Comparative Studies Of Different Type Hydrogels On Drought And Salt Resistance Of Metasequoia Glytostrob Oides Hu Et Cheng

The changes of physiological and biochemical mechanisms in growth, osmotic regulation, the production and elimination of reactive oxygen species, lipid peroxidation and antioxidant system in leaves of two-year-old Metasequoia glytostroboides Hu et Cheng were tested systematically under the stress of drought, salinity and drought plus salinity, taken by the method of plant-potted. The object of this study is to elucidate the the mechanism of salt and drought tolerance of hydrogel-treated plants. This study provides theoretical basis for the application of hydrogel polymers in arid and saline-alkali areas. The main results were as follows:1. The two polymers both postponed the occurrence of plant wilting and improved growth rate, root and shoot dry weight of stressed Metasequoia glytostroboides, although drought and salt stress significantly suppressed plant growth and biomass. Polymer-treated plants showed a stable transpiration during the period of drought and salinity treatments. Furthermore, hydrogel treatment significantly increased the biomass of stressed plants at the end of experiment. Compared to non-hydrogel treatment, granual polymer increased the growth rate by 170% (drought), 90% (salinity) and 390% (drought+salinity). Biomass of thess plants was 30% (drought), 19% (salinity) and 45% (drought+salinity) higher than non-hydrogel-treated ones. Powder polymer exhibited an effect similar to that of granual polymer, which increased growth rate by 170% (drought), 100% (salinity) and 390% (drought+salinity) and biomass by 30% (drought), 21% (salinity) and 40% (drought+salinity). There were no marked differences between the two type polymers. The enhancements of salt and drought tolerance of plants are mainly due to the water-retaining capacity of polymers, which increased the water available to plants and meanwhile decreased the salt concentrations in the soil solution.2. Drought, salinity and drought plus salinity markedly increased production rate of O₂^-·, MDA content and electrolyte leakage (EL) in leaves. However, polymer application reduced the production rate of O₂^-·, MDA content and EL in stressed plants. Granual and powder polymers lowered the level of O₂^-·by 1-17% (drought), 16-22% (salinity) and 18-22% (drought+salinity). MDA content was decreased by 6-19% (drought), 15-18% (salinity) and 17-34% (drought+salinity) in polymer-treated plants. Correspondingly, hydrogel application reduced EL by 8-21%, 15-38% and 17-41% in drought-, salt- and drought+salinity-treated plants. Collectively, results show that hydrogel application, on the one hand, down-regulated the production of active oxygen species (ROS), and on the other hand, accelerated the capacity to remove the ROS in stressed palnts.3. Under conditions of drought, salinity and drought+salinity, the levels of proline, soluble sugar, soluble protein in leaves were found to increase, although drought and salt treatment had a more pronounced effect on leaf proline. However, the stress-induced increase of proline, soluble sugar, soluble protein was reduced by the two polymers. Compared to non-hydrogel amendment, granual polymer lowered the proline level by 27% (drought), 67% (salinity) and 51% (drought+salinity), while powder polymer only reduced the proline content in drought stressed plants. Our results show that hydrogel alleviated the stress effects of drought and salinity, thus down-regulating the osmolyte solutes that enhanced by water and salt stress.4. Drought, salinity and drought+salinity up-regulated the production rate of O₂^-·in leaves, as well as the activity SOD, CAT, POD and APX during the period of treatment. The enhanced antioxidant system is thought to be helpful to control ROS homeostasis in stressed plants. However, the stress-induced increase of antioxidant enzyme was reduced by the two polymers, indicating that hydrogel alleviated the stress effects of drought and salinity. Hydrogel lowered the APX by 25% (granual polymer) and 16% (powder polymer) in drought-stressed plants. Under saline conditions, APX activity was reduced by 29% (granual polymer) and 20% (powder polymer) and CAT declined by 45% in granual polymer-treated plants. Under conditions of drought and salinity, the reductions of antioxidant enzymes in hydrogel-treated plants were: 45-51% (CAT), 33-58% (POD) and 14% (SOD, granual polymer).5. Cl^-, Na⁺ and K⁺ significantly increased in shoot and root tissues in drought and salinised plants. Noteworthy is that more Cl^- and Na+ accumulated in roots than in stem and leaves. Under conditions of water stress, concentrations of K⁺, Cl^- and Na⁺ in soil solutions markedly increased, while K⁺, Ca²⁺ and Mg²⁺ were remained unchanged over the observation period. Hydrogels reduced the concentration of toxic Cl^- and Na⁺ in root, stem and leaves, but increased K+ in drought and salt stressed plants. As a result, K⁺/Na⁺ in root and shoot tissues was improved, especially in powder polymer-treated plants. A much lower concentration of Cl^- and Na⁺ was found in gel matrix compared to soil solutions, but the level of K⁺, Ca²⁺ and Mg²⁺ showed an opposite trend. In conclusion, the two type hydrogels have a salt buffering capacity, which diluted the salt concentrations in the soil, thus restricting the salt uptake by roots. Moreover, the improved K/Na ratio indicates that the nutrient selectivity was enhanced by polymer treatment.