Drought-induced chilling tolerance in cucumber involves membrane stabilisation improved by antioxidant system

We assessed changes in ultrastructure, membrane lipid peroxidation and antioxidant systems for cucumbner seedlings subjected to low temperature stress (day/night temperature of 8 degrees C /5 degrees C) that had been either pre-treated with 10% PEG for 2 days or not. We found extensive cell structure damage in the non-treated seedlings, whereas the seedlings pretreated with PEG to simulate drought remained essentially undamaged, except for slight damage to plasma membrane lipids and alveolation in the mitochondria. Low temperature stress increased electrolyte leakage, MDA levels and H2O2; decreased the activities of SOD, CAT and APX, and AsA and GSH content. An increase in POD activity was observed in the PEG-pretreated seedlings during the chilling period, while non-treated seedlings showed an increase in POD activity only in the early days of chilling stress. PEG pre-treatment diminished the level of lipid peroxidation caused by chilling compared to the non-treated seedlings, possibly due to a decrease in electrolyte leakage and MDA content. Furthermore, PEG pre-treatment increased the activities of SOD, POD, CAT and APX and AsA and GSH content in the chilling-stressed seedlings. These results suggest that PEG pre-treatment stimulates the adaptation of cucumber to low temperature. This could be due to stabilisation of the cell structure, alleviation of lipid peroxidation as a result of the increased activity of antioxidant enzymes and contents of antioxidant metabolites.