EFFECTS OF EXCESS UV-B IRRADIATION ON THE ANTIOXIDANT DEFENSE-MECHANISMS IN WHEAT (TRITICUM-AESTIVUM L.) SEEDLINGS

In the present experiments, changes in the antioxidant defence systems of wheat (Triticum aestivum L. cv. Tiszataj) as affected by excess UV -B irradiation were investigated. Wheat seedlings were gl-own hydropon ically in phytotrons for 11 days. In one of the phytotrons 2.5 Wm(-2) excess UV-B irradiation was applied and samples were taken daily. Cata lase, guaiacol peroxidase, superoxide dismutase (SOD) and glutathione reductase (GR) activities were measured and changes in the level of gl utathione disulfide (GSSG) were determined by HPLC technique. It was f ound that catalase, guaiacol peroxidase and SOD activities decreased, while GSSG level and GR activity increased under excess W-B irradiatio n. In plant responses, the alarm and adaptation phases were recognised . During the alarm reaction, inhibition of stem and leaf elongation, a ccumulation of flavonoid pigments, decreased antioxidant enzyme activi ties, increased GSSG level and triggering of GR activity were observed , while normalisation of growth, disappearance of stem pigmentation, l evelling of antioxidant enzyme activities, high GR activity and decrea sed level of GSSG were found as parts of adaptation to UV-B irradiatio n. After 3 days of UV-B treatment, the activities of Ca2+-dependent pr otein kinases (52 and 65 kDa) increased, suggesting that they may part icipate in the UV-B signal transduction pathway.