Ultraviolet-B irradiation-induced freezing tolerance in relation to antioxidant system in winter wheat (Triticum aestivum L.) leaves

Seven-day-old seedlings of winter wheat (Triticum aestivum L.) in a growth chamber were exposed to ultraviolet-B (UV-B) irradiation for 20 days with daily biologically effective (BE) UV-B irradiation (UV-B-BE) at low (4.2 kJ m(-2) day(-1), L-UVB) and high (7.0 kJ m(-2) day(-1), H-UVB) levels. The UV-B irradiated seedlings and the control without UV-B irradiation were then subjected to freezing stress at -6 degrees C for 6 h and recovered to 20 degrees C with gradually increased temperature, to investigate the effects of UV-B irradiation on freezing tolerance. During the UV-B exposure, both L-UVB and H-UVB irradiated seedlings had lower half lethal temperature (LT50) values in comparison with the control, and L-UVB more effectively decreased the LT50 values than H-UVB. Moreover, foliar concentrations of thiobarbituric acid reactive substances (TBARS) in the UV-B irradiated seedlings were lower than that of control after recovery from freezing stress. Hydrogen peroxide (H2O2) rapidly increased after UV-B exposure, as did activity of superoxide dismutase (SOD). After recovery from freezing stress, activities of catalase (CAT), guaiacol peroxidase (GPX) and glutathione reductase (GR) increased in both L-UVB and H-UVB leaves, whereas activities of ascorbate peroxidase (APX) and monodehydroascorbate reductase (MDHAR) significantly increased only in the L-UVB leaves. Furthermore, the ascorbic acid (AsA) concentration and reduced-to-oxidized ascorbate ratio (AsA/DHA) increased in the L-UVB leaves both at the end of UV-B exposure and after recovery from freezing stress. However, the reduced glutathione (GSH) concentration, together with reduced-to-oxidized glutathione ratio (GSH/GSSG) increased in both L-UVB and H-UVB leaves after recovery from freezing stress. UV-B irradiation increased freezing tolerance in winter wheat seedlings, and this response appears to involve the scavenging enzymes and compounds in the antioxidant defense systems, particularly the ascorbate-glutathione cycle. (C) 2007 Elsevier B.V. All rights reserved.