Effects of magnetic field on tissue culture, biological activity, phenolic content and defense enzyme activities (sod, cat and pal) of Melissa officinalis L. (Lemon balm), a medicinal plant

Melissa officinalis L. (lemon balm) is a precious medicinal herb utilized in the remedy of depression, nervousness, anxiety, hypersensitivities, headache, colic, indigestion, cardiac problems, rheumatism, and insomnia in traditional medicine. The study's aim was to assess how three distinct magnetic fields (MF-50 mT, 100 mT, and 150 mT) with varying exposure periods affected seed germination and explant growth in vitro (shoot tip, axillary and hypocotyl buds). In vitro-propagated shoots and seedlings that had received MF treatments were also evaluated considerining their shoot and root length, fresh and dry weight and percentage of water content. Moreover, in vitro-propagated shoots acquired from 3 distinct MF applications with 1 h exposure and no MF-exposured shoots (control) were assessed in terms of individidual phenolic content, total content of phenol and flavonoid, biological effects (antibacterial and antioxidant) and enzyme activities [superoxide dismutase (SOD), catalase (CAT) and phenylalanine ammonium lyase (PAL)]. All evaluated MF treatments (50 mT, 100 mT, and 150 mT) improved the seed germination percentage and lowered the length of seed germination. It was also found that seedlings grown from MF-treated seeds had longer root and shoot lengths and greater water content than control seedlings (no MF exposure). The best regeneration occurred when axillary buds were exposed to 100 mT for 1 hour (15.8 shoots with a frequency of 100%). Additionally, low, and medium MF applications (100 mT and 150 mT) promoted the growth parameters as well as the water content of in vitro-propagated shoots. As opposed to that, quantity of rosmarinic acid, total content of phenol and flavonoid, antioxidant activities [enzymatic (SOD and CAT) and non-enzymatic (IC50)], and PAL enzyme activity involved with phenol synthesis were the highest with 50 mT MF application. Antibacterial capacity was observed only with no MF exposure shoots. The most effective shoot length and fresh-dry weight were achieved with 50 mT MF application for 1h, even though 100 mT MF for 1h exposure was indicated to be the most effective in terms of in vitro shoot propagation. Taking into account all of the findings regarding growth parameters, medicinal essence (rosmarinic acid quantity, total content of phenol-flavonoid, and antioxidant potential), and the capacity to cope with stress regarding PAL, SOD, and CAT levels, it is possible to deduce that 50 mT MF application was the most appropriate approach for M. officinalis propagation in vitro.