Response of in vitro-regenerated Myrtus communis L. shoots to PEG-induced water stress

Myrtle (Myrtus communis L.) is an evergreen and aromatic plant species with therapeutic and ornamental values. The effect of water stress induced by three concentrations of Polyethylene glycol (PEG), 0% (control), 3% (3%PEG) and 6% (6% PEG) was evaluated on in vitro-regenerated myrtle shoots. After 30 days of culture, all regeneration, growth and physiological parameters were measured at the end of the experiment. Murashige and Skoog (MS) medium contained 1.5 mgl-1 6-benzyladenine (BAP) and 0.2 mgl-1 Naphthaleneacetic acid (NAA) was optimized for the in vitro experiment. Increasing PEG concentration in the culture media resulted in fewer regenerated shoots, and higher rate of browning in the apical tissues. Chlorophylls and total carotenoids content significantly decreased in the shoots treated with PEG, however there were no significant differences in the pigment content under moderate or severe water stress conditions. PEG-induced water stress resulted in reduced plant growth and relative water content of newly-formed shoots. Lipid peroxidation and electrolyte leakage were lower in myrtle shoots treated with 6%PEG than 3%PEG, but they were still higher than that of the control. Proline accumulation significantly increased under 3%PEG treatment, while a little difference were observed between proline content under 6%PEG and the control. Apoptosis did not occur in the myrtle shoots grown under moderate or severe stress treatments which correlated with more increased Catalase and Superoxide dismutase activities in response to higher stress levels. The results showed a possible contribution of enzymatic antioxidant system in myrtle when counteracting severe water stress.