Metabolic Response Of Cotton Seedlings To Soil Salinity Under Drought Conditions

In this study,potted cotton was taken for experiments.The pots were divided into four treatments control?CK?,single salt stress?S?,single drought stress?D?and overlapped drought and salt stresses?D+S??where the salt concentration both in single salt stress and drought-salt overlapped stresses were 100 mmol/L NaCl,a range of moderate and mild salt stress for cotton,respectively?.The water status,growth,photosynthesis,osmotic regulation,antioxidant activity,material and energy utilization of cotton treated with different stress duration were measured and analyzed,and the metabolites and metabolic pathways were analyzed through LC-MS-based metabonomics to explore the effects of soil salinity on cotton metabolism under drought stress.The results are as follows:?1?The presence of appropriate soil salinity under drought conditions improved the photosynthetic performance and growth of cotton seedlings.With the prolongation of stress treatment,the intercellular CO₂ concentration of cotton leaves under single drought stress treatment first decreased and then increased,the maximum photochemical efficiency significantly decreased and the initial fluorescence significantly increased,that is,the single drought stress resulted in the damage of photosynthetic organs of cotton leaves,which resulted in a significant reduction of net photosynthetic rate,while the photosynthetic system of cotton leaves under overlapped drought and salt stresses treatment was more affected than that under overlapped drought and salt stresses treatment.Plant height and biomass were also significantly higher in the dry salt combination treatment than in the single drought.It indicated that the addition of proper amount of NaCl in soil could alleviate the adverse effects of drought stress on photosynthesis and growth of cotton seedlings.?2?The presence of appropriate soil salinity under drought conditions improved the antioxidant activity of cotton leaves.Compared with the cottons having normal water supply,drought stress significantly increased the activity of antioxidant enzymes and the content of malondialdehyde.However,under the same drought conditions,the changes of superoxide dismutase,peroxidase,catalase activity and malondialdehyde content in cotton leaves treated with single drought stress were significantly higher than those treated with the overlapped stresses of drought and salt,and the content of malondialdehyde in cotton leaves treated with single drought stress on the 30th day of drought treatment was about twice as much as those treated with combination of drought and salt stresses.The results showed that the presence of appropriate soil salinity under drought conditions could alleviate the degree of oxidative stress on cotton leaves,consequently alleviate the damage of drought on cell membrane system and photosynthetic apparatus.?3?The presence of appropriate soil salinity under drought changed the osmotic regulation of cotton leaves.Drought stress resulted in significant decreases in water potential,osmotic potential,cell swelling pressure and water content of cotton leaves,but the comparison of single drought stress and drought-salt combination treatment showed that the latter had significantly higher cell swelling pressure and water content than the former.Further analysis of the osmotic substances showed that the organic osmotic substance content and osmotic concentration of cotton leaves treated with single drought stress were significantly higher than those treated with combination of drought and salt stresses,while the inorganic osmotic substance content and osmotic concentration were opposite.There was no significant difference in K⁺,Ca²⁺ and Mg²⁺ contents in inorganic osmotic substances,but Na⁺ and Cl^- Contents in combination of drought and salt stresses treatment were significantly higher than that in single drought stress treatment.These results indicate that compared with single drought stress,the combination of drought and salt stresses can result in the absorption and accumulation of more Na⁺ and Cl^- from the outside for osmotic regulation,thus reducing the material and energy consumption of the synthesis of organic substances for osmotic adjustment,and it seems more economical and effective to increase the contribution of inorganic ions in osmotic regulation.?4?The existence of appropriate soil salinity under drought conditions reduced the construction cost of cotton leavesDrought stress reduced the leaf area and calorific value of cotton leaves.Under the same drought conditions,the specific leaf area and ash content of cotton leaves treated with combination of drought and salt stresses were significantly higher than those treated with single drought stress,which made the construction cost per unit leaf area of the former significantly lower than that of the latter.This implied that a proper amount of soil salt might have used more inorganic ions to save their organic assimilates and energy consumption under drought stress.?5?The presence of appropriate soil salinity under drought alleviated the interference of drought on cotton metabolism.LC-MS-based analysis of metabolites showed that single salt stress,drought and drought-salt combination significantly changed the metabolism of cotton leaves.Among them,there were 475 different metabolites between single drought and control,of which 163 were up-regulated and 312 were down-regulated,of which 9 metabolic pathways were significantly different between them;there were 460 different metabolites between drought-salt combination and control,of which 157 were up-regulated and 303 were down-regulated,where 4 metabolic pathways were very significantly different between the cotton seedlings treated with single drought and drought-salt,and 2 metabolic pathways were very significantly different within the 2 groups.It can be seen that the disturbance of single drought stress on cotton leaf metabolism was greater than that of combination of drought and salt,indicating that a proper amount of salt in soil can slow down the impact of drought on cotton leaf metabolism.In conclusion,the presence of appropriate NaCl in soil under drought stress could promote cotton to absorb and accumulate more Na⁺ and Cl^-,which not just enhanced the inorganic osmotic regulation,improved the water status in plant,reduced the membrane lipid peroxidation,thus alleviated the damage of cells and photosynthetic organs under drought stress,but also reduced the synthesis of organic osmolytes and their consumption.In this way,presence of appropriate NaCl could alleviate the damage of drought on metabolism and provide more organic substances and energy for growth and other defensive responses,ultimately enhance the growth of cotton seedling and drought resistance.