The Role And Mechanism Of Exogenous Plant Growth Regulators In Mitigating Salinity Stress Of Chickpea

Salinity is one of the most significant abiotic stresses affecting crop growth and productivity around the world.Salt stress is typically associated with increased toxic ion accumulation in plants,decreased photosynthetic rate,and subsequent growth.Salinity can influence crop growth at all stages,but salt tolerance varies greatly between crop species and growth stages.Salinity reduces seed water absorption at first and then causes toxicity and changes in enzyme activities during seed germination.After crop plant survival is assured,yield and growth reductions can be used as salt tolerance criteria.Chickpea(Cicer arietinum L.)is an important pulse crop for human consumption,animal feed,and nitrogen fixation in soil around the world.Chickpeas contain a high concentration of amino acids,vitamins,phosphorus,potassium,iron,calcium,and magnesium.However,chickpea is extremely sensitive to salinity stress and is generally grown in semi-arid areas prone to soil salinity.Plant growth regulators,when used correctly,can improve crop germination and emergence,seedling growth,plant growth,the antioxidant defense system,and yield under salinity stress.However,little is known about the effects of salinity stress on chickpeas and the effects of exogenous applications of β-aminobutyric,melatonin,and fulvic acid on chickpea germination,seedling growth,yield,and physiological traits.The studies of the present thesis consisted of four experiments that were conducted at Yangzhou University,Yangzhou,China(32.39°N,119.41°E)during the 2019 and 2022 cropping seasons of chickpeas.The first was a laboratory experiment done to examine the effects of seed priming with different levels of β-aminobutyric acid(BABA)under salinity stress.The seeds of three chickpea varieties(Jabelmarra,Porgeg,and Shendi)were primed in water solutions containing one of the three levels of BABA(0,0.25,and 0.50 mM)and then germinated in Petri dishes under controlled conditions.The seeds were germinated in Petri dishes at salinity levels of 0,50,100,and 150 mM NaCl.The experimental design was a factorial design with three factors(three varieties,three levels of β-aminobutyric acid,and four salinity levels),arranged in a completely randomized design with three replications.The seed priming process with BABA is conducted according to the following:Twenty-five seeds from each treatment were soaked in various concentrations of BABA solution for 12 hours at room temperature and then re-dried for 48 hours to near their original weight.The attributes of water uptake,germination,and early seedling growth were measured.The results of this experiment indicated that a high level of salinity(150 mM NaCl)increased the mean germination time and decreased the coefficient of germination velocity and root dry weight.’Jabelmarra’outperformed other studied varieties in germination characteristics,while ’Shendi’outperformed other varieties in root and shoot lengths and fresh and dry weights at seedling growth.Seeds treated with 0.25 mM BABA caused an increase in water uptake.Seeds primed with 0.50 mM BABA increased the germination percentage of’Jabelmrra’ and ’Porgeg’ at 150 mM NaCl.Root length,shoot length,shoot fresh weight,and root dry weight were all increased in the presence of 50 mM NaCl and 0.50 mM BABA.The current study’s findings suggested that the application of BABA as seed priming could be beneficial in alleviating chickpea salinity stress during germination and early seedling growth.The second experiment was done to determine the effects of foliar spray with exogenous melatonin(0.0,1.0 mM MT),BABA(0,1.0 mM BABA),and the combination of the two PGRs under different NaCl salinity levels(0.0,75.0,and 150.0 mM NaCl)on chickpea seedlings of the Sudanese variety "Salwa".The experiment was a factorial design arranged in a completely randomized design with three replications.This experiment was done twice.Seedling characteristics and physiological parameters were measured.The experiment results indicated that salinity gradually decreased length,project area,surface area,fresh weight,dry weight,and crossing of the root.Salinity stress significantly reduced root diameter,root volume,and root tips.Furthermore,salt stress caused reductions in shoot length,number of leaves,number of branches,chlorophyll content,and CAT activity.The activity of APX and content MDA increased with the increasing salinity.Moreover,salt stress of 150 mM NaCl substantially raised protein and proline contents.The combination of the two PGRs caused an increase in root volume and antioxidant enzyme activities(APX and CAT).While BABA increased chlorophyll a and b,MT increased