Do magnetic field applications affect in vitro regeneration, growth, phenolic profiles, antioxidant potential and defense enzyme activities (SOD, CAT and PAL) in lemon balm (Melissa officinalis L.)?

Melissa officinalis L. (lemon balm) is a valuable medicinal plant used in the treatment of nervousness, cardiac failure, hypersensitivities, depression, insomnia, indigestion and colic. Objective of the first part of the study was to determine the effect of 3 different magnetic fields (MF-50 mT, 100 mT and 150 mT) with 5 different exposure times (5 min, 15 min, 30 min, 1 h and 3 h) on in vitro regeneration of explants containing preexisting meristem (axillary, shoot tip and hypocotyl buds). Regenerated shoots were also compared in terms of shoot length, root length, fresh-dry weight and % water content. In the second stage, regenerated shoots obtained from 3 different MF treatments for 1 h and no MF-treated shoots (control) were compared according to their phenolic profile, total phenol-flavonoid content, free radical scavenging activity, superoxide dismutase (SOD), catalase (CAT) and phenylalanine ammonium lyase (PAL) activities. The best regeneration was obtained with axillary buds exposed to 100 mT for 1 h (15.8 shoots at 100 % shoot frequency). Furthermore, 50 mT and 100 mT MF treatments improved the growth parameter and water contents of regenerated shoots. Comparison of 4 different sources (3 MF exposure and no exposure-control) found out that 50 mT MF treatment was the most effective for the enhancement of rosmarinic acid quantity, total phenol-flavonoid content, non-enzymatic antioxidant activity (IC50), stress related enzymatic antioxidant activity (SOD and CAT) and phenol production related PAL enzyme activity. Although 100 mT MF for 1 h treatment was found to be the most potent in regard to number of shoot formation, the most productive shoot length and fresh-dry weight was obtained with 50 mT MF exposure for 1 h. Considering the all results about growth parameters, medicinal quality (rosmarinic acid amount, total phenol-flavonoid content and antioxidant activity) and endurance capacity against stress (SOD, CAT and PAL activities), it can be concluded that exposure to 50 mT MF was the most effective treatment for in vitro culture of M. officinalis.