The effects of hydrogel on growth and ion relationships of a salt resistant woody species, Populus euphratica, were investigated under saline conditions. The hydrogel used was Stockosorb K410, a highly crosslinked polyacrylamide with about 40% of the amide group hydrolysed to carboxylic groups. Amendment of saline soil (potassium mine refuse) with 0.6% hydrogel improved seedling growth (2.7-fold higher biomass) over a period of 2 years, even though plant growth was reduced by salinity. Hydrogel-treated plants had approximately 3.5-fold higher root length and root surface area than those grown in unamended saline soil. In addition, over 6% of total roots were aggregated in gel fragments. Tissue and cellular ion analysis showed that growth improvement appeared to be the result of increased capacity for salt exclusion and enhancement of Ca2+ uptake. X-ray microanalysis of root compartments indicated that the presence of polymer restricted apoplastic Na+ in both young and old roots, and limited apoplastic and cytoplastic Clˉin old roots while increasing Clˉ compartmentation in cortical vacuoles of both young and old roots. Collectively, radical transport of salt ions (Na+ and Clˉ) through the cortex into the xylem was lowered and subsequent axial transport was limited. Hydrogel treatment enhanced uptake of Ca2+ and microanalysis showed that enrichment of Ca2+ in root tissue mainly occurred in the apoplast. In conclusion, enhanced Ca2+ uptake and the increased capacity of P. euphratica to exclude salt were the result of improved Ca2+/Na+ concentration of soil solution available to the plant. Hydrogel amendment improves the quality of soil solutions by lowering salt level as a result of its salt-buffering capacity and enriching Ca2+ uptake, because of the polymer's cationexchange character. Accordingly, root aggregation allows good contact of roots with a Ca2+ source and reduces contact with Na+ and Clˉ, which presumably plays a major role in enhancing salt tolerance of P. euphratica.

The uptake and transport of salt ions (Na+, Clˉ), macronutrients (K+, Ca2+, Mg2+) and abscisic acid (ABA) response to increasing soil salinity were examined in 2-year-old seedlings of Populus euphratica and a hybrid, P. talassica Kom×(P. euphratica+Salix alba L.). Leaf burn symptoms appeared in the hybrid after 8 days of exposure to salinity when soil NaCl concentration increased to 206mM, whereas P. euphratica exhibited leaf damage after day 21 when soil NaCl exceeded 354mM. Leaf necrosis was the result of excess salt accumulation since the injury followed an abrupt increase of endogenous salt levels. Compared with the hybrid, P. euphratica exhibited a greater capacity to exclude salt ions from leaves under increasing salinity, especially Clˉ. Salt treatment altered nutrient balance of the hybrid, leaf K+, Ca2+ and Mg2+ concentrations significantly declined and the same trends were observed in roots with the exception of K+. Although K+ levels decreased in salinised P. euphratica, increasing salinity did not affect the levels of Ca2+ and Mg2+ in leaves, but did increase the uptake of these nutrients when salt stress was initiated. NaCl-induced increase of ABA concentration in xylem sap [ABA] was observed in the two tested genotypes, however xylem [ABA] increased more rapidly in P. euphratica and a fivefold increase of xylem [ABA] was recorded after the first day of exposure to salt stress. Therefore, we conclude that the increase of Ca2+ uptake may be associated with the rise of ABA, and thus contributes to membrane integrity maintenance, which enables P. euphratica to regulate uptake and transport of salt ions under high levels of external salinity in the longer term.