Effects Of Alkali Stress On Seed Germination And Seedling Growth Of Zhonghua 11 And Mitigation Effects Of Exogenous Substance

At present,alkali stress has become one of the main factors restricting regional agricultural development.The damage caused by alkaline salt(NaHCO3 and Na2CO3)to plants is different from neutral salt(NaCl)in that in addition to ion poisoning and osmotic stress,it also increases the solidification of soil elements caused by high pH outside the roots.Rice is one of the most important food crops in China and around the world.The traditional method of "treatment of alkali with rice" can reduce the content of alkaline salt in alkaline soil and increase the area of agricultural arable land.Therefore,constructing a rice alkali tolerance identification system,understanding and mastering the morphological and physiological adaptation mechanisms of rice seeds and seedlings to alkali stress have important practical significance for improving the utilization of alkali land and ensuring regional food security.In this study,a alkali-resistant rice cultivar,Zhonghua 11,was used to study the effects of alkali stress on rice seed germination,seedling morphology and physiological characteristics,and the mitigation effects of different exogenous substances on alkali stress.The main results are as follows:1.The seed germination test of Zhonghua 11 under alkali stress showed that alkali stress obviously inhibited the germination of rice seeds.With the increase of alkali solution concentration and pH value,the germination rate,germination potential and germination index showed a downward trend.When the alkali concentration reaches 30 mmol·L-1 and the pH exceeds 10.50,the germination rate of rice seeds is between 0～24%.In addition,alkali stress significantly reduces the activity of amylase in seeds,thereby inhibiting the growth of young shoot and roots.2.Alkaline stress significantly affected the morphological and physiological characteristics of rice seedlings,significantly reduced the survival rate and plant dry and fresh weight,increased leaf curling and membrane lipid peroxidation,resulting in malondialdehyde(MDA)content and hydrogen peroxide(H2O2)Content and superoxide anion generation rate(O2-·)increase.Under a certain concentration range of alkali stress(0～20 mmol·L-1),overexpression of OsAPx2,OsCu/Zn-SOD,OsLOL5 and OsPPa6 can increase the activities of antioxidant enzymes and the content of osmotic regulatory substances such as superoxide dismutase(SOD),peroxidase(POD),catalase(CAT),proline(Pro)and soluble sugar(SS),which can resist partial oxidative stress and osmotic stress;However,under high-concentration alkali stress(30 mmol·L-1),the antioxidant enzyme activity and the content of osmotic adjustment substances do not increase but decrease,and the degree of oxidative stress and osmotic stress deepens.At this time,the pH value in the mixed alkali stress becomes the dominant factor that determines the degree of stress to the plant,causing irreversible damage to the crop.3.Under normal conditions,compared with the control,foliar spraying of different exogenous substances such as betaine(GB),sodium nitroprusside(NO),and melatonin(MT)has no effect on the growth of seedlings,but had certain promotion effects on seedling biomass,antioxidant enzyme activity,osmotic regulation substances and so on.Under alkali stress(20 mmol·L-1,pH 8.67),leaf spraying of GB,NO,MT can effectively alleviate the effect of alkali stress on rice leaf curling,increase seedling survival rate and plant biomass,and reduce the accumulation of MDA,H2O2,O2-· and other reactive oxygen species.The activity of chlorophyll,antioxidant enzyme and contents of proline and soluble sugar were increased,which alleviated the damage of alkali stress to Zhonghua 11 rice seedlings.Among them,the mitigation effect is GB>MT>NO.In summary,alkali stress significantly inhibited the seed germination and seedling growth of Zhonghua 11 rice,reduced the dry and fresh weight of the plant,and caused the accumulation of reactive oxygen species and the increase of MDA content in the plant.At the same time,alkali stress induced the expression of OsAPX2,OsCu/Zn-SOD,OsLOL5 and OsPPa6 gene,which in turn increased the content of antioxidant enzymes and osmotic regulators in the body,and partially alleviating the damage of alkaline stress to rice seedlings.In addition,foliar spraying of GB,NO,MT can effectively improve the symptoms of alkali damage in rice seedlings,increase the accumulation of peroxidase activity and osmotic adjustment substances,and have a certain alleviating effect on alkali stress.