Alleviating Effects And Physiological Mechanism Of Exogenous Jasmonic Acid And Humic Acid On Sorghum[Sorghum Bicolor L.Moench] Plants Under Salinity Stress

Salinity is one of the major abiotic stresses that affect crop growth and productivity worldwide.Salinity stress reduces crop yield and growth by reducing osmotic potential and causing ion toxicity and/or nutritional imbalances.Salinity can influence all the growth stages of crop growth,but salt tolerance differs greatly at different growth stages of different crop species.During seed germination,salinity reduces seed water absorption at first and then makes toxicity and changes in enzyme activities.Salt tolerance during germination,emergence and early seedling growth is critically important for crop plant survival and establishment.After the survival of crop plants is guaranteed,the reductions in yield and growth can be used as criteria for salt tolerance.Forage sorghum(Sorghum bicolor L.Moench)is one of the basic feeds for livestock and especially valuable for feeding in the world.Sorghum fodder contains 70%carbohydrates,minerals,crude fat,and nitrogen free extract.Cured sorghum fodder,with a little protein supplement,can maintain cattle in good condition throughout the winter with little or no grain supplement.Forage sorghum is considered relatively more salt-tolerant than maize and has a huge potential as a fodder crop for salt-affected areas.Sorghum is more sensitive to salinity at germination,emergence and early seedling growth than other growth and developmental stages.The appropriate application of plant growth regulators and organic matter fertilizer is an effective way to enhance germination and emergence,seedling growth,plant growth,antioxidant defense system,and yield of crops under salinity stress.However,there is little knowledge available on the impacts of salinity stress on forage sorghum and on the effects of humic acid fertilizer,exogenous and foliar applications of jasmonic acid on germination,seedling growth,yield,and physiological traits of forage sorghum.The present thesis consisted of three controlled experiments conducted at Yangzhou University,Yangzhou,China(32.39°N,119.41°E)in the 2018 and 2019 cropping seasons of forage sorghum.The first experiment was a laboratory experiment to determine suitable jasmonic acid and salinity levels.Two varieties(Sudan grass and Abu 70)were soaked in water solutions containing one of the five levels of jasmonic acid(0,2.5,5,10,and 15mM JA)and then germinated under controlled conditions.The seeds were in Petri dishes at four salinity levels of 0,100,200,and 300 mM NaCl dissolved in distilled water.The experimental design was a 3-factorial design with three factors(two varieties,five levels of jasmonic acid,and four salinity levels),arranged in a completely randomized design with three replications.Seeds were imbibed for 12 h in 500 mL of one of the exogenous jasmonic acid solutions.After imbibition,the solution was decanted off,and the seeds were re-dried near to their original weight with forced air under shade for 48 h.Un-primed seeds were served as the control.The experiment results indicated that the highest germination percentage,seedling length,and seedling fresh and dry weight were found at the levels of 5 and 10 mM jasmonic acid.Both varieties at the high salinity level of 300 mM NaCl failed to germinate.At the three salinity levels(0,100,and 200 mM NaCl),as compared with Sudan grass,Abu 70 had higher germination percentage,seedling length,and total fresh and total dry weight.The second experiment was done to determine the effects of exogenous jasmonic acid(0,5,and 10 mM JA)and humic acid(HA)(0,3,and 6 g HA kg-1 soil)and different NaCl salinity levels(0,100,and 200 mM NaCl)on forage sorghum seedling variety Abu 70.Seedling emergence percentage,seedling emergence rate,seedling characteristics,salt tolerance traits,and physiological parameters were tested.The experimental design was factorial design with three factors,arranged in a completely randomized design with three replications.This experiment was done twice.The results indicated that the high salinity level of 200 mM NaCl reduced seedling emergence percentage(SEP),seedling emergence rate(SER),shoot and root length,fresh and dry weight,salt tolerance index,seedling vigor index,emergence seedling index,emergence stress tolerance index(ESTI)and dry weight stress tolerance index(DWSTI).The antioxidant enzyme activities of the peroxide(POD),catalase(CAT),and ascorbate peroxidase(APX)activity,and the total chlorophyll content decreased with salinity.Nevertheless,high salinity level increased proline and soluble protein content,superoxide dismutase activity(SOD),and malondialdehyde(MDA)content.The