The Relieving Effect Of Exogenous Spermidine On Saline-alkali Stress In Grape Seedlings

Soil salinization is an important environmental factor that affects the plant growth and agricultural productivity.Due to the unreasonable farming,fertilization,and irrigation,the problem of soil salinization is becoming increasingly serious.Grape（Vitis vinifera）is one of the most economically fruit crops and cultivated in a wide range of environments,which has certain salt-tolerance capabilities.However,high salinity could limit grape growth and quality most dramatically.Therefore,how to enhance the salt-alkali tolerance is an important issue facing the grape industry.Spermidine（Spd）,as a plant growth regulator,has been widely used to enhance the plant resistance to biotic and abiotic stresses.However,it is still unclear whether Spd can relieve the damage of saline-alkali stress on grape.In this study,one-year old grape?Kyoho?seedlings were used to explore the effects of exogenous Spd on grape seedlings under salt-alkali stress through evaluation of morphology,photosynthetic fluorescence and physiological and biochemical indexes.The results revealed that exogenous Spd could improve the salt-alkali tolerance of grape seedlings,which provided a corresponding theoretical basis for its application in grape production.The main results are given as follows:（1）After saline-alkali treatment,the growth and development of grape seedlings were inhibited,the fresh weight,dry weight,relative water content and root vigor were significantly reduced,and more leaves turned yellow and wilted.However,exogenous Spd increased the fresh weight,dry weight,relative water content and root vigor,and delayed leaf yellowing.In contrast,the application of DCHA,aggravated the damage to seedlings caused by salt-alkali stress.（2）Under salt-alkali stress condition,the content of photosynthetic pigments net photosynthetic rate（Pn）,stomatal conductance（Gs）,transpiration rate（Tr）,water use efficiency（WUE）and maximum photochemical efficiency（F_v/F_m）were all reduced.The application of exogenous DCHA aggravated this damage,but exogenous Spd increased the photosynthetic pigment content stabilized the photosynthetic parameters,thereby alleviating the damage of PSⅡreaction center and enhancing photosynthesis of grape seedlings under salt-alkali stress.（3）Saline-alkali stress induced the accumulation of proline and soluble sugar of grape seedlings,and the seedlings are forced to take in a large amount of Na⁺and excrete a large amount of K⁺.The application of exogenous Spd enhanced the water absorption and retention capacity of the roots through increasing the content of proline and soluble sugar.In addition,exogenous Spd up-regulated the expression of Na⁺/H⁺antiporter gene（VvNHXP）and K⁺transporter gene（VvHKT2）,which improved ion transport efficiency and ensures the efflux of Na⁺and the absorption of K⁺.In contrast,the application of exogenous DCHA has the opposite effect.（4）Saline-alkali stress significantly increased the contents of superoxide anion（O₂·^-）,hydrogen peroxide（H₂O₂）,malondialdehyde（MDA）and the relative electric conductivity（REC）of grape leaves.In addition,the expression of antioxidant enzyme genes（VvSOD,VvPOD,VvCAT and VvAPX）were all down-regulated,and the activities of SOD,POD,APX and CAT in grape leaves were all significantly reduced under saline-alkali treatment.However,the application of exogenous Spd reduced the accumulation of ROS and alleviated the oxidative damage caused by salt-alkali stress through up-regulating the expression of antioxidant enzyme genes,and increasing the activity of antioxidant enzymes.（5）Saline-alkali stress induced the synthesis and accumulation of endogenous polyamines in the leaves of grape seedlings.The application of DCHA decreased the content of free Spd and PAs and increased the content of free Put,thus aggravating the damage of saline-alkali stress to grape seedlings.On the contrary,the application of exogenous Spd significantly alleviated the excessive accumulation of Put induced by salt-alkali stress,and promoted the conversion of free Put to free Spd,which improved the salt-alkali tolerance of seedlings.