Sinusoidal vibration alleviates salt stress by induction of antioxidative enzymes and anatomical changes in Mentha pulegium (L.)

Effect of mechanical vibration on salt stress alleviation was studied in pennyroyal (Mentha pulegium L.) seedlings under in vitro and greenhouse conditions, and changes in growth, anatomical, and antioxidative responses were investigated. To determine the optimum frequency of vibration, the sterilized seeds were cultivated on Murashige and Skoog (MS) basal medium. The 14-day-old seedlings were exposed to various frequencies of sinusoidal vibration (0, 50, 100, and 150 Hz) for 30 min, and the optimum frequency was determined based on some growth, physiological, and anatomical parameters. Vibrations up to 100 Hz increased growth, shoot length, chlorophyll content, and relative water content (RWC). Vibration-induced aerenchyma formation, stele diameter, phloem, and suppressed xylem and only a few xylem vessels can be observed at 100 Hz. For evaluation of vibration effects on salt stress alleviation, the seedlings were treated by optimum vibration frequency (100 Hz) and then were subjected to salinity stress (0, 50, 100, and 150 mM NaCl). In salt-stressed plants, vibration decreased malondialdehyde (MDA), H2O2, electrolyte leakage, total carbohydrates, polyphenol oxidase (PPO) activity, and induced proline, protein, ascorbate peroxidase (APX) peroxidase (POX), and superoxide dismutase (SOD) activities. The lower contents of MDA, H2O2, and electrolyte leakage under in vitro grown seedlings comparing to that of whole plants could be associated with the higher levels of in vitro antioxidative enzyme activities. However, vibration induced more antioxidative enzyme activities.