Effects of CO2 enrichment on non-structural carbohydrate metabolism in leaves of cucumber seedlings under salt stress

The effects of CO2 enrichment (800 +/- 40 mu mol.mol(-1)) on sucrose metabolism, the glycolytic (EMP) pathway, and tricarboxylic acid (TCA) cycle intermediates and their key enzyme activities in leaves of cucumber (Cucumis sativus L. cv. 'Jinyou No. 35') seedlings under salt stress (80 mmol.L-1 NaCl) in solution cultures were investigated. After 7 d of the experimental treatments, salt stress 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. Enriched [CO2] regulated the invertase (soluble acid and neutral invertase) activity levels in 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 to prevent salt-induced injury. Therefore, the negative effects of salt stress were alleviated under enriched [CO2] conditions and resistance was enhanced.