| Wheat is a major staple crop and the most widely cultivated cereal crop in the world,suitable for being grown in nearly every region.However,the production of wheat has been hampered by various abiotic stresses that frequently occurred in the world.Drought and high temperature are the most critical abiotic stresses affecting crop growth and productivity around the world.The appropriate application of plant growth regulators and other substances is an effective way to enhance germination,seedling growth,plant growth,osmotic adjustment,antioxidant defense system,and yield of crops under high temperature and drought stresses.In many tropical and dry regions in the world(like Sudan),wheat is seriously inhibited by heat and drought,resulting into the failure in germination,early seedling growth and crop establishment.The study of the present thesis consisted of four controlled experiments that were conducted at Yangzhou University,Yangzhou,China(32.39°N,119.41°E)in the 2019-2020 and 20202021 cropping seasons of wheat.The first experiment was a laboratory experiment to examine the effects of seed priming with different levels of 5-ALA at high temperature and drought stresses.The seeds of three Sudanese wheat varieties(Emam,Gomria,and Zakia)were primed in water solutions containing one of the four levels of 5-ALA(0,25,50,and 100 mg L-1)and then germinated in petri dishes under three temperature levels(25,30,and 35℃)and two water potential levels(0.0 and-0.5 MPa).The characteristics of water uptake,germination,and early seedling growth were measured.The results of this experiment indicated that heat and water potential stress significantly reduced seed water imbibition and the characteristics of germination and seedling establishment.Lower germination percentage,germination index,seedling vigour index,length,and biomass of radicle and shoot were obtained under drought and high temperature stresses.Whereas these attributes were considerably decreased by the combined effect of these two factors.When compared with the control,the temperature of 30℃decreased mean seed germination time,whereas the temperature of 35℃ significantly increased the time of germination;and the greatest increase in time was at high temperature of 35℃ and drought stress.Seed priming with 5-ALA at suitable levels significantly improved germination percentage,germination index,seedling vigour index,lengths of radicle and shoot,and fresh and dry weights of radicle and shoot.Under high temperature and/or drought stresses,Emam variety had a higher seed water uptake amount,germination percentage,and germination index.This study suggested that seed priming with 5-ALA is an effective strategy for mitigating the detrimental effects of drought and temperature stress on wheat imposed at germination and early seedling growth.The second experiment was carried out to the determine impact of foliar spray with exogenous 5-ALA(0,25,50,100,and 150 mg L-1)application on wheat seedlings of Sudanese variety"Emarm" grown under various temperature(25,30,and 35℃)and water regime(normal and 65%watering)conditions.5-ALA was applied to seedlings until the leaves dripped on the 6th day after seed sowing,and then repeated every three days until the seedlings were sampled for determination.Leaf area,stem diameter,shoot and root length,shoot and root fresh and dry weight,and physiological parameters were measured.The experiment results indicated that both high temperature and drought stresses significantly declined shoot length,stem diameter,leaf area,and shoot fresh weight.The highest malondialdehyde(MDA)content was recorded at the high temperature of 35℃.Under the combined stress of a high temperature of 35℃ and drought,the dry weights of root and shoot and SPAD substantially declined.The highest shoot length,leaf area,and root dry weight were obtained at a concentration of 25 mg L-1.In terms of the interaction between temperature and 5-ALA,25 mg L-1 5-ALA had a maximum shoot dry weight at 25℃ and 35℃,respectively.Foliar spray with 150 mg L-1 5-ALA increased root fresh weight by 101.0%compared with 0 mg L-1 5-ALA at normal watering and high temperature of 35℃.Furthermore,spraying of 50 mg L-1 5-ALA resulted in highest soluble protein content at water deficit and high temperature of 35℃,whereas spraying of 150 mg L15-ALA increased proline content by 66.5%relative to 0 mg L-1.The findings of this experiment indicated that foliar application of 5-ALA at appropriate levels is beneficial in alleviating temperature and/or drought stress in wheat seedlings.The third experiment was conducted to determine the role of glutathione(GSH)amendment at various concentrations(0.0,0.5,1.0,and 2.0 mM)to soil in mitigating the negative impact of High temperature(33℃,with 25℃ as control),water deficit(60%of field capacity,with full field capacity as control),and their combinations on the seedling stage of Sudanese wheat variety "Gomria".This experiment was conducted twice.Root proliferation,shoot growth,physiological and biochemical parameters were assessed in this experiment.High temperature decreased all root proliferation characteristics(length,project area,surface area,volume,and forks of the root),except for the root tips.With the exception of root diameter,drought stress increased all of these characteristics.High temperature and drought significantly declined shoot length,leaf area,leaf relative water