Mechanisms of tolerance differences in cucumber seedlings grafted on rootstocks with different tolerance to low temperature and weak light stresses

Stresses of low temperature and weak light are major environmental limiters for horticultural production in greenhouses in northern China during winter and early spring. Grafting is regarded as a promising approach to enhance cucumber tolerance to these environmental stresses. However, it is still unclear why cucumber plants grafted on different rootstocks usually exhibit different tolerance. The responses of cucumber (Cucumis sativus L. Cv. 'Xintaimici') seedlings that were grafted on three rootstock cultivars ('Kilameki', 'Tielizhen', and 'Figleaf gourd') to low temperature (5 °C) and weak light (100 µmol m-2 s-1) were investigated in this study. The results showed that, compared to ungrafted seedlings, the electrolytic relative leakage rate and malondialdehyde (MDA) content were significantly reduced in grafted cucumber, especially in 'Figleaf gourd'. Seedlings grafted on 'Figleaf gourd' had the highest proline content, as well as activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX), followed by 'Tielizhen' and 'Kilameki'. Relative expression of Cu/Zn-SOD, Mn-SOD, and CAT mRNAs was consistent with activities of the corresponding enzymes, and higher levels were found in grafted seedlings, especially in 'Figleaf gourd'. APX activities and the relative expression of APX were also stimulated in grafted seedlings, although their changes were not completely consistent. All evidence indicated that the differences in rootstock-mediated tolerance improvement in cucumber were mainly attributed to the different stimulation of antioxidative defense systems under low temperature and weak light stresses.