Effect of different methyl jasmonate concentrations on pumpkin (Cucurbita pepo L.) seedling tolerance to chilling stress

Although early sowing the summer plants in the spring season is important for the use of moderate conditions as well as proper vegetative growth, but the incidence of low temperatures at the beginning of the growing season may be caused the problem for good establishment of seedlings. In temperate regions, chilling stress especially where early sowing was done, is very harmful for most summer plants. Seed germination is the most sensitive phase to chilling stress. Seed germination is slowed down or decreased in cold-sensitive species at temperatures below 20 °C, resulting in poor establishment and is generally prevented from germinating at temperatures below 15 °C. In order to use more of the spring growing season, improving the germination of summer plants is important at lower temperatures. Since one of the ways to reduce the low temperature damage is the upgrade of the antioxidant system of seedlings, in this study the effect of different methyl jasmonate concentrations on germination and antioxidant system of hull less seed pumpkin were studied under chilling stress. A factorial experiment was conducted in a completely randomized design under control conditions at Shahrekord University in 2017. Treatments were included six concentrations of methyl jasmonate (0, 1, 5, 50, 100 and 150 μM ) and three temperature levels (8, 11 and 14 °C). After eight days, the temperature gradually increased to reach the optimum temperature of the hull less seed pumpkin (25 ° C). On the 11 day, after separation of normal and abnormal seedlings, 20 normal seedlings were selected from each petri dish and then traits such as radicle length, plumule length, radicle dry weight, plumule dry weight, seed vigor index, soluble protein, content of malondialdehyde , superoxide dismutase enzyme activity, guaiacol peroxidase enzyme, catalase enzyme were measured. Data were analyzed by SAS V9 software. Means comparisons, were done by least significant difference (LSD) test at the 1% probability level. The results showed that the maximum radicle length (76.45 mm) was recorded at 50 μM concentration and 11 °C temperature (on an average 55.6% over the control). At 8, 11, and 14 °C, the unprimed seeds had not plumule. At 8 °C, the highest radicle length, plumule length, plumule dry weight were odserved in 5 μM methyl jasmonate. The greatest guaiacol peroxidase and catalase activity were recorded in 5 μM methyl jasmonate as well. At 11°C, the plumule length, radicle dry weight, and catalase activity in 5 μM methyl jasmonate were more than the others methyl jasmonate concentrations. However, radicle length and superoxide dismutase activity of 5 μM methyl jasmonate was second rank. At 14 °C, the highest radicle dry weight, plumule dry weight, superoxide dismutase, guaiacol peroxidase and catalase activity were obtained in 50 μM methyl jasmonate. The greatest radicle length and seed vigour were observed in in 5 μM methyl jasmonate and the in 50 μM methyl jasmonate was second rank. Methyl jasmonate, due to the polyacetylation nature at the physiological pH, can strongly adhere to the anionic position in the cellular components and cause the membrane to stabilize and thus contribute to the environmental stresses in the plant's response. Seedling tolerance to low temperatures was proved by methyl jasmonate treatments. Based on the results of this experiment, it can be said that the use of methyl jasmonate increases the activity of antioxidant enzymes and, therefore, the resistance of the seed germinator to the low temperature stress. Therefore, appllication this substance, we can modify the effect of probable fever at the beginning of the growing season.