Influence of pH and plant growth regulators on secondary metabolite production and antioxidant activity of Stevia rebaudiana (Bert)

Background and purpose: Beside being rich with sweet glycosides, Stevia rebaudiana Bertoni is an interesting source of flavonoids and phenols. The current study aimed to assess the potential for increasing total phenols and flavonoids in S. rebaudiana tissue by varying concentrations of plant growth regulators (PGR) and pH levels of media. Materials and methods: The culture was established from seeds and propagated shoots were cultured on media of different pH levels (4.6, 5.8 and 7.4) and PGR. Total polyphenolics and free radical scavenging capability of leaf, callus and root extracts were determined. Shoot height, root length, shoot and root number were also recorded. Results: Shoot elongation and root development was stimulated by singly applied growth regulators though the values of both parameters were also satisfactory in PGR-free media. In the latter media, pH value of 4.6 was a main factor for increasing leaf metabolite levels. A most significant rise in phenols and flavonoids was evident in response to combination of BA either with GA3 or IAA compared to singly applied regulators indicating synergistic effects of PGR (especially of auxins and cytokinins). However, polyphenolics levels and their distribution between different tissues were also influenced by medium pH value. A positive correlation was found between antioxidant activity of S. rebaudiana extracts and phenols and flavonoids. Conclusions: The results show that PGR and medium pH value strongly affect accumulation of secondary metabolites and can lead to significant enhancement in productivity of bioactive polyphenolics in S. rebaudiana plant cultures