Effects Of CO₂ Enrichment On Seedling Growth And Physiological And Biochemical Characteristics Of Tomato Under Salt Stress

With the development of facility agriculture,tomato production technology system has been constantly improved.However,the secondary salinization of facility soil and CO₂ deficiency have seriously affected the yield and quality of tomato,which has become the bottleneck problem of improving quality and efficiency.It is an effective way to improve the quality and efficiency of greenhouse vegetables to regulate the concentration of CO₂ in cultivation facilities and enhance crop photosynthesis through CO₂ fertilization?i.e.CO₂ enrichment?.At present,the regulation mechanism of CO₂ enrichment on crop growth and physiological and biochemical characteristics under secondary salinization of protected soil is still unclear.Therefore,the growth,water metabolism,photosynthetic fluorescence,mineral nutrients,antioxidant,osmotic regulation and endogenous polyamines content of Tomato Seedlings under Ca?NO₃?₂ stress were studied with 80 mmol/L Ca?NO₃?₂ Stress Simulated facility soil secondary salinization and 800⁺40 mmol/mol simulated CO₂ enrichment environment.The aim of this study was to explore the mitigation mechanism of CO₂ enrichment on Ca?NO₃?₂ stress injury in tomato,and to provide appropriate reference for CO₂ fertilization to deal with secondary salinization of greenhouse soil and salt-resistant cultivation of greenhouse vegetables.The main results are as follows:1.Ca?NO₃?₂ stress significantly inhibited the growth of tomato plants.The biomass accumulation of tomato seedlings,leaf water potential,relative water content of leaves,total root length,total root surface area and hydraulic conductivity of roots were significantly reduced.CO₂ enrichment significantly promoted the accumulation of plant biomass,increased the total root length and total surface area of salt-stressed plants,effectively alleviated the decline of relative water content and water potential of leaves,promoted the absorption of water by roots,enhanced the water-holding capacity of salt-stressed plants to a certain extent,improved the water status of plants,and enhanced their tolerance to Ca?NO₃?₂ stress.2.Under Ca?NO₃?₂ stress,the photosynthetic pigments content,net photosynthetic rate,stomatal conductance,transpiration rate,actual photochemical efficiency of PS II,apparent photosynthetic electron transfer rate,photochemical quenching coefficient and K⁺,Na⁺,Mg²⁺ content of tomato leaves decreased,while water use efficiency,maximum photochemical efficiency of PS II,non-photochemical quenching coefficient and Ca²⁺ content increased,and the light intensity of plants under salt stress was increased.Stomatal factors were the main limiting factors.CO₂ enrichment can improve photochemical efficiency and electron transfer efficiency,ensure the normal formation of photosynthetic assimilation,accelerate the primary reaction process of PS II,thus alleviating stomatal limitation of photosynthetic rate reduction of tomato leaves under Ca?NO₃?₂ stress,and maintain the relative balance of main mineral nutrients in plants.3.Ca?NO₃?₂ stress caused serious oxidative damage to cell membrane lipids of tomato leaves and roots.The electrolyte permeability,malondialdehyde,superoxide anion and hydrogen peroxide content increased significantly.The activities of main antioxidant enzymes?superoxide dismutase,peroxidase and catalase?decreased significantly,which destroyed the dynamic balance of production and elimination of reactive oxygen species in tomato plants.CO₂ enrichment increased the activity of major antioxidant enzymes,reduced the accumulation of reactive oxygen species,and alleviated the membrane lipid peroxidation injury caused by Ca?NO₃?₂ stress.4.Ca?NO₃?₂ stress induced the accumulation of osmotic regulators such as soluble sugar,free amino acid and proline in tomato leaves and roots.The osmotic potential of tomato plants was reduced and the damage caused by salt stress was alleviated.Ca?NO₃?₂ stress inhibited the synthesis of soluble protein and significantly reduced the soluble protein in tomato leaves.CO₂ enrichment significantly promoted the synthesis and accumulation of soluble protein and proline in leaves of salt-stressed plants,contributed to the recovery of root function,improved the water absorption capacity of cells,thus enhancing the salt tolerance of tomato plants;CO₂ promoted plant growth,more soluble sugar was used for the formation of carbon skeleton,and reduced the soluble sugar content in plants.5.Under Ca?NO₃?₂ stress,the contents of free form,bound polyamines and bound spermidine in tomato leaves decreased to varying degrees,while the contents of bound putrescine and spermidine increased.It can be seen that salt stress induced the accumulation of bound polyamines in tomato plants to resist the damage caused by salt stress.CO₂ enrichment can reduce the damage of putrescine to plants by promoting the transformation of spermidine and spermine from binding state to free state,and inhibiting the transformation of putrescine from binding state to free state.At the same time,it can promote the accumulation of bound polyamines,maintain the integrity of cell membrane structure and function,and enhance the adaptability of salt-stressed plants.These results indicated that CO₂ enrichment could enhance the tolerance of tomato seedlings to Ca?NO₃?₂ stress by alleviating oxidative damage of cell membrane,improving osmotic regulation ability,promoting photosynthesis,maintaining water and main mineral elements homeostasis,and regulating the accumulation levels of polyamines of different species and forms.