Physiological Mechanisms Underlying Nitrogen Fertilizer Alleviated Salinity Stress In Cotton

Salinity is an abiotic stress factor responsible for limiting crop growth and yield throughout the world.To overcome the global challenge caused by salinity stress is reasonably important for sustainable agricultural development.The most effective approach for preventing crop losses due to salinization is to induce crop cultivars that are salt resistant or salt-tolerant.Cotton?Gossypium hirsutum L.?is a major economic crop,widely uses for fiber and oil.Even though it is ranked as moderately salt-tolerant crop among the glycophytes,however the growth and yield being severely reduced at high salinity and the tolerance variations varying among the cultivars.Therefore,it is a fundamental need to identify salt-tolerant genetic resources or germplasm and also to find out an effective management practice in combating salinity stress to increase cotton growth,yield and fiber quality.In view of that,the present study was carried out to screen out the salt-tolerant upland cotton genotypes and to reveal the salt tolerance mechanism through nutrient supplementation.The major findings of this study were summarized as follows:1.Screening and evaluation of reliable traits of upland cotton genotypes for salt tolerance at the seedling growth stageThis study subjected 11 upland cotton genotypes at the seedling growth stage to five different NaCl concentrations(0,100,150,200,and 250 mmol·L^-1)and evaluated their salt tolerance and reliable traits under hydroponic culture.The optimal NaCl concentration for the evaluation of salt tolerance was 200mmol·L^-1.Leaf relative water content and photosynthesis were identified as reliable indicators for salt tolerance at the seedling stage.All considered traits related to salt tolerance indices were significantly and positively correlated with each other except for malondialdehyde.Cluster heat map analysis based on the salt tolerance-indices clearly discriminated the 11 cotton genotypes into three different salt tolerance clusters.Cluster I represented the salt-tolerant genotypes?Z9807,Z0228,and Z7526?whereas clusters II?Z0710,Z7514,Z1910,and Z7516?and III?Z0102,Z7780,Z9648,and Z9612?represented moderately salt-tolerant and salt-sensitive genotypes,respectively.2.Effect of nitrogen application on cotton growth under salt stress and its physiological mechanismThis study assessed two cotton genotypes with different salt sensitivities to investigate the possible role of nitrogen supplementation(0.25 and 2.5 mmol·L^-1N)at the seedling stage.Salt stress-induced by sodium chloride(NaCl,200 mmol·L^-1)decreased the growth traits and dry mass production of both tolerant and sensitive genotypes.Nitrogen supplementation increased the plant water status,photosynthetic pigment synthesis,and gas exchange attributes.Addition of nitrogen to the saline media significantly decreased the generation of lethal oxidative stress biomarkers.The activity of the antioxidant defense system was up-regulated in both saline and non-saline growth media as a result of nitrogen application.Furthermore,nitrogen supplementation enhanced the accumulation of osmolytes,such as soluble sugars,soluble proteins,and free amino acids.This established the beneficial role of nitrogen by retaining additional osmolality to uphold the relative water content and protect the photosynthetic apparatus,particularly in the salt-sensitive genotype.3.Effect of nitrogen application on cotton growth and yield grown under salt stressA hydroponic experiment was conducted on cotton?Z9807,salt-tolerant and Z0102,salt-sensitive?plants to find out the effects of different levels of nitrogen(0.25,2.5 and 5 mmol·L^-1 N)on the growth and yield of upland cotton genotypes exposed to salt stress(200 mmol·L^-1 NaCl).Salt stressed cotton genotypes exhibited a marked reduction in growth traits,fresh and dry biomass accumulation.Nitrogen confirmed its effectivity by ameliorating the detrimental effects of salt stress on significant levels when supplied at 2.5 mmol·L^-1 as compared to 5 mmol·L^-1.Nitrogen application at 5 mmol·L^-1 concentration significantly increases all the morphological and physiological traits under non-saline environment.Nitrogen at 2.5 mmol·L^-1 progressively improves the root morphology of both tolerant and sensitive genotypes and ensures better nitrogen uptake under saline conditions.In addition,nitrogen supplementation at 2.5 mmol·L^-1 proved beneficial in positively enhancing the photosynthetic pigments and leaf gas exchange attributes and enhanced seed cotton yield and lint yield of cotton genotypes.Conclusively,this study result evidenced that nitrogen at moderate concentration can be advantageous in preventing NaCl induced damage.