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 Cl- in old roots while increasing Cl- compartmentation in cortical vacuoles of both young and old roots. Collectively, radical transport of salt ions (Na+ and Cl-) 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 cation-exchange character. Accordingly, root aggregation allows good contact of roots with a Ca2+ source and reduces contact with Na+ and Cl-, which presumably plays a major role in enhancing salt tolerance of P. euphratica.
