Exogenous 24-Epibrassinolide Enhances NaHCO3 Tolerance in Cucumber Seedlings Associated with Changes in Carbohydrate Metabolism, Glycolysis, and the Tricarboxylic Acid Cycle

This study evaluated the role of exogenous 24-epibrassinolide (EBR) in mitigating NaHCO3 stress in cucumber (Cucumis sativus L.) seedlings, with particular attention to carbohydrate metabolism, glycolysis, and the tricarboxylic acid (TCA) cycle. Seedlings were exposed to 30 mM NaHCO3 with or without 0.2 μM EBR, and physiological and metabolic responses were evaluated during a 15-d treatment period, with gene expression analyzed in roots at 24 h. EBR treatment substantially mitigated the adverse effects of NaHCO3, as evidenced by improved cucumber growth, reduced inhibition of photosynthetic rate (Pn) and Fv/Fm, and increased biomass accumulation. NaHCO3 stress caused excessive starch accumulation in leaves while reducing starch levels in roots, indicating marked alterations in carbohydrate accumulation in the two tissues. EBR partially mitigated these changes by reducing starch accumulation in leaves while increasing starch content in roots. In addition, EBR also altered the activities of sucrose-metabolizing enzymes. NaHCO3 stress inhibited key glycolytic enzymes, including phosphofructokinase (PFK) and pyruvate kinase (PK), and suppressed the activities of isocitrate dehydrogenase (IDH) and succinate dehydrogenase (SDH) during prolonged stress. These inhibitory effects were partially alleviated by EBR, which also altered organic acid accumulation under NaHCO3 stress. Transcript analysis further showed that EBR altered the expression patterns of genes associated with glycolysis and the TCA cycle, including HK, PFK, PK, IDH, SDH, MDH, and FUM. Taken together, these findings indicate that EBR improves cucumber tolerance to NaHCO3 stress in association with adjustments in carbohydrate, glycolytic, and TCA-cycle metabolism, together with improved biomass accumulation.