Optimized Nitrogen Topdressing Strategies Enhance Steviol Glycoside Productivity in Stevia ( Stevia rebaudiana Bertoni) Plants

The nitrogen (N) management is crucial to stevia growth and steviol glycoside (SG) production. Although previous studies have concentrated on the impacts caused by N fertilization rates, very few studies are concerning N topdressing strategies. We hypothesized that stevia biomass and SG productivity can be effectively increased by altering the ratio of base fertilization:first topdressing at the seedling stage:second topdressing at the branching stage. The current plot experiment was undertaken to evaluate such effects. Compared with one-time basal fertilization, optimized N topdressing ratios (especially the 5:3:2 and 4:3:3 ratios) significantly enhanced the leaf biomass formation and aboveground N accumulation. Different results were observed for the leaf SG concentrations, and increasing the topdressing proportions critically reduced the total leaf SG concentrations (3.3–24.7%) at the harvest stage. The highest values for total SG accumulation were exhibited under the 6:4:0 and 5:3:2 topdressing strategies, which depend on the changes in biomass and SG concentration. Moreover, a significant negative correlation between leaf SGs and the N concentration was observed at all growth stages, but a positive correlation was observed with the leaf C/N ratio and soluble sugar concentrations. Our results suggest the potential for increasing SG productivity by optimizing N fertilization regimes, especially with an appropriate reduction in the topdressing ratio after the vegetative growth stage. Our findings further expanded the effect of topdressing strategies on SG production based on the N fertilization levels.