DROUGHT AND EXCESS UV-B IRRADIATION DIFFERENTIALLY ALTER THE ANTIOXIDANT SYSTEM IN CUCUMBER LEAVES

The combined effects of enhanced UV-B radiation and soil drought off antioxidant enzyme activity were investigated in cucumber leaves. One-month-old cucumber plants (Cucumis sativus cv. Dar) were exposed to UV-B irradiation and water deficit alone or combined. Physiological measurements were made in seedlings kept tinder stress conditions for nine days and then two more days with stresses withdrawn. Generally a decrease in relative water content and an increase in dry weight content were recorded. The more significant changes were observed tinder drought than tinder UV-B radiation and or combined UV-B and drought. Both stresses stimulated antioxidant enzyme activity. Superoxide dismutase activity increased earlier (day 2) than guaiacol peroxidase and glutathione reductase activity (days 5 and 7). Elevation of enzyme activities was higher tinder drought than tinder UV-B. Combined UV-B and drought functioned synergistically: one of the stresses reduced the effects caused by simultaneous application of the other.