Effects of cytokinins, gibberellic acid 3, and gibberellic acid 4/7 on in vitro growth, morphological traits, and content of steviol glycosides in Stevia rebaudiana

Steviol glycosides (SGs) and gibberellic acids share a part of their biosynthesis pathways. Despite the widespread studies on the effect of gibberellic acid 3 (GA(3)), the effect of gibberellic acid 4 and 7 (GA(4/7)) on Stevia rebaudiana has never been studied. This study aimed at a comparative evaluation of different hormone effects, i.e., 1 mg L(-1)GA(4/7), 1 mg L(-1)GA(3), or 0.5 mg L(-1)kinetin and 0.5 mg L-1 6-benzylaminopurine (BAP) (KB 0.5), on in vitro propagation, growth, morphological properties, and content of SGs in leaf samples of stevia. In comparison with the control group (hormone-free), the treatments of KB 0.5 or GA(3) produced the highest biomasses and largest leaf areas. The three hormonal treatments produced a similar number of leaves, the ratio of fresh to dry weight, and leaf length. GA(4/7) treated explants produced the highest ratio of leaf area to leaf length. The effect of Gk(4/7) on shoot elongation was greater than that of the control or even GA(3). While the effect of GA(3) on rebaudioside-A (Reb-A) production was similar to that of the control (16.2 and 18.04 mg g(-1), respectively), GA(4/7) resulted in a lower amount of it (13.31 mg g(-1)). Except for GA(4/7), which induced more stevioside accumulation, the treat.ments' effects were comparable to that of the control. The ratio of stevioside to Reb-A was the highest for GA(4/7) (2.62), followed by GA(3)(1.93), and then the two others. Sum of Reb-A and stevioside content was not changed by the use of any of the treatments.