Florina O. Pacaldo and Catherine C. Arradaza
This study was conducted to evaluate the effect of different combinations and levels of plant growth regulators, 2,4-dichlorophenoxy acetic acid (2,4-D) and 6-benzylaminopurine (BAP), on callus induction of Stevia rebaudiana Bertoni leaf and internode explants, and the total phenolic content of Stevia callus as influenced by the length of exposure to ultraviolet (UV) light radiation. Early callus initiation was recorded in leaf explants inoculated in Murashige and Skoog (MS) medium supplemented with 2,4-D and BAP. After a week of incubation, leaf explants showed callus formation while in internode explants, callus formation was observed 2 weeks after inoculation. The culture medium supplemented with 1.5mg L-1 2,4-D + 2.0mg L-1 BAP (T4) was the best treatment for leaf explants with 95% callus formation and the addition of 1.0mg L-1 2,4-D + 4.0mg L-1 BAP (T3) was the best treatment for internode explant with 90% callus formation. After 4 weeks of incubation, the leaf and internode calli were observed as compact, non-embryogenic, and yellowish green in all treatments except those inoculated in MS medium alone (control). Stevia leaf callus was subjected to UV radiation after 4 weeks at varying time of exposure. The exposed calli turned brown signifying a possible increasing production of secondary metabolites. Furthermore, combination of 1.5mg L-1 2,4-D + 2.0mg L-1 BAP (T4) and 3h of UV light exposure yielded the highest phenolic content of 87.71mg GAE per g callus. The results further revealed that both medium composition and time of exposure affect the production of phenolic content of Stevia leaf callus.
Keywords: Stevia rebaudiana Bertoni, Phenolic Content, UV light exposure, Callus, Plant Growth Regulators