Enhancement of hydrogel on salt resistance of Populus popularis ‘35-44’ and its mechanism

The effects of hydrogel on salt resistance of a salt-sensitive woody species, Populus popularis ‘35-44’, were investigated under saline conditions.The hydrogel used was Stocksorb 500 XL (crosslinked poly potassium-co- (acrylic resin polymer) -co-polyacrylamide hydrogel) manufactured by Stockhausen GmbH Krefeld (Germany).Amendment of saline soil (dry weight) with 0.5% Stocksorb 500 XL enhanced salt tolerance of the tested poplar genotype.Hydrogel-treatment markedly limited the buildup of salt ions (Na⁺ and Cl^-) in leaves of P.popularis ‘35-44’, but increased stomatal conductance (Gs), transpiration rate (TRN) and net photosynthetic rate (Pn) of the genotype.Moreover, application of hydrogel assisted P.popularis ‘35-44’ to maintain the activity of superoxide dismutase (SOD) and peroxidase (POD) isoenzymes, thus remaining the capacity to eliminate salt-induced reactive oxygen species (ROS) under salt stress.NaCl-treated P.popularis ‘35-44’ plants exhibited leaf necrosis after 25 days of salt stress, whereas the hydrogel amendment delayed the occurrence of leaf injury for 20 days.The reduction of salt uptake by hydrogel amendment was the results of salt-buffering and ion-selective uptake capacity:1) Stocksorb 500 XL diluted Na⁺ and Cl^-concentration in the gel matrix due to the water-holding capacity of hydrogel, allowing roots to grow in an environment of lower salinity;2) the enrichment of nutrients Ca²⁺ and Mg²⁺, instead of Na⁺, was accounted for the cation-exchange character of Stocksorb 500 XL.Furthermore, Stocksorb 500 XL contains K⁺ that can be taken up by plant roots.Collectively, root aggregation in hydrogel allows good contact with a source of lower salinity but with higher K⁺, therefore, the absorption, transportation and accumulation of salt ions in plant tissues were reduced, but the uptake of nutrients was increased.As a result, the salt resistance of plants is consequently increased.