Branch growth and leaf gas exchange of Populus tremula exposed to low ozone concentrations throughout two growing seasons

During two consecutive growing seasons, the same potted individuals of European aspen (Populus tremula), grown from root cuttings of one clone, were fumigated with either ambient air or ozone concentrations of 0 (control), 0.05 or 0.1 μlitre litre^-1. Structure and biomass of the annually formed branches were analysed after excision at the end of each season. Only at 0.1 μlitre litre ^-1 was branch weight reduced, and crooked axes occurred in each season. During the second season, branch length and leaf sizes were strongly reduced, while many leaves displayed yellowish deficiency symptoms and lowered cation concentrations. Such leaves contrasted to those showing characteristic O₃-bronzing. Although foliage density was enhanced due to reduced branch length, the area of attached foliage was limited by the small leaf sizes, necrotic leaves and premature leaf loss. During mid-summer of the second fumigation period, photosynthetic capacity, carboxylation efficiency and water-use efficiency (WUE) declined in (attached) yellowish and bronze leaves at 0.1 μlitre litre^-1, whereas green leaves at 0.05 μlitre litre^-1 displayed accelerated senescence in late summer while maintaining WUE. It is concluded that the differences in branch growth between the two growing seasons were caused in part by internal changes in those plant organs (root and basal stem), which had experienced both fumigation periods.