Grafting Increases the Copper Tolerance of Cucumber Seedlings by Improvement of Polyamine Contents and Enhancement of Antioxidant Enzymes Activity

The aim of the study is to determine whether grafting could improve antioxidant enzyme activities and polyamine contents in leaves of cucumber plants (Cucumis sativus L. cv. Xintaimici) under copper stress. Grafted (using Cucurbita ficifolia as rootstock) and ungrafted cucumber seedlings were cultured in deep flow technique (DFT) with the Cu2+ concentration of 40 μmol L−1. The results showed that on the 9th day of copper stress treatment, the contents of malondialdehyde (MDA) and hydrogen peroxide (H2O2), superoxide radical ( ) producing rate, and electrolyte leakage percentage were significantly lower in grafted seedlings in comparison to those of the ungrafted seedlings, whereas the activities of antioxidants such as superoxide dismutase (SOD, EC 1.15.1.1), peroxidase (POD, EC 1.11.1.7), ascorbate peroxidase (APX, EC 1.11.1.11), catalase (CAT, EC 1.11.1.6), glutathione reductase (GR, EC 1.6.4.2), and monodehydroascorbate reductase (MDHAR, EC 1.6.5.4) of grafted seedlings were also significantly higher than that of ungrafted seedlings, and the contents of free proline and soluble protein of grafted seedlings were significantly higher than that of ungrafted seedlings. Cu2+ treatment increased the putrescine (Put) level and lowered the spermidine (Spd) and spermine (Spm) levels, thereby reducing the Put/(Spd + Spm) ratio in leaves of grafted and ungrafted seedlings. Grafting markedly reversed these Cu-induced effects for all three PAs and partially restored the Put/(Spd + Spm) ratio in leaves. These results suggest that grafting can enhance the tolerance of cucumber seedlings to Cu2+ stress by increasing the activities of antioxidants and the levels of endogenous Spd and Spm, decreasing the Put/(Spd + Spm) ratio and the levels of ROS, promoting free proline and soluble protein synthesis in cucumber seedling leaves.