Physiological And Biochemical Mechanism Of Geniposide Enhancing Tolerance Of Maize To Saline-alkali Stress

Saline-alkali stress is one of the main abiotic stresses that limit the growth and development of plants.The concentration of soluble saline-alkali in soil beyond a certain range will affect the physical and chemical properties of soil and cause soil salinization.Located in the northeast of my country,the Songnen Plain is one of the most widely distributed areas of saline-alkali land in the world.The area of saline-alkali land has reached 3.73 million hm²,and it is increasing year by year at a rate of 1.4%.Geniposide（GD）is an iridoid glucoside extracted from the gardenia.GD has been shown to promote the growth and yield of wheat,cotton,cucumber and cowpea.However,the effect of GD on maize seed germination and seedling growth and development under saline-alkali stress is still unclear,and there is a lack of in-depth research.Maize is one of the most important cereal crops in the world.It is not only an excellent food crop with high nutritional value,but also has excellent environmental adaptability.Asa high-yield food crop in China,maise is widely used and is an indispensable food crop.Maize is a moderately saline-alkali tolerant crop and is more sensitive to saline-alkali stress at the seedling stage.The study of the adaptability and protective mechanism of GD maize to saline-alkali stress has important theoretical and practical value for maise production.In this paper,（1）seed germination experiment,using two different saline-alkali-tolerant types of maize‘Jilong 2’（saline-alkali insensitive type）and‘Xinxuan 58’（saline-alkali sensitive type）as exterimental,setting up the Hoagland nutrient solution and 150 mmol·L^-1mixed saline-alkali solution（molar concentration ratio,NaCl:Na₂SO₄:NaHCO₃:Na₂CO₃=1:9:9:1）to simulated saline-alkali stress,and combination of exogenous GD,to investigate the effects of germination rate,germination potential and germination index of maize seeds under saline-alkali stress;（2）a hydroponics experiment,using two different saline-alkali-tolerant types of maize‘Jilong 2’（saline-alkali insensitive type）and‘Xinxuan 58’（saline-alkali sensitive type）as exterimental,setting up the Hoagland nutrient solution and 150 mmol·L^-1mixed saline-alkali solution（molar concentration ratio,NaCl:Na₂SO₄:NaHCO₃:Na₂CO₃=1:9:9:1）to simulated saline-alkali stress,and combination of exogenous GD,to investigate the effects of growth morphology and biomass of maize seedlings,antioxidant enzyme（APX,GR,MDHAR,DHAR）activity in the ascorbic acid-glutathione cycle（AsA-GSH）cycle,leaf photosynthetic physiology,antioxidants（AsA,GSH）,root osmotic regulation and endogenous hormone content.The aim of this study was to the physiological ways to improve the ability of saline-alkali tolerance in maize.The aim of this study was to find out the physiological ways of GD in improving the ability of maize to resist saline-alkali stress,The results show that:（1）Saline-alkali stress inhibited the germination of the two varieties of maize seeds,and reduced the seed germination rate,germination potential and germination index.Exogenous GD treatment can promote the germination of maize seeds under saline-alkali stress,and is relatively weak to saline-alkali-tolerant varieties.The mitigation effect of Xinxuan 58 was more obvious.This study showed that GD significantly improved the germination rate,germination potential and germination index of Xinxuan 58,and only increased germination index of Jilong 2.（2）Saline-alkali stress inhibited the growth of the two varieties of maize seedlings,decreased the chlorophyll content of the seedlings,and affected the photosynthesis of the seedlings,which resulted in a significant decrease in the dry and fresh weight,plant height and leaf area of the seedlings,and an increase in the root-to-shoot ratio.The treatment with exogenous GD can significantly increase the dry and fresh weight,plant height,leaf area and total chlorophyll content of seedlings,reduce the root-to-shoot ratio,and alleviate the photosynthesis of maize seedlings under saline-alkali stress.