Exogenous Si Alleviation of Autotoxicity in Cucumber (Cucumis sativus L.) Seed Germination is Correlated with Changes in Carbohydrate Metabolism

Benzoic and cinnamic acid derivatives such as 3-phenylpropionic acid (PA) are the main autotoxins identified from cucumber root exudates. In this study, we investigated the effects of silicon (Si) supplemented on cucumber seed germination under PA-induced stress. Cucumber bud seedlings were harvested at 0, 4, 8, 12, 24, 36, and 48 h after treatment and assessed for plant growth, amylase activity and gene expression, and starch metabolism. Results revealed that PA significantly reduced the seed germination rate, seed vigor, radicle length, number of lateral roots, fresh weight, and soluble sugar content of cucumber bud seedlings. However, PA increased the starch content. The supplement of silicon promoted the growth of PA-treated plants and significantly increased the germination rate, seed vigor, and soluble sugar content of cucumber bud seedlings. Compared with those of the control plants, the levels and activities of α- (AMY) and β-amylase (BMY) during germination significantly decreased after PA treatment. The supplement of silicon significantly improved amylase activity, and total amylase and β-amylase activities reached maximum values at 36 h. PA significantly down-regulated the relative expression levels of AMY and BMY, whereas exogenous silicon significantly increased their transcript levels. These results suggest that supplement of Si alleviated cucumber autotoxicity caused by PA during seed germination at both the physiological and molecular levels.