Effects of 24-epibrassinolide on ascorbate-glutathione cycle and polyamine levels in cucumber roots under Ca(NO3)2 stress

The effects of 24-epibrassinolide (EBL) applied by spraying the leaves of Cucumis sativus L (cv. Jinyou No.4) on the roots of ascorbate-glutathione (AsA-GSH) cycle and levels and forms of polyamines were investigated under Ca(NO3)(2) stress. Ca(NO3)(2) stress caused significant decreases in ascorbate acid (AsA) and glutathione (GSH) levels and the activities of antioxidant enzymes involved in the AsA-GSH cycle, including ascorbate peroxidase (APX, EC 1.11.1.11), glutathione reductase (GR, EC 1.6.4.2), monodehydroascorbate reductase (MDAR, EC 1.6.5.4) and dehydroascorbate reductase (DHAR, EC 1.6.5.4). In contrast, exogenous EBL in NB combination increased AsA and GSH levels through enhancing activities of APX, GR, MDAR and DHAR. This alleviated the declines of AsA/DHA and GSH/GSSG ratios, which could maintain cellular homeostasis. In addition, EBL application reduced the accumulation of putrescine (Put) induced by excess Ca(NO3)(2), and promoted the conversion of Put into spermidine (Spd) and/or spermine (Spm) able to bind to cell membrane structures. These results showed that EBL could alleviate the damage from oxidative stress by up-regulating the capacity of the AsA-GSH cycle and altering polyamine levels and forms.