Effects of External Melatonin Treatments on Morphological and Physiological Changes in Cucumber (Cucumis sativus L.) Seedlings Against Chilling Stress

This study aimed to determine chill resistance and to explain how chilling stress affects metabolic processes by applying different doses of melatonin to Beith F1 hybrid cucumber (Cucumis sativus L.). In addition, experiments were conducted to understand the response of plants with and without melatonin to chilling stress and their adaptation mechanisms against chilling. Melatonin-containing (0, 1, 10, 20, 30 and 40 μM) distilled water was sprayed to the leaves of chilling-treated seedlings with 3–4 true leaves. One full day after melatonin treatments, half of the plants were exposed to chilling stress in a climate chamber for 15 days and the other half was kept in the climate chamber under normal conditions (25/20 °C, 12/12 light/dark photoperiod). Plants exposed to chilling stress were kept in an incubator for 15 days at 5 ± 1 °C dark (12 h)/10 ± 1 °C light (12 h), then samples were taken. Some plant growth parameters were measured, and some biochemical analyses were performed. In terms of plant total fresh weights, it was observed that chilling stress and plants treated with 30–40 µM melatonin showed limited plant growth and development and thus were less influenced by chilling stress. Melatonin treatments slowed down the growth rate of plants under cold stress, but, as it increased antioxidant enzyme activities, it protected plants from the destructive effect of stress and reduced the formation of malondialdehyde, a product of lipid peroxidation. In addition, there was no decrease in the amount of chlorophyll of the plants since malondialdehyde prevents the degradation of chlorophyll pigments. It was concluded based on present findings that external melatonin treatments could be used as a physiologically effective auxiliary application to reduce harmful effects of chilling stress.