effect of jasmonic acid on ESP,ESR and seedling growth traits and physiological parameters in salinity was positively affect except CAT and MDA.At the high salinity level of 200 mM NaCl,plants treated with the high humic acid rate of 6 g HA kg-1 dry soil positively affected seedling characteristics and physiological parameters compared to the control.The application of humic acid at 6 g HA kg-1 dry soil decreased SOD activity and ESI relative to the control at the high salinity level.The effect of interaction between jasmonic acid and salinity was significant in most parameters measured except root length,seedlings vigor index,SOD,POD and MDA.The ESP,ESR,shoot length,total fresh weight,seedling vigor index,salt tolerance index,POD,SOD,CAT,APX and MDA and total chlorophyll content were significantly affected by the interaction between jasmonic acid and humic acid.Most growth parameters,soluble protein content and chlorophyll content of forage sorghum seedling were significant affected by the interaction between salinity and humic acid.All seedling growth parameters,seedling emergence index,seedling emergence rate,soluble protein content,proline content and total chlorophyll content were significantly affected by salinity stress.The seeds priming on jasmonic acid did not significantly affect SER,seedling shoot length,seedling vigor index,salt tolerance index and dry weight salt tolerance index.Humic acid application showed a significant difference in many parameters tested except emergence seedling index,emergence seedling rate,emergence stress tolerance index,total fresh weight,salt tolerance index,and proline content.SEP,SER,root length,and DWSTI were significantly affected by the interaction among three experiment factors.The combination of jasmonic acid at 10 mM JA and humic acid at 6g HA kg-1 dry soil was most effective in alleviating salinity stress at seedling stage.The third experiment was a pot study conducted under greenhouse conditions in the two cropping seasons of 2018 and 2019.It was done to assess whether the exogenous application of jasmonic acid(0,5,and 10 mM JA)as a foliar spray and humic acid fertilizer(0,3 and 6 g HA kg-1 dry soil)could alleviate the adverse effects of salinity stress(0,2,and 4 g NaCl kg-1 dry soil)on forage sorghum variety Abu 70.The experimental design was factorial design with three factors,arranged in a completely randomized design with three replications.At the vegetative growth stage and booting stage,stem diameter,leaf area/plant,physiological responses in terms of photo synthetic rate,stomatal conductance,transpiration rate,and chlorophyll content(as SPAD reading)were measured.When plants reached the boot and milk stages the parameters of plant height,elongation percentage,leaf area,relative growth rate,plant fresh and dry weight were measured.At boot stage,soluble protein content and proline content,chlorophyll a,b,carotenoid content,SOD,APX,POD,and CAT were measured.At soft dough stage,nitrogen uptake and sugar content were measured.Salinity stress and humic acid application showed significant effects on all growth parameters.In addition,jasmonic acid had a significant effect on most growth traits except elongation percentage,relative growth rate,dry weight in the second season(2019)and stem dimeter in the first season(2018).Interaction between salinity and humic acid significantly affected plant height,leaf area,relative growth rate(2018),stem diameter,fresh weight and dry weight(201 8).The interaction between humic acid and jasmonic acid showed a significant effect on most growth parameters except leaf area,relative growth rate in the two growing seasons and stem dimeter in the first season.The interaction between interaction between salinity and jasmonic acid showed a significant effect on most growth parameters.However,significant differences were observed regarding the effect of interaction among salinity stress and jasmonic acid and humic acid treatments on elongation percentage,relative growth rate,dry weight in the two seasons.Results showed that jasmonic acid significantly reduced the negative effects of NaCl salinity stress.Humic acid application improved plant height,stem dimeters,leaf area,and dry weight significantly affected under saline stress.Conversely,salinity stress had harmful effects on all parameters,gradually reduced with increased salinity stress.Humic acid and jasmonic acid alleviated the negative effects of salinity stress on the plant growth parameters.Salinity stress increased carotenoid content,protein content,and antioxidant enzymes of SOD.In contrast,salinity stress reduced proline content,MDA,POD,CAT,and APX.Under salinity stress,JA and HA significantly improved all the parameters tested.Moreover,10 