Physiological Mechanism Of Differential Responses Of Elaeamus Angustifolia To NaCl And NaSO₄Stress

There are complex and diverse types of saline soil with a large total area in China. As oneof the first-choice tree species which are widely and extensively used for land afforestation innorthern China’s ecologically fragile regions, Elaeagnus angustifolia L. seedlings stressed byNaCl or/and Na₂SO₄were used as experimental materials in this research. Under the controlledconditions of potted culture and nutrient solution culture in greenhouse, the following fouraspects were investigated by Inductively Coupled Plasma Optical Emission Spectrometer（ICP-OES）, Scanning Ion-selective Electrode Technique （SIET）, and other conventionalinstruments and methods, those are,（1） salt-damaged symptoms, plant growth performanceand salt tolerance;（2） photosynthetic gas exchange parameters, net photosynthetic rate （Pn）^light intensity response curves, Pn^CO₂concentration response curves, and their characteristicparameters;（3） the absorption, transportation and allocation of salt ions (Na⁺, Cl-and SO₄^2-),and the status of mineral nutrition elements (K⁺, Ca²⁺, Mg²⁺, NO₃^--N, P, etc.); and （4） dynamicchanges in ion fluxes （Na⁺, K⁺and H⁺） in root axial direction or root meristematic zone aftervarious salt treatments for24h or different chemical shocks for25min.Based on the above-mentioned four research aspects, firstly, the salt tolerance and thecritical growth Na⁺concentration （C50） were comprehensively analyzed and compared betweenNaCl-and Na₂SO₄-treated E. angustifolia seedlings. Secondly, the adaptation mechanisms tosalt （NaCl） of E. angustifolia were elucidated clearly. Furthermore, the physiologicalmechanisms （including photosynthesis, ionic metabolism and electrophysiology） of differentialresponses of E. angustifolia seedlings to NaCl and Na₂SO₄stress were revealed and discussed.The aims of this current study were to provide theoretical basis for the large-scale extensionand utilization of E. angustifolia resources and for the selection and breeding of new E.angustifolia germplasms suitable for plantation and utilization in different types ofsaline-alkaline lands, and to provide reference for plant material selection in the process ofbiological control and development of different types of saline-alkaline lands. The main results of this study were summarized as follows:1. Plant growth was significantly inhibited by both types of salt stress. There was typicalsalt-damaged symptoms （such as abscised and turning-yellow leaves）, to a different extent, inNaCl-and Na₂SO₄-stressed E. angustifolia seedlings. The plant height, the growth of grounddiameter and branch, the various leaf growth parameters, and the biomass accumulation ofvarious plant tissues （root, stem, leaf, shoot and even whole-plant） of two types of salt-stressedseedlings were lower or significantly lower than the corresponding parameters of no saltcontrol. With the increase of stressed solution [Na⁺], the above-mentioned salt-damagedsymptoms and various growth parameters were generally exhibited an aggravated anddecreasing trend, respectively, whereas, the root biomass allocation ratio and root to shoot ratiowere both presented an increasing trend.2. The salt tolerance of Na₂SO₄-stressed E. angustifolia is higher than that ofNaCl-stressed. Under the conditions of equal [Na⁺](100or200mmol· L^-1), compared withNaCl-stressed E. angustifolia seedlings, Na₂SO₄-stressed seedlings had a lower leaf-damagedpercentage, a better growth performance and a higher tolerance, and the differences inleaf-damaged percentage, growth performance and salt tolerance between seedlings stressed byNaCl and Na₂SO₄enlarged gradually with the solution [Na⁺] increased. E. angustifolia is highsalt tolerance, its critical growth Na⁺concentration （C50） is180and280mmol· L^-1under thestress of NaCl and Na₂SO₄, respectively.3. Photosynthetic capacity was significantly decreased by both types of salt stress. Netphotosynthetic rate （Pn）, stomatal conductance （Gs）, intercellular CO₂content （Ci） andtranspiration rate （Tr）, and stomatal limitation value （Ls） and water use efficiency （WUE） of twokinds of salt-stressed seedlings was significantly lower, and higher than those of no salt control,respectively. The decreased Pnof stressed seedlings was mainly attributed to stomatalrestrictions. The effects of salt stress on photosynthesis and various leaf growth parametersultimately induced a decrease in plant growth