proline content.Exogenous MT and BABA applications successfully improved salt stress damage on chickpea seedlings.The third experiment was carried out to determine the impact of melatonin(MT)(1.0 and 2.0 mM),fulvic acid(FA)(2.0 and 4.0 g FA kg-1 dry soil),and the combination of(2.0 mM MT with 4.0 g FA)application on chickpea seedlings of the Sudanese variety "Salwa"grown under two salinity stresses(4.3 and 7.7 dS m-1 NaCl).The study was designed as a 2-factorial experiment arranged in a completely randomized design with three replications for each experimental unit.Salinity stress significantly reduced root proliferation characteristics(length,project area,surface area,volume,tips number,crossing,forks,and fresh weight of the root),shoot features(shoot length,branches number,leaves number,and shoot fresh and dry),leaf relative water content,and chlorophyll a and b content.Moreover,salt stress increased the activity of antioxidant enzymes and contents of soluble protein and proline,as well as MDA content.The application of two PGRs and their combination treatment significantly improved the above-mentioned parameters under salinity stress.The fourth experiment was designed to evaluate the growth and yield under different levels of salinity(1.2,1.8,and 3.1 dS m-1NaCl)and foliar sprays with BABA(0.0,0.25,and 0.75 mM)on two Sudanese chickpea varieties(Shendi and Salwa).BABA was applied as a foliar spray on the 21th day after seeding and repeated every 10 days until the anthesis stage.This study was a 3-factorial experiment in a split-split plot design with three replications for each treatment,with BABA as the main factor,salinity as the sub-plot,and variety as the sub-subplot.This experiment was conducted twice during the chickpea growing seasons of 2019-2020 and 2020-2021.Plant height,number of branches,and stem diameter were measured on the 40th day after seeding.Physiological attributes were determined three months after seeding,while yield characteristics were measured after field maturity.Salinity stress reduced plant height,number of branches,stem diameter,shoot dry weight,and chlorophyll a and b.Moreover,salt stress declined yield traits such as pod number,hundred-grain weight,and plant yield and increased the percentage of empty pods in seasons 2019-2020 and 2020-2021.However,the application of BABA significantly increased plant height,branches number,stem diameter,shoot dry weight,and chlorophyll content of the two varieties in both seasons.Furthermore,spraying with BABA reduced MDA content and increased antioxidant enzyme activity.BABA increased CAT and APX activities of the two varieties in both seasons.At 1.8 dS m-1 NaCl,0.25 mM BABA significantly enhanced the SOD and POD activities of Shendi in season 2019-2020.Exogenous BABA treatment significantly increased the number of pods,grain weight,and plant yield.The results of this experiment demonstrated that salinity stress negatively affected the vegetative traits of chickpea plants,resulting in a decrease in plant yield.On the other hand,plants treated with BABA had better vegetative characteristics and yield components,leading to an improvement in plant production.In general,exogenous application of BABA at appropriate doses as a seed priming successfully alleviated the adverse effects of salinity stress on seed germination and early seedling growth,such as water uptake,germination percentage,germination index,seed vigor index,coefficient velocity of germination,and allometry.At the seedling stage,the application of MT and BABA and their combination at suitable levels by spraying was successful in alleviating the harmful effects of salinity stress on root and shoot characteristics,chlorophyll a and b,antioxidant enzyme activity(CAT and APX),MDA,soluble protein,and proline contents.Moreover,the appropriate application of FA as soil supplementation and MT by seeds soaking and their combined treatment successfully mitigated the negative impact of salt stress on seedling growth,photosynthetic pigments,MDA content,and antioxidant enzyme activity,including SOD,POD,and APX in chickpea seedlings.The application of BABA as a foliar spray during the vegetative and reproductive stages effectively enhanced vegetative growth and yield components of chickpea plants exposed to salinity stress.The appropriate application of exogenous PGRs through seed priming,soil supplementation,and foliar spraying can serve as a feasible way to promote seed germination,seedling growth,and yield development.The findings of this thesis provided a practical strategy for mitigating the negative effects of salinity stress on the growth and development of the chickpea crop.It is an effective strategy for improving chickpea production in saline soil regions.