Prolonged low dose of ultraviolet-B radiation does not activate antioxidant defence in young pea plants

The behavior of the antioxidant defense system was studied in pea (Pisum sativum L., cv. Scinado) leaves after exposure of intact plants to short pulses of ultraviolet-B (UV-B) radiation. Low and moderate regimes were used, B1 (4.4 kJ m-2 d-1) and B2 (13.3 kJ m-2 d-1), for two weeks. Hydrogen peroxide (H2O2) levels increased moderately after 7 and 10 days of B1 and B2 treatments. At the end of experiment (14d UV-B) accumulation of H2O2 was found only in B2-irradiated plants. The activities of superoxide dismutase (SOD) and glutathione-reductase (GR) were not changed noticeably. A substantial increase was found in guaiacol-peroxidase (GPX) and glutathione-S-transferase (GST) (only in B2 treated plants), whereas catalase (CAT) and ascorbate peroxidase (APX) decreased. In conclusion, the activation of enzymatic antioxidant defence was not noticeable when plants were exposed to prolonged and low levels of UV-B radiation. The result was opposite to the researches where plants were treated with single and high UV-B radiation doses.