Ethephon, as an ethylene-releasing compound, is frequently used for the regulation of plant growth and metabolism. The present study aimed to investigate the impact of ethephon application at various concentrations (0, 0.37, 0.75, 1.5, and 3 mM) and stages [(treatment on seed, and foliar application on seedling (6-leaf stage) and full-grown plant)] on growth and metabolites biosynthesis of stevia (Stevia rebaudiana L.). It was observed that ethephon had no significant effects on seed germination, while seed treated by 0.37 mM ethephon produced stevia plants with significant leaf biomass. The foliar application of ethephon on full-grown plants caused a significant loss in leaf production compared to non-ethephon-treated plants (control). The most significant increase (39% compared to control) in leaves dry weight was recorded at 0.75 mM ethephon applied on the seedling stage. A significant increase in steviol glycosides (SVglys) content was observed when 0.37 and 0.75 mM ethephon were applied to stevia seeds and seedlings, respectively. This result was equally certain regarding Rebaudioside (Reb) A and Stevioside (Stev). The SVglys production in stevia was significantly increased (50%, compared to control) when ethephon was applied to seeds and seedlings at 0.75 mM concentration. However, ethephon foliar application on full-grown plants sharply decreased the SVglys biosynthesis. The ethephon concentrations of 0.37 and 0.75 considerably increased the sucrose, glucose, and fructose content of the leaves. Based on the results presented, it can be concluded that stevia response to exogenous ethephon depends on ethephon concentration and time of application. In this respect, the application of 0.75 mM ethephon on seeds and seedlings could be an appropriate strategy for biosynthesis and accumulation of metabolites in stevia.
