Effects Of Exogenous Spermidine On Physiological And Biochemical Characteristics Of Grape Seedlings Under High Temperature Stress

Grape（Vitis vinifera L.）is an important fruit tree crash crop.At present,the cultivated area and yield are among the highest in the world.In recent years,the emission of CO₂ and other gases has led to the rising of global temperature and the frequent occurrence of extreme high temperature weather.Polyamines（PAs）is a kind of bioactive low molecular weight aliphatic nitrogen-containing alkalis,which extensively exists in various organisms andhas a protective role in biological stress response.In this study,the wine variety’Vitis vinifera L.cv.Marselan’was used as the material,and spermidine（Spd）was used as the experimental reagent.By examining the effects of exogenous Spd on photosynthesis,antioxidant system and heat shock related gene expression,to explore whether exogenous Spd can improve the heat resistance of grape seedlings.The results can provide a theoretical basis for the practical application of exogenous Spd in grape heat tolerance.The main results are as follows:（1）Leaf spray of 1.0 mmol·L^-11 Spd alleviated the growth inhibition of high temperature stress on grape seedling.Under high temperature stress,the growth of grape seedlings was significantly inhibited.The third functional leaf of grape seedling was seriously damaged by high temperature,the leaf margin was all yellow,and the other leaves also showed signs of yellow.However,only the tip of the third functional leaf turned yellow in grape seedlings treated with exogenous Spd.At the same time,the plant biomass also was increased compared with the high temperature control plants.These results indicated that exogenous Spd alleviated the growth inhibition of grape seedlings under high temperature stress.（2）Exogenous Spd reduced the damage to photosynthesis of grape seedling leaves by high temperature stress.High temperature treatment for 5 days resulted in the damage of photosystem II（PSII）and serious deformation of OJIP curve,and maximum fluorescence（F_m）,maximum photochemical efficiency of PSⅡ（F_v/F_m）,and potential PSⅡactivity（F_v/F_o）were significantly decreased.Under high temperature stress,exogenous Spd helped to maintain the OJIP curve line shape and relieved the damage of PSⅡcaused by high temperature stress.After 5 days of high temperature treatment,the photosynthetic pigment content in grape seedling leaves was decreased,but leaf spraying with Spd inhibited the degradation of photosynthetic pigment.At the same time,exogenous Spd increased Pn,Gs,Tr and decreased Ci by regulating non-stomatal limiting factors,which suggested that the exogenous Spd reduced the inhibition of photosynthesis under high temperature stress.（3）Exogenous Spd enhanced the antioxidant capacity of grape seedling leaves under high temperature stress.After 5 days of high temperature treatment,the relative conductivity,MDA and ROS contents of grape seedling leaves were significantly increased and the content of soluble sugar was decreased sharply,which aggravated the degree of lipid peroxidation.However,after 5 days of high temperature stress,exogenous spd-spraying significantly improved the activity of SOD,POD and CAT and the relative expression of antioxidant enzyme genes（VvSOD,VvPOD and VvCAT）compared with the high temperature control,which indicated tha Spd enhanced the antioxidant capacity of grape seedlings.（4）Exogenous Spd increased the expression of heat shock related genes in the leaves of grape seedlings under high temperature stress.Six heat shock related genes（VvHsfA1,VvHsfA2,VvHsfB1,VvHsp23.6,VvHsp17.9 and VvGOLS1）were selected in this experiment.The relative expression of these heat shock genes was increased significantly under high temperature stress.Especially,when exogenous Spd was applied under high temperature stress,the expression levels of other 5 genes except VvHsfA1 were significantly higher than those of the high temperature control.These data indicated that exogenous Spd can regulate the expression of heat shock genes,thus enhancing the heat tolerance of grape seedlings.