Studies On Physiological Regulation Mechanism Of Exogenous ALA To Cucumber Seedlings Under Salt Stress

For the highest benefits, farmers and producers tend to applying excess chemicalfertilizer to get a high yield. However, a long period of over-fertilizer application led to soilsalinity in the greenhouse which has been becoming a limitation of further increase ofvegetables yeild. ALA is a kind of biological active non-protein amino acid and widespread inorganism including plant, animal and bacteria and so on. In recent years, the study on therelationship between ALA and plant resistance to abiotic stress is becoming a hot topic. Thesestudies mainly focus on how to improve and increase plant photosynthetic rate, but thephysiological mechanism on relieving salt stressed plants when applying ALA is still limited,particularly on cucumber.In this study, we screened two cultivated varieties of cucumber, salt-tolerant (JinchunNo.4) and salt-sensitive cucumber (Jinyou No.1) which showed a big difference of saltresistance under salt stress. Both of them are conducted hydroponics to in vestigate theresponse of application ALA on two cultivars seedlings under salt stress. We mainlydiscussed the seeds germination rate, plant growth, osmoregulation substance, mineralelement absorption, active oxygen scavenging system, photosynthetic and chlorophyIIfluorescence characterisitic, regulation of endogenous and the gene expression of aquaporinsin two cultivars under different treatments, and explicated thephysiological mechanism of theeffects exogenous of ALA on plants salt-tolerance.The main results are as follows:1. The sprout and development of cucumber seedlings after applying exogenous ALAunder salt stress.Seeds properties such as germination rate, germination potential, fresh weight and lengthof hypocotyls are markedly decreased under NaCl treatment. The height, fresh weight, rootactivity, the absorption area of root and effective absorption area of root also decreased afterNaCL treatment. However, only appropriate concentration of ALA,50mg.L-1, can enhancedthose negative effects mentioned above.2. The osmolyte and leaf water potential of cucumber seedlings after root applyingexogenous ALA under salt stress. NaCl treatment significantly induces the accumulation of the MDA content, intensifiesthe extent of membrane lipid peroxidation and alsoincreases the permeability of electrolyte,finally leading to leaf water potential leaf reduced, which further induces the synthesis ofosmolyte, including proline，the soluble sugar and soluble proteins. Therefore, exogenousapplying ALA has the ability to increase the amount of osmolyte to maintain high osmoticpotential in cells. In this case, the accumulation of MDA is decreased, membrane lipidperoxidation is released, maintaining the integrity of cell membrane, declining thepermeability of electrolyte and increasing leaf water potential. These results show that ALAhas the function to induce the synthesis of osmolyte to maintain the integrity of cellmembrane and further keep cell function operating properly.3. The mineral element absorption of cucumber seedlings after root applying exogenousALA under salt stress.Salt stress treatment increases the Na+concentration and decreses the content of K+, Ca2+and Mg2+because of antagonism of ions. Na+/K+, Na+/Ca2+and Na+/Mg2+are higher thancontrol which leads to stress symptoms of seedlings. Applying exogenous ALA can increasethe concentration of K+, Ca2+and Mg2+which maintain cell turgor, increase transport proteinactivity and upregulate chlorophyll content. As a result, Na+effluxes from cell whichincreases the resistance of seedlings to salt stress.4. The active oxygen scavenging system of cucumber seedlings after root applyingexogenous ALA under salt stress.Under the NaCl treatment, Na+is accumulated substantially in the cell which resulted inacid base equilibrium of cell is broken, photosynthetic electron transport is disordered,superoxide anion and hydrogen peroxide are over-accumulated and membrane lipidperoxidation is aggravated. Accumulation of hydrogen peroxide induces the increase ofactivity of SOD and APX which further active the POD, CAT, GR, AsA and GSH which havefunction to degrade the reactive oxygen species to make plants gain the ability to resist to saltstress. Applying exogenous ALA can further increase the amount of antioxidants and theactivity of antioxidase, stablizing the circle of AsA-GSH, decreasing the content ofsuperoxide anion and hydrogen peroxide, reducing the damage of membrane lipidperoxidation and maintaining the integrity of cell structure and function. These resultsindicate that ALA can be used to raise the resistance of plant to salt stress by increasing theamount of antioxidants and the activity of antioxidase.5. The chlorophyll content and photosynthsis system of cucumber seedlings after rootapplying exogenous ALA under salt stress. Salt stress inhibits the synthesis of precursor of chlorophyll UroIII, ProtoIX, Mg-ProtoIXand Pchl and reduced the content of chlorophyll. Meanwhile, net photosynthetic rate, stomatalconductance, intracellular CO2concentration, transpiration rate, quantum efficiency, electrontransport rate, photochemical quenching and photosynthetic rate are declined.ALA is the firstprecursor in the biosynthesis of all porphyrins compounds. Applying exogenous ALA canincrease the precursors of chlorophyll and further increase the content ofchlorophyll.Cucumber seedlings can get more photosynthetic rate than under salt stresscondition. Besides that, ALA can also increase the amount of antioxidants and the activity ofantioxidase to reduce the inhibition of superoxide anion and hydrogen peroxide on electrontransport, increaseing the photosynthetic activity and resistance of plant to salt stress.6. The endogenous hormone content of cucumber seedlings after root applying exogenousALA under salt stress.Under the NaCl treatment, ABA concentration in leaves and root goes up and content ofGA3and IAA fall down, which lead to growth inhibition of seedlings. Applying exogenousALA can increase the content of GA3and IAA. Further analysis indicates that ALA raises theratio of IAA/ABA and GA3/ABA which benefits the growth of seedlings and increases theresistance to salt stress.7. Gene expression of aquaporins in cucumber seedlings after root applying exogenousALA under salt stress.Under NaCl treatment, the expression level of CsPIP1;1and CsNIP is up-regulated andthe expression of CsPIP1;1is much higher than CsNIP. Applying exogenous ALA candown-regulate the expression of these two genes. This indicates that ALA can induce theexpression of aquaporin which maintains the stability of cell structure and function andincreases the resistance of seedlings to salt stress.