Physiological and antioxidant defense mechanisms of Nepeta nervosa plants against polyethylene glycol (PEG)-mediated drought stress

Abstract

Drough-stress conditions lead to excessive production of reactive oxygen species (ROS) and oxidative damage in plants. Plants display various defense mechanisms for survival, including the activation of antioxidant enzymes and the production of low molecular weight non-enzymatic antioxidants like phenolics. In this work, roots of four weeks old Nepeta nervosa plants were exposed to PEG solutions with osmotic pressure of −3 MPa for one, three and six days. Fresh weight (FW) and relative water content (RWC) of N. nervosa plants were significantly reduced compared to the corresponding control - up to 68% (FW) and up to 35% (RWC). In leaves, the content of rosmarinic and chlorogenic acids, dominant phenolic compounds, as well as of less abundant ferulic and caffeic acids, was dependent on the duration of PEG-induced drought stress. The content of rosmarinic acid in control plants achieved an average value of 18.27 µg/mg FW, while at PEG-treatments it increased by 355% (3 days of PEG-treatment). Among all examined antioxidative enzymes in leaves of PEG treated N. nervosa plants, superoxide dismutases (SOD) were the most active (an increase of 853% on day 6). PEG treatment also caused an increase in peroxidase (POX) activity in leaves of N. nervosa plants (108% increase on day 3). Since catalase (CAT) activity was significantly decreased on PEG treatment, except on day 1, it could be presumed that H2O2 elimination is performed mostly by POX. This work is a useful contribution to the development of a strategy for overcoming oxidative stress in Nepeta species, especially in arid and semiarid regions.