Viabilidade de sementes e perfis bioquímico e metabolômico de plantas de stevia rebaudiana (BERT.) Bertoni cultivadas sob diferentes fotoperíodos

Abstract

Stevia rebaudiana (Bert.) Bertoni has stood out in recent years due to the sweetening characteristics provided by the presence of steviol glycosides (StGly - Steviol Glycosides), mainly in its leaves. Due to its rich nutritional and phytochemical profile, stevia also provides beneficial effects against a multitude of health conditions, making it a biomaterial of interest in the field of plants with medicinal properties. However, the large-scale cultivation of stevia faces some obstacles, since this species is highly responsive to environmental stimuli, among which the photoperiod is one of the most relevant, affecting its morphophysiological and biochemical characteristics. Therefore, it is of fundamental importance to understand the mechanisms underlying stevia's responses to different environmental conditions. In this sense, the present thesis aimed to (i) characterize the morphoanatomical profile of seed germination and seedling enzymatic, in addition to (ii) perform a biochemical and metabolomic characterization of the development of Stevia rebaudiana (Bert.) Bertoni plants cultivated under different photoperiods. In CHAPTER 1, we report on the characterization of the morphophysiological profile of germination of the different achenes normally produced by the plants, where we could define a viability pattern for Stevia seeds based on tetrazolium test analysis. Further on, we report on the profile of antioxidant enzymes in seedlings submitted to different photoperiod conditions. The results showed that antioxidant enzymes can be an important tool for evaluating the physiological state of stevia seedlings/plants under the different conditions tested, providing very useful information for seed-based cultivation. In CHAPTER 2 we reported on the growth of stevia plants under different photoperiod regimes, where we evaluated the StGly content and the general chemical composition by Nuclear Magnetic Resonance, antioxidant activity (SOD / DPPH) and antimicrobial activity of phytochemical extracts, as well as the total phenolic content. of extracts obtained from leaf and stem samples. We noticed that photoperiods with less than 12h of light induce unwanted early reproductive mode by reducing the StGlys content in their leaves. In contrast, long days (vegetative mode) increased the production of StGly and other bioactive compounds produced by plants. Thus, providing valuable information will potentially serve for the development of a large-scale stevia cultivation protocol. In CHAPTER 3, we report on the phytochemical profile with a more detailed focus on steviol glycosides (Stevioside and Rebaudioside A), in which we promote the extraction and quantification of these metabolites via RP–HPLC–UV (Reversed-Phase High-Performance Liquid Chromatographic) for stevia leaf and stem samples. As expected, the leaf being the raw material of greatest interest in the stevia culture, showed higher levels of StGly, and their synthesis depends on the light conditions tested, where plants grown under 16/8h of light have a greater potential for the synthesis of the main glycosides. Among all, it is worth mentioning the presence of relevant amounts of the metabolites of interest also in the stem samples, which may also present potentially useful properties for future exploration.