Studies On Physiological Mechanism Of CO₂ Enrichment Improved Salt Tolerance In Cucumber Seedlings

We investigated the physiological mechanism of CO₂ enrichment improved salt tolerance in cucumber?Cucumis sativus L.‘Jinyou No.35'?seedlings.In the split-plot design used,the main treatment consisted of two CO₂ concentration levels[ambient[CO₂]?400?mol·mol^-and enriched[CO₂]=?800±40??mol·mol^-1]and the subplot had two levels of salinity treatment(0 and 80 mmol·L^-1 NaCl).The main results were as follows:1.After 7 days of experimental treatment,salt stress caused a significant reduction in growth rate?i.e.,increase in plant height,stem thickness,leaf area,and the growth of both shoot and root?,ribulose-1,5-bisphosphate carboxylase?RuBPCase?activity,net photosynthetic rate?P_n?of the leaves,stomatal conductance?G_s?,and transpiration rate?T_r?of cucumber seedlings.Salt stress decreased the chlorophyll[chlorophyll a,chlorophyll b,carotenoids,and chlorophyll?a+b?]content,electron transport rate?ETR?,quantum of PSII(?_PSII),and photochemical quenching?qP?but markedly increased non-photochemical quenching?NPQ?;there were no significant differences in chlorophyll a/b and maximum quantum yield of PSII?F_v/F_m?between control and salt stress treatments.Enriched[CO₂]increased the plant height,stem thickness,leaf area,and shoot fresh weight of cucumber seedlings significantly under salt stress.Additionally,enriched[CO₂]decreased the content of chlorophyll[chlorophyll a,chlorophyll b,carotenoid,and chlorophyll?a+b?],stomatal conductance,and transpiration rate but markedly increased P_n,ETR,?_PSII,and RuBPCase activity in leaves of cucumber seedlings under salt stress.2.The leaf respiratory rates significantly decreased when exposed to salt stress.Salt stress also significantly reduced hexokinase?HK?,phosphofructokinase?PFK?,pyruvate kinase?PK?,isocitrate dehydrogenase?IDH?,succinate dehydrogenase?SDH?and malate dehydrogenase?MDH?activities,causing decreases in the pyruvate,citrate,nicotinamide adenine dinucleotide?NADH?,adenosine triphosphate?ATP?and adenosine diphosphate?ADP?contents in leaves.However,enriched[CO₂]regulated the invertase?soluble acid and neutralinvertase?activity levels in cucumber seedlings leaves under salt stress.This facilitated the normal metabolism of sucrose,providing more substrates?fructose and starch?for respiratory metabolism,effectively participating in the glycolytic pathway and tricarboxylic acid cycle,and providing more energy for the defense of salt-induced injury.3.Moreover,the portion of excessive energy in the photosystem II?PSII?reaction center was enhanced by salt stress that increased the rate of production of the superoxide anion radical?O₂.^-?and hydrogen peroxide?H₂O₂?content in the leaves of cucumber seedlings.Subsequently,this resulted in an increase in malondialdehyde?MDA?content,permeability of cell membranes,and osmoticum?proline?content.Changes in antioxidative enzyme activities of cucumber seedling leaves differed between the salt stress and enriched[CO₂]treatments.Superoxide dismutase?SOD?and catalase?CAT?activities increased while peroxidase?POD?activity decreased in cucumber seedling leaves treated with salt stress.However,the activity of antioxidative enzymes?such as SOD,POD,and CAT?and proline content were enhanced by enriched[CO₂].Therefore,we can speculate that reactive oxygen species?ROS?quenching was limited under salt stress,leading to their accumulation and serious lipid peroxidation.However,enriched[CO₂]reduced the content of MDA,H₂O₂,and the rate of O₂.^-production in cucumber seedling leaves under salt stress.ROS was eliminated by antioxidative enzymes under enriched[CO₂],therefore,lipid peroxidation damage was less than that caused by salt stress.4.Under the experimental conditions,enriched[CO₂]and salt stress significantly inhibited the nitrogen assimilation process in cucumber leaves;however,enriched[CO₂]had no effect on the nitrate reduction and ammonium assimilation of leaves under salt stress,only inhibited the transamination.At the same time,enriched[CO₂]enhances the plasma membrane H⁺-ATPase activity,vacuolar membrane H⁺-ATPase activity and root hydraulic conductivity under salt stress,thereby enhancing the ions selective absorption and water absorption capacity.To a certain extent,it promotes the accumulation of K⁺in plants,which significantly reduces Na⁺/K⁺,and finally improves the water state conditions and maintains the ion balance in plants under stress,ensuring the normal enzymatic reaction.