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.

Grafting Enhances Copper Tolerance of Cucumber Through Regulating Nutrient Uptake and Antioxidative System

An experiment was carried out to determine plant growth, mineral uptake, lipid peroxidation, antioxidative enzymes, and antioxidant of cucumber plants （Cucumis sativus L. cv. Xintaimici） under copper stress, either ungrafted or grafted onto the rootstock （Cucurbitaficifolia）. Excess Cu inhibited growth, photosynthesis, and pigment synthesis of grafted and ungrafted cucumber seedlings and significantly increased accumulation of Cu in roots besides reducing mineral uptake. Cu concentration in roots of grafted cucumber plants was significantly higher than that of ungrafted plants and obviously lower in leaves. The accumulation of reactive oxygen species （ROS） significantly increased in cucumber leaves under Cu stress and resulted in lipid peroxidation, and the levels of ROS and lipid peroxidation were greatly decreased by grafting. Activities of protective enzymes （superoxide dismutase, SOD; peroxidase, POD; catalase, CAT; ascorbate peroxidase, APX; dehydroascorbate reductase, DHAR; glutathione reductase, GR） and the contents of ascorbate and glutathione in leaves of grafted plants were significantly higher than those of ungrafted plants under Cu stress. Better performance of grafted cucumber plants were attributed to the higher ability of Cu accumulation in their roots, better nutrient status, and the effective scavenging system of ROS.

Grafting Raises the Cu Tolerance of Cucumber Through Protecting Roots Against Oxidative Stress Induced by Cu Stress

A greenhouse experiment was carried out to determine plant growth, reactive oxygen species （ROS） metabolism in roots and functions of plasma membrane （PM） and tonoplast in cucumber seedlings （Cucumis sativus L. ev. Xintaimici） treated with 40 μmol L^-1 CuSO4·5H2O, which were either ungrafted or grafted onto the rootstock （Cucurbitaficifolia）. Cu treatment inhibited growth, induced significant accumulation of H2O2 and led to serious lipid peroxidation in cucumber roots, and the ROS-scavenging enzymes activities in grafted seedlings roots were significantly higher than that of ungrafted plants, thus less accumulation in grafted cucumber roots induced by Cu. As a result, lipid peroxidation in roots decreased. Furthermore, the activities of H^-ATPase, H＋-PPase and Ca^2＋-ATPase in PM and/or tonoplast in grafted cucumber seedlings under Cu stress were obviously higher than that in ungrafted plants, resulting into higher ability in grafted plants to expulse the excess H＋, promote the cytoplasm alkalinization, regulate the intracellular Ca^2＋ concentration and brought the cytoplasma concentration of free Ca2＋ to extremely low level under Cu stress.