Effects of exogenous melatonin on physiological characteristics of leaves under low-temperature stress in red raspberr

Abstract: 【Objective】Red raspberry, the third generation golden fruit, is rich in a variety of vitamins and mineral elements, and the root, stem and fruit can be used as medicine. Red raspberry not only owns nutritional and medicinal value, but also has high economic values. However, due to the falling temperature and leaf frostbite, most of the red raspberry cannot ripen in the northern regions of China,which seriously affects the yield of the red raspberries, thus reducing the economic return. To alleviate the chilling injury to red raspberry in early winter, the effects of exogenous melatonin (MT) treatment on cold hardiness of red raspberry under low-temperature stress were studied, and the effects of exoge-nous melatonin treatment on cold hardiness and the appropriate concentration of MT were determined.【Methods】The 2-year-old potted red raspberry cultivar Polka was used as experimental material. Pot-ted Polka red raspberry seedlings with the same growth vigor were selected. The upper diameter of the pot was 44 cm, the lower diameter was 25 cm and the height was 30 cm. Routine management was ap-plied before treatment to ensure normal growth. On October 20, 2020, the maximum temperature was 18 ℃, and the minimum temperature was 4 ℃. The treatments were designed as 50, 100, 150, and 200μmol∙L^-1 melatonin leaf spraying with an equal amount of distilled water as the control (CK), and each treatment was repeated for 3 times. At 5:30 p.m., the corresponding concentration of melatonin was ful-ly sprayed on the front and back sides of each leaf, until the liquid drips off the leaves. It was treated once every other day for 4 times. On the first, third and fifth days after the last foliar spraying with mel-content, antioxidant enzyme activity and osmotic regulatory substances in the leaves of red raspberry.【Results】MT treatment could significantly reduce the relative electrical conductivity and MDA con-tent of leaves compared with CK. The contents of chlorophyll, soluble protein and soluble sugar in-creased. The activity of SOD (superoxide dismutase), POD (peroxidase) and CAT (catalase) increased significantly, and the effect of 150 μmol ·L-1 MT treatment was the most obvious. The relative conduc-tivity, MDA content and soluble sugar content in leaves increased with the extension of low-tempera-ture stress, and the lowest values were 19.26%, 2.248 9 umol·g^-1 and 50.22 mg·g^-1 on the 5th day with 150 μmol ·L^-1 Mt treatment, respectively. POD and CAT activity increased with the extension of low-temperature stress, and the highest values were 86.7833 U·g^-1·min^-1 and 82.044 4 U·g^-1·min^-1 with 150μmol · L^-1 MT on the 5th day, respectively. The contents of starch, chlorophyll a and total chlorophyll decreased with the extension of low-temperature stress, and the highest values were 48.42 mg·g^-1, 1.23mg·g^-1 and 1.6 mg·g^-1 on the 5th day with 150 μmol ·L^-1 MT treatment, respectively. The soluble pro-tein content increased firstly and then decreased with the extension of low-temperature stress. On the 5th day, the value of 150 μmol ·L^-1 MT treatment was 2.973 2 mg · g^-1, which was higher than that of other treatments. The SOD activity with 150 μmol·L^-1 MT was 4.935 1 U·g^-1·min^-1, which was higher than that with other treatments.【Conclusion】Low-temperature stress will cause the disorder of physio-logical and biochemical systems in plants, affect the normal growth and development of plants, and even lead to production reduction and inevitable economic loss. When plants are damaged by low-tem-perature stress, the plant cell membrane system will be impacted, resulting in the change or loss of the membrane integrity, and the rapid accumulation of reactive oxygen species and free radicals in cells.However, plants also have a self-preservation mechanism. At low-temperature, plants usually accumu-late osmotic regulation substances to reduce the water potential in cells and promote the absorption of water by plant cells to maintain normal physiological functions. It can also mobilize the close connec-tion among the three enzymes of SOD, CAT and POD in the antioxidant system within the body to form part of the defense system in the plant body, which plays an important role in maintaining ROS level. In this experiment, different concentrations of MT (0, 50, 100, 150, 200 μmol ·L^-1) were sprayed on the leaves of red raspberry under low-temperature stress, which significantly reduced the membrane leakage and malondialdehyde content, and reduced the degree of membrane lipid peroxidation in red raspberry leaves under low-temperature stress. The total chlorophyll, soluble protein and soluble sugar contents in red raspberry leaves increased, and the intracellular osmotic balance was maintained. The antioxidant enzyme activity system of SOD, POD and CAT was activated to alleviate the oxidative damage caused by reactive oxygen species and free radicals. According to the effects of different concentrations, the 150 μmol·L^-1 MT concentration was suggested to alleviate the low-temperature stress of red raspberry leaves.