（3）Saline-alkali stress caused changes in osmotic adjustment substances in maise roots.The content of proline,soluble sugar and soluble protein in Jilong 2 increased significantly,while the content of proline in Xinxuan 58 increased,the content of soluble sugar and soluble protein in Xinxuan 58 obvious decreased.Exogenous GD can alleviate the damage caused by osmotic stress and improve the adaptability of maize seedlings to saline-alkali stress by regulating the further synthesis and accumulation of osmotic regulators.（4）Under saline-alkali stress,the photosynthetic electron transfer in maize seedlings is disturbed,and the production rate of superoxide anion（O₂^-.）and the content of hydrogen peroxide（H₂O₂）are both significantly increased.The accumulation of reactive oxygen species resulted in the increase of membrane peroxidation,the content of malondialdehyde（MDA）increased significantly,the membrane permeability increased,and the activities of superoxide dismutase（SOD）,peroxidase（POD）and catalase（CAT）increased.Exogenous GD increased the activities of SOD,POD and CAT in the roots of maize seedlings under saline-alkali stress,the O₂^-.production rate,H₂O₂content and the MDA content were significantly reduced,which indicated that GD can effectively remove ROS and reduce the degree of membrane lipid peroxidation.To maintain the integrity of the cell membrane structure and function,which is conducive to improving the saline-alkali tolerance of maize seedlings.（5）Under saline-alkali stress,auxin（IAA）and gibberellin（GA）content in the roots of Jilong2 significantly decreased,and abscisic acid（ABA）content increased significantly,the IAA/ABA and GA/ABA ratio is significantly reduced.And the IAA content in the root system of Xinxuan 58increased significantly,while the contents of GA,ZR and ABA decreased significantly,and the ratio of IAA/ABA increased significantly.Exogenous GD can increase the IAA,GA and ZR content in the root system of Jilong 2,reduce the ABA content.And GD can increase the IAA、GA and ZR content in the root system of Xinxuan 58,reduce the ABA content.The ratio of IAA/ABA、GA/ABA and ZR/ABA in the root system of two varieties of maize was significantly increased.GD has a significant regulatory effect on endogenous hormone balance,and the mediating balance of endogenous hormone may be closely related to the ability of maize to resist saline-alkali resistance under saline-alkali stress.（6）The photosynthetic enzyme phosphoenolpyruvate carboxylase（PEPCase）,pyruvate dikinase（PPDK）,NADP-malase（NADP-ME）,malate dehydrogenase（NADP-MDH）and ribulose diphosphate carboxylase（Ru BPCase）in the leaves of maize seedlings decreased significantly under saline-alkali stress.Exogenous GD can promote the related photosynthetic activity and effectively alleviate the damage of saline-alkali stress to photosynthesis.（7）Reactive oxygen species（ROS）in plants are in dynamic equilibrium under normal plant growth.The balance of ROS under saline-alkali stress was destroyed under saline-alkali stress,the activity of glutathione reductase（GR）and dehydroascorbic acid reductase（DHAR）in the leaves of Jilong 2 were reduced,increased monodehydroascorbate reductase（MDHAR）activity,reduced ascorbic acid（AsA）content,reduced glutathione（GSH）content,AsA/DHA and GSH/GSSG ratios were all significantly reduced.In the leaves of Xinxuan 58,MDHAR activity decreased,DHAR activity increased significantly,and AsA content increased significantly.Exogenous GD can effectively increase the activities of APX,GR,MDHAR and DHAR,AsA content,GSH content,AsA/DHA and GSH/GSSG ratios in Jilong 2.The GR and DHAR activity in Xinxuan 58 decreased,and the MDHAR activity,GSH content and GSH/GSSG ratio increased significantly.The levels of antioxidant substances and antioxidant enzyme activity in the ascorbic acid-glutathione cycle（AsA-GSH cycle）contribute to maintaining cycle stability,effectively removing ROS,mitigating oxidative stress damage,maintaining the structural and functional integrity of and functional integrity of the cell membrane,and improving the saline-alkali resistance of plants.