The Response Of Two Representative Desert Shrubs To Salt Stress In Northwest Arid Region

Establishing sand break forest in arid areas of northwest China is an effective way tocurb the expansion of land desertification and protect oasis agriculture from the hazardsof sandstorm. Natural rainfall in the area is less and difficult to satisfy the demand of sandbreak forest for water. In order to ensure young sand break forest survival, Supplementaryirrigation with underground brackish water must be performed. However, the young sandbreak forest will be harmed by high salinity brackish water. So, mechanisms of salinitytolerance in plants must be studied firstly before the utility of brackish water irrigation.In order to understand effects of salinity on physiological traits of Caraganakorshinskii Kom and Haloxylon ammodendron Bunge, the two typical sand-fixing plants,in arid areas of northwest China. Leaf photosynthesis, ion content, antioxidant enzymeactivities, free amino acid and endogenous hormones in Caragana korshinskii Kom andHaloxylon ammodendron Bunge exposed to NaCl stress was studied in this paper.Furthermore, differences in physiological traits of Caragana korshinskii Kom andHaloxylon ammodendron Bunge under salt stress were compared. Theoretical basis wasprovided for differences in salt resistance of Caragana korshinskii Kom and Haloxylonammodendron Bunge.This study has obtained the following results.（1） Stomatal conducance of Caragana korshinskii Kom is significantly reduced dueto mild and moderate NaCl stress. CO2which entered cell gap could not satisfy thedemand of photosynthetic carbon assimilation, and stomatal limitation became the majorfactor in reducing the photosynthesis rate. At18days with300mM NaCl treatment, theinhibition of photosynthetic carbon assimilation increased due to non-stomatal factors,and non-stomatal limitation becomes the major factor in reducing the photosynthesis rate.During the entire experiment, photosynthesis rate, stomatal conductance and intercellularCO2concentration of Haloxylon ammodendron Bunge reduced synchronously, whichindicated stomatal limitation was the major factor in reducing the photosynthesis rate.（2） Salt stress increased Na⁺content in leaves of Caragana korshinskii Kom andHaloxylon ammodendron Bunge, but had no effect on K⁺content. The phenomenon of alack of competition between K⁺and Na⁺absorption confirms a unique pathway of Na⁺ absorption in the plants which can grow well under salt environment. The ratio of Na⁺toK⁺in salt-treated leaves of Caragana korshinskii Kom was above0.6（Maximum for anoptimal metabolic efficiency in non-halophytic plants）, suggesting the metabolicdisorders in leaves of Caragana korshinskii Kom occurred. During the entire experiment,Ca²⁺and Mg²⁺contents in leaves of Caragana korshinskii Kom reduced but Ca²⁺andMg²⁺contents in leaves of Haloxylon ammodendron Bunge was almost constant.（3） Oxidative damages to leaves of Caragana korshinskii Kom and Haloxylonammodendron Bunge were observed under salt stress. The increases of production rate ofsuperoxide radical, content of Hydrogen peroxide and content of Maloniedialdehyde inleaves of Caragana korshinskii Kom were significantly higher than Haloxylonammodendron Bunge. To reduce the physiological damages caused by salt stress, SOD,CAT and POD activity in leaves of Caragana korshinskii Kom increased with lowconcentrations of NaCl. It is helpful to eliminate excess free radicals and reactive oxygenspecies. However, with high concentrations of NaCl, enzyme activity in leaves ofCaragana korshinskii Kom reduced. SOD, CAT and POD activity in leaves of Haloxylonammodendron Bunge increased with salt stress. It indicated that Haloxylon ammodendronBunge has greater salt tolerance than Caragana korshinskii Kom.（4） In seventeen free amino acids of Caragana korshinskii Kom and Haloxylonammodendron Bunge leaves, proline, serine, glutamic acid, aspartic acid, alanine andthreonine are most abundant in both salt-treated and control leaves. The rest of elevenfree amino acids only accounting for20%of total amino acids. The increases of freeamino acid always happen after a certain stage of NaCl stress, which indicates theexistence of a critical salinity level in Caragana korshinskii Kom and Haloxylonammodendron Bunge. Free amino acids in salt-stressed leaves slowly accumulated belowthe critical salinity level, but rapidly accumulated when above the level.（5） NaCl treatment would lead to the accumulation of ABA in leaves of Caraganakorshinskii Kom and Haloxylon ammodendron Bunge. But ABA content in leaves ofHaloxylon ammodendron Bunge is more stable than Caragana korshinskii Kom.Endogenous hormones which are able to promote growth in leaves of Caraganakorshinskii Kom decreased with increased salinity, but contents of IAA, GA and ZR inleaves of Haloxylon ammodendron Bunge was constant or increased significantly. Duringthe entire experiment, the ratio of IAA/ABA, GA/ABA, ZR/ABA and （IAA+GA+ZR）/ABA in salt-treated leaves of Caragana korshinskii Kom decreased with the increase ofNaCl stress. While, the ratio of growth-promoting hormones to ABA in leaves ofHaloxylon ammodendron Bunge was constant. It is indicated that Haloxylon ammodendron Bunge could regulate the hormonal balance effectively to adapt theadverse salt environment.