of salt concentration, the order of ionic content in seedling was leaf > root > stem, leaf was the main part of ionic accumulation. Increasing salinity stress significantly elevated Na⁺ content and Cl^- content in root and shoot tissues (stem+leaf) of P. euphratica seedlings, while K⁺, Ca²⁺ and Mg²⁺ contents showed reverse trends. Similar patterns as K⁺ content were also observed across soil salinity gradients in K⁺/Na⁺, Ca²⁺/Na⁺, Mg²⁺/Na⁺ ratio of root and shoot. The significant decrease of S_{K, Na}, S_{Ca, Na} and S_{Mg, Na} of mild (50 mol·L^-1) and moderate (100 mol·L^-1) salinity stress, and the transport and absorption of nutrients were inhibited. Under severe (200 mol·L^-1) salinity stress condition, a marked drop of K⁺ content in root and shoot tissues upon salt accumulation and a third of K⁺/Na⁺ ratio of seedlings those under control conditions suggests that decreasing osmotic pressure of seedling, and the seedlings suffered the severe stress. However, seedlings had higher root/shoot ratio (R/S) to limit water lose, and accumulated Na⁺ in leaves to promote water transport and meliorate water situation of shoots. Under severe (200 mol·L^-1) salinity stress, P. euphratica seedlings maintained strong Ca²⁺ absorption capacity in root to insure the cell membrane stability was high that effectively preventing an excessive amount of Na⁺ into seedling and keeping proper ion homeostasis, it was also associated with high S_{K, Na}, of leaves to avoid further harm on seedling.

Key words: Populus euphratica seedling, salt treatment, ionic homeostasis, ionic distribution and transportation