Flux-based response of sucrose and starch in leaves of adult beech trees (Fagus sylvatica L.) under chronic free-air O₃ fumigation

Investigations on sucrose and starch contents in leaves of 60-year-old beech trees (Fagus sylvatica L.) are the focus of the present study. Five trees were exposed to a twice ambient ozone regime (2 x O₃) with a free-air canopy exposure system throughout the seasons and five trees under the prevailing ambient ozone regime served as controls (1 x O₃). In order to examine chronic ozone (O₃) effects, leaf samples from the sun and shade crowns of the trees were analyzed five times throughout the growing seasons in 2003 and 2004. Sucrose concentrations of leaves collected in 2004 were consistently lower than those taken in 2003, regardless of the O ₃ treatment and crown position. However, the opposite was found for starch. O₃ caused a reduction of sucrose and starch contents of sun leaves in both years. Due to the fact that O₃-responsiveness depends on the O₃ uptake through stomata during the season, all carbohydrate data were related to the cumulative O₃ uptake (COU). Little differences were found comparing sucrose and starch contents in leaves of trees grown under ambient or elevated O₃ regimes, possibly indicating the high capacity of leaves of adult beech to cope with rising O₃ exposure. Even under 2 x O₃, leaves were still able to regulate the O₃ intake by narrowing their stomata at the cost of CO ₂-uptake and sugar synthesis. In order to clarify whole-tree response patterns carbohydrate data were compared with photosynthesis, stomatal conductance and electron transport rates. In 2004 all parameters revealed a significant common response pattern to COU that indicated a reduction for all parameters under 2 x O₃.