content,and shoot and root weights,all of which were substantially reduced when high temperature and drought were combined.GSH significantly promoted the root system,shoot growth,and leaf relative water content of seedlings,as compared with non-treated seedlings.Heat stress lowered the contents of chlorophyll a,chlorophyll b.and total chlorophyll much more than drought stress,and their combined effect was greater than their individual effects.Exogenous application of GSH significantly increased the contents of chlorophyll a,chlorophyll b,and total chlorophyll.The content of MDA and the content of soluble protein was increased gradually under high temperature,drought,and their combination stresses,respectively.On the other hand,GSH positively reduced MDA content and increased soluble protein content of seedlings under stress and normal conditions.Moreover,seedlings treated with 2.0 mM GSH showed the highest soluble protein content at different treatment conditions.The activities of antioxidant enzymes were significantly increased by the exogenous application of GSH.Superoxide dismutase(SOD)and catalase(CAT)activity were increased with application of GSH at high temperature and/or drought stresses.According to the results of this experiment,GSH at suitable concentrations can alleviate wheat seedlings from the negative effects of High temperature and/or drought stress.The fourth experiment was designed to evaluate the growth and yield under different levels of temperature(25℃,30℃,and 35℃)and foliar sprays with 5-aminolevulinic acid(0 and 25 mg L-1).Three Sudanese wheat varieties(Emam,Gomria,and Zakia)were exposed to different temperatures for one week on the 15th day after seedling growth.This experiment was conducted twice in the two growing seasons of wheat.Plant height,leaf area,tiller numbers,and leaf numbers were measured at the tillers formed stage(60th day after seeding)and at the flowering stage(120th day after seeding).Physiological attributes were determined at the booting stage(100th day after seeding),while yield characteristics were measured after field maturity.The 5-ALA was applied as a foliar spray on the fifth week of the plant’s life and repeated every 15 days until the anthesis stage.This study was a 3-factorial arranged in a splitsplit plot design with three replications for each treatment,with 5-ALA as the main factor,the temperature as the sub-plot,and variety as the sub-sub plot.Seedlings exposure to high temperature of 35℃ considerably reduced plant height,leaf area,number of tillers and leaves on the 60th day after seeding as compared with 25℃.While on the 120th day after seeding,35℃ at the early seedling stage had a slight effect on plant height and leaf area,and had a significant effect on the number of tillers and leaves.When compared with the control plants,sprayed 5-ALA significantly increased the number of tillers and leaves of the plants on the 60th day after seeding,and leaf area on the 120th day after seeding.The fresh and dry weights of the three varieties were raised by the spraying of 5-ALA at different temperature regimes.On the 100th day after seeding,photosynthetic attributes such as photosynthesis rate,CO2 concentration,stomata conductance,and photorespiration rate of the three wheat varieties were unaffected significantly by the temperature treatment and hormone application.The antioxidant enzyme activity of varieties showed variation in response to temperature and 5ALA.As compared with 25℃,the activity of POD in Emam was decreased with the temperature of 30℃ and increased with the temperature of 35℃.Plants treated with 5-ALA had higher SOD and POD activity than untreated plants.When seedlings were subjected to both high temperatures,the spike numbers and plant yield were significantly reduced as compared to control.However,the grain weight increased gradually as the temperature increased.Exogenous spraying of 5-ALA boosted spike numbers,thousand-grain weight,and plant yield in all three varieties.The results of this experiment demonstrated that early exposure to high temperatures negatively affected the vegetative traits of wheat plants.resulting in a decrease in plant yield.Plants treated with 5-ALA had better vegetative characteristics and yield components,leading to an improvement in plant production.In general,exogenous application of 5-ALA at appropriate doses via seed priming or spraying was successful in alleviating the adverse effects of high temperature and/or drought stresses on germination,seedling growth,fresh and dry weights,and osmatic adjustment of wheat.Application of GSH at suitable levels as soil supplementation successfully ameliorated the negative impact produced by high temperature and/or drought stresses on seedling growth,root system,fresh and dry weights,photosynthetic efficiency,and antioxidant defense system of wheat.Application of 5-ALA as a foliar spray during the vegetative stage effectively enhanced the vegetative growth and yield traits of wheat plants exposed to early high temperature stress.The results of this thesis provided a practical strategy for mitigating the detrimental effects of high temperature and drought on wheat imposed at germination and seedling stages.It is a useful strategy to alleviate abiotic stresses on wheat production in the areas affected by high temperature and drought. |
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