Effect of abscisic acid on active oxygen species, antioxidative defence system and oxidative damage in leaves of maize seedlings

Leaves of maize (Zea mays L.) seedlings were supplied with different concentrations of abscisic acid (ABA). Its effects on the levels of superoxide radical (O₂^-), hydrogen peroxide (H₂O₂) and the content of catalytic Fe, the activities of several antioxidative enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR), the contents of several non-enzymatic antioxidants such as ascorbate (ASC), reduced glutathione (GSH), α-tocopherol (α-TOC) and carotenoid (CAR), and the degrees of the oxidative damage to the membrane lipids and proteins were examined. Treatment with 10 and 100 μM ABA significantly increased the levels of O₂^- and H₂O₂ followed by an increase in activities of SOD, CAT, APX and GR, and the contents of ASC, GSH, α-TOC and CAR in a dose- and time-dependent pattern in leaves of maize seedlings. An oxidative damage expressed as lipid peroxidation, protein oxidation, and plasma membrane leakage did not occur except for a slight increase with 100 μM ABA treatment for 24 h. Treatment with 1,000 μM ABA led to a more abundant generation of O₂^- and H₂O₂ and a significant increase in the content of catalytic Fe, which is critical for H₂O₂-dependent hydroxyl radical production. The activities of these antioxidative enzymes and the contents of α-TOC and CAR were still maintained at a higher level, but no longer further enhanced when compared with the treatment of 100 μM ABA. The contents of ASC and GSH had no changes in leaves treated with 1,000 μM ABA. These results indicate that treatment with low concentrations of ABA (10 to 100 μM) induced an antioxidative defence response against oxidative damage, but a high concentration of ABA (1,000 μM) induced an excessive generation of AOS and led to an oxidative damage in plant cells.