Stimulation of antioxidant system and lipid peroxidation by abiotic stresses in leaves of Momordica charantia

The purpose of the present work was to evaluate both the antioxidant response system and oxidative stress in leaves from bitter gourd (Momordica charantia L) subjected to NaCl, UV-B and water stresses at three different stages of plant growth: pre-flowering, flowering and post-flowering. Except for peroxidase (POX), all enzyme activities including superoxide dismutase (SOD), catalase (CAT), polyphenol oxidase (PPO), glutathione reductase (GR), as well as concentrations of ascorbate (ASA), hydrogen peroxide (H2O2) and thiobarbituric acid reactive substances (TBARS) exhibited maximum values at the flowering stage under all three stresses. All the enzyme activities, SOD, CAT, POX, PPO, GR and the concentrations of ASA, H2O2 and TBARS were elevated under NaCl and UV-B stresses at all growth stages with the exception of H2O2 concentration at the post-flowering stage under UV-B radiation. Greater quantities of the inorganic ions Na+ and Cl- were accumulated at all growth stages under salt stress. Drought led to decreases in the concentrations of H2O2, ASA and activities of PPO and GR; conversely, it led to elevated concentrations of TBARS and activities of SOD, CAT, POX at all three stages in comparison with control values. The POX activity at the third stage was, however, reduced. The photosynthetic pigments decreased at all stages under all stresses. The chlorophyll stability index decreased under NaCl stress, accelerated only at the post-flowering stage under UV-B radiation and significantly increased at pre- and post-flowering stages under water stress. The protein concentration under NaCl stress decreased except at the pre-flowering stage and under UV-B and water stresses the protein concentration was higher than that of the control and NaCl treatment. It is suggested that M. charantia exhibits a protection mechanism against oxidative damage by maintaining a highly induced antioxidant system under all three stresses.