and biomass accumulation. Under the conditionsof equal solution [Na⁺], the inhibitory effects of NaCl stress on photosynthesis were strongerthan those of Na₂SO₄stress, while the Na₂SO₄-stressed seedlings presented a higher maximum net photosynthetic rate （Pn-max）, a larger apparent quantum yield （AQY） and carboxylationefficiency （CE）, a wider CO₂utilization range, and a higher light eco-adaptability.4. The NaCl-adaptation mechanisms of E. angustifolia are root salt-rejection and shootsalt-tolerance. The Na⁺and Cl-allocation model were changed by salt stress,36.2%and42.3%of net Na⁺accumulation allocated in roots and leaves of seedlings stressed by200mmol· L^-1NaCl, respectively,58.9%of net Cl-accumulation was restricted in stems, and the abilities inK⁺and Ca²⁺selective transportation from root to leaf were enhanced substantially. Stem [Na⁺],leaf [Na⁺], net shoot Na⁺accumulation and net shoot Na⁺flux （JNa⁺shoot） of200mmol· L^-1NaCl-stressed seedlings were7.22,9.58,5.45and5.36times that of control seedlings,respectively. For Cl-, it was2.27,3.70,2.03and2.01times in the same tissue order,respectively. Even stressed by200mmol· L^-1NaCl, the seedlings still grew well, and there wasno typical salt-damaged symptoms, succulent characteristics and salt-avoidance mechanism.5. There were great differences in salt-adaptation mechanisms and mineral nutrientstatuses between NaCl-and Na₂SO₄-stressed E. angustifolia seedlings. Under the conditions of200mmol· L^-1equal [Na⁺], there was a larger proportion of root Na⁺retention and a largeramount of Na⁺transportation from stem to functional leaves in NaCl-stressed E. angustifolia,whereas for Na₂SO₄-stressed seedlings, it held a higher capacity in stem Na⁺retention, leaf K⁺selective transportation and leaf Na⁺avoidance, thus kept a better K⁺-Na⁺homeostasis. Net Cl-accumulation in leaves of NaCl-stressed seedlings was the largest among the three plant tissues,accounting for approximately60%of the total net Cl-accumulation. While for Na₂SO₄-stressedseedlings, the absorbed SO₄^2-was mainly distributed in root, approximately accounting for halfof the total net SO₄^2-accumulation, and the leaf [SO₄^2-] maintained an equivalent level withthat of control seedlings. The Cl-absorption or transportation rate in NaCl-stressed seedlingswas substantially larger than SO₄^2-rate in Na₂SO₄-stressed seedlings. The inhibitory effects ofNaCl stress on mineral nutrient （such as NO₃^--N and P） absorption were stronger than those ofNa₂SO₄stress.6. The short-term steady and initially dynamic response mechanisms of root ion fluxes aresalt-type heterogeneity. Salt stress induced a changed root functional structure, the optimal scanning position in the process of ion flux measurement are600μm (for control or50mmol· L^-1Na₂SO₄-stressed seedlings) and300⁴50μm (for100mmol· L^-1NaCl-stressedseedlings) from root tip, respectively. After stressed by no salt control,50mmol· L^-1Na₂SO₄or100mmol· L^-1NaCl for24h, the steady Na⁺and K⁺were all efflux. There was nosignificant difference in K⁺efflux between control and Na₂SO₄stress, and both weresignificantly lower than NaCl-induced K⁺efflux, while the difference in Na⁺efflux betweenany two of the three treatments reached a significant level. For steady H⁺flux, it was influx incontrol roots, and efflux in NaCl-and Na₂SO₄-stressed roots, and the NaCl-induced H⁺effluxwas significantly higher than Na₂SO₄-induced. The mean K⁺and H⁺fluxes within themeasuring periods of the initial5min and the whole25min after the transient addition ofvarious chemicals (50mmol· L^-1Na₂SO₄,100mmol· L^-1NaCl,100mmol· L^-1Choline Cl,and50mmol· L^-1Na₂SO₄+100mmol· L^-1Choline Cl) presented a great difference, of them,the difference in K⁺and H⁺fluxes between NaCl and Na₂SO₄treatment arrived a significantlevel.In conclusion, differences in microscopical root structure, short-term （24h） and initial （10²5min） ion flux dynamic exchange characteristics of Na⁺, K⁺and H⁺between E. angustifoliaseedlings stressed by NaCl and Na₂SO₄conferred the long-term （30d） stressed seedlings witha different salty ions tissue-level compartmentation and a different mineral nutrient status,which further induced a different leaf inhibitory growth, a differently reduced leafphotosynthetic capacity, and a different photosynthetic apparatus dysfunction. All thesedifferences between NaCl and Na₂SO₄stressed seedlings finally exhibited a different plantgrowth performance and biomass accumulation, and a different salt tolerance.