Alleviating Effect of Melatonin on Melon Seed Germination Under Autotoxicity and Saline-Alkali Combined Stress

Autotoxicity and soil salinization are the main causes of continuous cropping obstacles of melon, which often occur at the same time and aggravate with the extension of cultivation years. Melatonin (MT) has a broad application prospect in improving plant resistance. The effects of exogenous MT on seed germination of melon (Cucumis melo L.) under autotoxicity and saline-alkali combined stress were observed in this paper. Starch and reactive oxygen species metabolism were determined to analyze the physiological and biochemical basis of MT function. Cinnamic acid was used to simulate melon autotoxicity. The mixed solution of NaCl and NaHCO3 was used to simulate saline-alkali stress. The effect of MT showed a concentration-dependent manner. 16 μM exogenous MT can effectively improve the germination rate, germination potential, and vigor index of melon seeds under combined stress, and increase radicle length and fresh weight. Seed vigor index was the most sensitive index, and MT treatment increased by 102.22% compared with combined stress. However, at high concentration, this stress relief effect can be offset or even reversed. MT can significantly shorten the time of seed breaking through the seed coat and improve the root growth inhibited by stress. MT enhanced α-amylase and β-amylase gene expression, antioxidant enzyme, and amylase activity. The contents of starch and soluble sugar changed accordingly. At the same time, the contents of malondialdehyde and reactive oxygen species were reduced. The expression analysis of 10 MT synthesis genes showed that MT treatment up-regulated eight genes. In general, by up-regulating the expression of amylase and some MT synthesis genes, enhancing the activities of antioxidant enzymes and amylase, reducing the content of malondialdehyde and reactive oxygen species, exogenous MT effectively increased the germination rate and germination potential of melon seeds under autotoxicity and saline-alkali combined stress, and significantly improved the quality of seed germination.