One of the most important factors limiting plant growth and crop production in many parts of the world has been considered to be low temperatures. Cucumber is a warm season crop and is sensitive to cold. With exception of southern and central part of the country, the rest are subjected to lower temperatures which cause chilling injury on warm season crops early in the season. There are different methods for controlling chilling injury, one of this methods is application of plant growth regulators. Spermidine, from polyamine group is one of the growth regulators that in the recent years its use in controlling chilling injury has been studied. Therefore, a greenhouse study includes several experiments were performed to determine the effects of spermidine on the cold tolerance of cucumber cultivar “Rashid” in the early stages of growth. All experiments were done as completely randomized design withhormone levels, included zero, 0.1, 0.5 and 1 mM spermidine with four replications at College of Agriculture of Isfahan University of Technology. Experiments were performed in the imbibition, completing germination and seedling stages. In imbibition stage, seeds after treatment with the hormone were incubated for 15 days in 13°C. In completing germination stage, seeds were exposedto 20°C for two days and then treated with hormones, the remaining 13 days theywere kept at temperatures 15, 13, 11 or 9°C. For seedling stage seeds were germinated for 7 days at 20°C and then treated with hormones and incubated in 9 or 6 °C temperatures under light photoperiod(8 hours light and 16 hours dark) for remaining 8 days. During and at the end of each experiment measures such as germination percentage, the average days to germination, length of shoot and root, fresh and dry weight of shoot and root, ion leakage of shoot and root, proline of shoot and root was measured. Briefly, the findings in this study showed that application of spermidine in low concentrations increased growth at low temperatures compared with control. It also became clear that the imbibition step towards completing germination and seedling stages is relatively more sensitive to chilling injury. Overall, the experiments showed that concentration of 0.5 mM spermidine in most cases increased cold resistance (improved growth, etc.) in cucumber seedlings. Also in most cases, concentrations of more than 0.5 mM spermidine demonstrated inhibitory effect on cucumber growth. To compare the effects of temperature and its interaction with the hormone data of four experiments of completing germination stage analysed as split plot experiment (four main plots and four sub plots). The results showed that reducing the temperature in this stage, redused most of the growth parameters significantly.
