Effects Of Exogenous ALA On Antioxidant System And Chlorophyll Biosynthesis In Wild Jujube Seedlings Under NaCl Stress

Salt stress was one of the main abiotic stresses affecting plant growth,which severely restricted the development of the jujube industry.Wild jujube had resistant to drought and salinity,and was often used as the rootstock of jujube trees.In recent years,due to the increased area of saline-alkali in Xinjiang,the benefits of grafting to increase the yield of cuttings have gradually decreased.Therefore,the way of using external regulators to improve the salt tolerance of jujubes had an important role in increasing the income of farmers.Exogenous 5-aminolevulinic acid（ALA）played a role in regulating plant growth by participating in various mechanisms in plants,enhancing plant’s ability to resist external adverse factors.It had been considered as a potential plant growth regulator.In this experiment,wild jujube seedlings were used as experimental materials by hydroponic to study the morphological indexes,antioxidant system,chlorophyll biosynthesis and photosynthetic fluorescence characteristics of wild jujube seedlings under external spraying of 100 mg·L^-1 ALA against 150 mmol·L^-1 NaCl.Real time fluorescent quantitative PCR（qRT-PCR）was used to detect the expression levels of antioxidant enzyme-related genes and chlorophyll biosynthesis-related genes to provided theoretical reference for the alleviation of exogenous ALA on jujube under NaCl stress.The main findings were as follows:（1）Under the NaCl treatment,exogenous ALA can significantly improved the plant height,dry and fresh weight,total root length,root area,root volume and root tip number of wild jujube seedlings,indicating that exogenous ALA can alleviate the effects of NaCl stress on the growth of wild jujube seedlings.（2）Under the NaCl treatment,the application of exogenous ALA regulated the REDOX equilibrium state by increasing SOD,CAT,APX,GR,DHAR and MDHAR activities and gene expression levels of leaves of wild jujube seedlings,increasing the content of ASA and GSH,as well as the ratio of ASA/DHA and GSH/GSSG,and reducing the accumulation of ROS and MDA,removed active oxygen,reduced the degree of membrane lipid peroxidation,and alleviated the NaCl stress on the leaves of wild jujube seedlings benefit impact.（3）Exogenous spraying of ALA can up-regulate the expression of HEMA gene and stabilize the transformation of endogenous ALA in wild jujube seedlings under the NaCl treatment.At the same time,exogenous ALA can up-regulate the expression of chlorophyll biosynthesis-related enzyme genes（HEMB,HEMY,chlM,por,and chlG）,increase the content of PBG,Proto IX,Mg-Proto IX,Pchl,and Chla,and promote chlorophyll biosynthesis,avoid the damage of photooxidation,maintain the integrity of chloroplast structure,increasing photosynthetic rate of the leaves of wild jujube seedlings under salt stress.This study also found that the conversion process of chlorophyll synthesis blocked the site in ALA→PBG.（4）Exogenous spraying of ALA slowed down the decrease of Pn,Gs and Tr in the leaves of wild jujube seedlings under the NaCl treatment,and the increase of Ci was also significantly reduced,indicating that ALA can alleviate the non-stomatal limitation of photosynthesis which contribute to regulate the normal opening and closing of stomata in plant leaves,so as to promotethe exchange of CO₂and enhance the assimilation capacity of CO₂.At the same time,exogenous spraying of ALA can significantly increase the levels of Fm,Fv/Fm,Y（Ⅱ）and qP,and reduce the levels of F₀ and NPQ under the conditions of NaCl stress.It showed that exogenous ALA protected the photosynthetic mechanism of wild jujube seedlings under NaCl stress,promoted the electron transfer activity,increased the photochemical efficiency,and effectively alleviated the damage of PSII underNaCl stress on wild jujube seedling,so as to increase the photosynthetic rate and maintaining the stability of the photosystem.