Research On The Mechanism Of Exogenous Spermidine Alleviating The Effect Of Salt Stress On Photosynthetic Performance Of Rice

Soil salinization seriously restricts the sustainable development of agriculture and is one of the main stresses faced by crops.The environmental problem caused by soil salinization has become a worldwide problem.Previous studies have shown that spermidine（Spermidine,Spd）plays an important role in plant abiotic stress tolerance.Under salt stress,external application of Spd can alleviate plant growth inhibition or increase plant photosynthesis,but its mechanism has not been reported.In order to explore the effects of exogenous Spd on photosynthetic physiology of rice seedlings under salt stress,"Nnipponbare"was used as the experimental material.After being cultured in Hogland nutrient solution for 30 days,100m M Na Cl was used to simulate moderate salt stress,and 1m M Spd was used to carry out salt stress tolerance experiment.Through the physiological characteristics of rice seedlings,the characteristics of photosynthetic fluorescence,the structure and function of chloroplasts and the changes of thylakoid protein complexes,the effects of exogenous Spd on chloroplast and photosynthetic performance of rice seedlings were analyzed,and the mechanism of exogenous Spd enhancing rice photosynthetic performance under salt stress was clarified,which provided a theoretical basis for the rational utilization of plant growth regulators in agricultural production.The contents of the study are as follows:（1）The effect of exogenous Spd on the apparent characteristics of rice under salt stress,rice seedlings showed obvious curl and wilting,and the leaf color lost green,which seriously inhibited the normal growth of rice.After the application of Spd in the nutrient solution of Na Cl,the contraction degree and contraction area of rice leaves decreased relatively,only the leaf edge was curly,and the leaf color was bright green.The rice plants treated with Spd alone were sturdy,and there was no significant difference in plant height and leaf color between the control group.At the same time,under the condition of no salt,the photosynthetic pigment content of rice seedlings treated with Spd increased slightly,salt stress led to the decrease of Chl a,Chl b,Total Chl and Chl a/b content in varying degrees,and the application of Spd significantly alleviated the damage to photosynthetic pigment.（2）Effects of exogenous Spd on ROS production and lipid peroxidation in rice chloroplasts under salt stress,the contents of O₂.^-,H₂O₂and MDA in rice chloroplasts were significantly increased.Spd alone had no significant effect on ROS production and MDA content in rice seedlings,but the combined application of Spd and 100m M Na Cl prevented the effect of Na Cl alone,indicating that exogenous Spd reduced the lipid peroxidation of chloroplasts and alleviated the inhibited growth.Antioxidant enzyme system is an important defense system produced by plants under environmental stress.Spd alone could increase the activities of SOD,CAT,APX and GR,but there was no significant difference.Under salt stress,the activities of SOD,APX and GR increased significantly,but the activity of CAT decreased.Exogenous Spd treatment further increased the activities of SOD,CAT and APX,and alleviated the inhibitory effect of salt stress on CAT activity.It is suggested that Spd may be involved in the redox dynamic balance of chloroplasts under salt stress.（3）The effect of exogenous Spd on photosynthetic fluorescence of rice seedlings was measured by portable fluorometer.OJIP curve and JIP-test analysis showed that the curve was typical OJIP shape and was composed of O-J-I-P.The curve showed K point after salt stress,and the main changes were the increase of J phase and the decrease of I-P segment.At the same time,the values of V_Kand V_Jincreased significantly,and V_Kand V_Jdecreased significantly after exogenous Spd treatment.It is suggested that the accumulation of Q_A^-is also the evidence of damage to the donor side and electron transport chain of PSII.Among the fluorescence parameters,ψE₀,φE₀,δR₀,φR₀,ET₀/RC,ABS/CSm,TR₀/CS_mand ET₀/CS_m,K_p,S_m/t _Fmaxdecreased significantly,while ABS/RC and TR₀/RC had no significant change,while exogenous Spd could effectively alleviate these negative effects of salt stress,while K_n,PI_total,SFI_totaland DF_totalincreased significantly.Compared with the control group,the application of Spd alone increased M₀,S_m,N,δR₀andφR₀slightly,but there was no significant change in other parameters.The results showed that exogenous Spd effectively maintained the activity of PSII and the stability of electron transport chain.（4）The effect of exogenous spermidine on the structure and function of chloroplast chloroplast is one of the most sensitive organelles of green plants to environmental changes.the ultrastructure of chloroplast was observed by transmission electron microscope.the experimental results showed that salt stress seriously damaged the internal structure of chloroplast,thylakoid membrane dilated,grana stacked and disintegrated,a large number of membrane structure and a large number of osmiophilic droplets were dispersed in the matrix.In order to further study the changes of thylakoids,we used Blue-Native PAGE technique to quantitatively and qualitatively analyze thylakoid membrane proteins.The results showed that under salt stress,the contents of PSII SC,PSI SC,PSI M,PSII D,PSII M,Cytb6/f and LHCII-M decreased significantly,while LHCII-M did not change significantly.The application of exogenous Spd alleviated these negative effects...