mM JA and 3g HA kg-1 dry soil is more efficient in most of the parameters than the 5 mM JA level and 6 g Ha kg-1 dry soil.Effects of variables of salinity,humic acid and jasmonic acid on chlorophyll a and b,carotenoid contents and antioxidant activity were analyzed for significance at the P<0.05,P<0.01 or P<0.001 levels.Significant effects of salinity and humic acid were observed on all the parameters except chlorophyll b in the two seasons,soluble protein and proline content in the 2019 season.Only POD activity was not significantly affected by humic acid.However,no significant effects were exerted by jasmonic acid on POD activity in the two seasons,and chlorophyll b,SOD,and CAT in the second season.Significant effects of salinity on most of parameters except chlorophyll a,proline content,CAT in the 2019 and SOD in the 2018.The interaction of salinity with jasmonic acid was revealed significant effects on chlorophyll a,carotenoid content,protein content,proline content(2018),SOD(2019),POD(2019),CAT and MDA.While the interaction between jasmonic acid and humic acid exerted significant effects on all parameters(P<0.05).The interaction among three experiment factors(salinity x humic acid x jasmonic acid)had a significant effect(p<0.05)on proline content,SOD,POD,CAT,MDA in the growing seasons(2018 and 2019)and APX for second season(2019).In the present study,chlorophyll content,photosynthesis parameters,sugar content,starch content,and glucose content of forage sorghum were measured.The results showed that high salinity caused significant reductions in nitrogen uptake,chlorophyll content,photosynthesis parameters,sugar content,and glucose.High salinity level increased starch content in the dry weight of the forage sorghum.The interactive effect between jasmonic acid and salinity was significant in most parameters except photosynthesis rate.The chlorophyll content,photosynthesis rate,stomatal content,starch content and glucose content were significantly affected by the interaction between jasmonic acid and humic acid.All photosynthesis parameters and sugar content were significantly affected by the interactive impact between humic acid and salinity.The humic acid and jasmonic acid application were successfully mitigating the adverse effects of salinity.The 5 mM JA with 3 HA g kg-1 dry soil rate was most influential on many of the attributes studied.Salinity stress,jasmonic acid and humic acid application had significant effects on all the physiological and biochemical parameters except chlorophyll content(SPAD reading)and photosynthesis parameters not affected by the jasmonic acid.All parameters significantly affected by the interaction between salinity and jasmonic acid.The interactive effect between humic acid and salinity was significant on the all the photosynthesis parameters,sugar content and nitrogen content.The chlorophyll content,photosynthetic rate,stomatal content,starch content and glucose content were significantly affected by the interaction between humic acid and jasmonic acid.All photosynthesis parameters and most of biochemical traits were significant difference by the interaction among three experiment factors except glucose content.In this study,the application of humic acid as fertilizer and jasmonic acid was successful in alleviating the adverse effects of salinity.Our study suggested that exogenously applied jasmonic acid and humic acid fertilizer mitigated salt stress inhibitory effect on these variables at all salinity levels.Exogenously applied JA counteracted the inhibitory effects of NaCl by increasing all parameters tested.The humic acid significantly affected all the parameters in the first and second seasons by the enhancement of chlorophyll a,b,photosynthetic rate,stomatal conductance,and nitrogen uptake,chlorophyll content,sugar content,glucose content,and starch content.Moreover,3 HA g kg-1 dry soil rate was more efficient in most parameters than the control and 6 g HA kg-1 dry soil rate.The application of 10 mM JA combined with 6 g HA kg-1 soil was most effective in alleviating salinity stress.The application of jasmonic acid at appropriate levels either by seed priming or by spraying was successful in ameliorating the negative effects of salinity stress on germination,emergence,seedling growth,photosynthetic efficiency,antioxidant defense system,and fresh and dry yield of forage sorghum.The appropriate application of humic acid fertilizer could relieve the detrimental effects of salinity stress on forage sorghum.The appropriate application of jasmonic acid and humic acid can serve as a feasible way to promote germination,emergence,seedling growth and development,photosynthesis and optimize physiological and biochemical attributes.