Study On The Physiological Response Mechanism Of Nitraria Tangutorum To NaCl Stress

Nitraria tangutorum is one of dominant or constructive plant species of desert vegetation innorthwest inland saline area and it is also one of the endemic plant species in China with featuresof psammophytes and halophytes. The study on resource characteristics and salt resistance ofNitraria tangutorum was conducted through measuring the microelement content, root systemdistribution, soil moisture in rhizosphere, physiological index variation of seedling under saltstress, salt ion absorb-transportation and distribution, light response and chlorophyll fluorescencechange, daily variation of photosynthesis in order to provide the basis for N. tangutorumutilization in ecological protection in saline and arid area. The main results are as follows:1. The soil moisture in rhizosphere showed a remarkable variation pattern. The variationwithin0to40cm soil layer was large and it was small within160to300cm soil layer. The soilmoisture within40to80cm layer was the highest. The root length and diameter of root systemwere100cm and300cm respectively. The root length was1.32times of plant height. Thediameter of root system was3.22times of plant crown. The aboveground biomass was1.46times of underground. And the alive root weight within0to20cm and20to40cm layersaccounted for58.69%and22.96%of the total respectively, and for the alive root length, theywere59.65%and23.20%respectively. Root system distributed horizontally and mainly inuplayer in order to effectively absorb the rainfall to surface and this was the important strategyfor plants growing in desert area to survive.2. The activity of protective antioxidant enzyme in leaf of seedling under NaCl stress for10,20and30days showed a same pattern. The activity of superoxide dismutase （SOD）, catalases（CAT） and peroxidase （POD） increased under the low concentration stress (25to100mmol·L^-1)and decreased under high concentration (200to400mmol·L^-1). The content of malonate （MDA）kept increasing under stress. The proline （Pro） content increased along with the stressconcentration. And the increasing amount was higher within10to20days compared to thatwithin20to30days under same stress concentration. Soluble protein （SP） content showed adecreasing pattern in general. Soluble sugar （SS） content increased under low concentrationstress and decreased under high concentration stress. This might explain that the membrane lipidperoxisome degree of seedling increased along with NaCl concentration and the protective enzyme played an important role for clearing reactive oxygen under low NaCl concentratin. Procould keep seedling osmoregulation balance as the major material, SS and SP behaved as theassistant matter.3. The absorptance of root system for K⁺was the highest, and followed by Cl^-and Na⁺. Thetransportation rate for Ca²⁺was the highest, and followed by Cl^-and Na⁺. The absorbtion andtransportation amount of Cl^-and Na⁺were the highest because of their high proportions in soil,and followed by K⁺, Ca²⁺and Mg²⁺. The leaf priorly absorbed and accumulated a large amountof action for adapting the environment under NaCl stress, and the contents of Cl^-, Na⁺and totalions in leaf, stem and root increased with NaCl concentration. The accumulation amount of Cl^-,Na⁺and total inorganic ions in leaf were higher than those in stem and root, and leaf and stemcould store a large amount of salt ions for maitaining the osmosis pressure and protoplast of highhydrophilic property, increased over ground part and root system osmosis pressure difference,keeping water towards over ground part transportation.4. The variation of net photosynthetic rate （P_n） and transpiration rate （T_r） of seedling underNaCl stress with different concentrations showed a same pattern and the order of P_n was25>CK>50>100>200>400mmol·L^-1. The water use efficiency （WUE） under treatments of200and400mmol·L^-1NaCl was higher that other treatments and CK.With NaCl concentration increased, the apparent quantum yield （a）, dark respiratory rate（R_d）, maximum photosynthesis rate (P_nmax), light saturation point （LSP） and light compensationpoint （LCP） of seedling performed a decreasing trend. And its P_nmax, LSP and LCP were thehighest under25mmol·L^-1stress out of all treatment concentrations. The weak light utilizationability of seedling was increased along with the NaCl stress concentration and meanwhile thestrong light utilization ability was decreased. The photosynthesis assimilatory potential ofseedling reached the highest under25mmol·L^-1NaCl concentration and the lowest under400mmol·L^-1NaCl concentration.With NaCl concentration increasing, the minimal fluorescence （F_o） of seedling showed anincreasing trend, however, the maximal fluorescence （F_m）, maximal quantum yield ofPSⅡ（F_v/F_m）, PSⅡLatent photosynthesis efficiency （F_v/F_o）, photochemical quench coefficient（qP）, non photochemical quench coefficient （NPQ）, PSⅡreality photosynthesis efficiency（ФPSⅡ） decreased. It could be concluded that the PSⅡlight reaction center was damaged by NaCl stress and it was also the cause of P_n decreace.5. The P_n daily change of seedling showed a single peak pattern and the peak valueappeared at10:00, and then it decreased along with the increase of photosynthesis activeradiation. No photosynthesis noon break phenomenon was found. T_r increased greatly from8:00to10:00along with PAR increase and it steadily increased from10:00to16:00, and then it beganto decrease. The daily change of WUE showed the following pattern: it rapidly decreased from8:00to14:00and was stable after14:00or a small amount of increasing. The stomata limitationvalue （L_s） from8:00to14:00was rising in general and dropping after14:00.Multiple regression indicated that PAR partial regression coefficient was notable largerthan other environment factor, and it suggested that PAR greatly correlated to P_n. Pathanalysis showed that the order of path coefficient of environmental factors was PAR> airCO₂concentration （C_a）> air relative humidity （RH）> air temperature （T_a） except800mmol·L^-1treatment, and this meant that the contribution of PAR to P_n was the largest. PARpath coefficient under stress of different NaCl concentrations increased first and thendecreased along with the increase of NaCl concentration. And it suggested that the impact ofenvironmental factors on P_n was most significant under100mmol·L^-1and PAR and C_autilization efficiency were the highest, however, they were the lowest under800mmol·L^-1.