Evidence that branch cuvettes are reasonable surrogates for estimating O₃ effects in entire tree crowns

Within the scope of quantifying ozone (O₃) effects on forest tree crowns it is still an open question whether cuvette branches of adult trees are reasonable surrogates for O₃ responses of entire tree crowns and whether twigs exhibit autonomy in defense metabolism in addition to carbon autonomy. Therefore, cuvette-enclosed branches of mature beech (Fagus sylvatica) trees were compared with branches exposed to the same and different ozone regimes by a free-air fumigation system under natural stand conditions by means of a wee versa experiment. For this purpose, cuvettes receiving 1 x O ₃ air were mounted in trees exposed to 2 x O₃ and cuvettes receiving 2 x O₃ air were mounted in trees exposed to 1 x O ₃ in the upper sun crown. At the end of the fumigation period in September 2004, leaves were examined for differences in gas exchange parameters, pigments, antioxidants, carbohydrates, and stable isotope ratios. No significant differences in foliar gas exchange, total carbohydrates, stable isotope ratios, pigment, and antioxidant contents were found as a consequence of cuvette enclosure (cuvette versus free-air branches) of the same O₃ concentrations besides increase of glucose inside the cuvettes and reduction of the de-epoxidation state of the xanthophyll cycle pigments. No significant ozone effect was found for the investigated gas exchange and most biochemical parameters. The total and oxidized glutathione level of the leaves was increased by the 2 x O₃ treatment in the cuvette and the free-air branches, but this effect was significant only for the free-air branches. From these results we conclude that cuvette branches are useful surrogates for examining the response of entire tree crowns to elevated O₃ and that the defence metabolism of twigs seems to be at least partially autonomous.