Influence of exogenous spermidine on carbon–nitrogen metabolism under Ca(NO3)2 stress in cucumber root

The present study aimed to investigate the effect of exogenous spermidine (Spd) on cucumber (Cucumis sativus L. cv. Jinyou No. 4) growth and carbon–nitrogen balance under 80 mM Ca(NO3)2 stress. The result showed that leaf-applied Spd (1 mM) treatment alleviated the growth inhibition caused by Ca(NO3)2 stress by regulating the carbon–nitrogen balance in cucumber seedlings. The application of exogenous Spd effectively regulated the transcription levels and activities of major carbon–nitrogen metabolism enzymes, resulting in a significant decrease of NO3− and NH4+ contents under Ca(NO3)2 stress. In addition, Spd treatment remarkably increased the accumulation of soluble carbohydrates (sucrose, fructose and glucose), thus protected enzyme activities related N metabolism and effectively promoted NO3− assimilation under Ca(NO3)2 stress. Exogenous Spd also enhanced total amino acids, which serve as the building blocks of protein, and promoted the biosynthesis of soluble protein. In the presence of Spd, total C content and the C/N ratio increased significantly, while total N content decreased in response to Ca(NO3)2 stress. Based on our results, we suggested that exogenous Spd could effectively accelerate nitrate transformation into amino acids and improve the accumulation of carbon assimilation production, thereby enhancing the ability of the plants to maintain their C–N balance, and eventually promote the cucumber Ca(NO3)2 stress tolerance.