Effects Of Osmotic Stress And Ion Toxicity On Rice Germination During Salt Stress

Rice（Oryza sativa）,as one of the most important food crops,is widely cultivated all over the world.Seed germination is a critical period of plant growth and development,which is easily affected by environment.Therefore,it is of great theoretical and practical significance to study the mechanism of rice seed germination and seedling formation under different stress.Soil salinization is one of the main abiotic factors affecting plant growth and reducing crop yield.Rice seed germination under salt stress is influenced by both osmotic stress and ion stress.However,the effects of osmotic stress and ionic stress on the germination and seedling morphogenesis of rice seeds under salt stress are rarely discussed.Li⁺,belonging to the same main group as Na⁺,has stronger ionic toxicity.In this study,low concentration Li Cl was used for ion stress treatment instead of high concentration Na Cl,which could avoid osmotic stress accompanied by high concentration Na Cl treatment.rice varieties 58M and 58L with different salt tolerance were selected as experimental materials.Rice lines 58m and 58L with different salt tolerance were treated with Na Cl(100mmol·L^-1)solution,isotonic PEG and PEG+Li Cl solution and hypotonic Li Cl(20mmol·L^-1)solution during seed germination,respectively.The morphological characteristics and physiological indexes during seed germination and seedling morphogenesis were observed and measured,and the samples on the second day of treatment were taken for metabolic analysis.The main results are as follows:1.All stress treatments could inhibit or delay the germination of rice seeds in varying degrees,among which PEG and PEG+Li Cl treatments had the most obvious effect,followed by Na Cl treatment,and Li Cl treatment had the least effect.As a result,osmotic stress was the main factor affecting seed germination potential.There were significant differences in morphogenesis of seedlings under different treatments.PEG mainly affected the growth of buds,Li Cl mainly inhibited the growth of roots,and Na Cl and PEG+Li Cl significantly inhibited the growth of roots and buds.2.In the early stage of seed germination,Li Cl and Na Cl had little effect on the activity ofα-amylase,while the effect of PEG treatment was more obvious.With the extension of treatment time,the effect of Na Cl,Li Cl and PEG+Li Cl onα-amylase activity increased,while under PEG treatment,α-amylase activity was significantly inhibited in the early stage of seed germination,and thenα-amylase activity increased rapidly,even higher than that of the control.3.According to the analysis of physiological indexes related to stress,it was found that in the process of seed germination,the oxidative damage of seeds treated with PEG was mild,while that of seeds under other stresses was serious.Histochemical examination showed that H₂O₂and O₂^-jointly participated in regulating the effect of ion stress on root,and the accumulation of both of them could significantly inhibit the growth of root,while the effect of each stress on bud may be mainly caused by O₂^-.The accumulation of O₂^-and H₂O₂under salt stress was significantly higher than that under osmotic,ion and compound stress.Plants have different response mechanisms of antioxidant enzymes under different stresses.The response of proline to osmotic and compound stress was more significant,but the response to ion and salt stress was not significant.4.The metabolites of rice seeds germinated for 48 hours under different treatments were detected.Through the screening of significant differences in metabolites,it was found that the metabolic pathways of starch and sucrose were down-regulated in Na Cl and PEG treatment,and amino acid biosynthesis was down-regulated in Na Cl and Li Cl treatment.Through the analysis of the secondary metabolic pathway,it was found that the flavonoid biosynthesis pathway of salt-tolerant variety 58M was significantly up-regulated under salt stress,while the flavonoid biosynthesis pathway of rice 58M was not significantly enriched under osmotic and ion stress,indicating that in the early stage of germination,flavonoids are more involved in the resistance response of rice to salt stress.In addition,the stronger salt tolerance of rice 58M may be related to the biosynthesis of phenylpropane and flavonoids.