Mechanisms of Low-temperature Tolerance in Cucumber Leaves of Various Ages

To determine whether differential tolerance to paraquat among cucumber (Cucumis sativus L.) leaves of various ages correlates with low-temperature tolerance, leaves of three cultivars at the four-leaf stage were exposed to 5 °C. Leaves tolerant to paraquat were also tolerant to low temperature as indicated by electrolyte leakage changes in leaf tissue, lipid peroxidation, and photosynthesis measurements. The youngest leaf (leaves were numbered 1 to 4 with the youngest leaf given 4) on the same plant was more tolerant than older leaves to low temperature. During chilling, lipid peroxidation and H2O2 content increased in the oldest leaf when compared with the three younger leaves. Photosynthetic activity and chlorophyll a fluorescence (Fv/Fm) were significantly lower in leaf 1 than in leaf 4. There was no difference in superoxide dismutase, catalase, ascorbate peroxidase (APX), and glutathione reductase activities among various leaf ages in untreated plants. Induction of antioxidant activity in response to low temperature was higher in the youngest leaf than in the oldest leaf. Expression of APX isozymes increased in response to low temperature, and their expression was the highest in the youngest leaf. Current results support earlier findings that antioxidant protection also was a mechanism for paraquat tolerance.