SEASONAL-CHANGES OF THE ANTIOXIDATIVE SYSTEMS IN FOLIAR BUDS AND LEAVES OF FIELD-GROWN BEECH TREES (FAGUS-SYLVATICA, L) IN A STRESSFUL CLIMATE

The activities of superoxide dismutase, ascorbate peroxidase, monodehy droascorbate radical reductase, and dehydroascorbate reductase and the contents of ascorbate, chlorophyll and soluble protein were determine d in beech (Fagus sylvatica, L.) foliage over two or three seasons. Fo ur important stages of leaf development were distinguished: resting bu ds, emerging, mature and senescent leaves. Foliar buds in spring, prio r to the emergence of new leaves, contained a lower chlorophyll conten t but a higher protein content and higher activities of ascorbate pero xidase and monodehydroascorbate radical reductase than mature leaves i n summer. By contrast, superoxide dismutase and glutathione reductase activities and ascorbate contents were higher in mature leaves than in swollen foliar buds. Dehydroascorbate reductase activity was low in a ll developmental stages. Resting buds in winter contained activities o f superoxide dismutase, ascorbate peroxidase and monodehydroascorbate radical reductase that were similar to those found in mature leaves in summer, whereas the contents of total and reduced ascorbate were 6- a nd 20-times lower, respectively, in buds than in mature leaves. The lo w foliar concentration of reduced ascorbate in resting buds, despite h igh monodehydroascorbate radical reductase activity, suggests that the regeneration of ascorbate might be limited by the availability of red uctant. High anti-oxidative capacity was conferred by mature beech lea ves and may be an important protection measure for coping with the lar ge fluctuations in temperature and exposure to elevated ozone concentr ations in summer.