Physiological And Biochemical Characteristics Of Cotton(Gossypium Hirsutum L.)Seedlings In Response To Exogenous Glycine Betaine And Salicylic Acid Under Salinity Stress

The elevated concentration of salt in the soil（mainly provoked by NaCl）is the most violent abiotic stress,restricting plant productivity for about 20%of irrigated lands worldwide.Glycine betaine（GB）is an osmotically dynamic compound that plays a key role in plant adaptations to various abiotic stresses,including cold stress and high saline conditions.Salicylic acid（SA）is one of the most important phytohormones that keeps a considerable place in adjusting several physiological and biochemical characteristics.SA also has diverse impacts on plant’s responses to abiotic stress.Cotton is nowadays recognized as the world’s most abundant textile fiber and known to be a remarkable salt-resilient crop.However,cotton growth under saline conditions still faces several negative influences of salt on their growth parameters,fiber quality,yield,mainly affects the germination and seedling stage.Therefore,a great understanding of cotton responses to salinity stress is necessary.It is also essential to improve cotton resistance to salinity stress through foliar supplementation of exogenous substances such as GB and SA.The main goal of the current research was to investigate the beneficial impacts of foliar sprayed GB and SA on physiological and biochemical characteristics of cotton seedling subjected to high NaCl stress.Under the condition of 150 mM NaCl,there were three levels of exogenous GB,i.e.2.5,5.0 and 7.5 mM as well as three levels of exogenous SA,i.e.1.0,1.5 and 2.0 mM.The control treatment（CK）was free of salt and without exogenous substances.The main findings are as follows:（1）Salinity stress of 150 mM NaCl not only significantly reduced leaf gas exchange and chlorophyll fluorescence parameters but also the photosynthetic pigments.The foliar spray with especially the medium concentration（5.0 mM）of GB and the lowest concentration（1.0 mM）of SA positively affected cotton gas exchange characters,chlorophyll pigments and fluorescence parameters,antioxidant enzymes activities as well as endogenous GB and SA contents when compared the NaCl-stressed alone to CK.Despite the NaCl-induced elevation of antioxidant enzymes,the above levels of exogenous GB and SA significantly improved glutathione reductase（GR）,ascorbate peroxidase（APX）,superoxide dismutase（SOD）,catalase（CAT）,peroxidase（POD）activities and decreased malondialdehyde（MDA）content under 150 mM NaCl stress.Across all levels of foliar GB and SA,the maximal photochemical efficiency of photosystem（F_V/F_M）reached its peak with the medium dose（5.0 mM）of GB.Photosynthetic rate and F_V/F_M positively correlated with chlorophyll a and b contents as a response to foliar sprayed GB and SA under the NaCl regime.Based on our results,we postulated that 5.0 mM GB and 1.0 mM SA could be optional choices for mitigating NaCl-induced damage through enhancing leaf photosynthesis,photosynthetic pigments and antioxidant enzymes activities.（2）Compared with CK,the 150 mM NaCl alone resulted in Na⁺accumulations in both root and shoot and reductions of K⁺,Ca²⁺,Mg²⁺,and biomass of roots and shoot.All levels of exogenous GB and SA markedly inversed these trends by positively regulating ions uptake as well as biomass of roots and shoot compared to the saline treatment alone.The medium concentration of GB（5.0 mM）and the lowest concentration of SA（1.0mM）were more effective in mitigating Na⁺toxicity and enhanced biomass accumulation under high NaCl regime.Plant growth characteristics including plant height,leaf area,leaf water potential as well as roots and shoot total nitrogen content were also decreased by NaCl stress,but GB of 5.0 mM and SA of 1.0 mM improved all of them under high NaCl stress.Overall,the exogenous GB and SA levels,GB of 5 mM was more active to control the percent loss of conductivity（PLC）under high NaCl stress.Under the NaCl stress of150 mM,foliar sprayed SA significantly increased both endogenous GB and SA concentrations,while exogenous GB only increased endogenous GB concentration.