Antioxidative defence of old growth beech (Fagus sylvatica) under double ambient O3 concentrations in a free-air exposure system

In this study the influence of chronic free-air ozone exposure and of different meteorological conditions in the very dry year 2003 and the more humid year 2004 on the antioxidative system in sun and shade leaves of adult Fagus sylvatica trees were investigated. Contents of ascorbate, glutathione, and a-tocopherol, as well as chloroplast pigments were determined under ambient (1 x O-3) and double ambient (2 x O-3) ozone concentrations. Ozone affected the antioxidative system in June and July, causing lower ascorbate contents in the apoplastic space, a more oxidized redox state of ascorbate and glutathione and an increase in pigment contents predominantly in the shade crown. For all measured parameters significant differences between the years were observed. In 2004 the redox state of ascorbate and glutathione was in a more reduced state and leaf contents of alpha-tocopherol, pigments of the xanthophyll cycle, beta-carotene, lutein, neoxanthin, and alpha-carotene were lower compared to 2003. Contents of total glutathione and chlorophyll a + b were increased in the second year. These results indicate a strong influence of the drought conditions in 2003 on the antioxidative system of beech overruling the ozone effects. Shade leaves showed lower contents of ascorbate in both years and the redox states of ascorbate and glutathione were more oxidized compared to sun leaves. Contents of photoprotective and accessory pigments generally were enhanced and the de-epoxidation state of the xanthophyll cycle was lower in the shade compared to the sun crown. Exhibiting less antioxidants shade leaves seem to be more sensitive against ozone than sun leaves.