Impact of cold plasma seed treatment on phytochemical content of Stevia rebaudiana Bertoni cultivars

Abstract

Stevia rebaudiana Bertoni is a zero-calorie natural sweetener, being 10-15 times sweeter than sucrose. When consumed, it has shown to have therapeutic benefits [1]. Apart from steviol glycosides, the leaves of stevia are also known for their antioxidant properties, which are possibly related to a rich profile of various phenolic compounds [2]. The highest economic importance lies in steviol glycosides (SGs): stevioside (Stev) and rebaudioside A (RebA), being the most abundant. Increase in Stev, RebA and their overall ratio is closely linked to sweetness intensity and pleasantness of taste. The aim of this study is to determine the effect of seed treatment with two types of cold plasma on phytochemical biosynthesis in two S. rebaudiana cultivars (SHUG A3-6 and SHUG HIGH A3, EverStevia, Canada). Previous studies indicate that the effect varies in cultivars. Seeds were treated with plasma using low-pressure capacitively-coupled (CC) and atmospheric-pressure dielectric barrier discharge (DBD) systems for durations of 2, 5, and 7 minutes prior to sowing. Eight-week-old plant leaves were collected and dried, and extracts were prepared for the analysis of steviol glycosides, total phenolic compounds (TPC) determined using Folin-Ciocalteu (FC) reagent, total flavonoid compounds (TFC) determined using the aluminum compound formation reaction, and antioxidant activity determined as DPPH radical scavenging capacity. Stev, RebA, and other SGs were measured using High-Performance Liquid Chromatography (HPLC). Our results indicate that treatment with cold plasma (CP) can enhance secondary metabolite biosynthesis and therefore improve stevia traits. A two-fold increase in stevioside, as well as a statistically significant increase in TPC and antioxidant activity, was observed in SHUG A3-6 treated with CC for 5 minutes. SHUG HIGH A3 showed an increase in stevioside in all experimental groups, but it was not as drastic as in SHUG A3-6. The obtained results correlate with each other, where an increase in Stev is followed by a decrease in RebA and an increase in TPC, TFC and antioxidant activity in both cultivars, respectively. It seems that CP treatment affects the overall biochemical composition. It is important to understand that numerous factors influence the biosynthesis of phytochemicals, including the specifics of the plasma system, duration of plasma exposure, growth conditions, cultivar characteristics, among many others. Further research is needed to confirm specific influencing factors of CP treatment.