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.
INTRODUCTION
S tevia rebaudianaBertoni is a small perennial shrub of the Asteraceae family, native to South and Central America. S. rebaudiana occurs naturally on infertile, acid (pH 4 to 5) soils, but thrives with soil pH as high as 7.5 [1,2]. The plant is known worldwide principally for its high content of sweet diterpene glycosides, rebaudioside A and stevioside (the most prevalent glycoside) in leaf tissue and is being used as a low-calorie and non-toxic sweetener. Moreover, antimicrobial, antiviral, anticarcinogenic and antioxidant activity of S. rebaudiana leaf extracts was demonstrated by a number of studies. S. rebaudiana is also a good source of inulin-type carbohydrates, proteins, essential oils, minerals, vitamins as
S. Radić et al.
Influence of pH and plant growth regulators on Stevia rebaudiana
10Period biol, Vol 118, No 1, 2016.well as flavonoids and phenolic compounds [3]. Similar to other plant secondary metabolites, polyphenols have roles in protection against infections, attraction of pollinators and seed-dispersing animals, as allelopathic agents and are generally involved in plant defense against ultraviolet radiation. However, these bioactive compounds have attracted much scientific attention due to their beneficial effects on human health, notably in the prevention of various diseases associated with oxidative stress. The antioxidant effects of polyphenols are co...
