Photosynthetic parameters of birch (Betula pendula Roth) leaves growing in normal and in CO2- and O3-enriched atmospheres

Two silver birch (Betula pendula Roth) clones K1659 and V5952 were grown in open-top chambers over 3 years (age 7-9 years). The treatments were increased CO2 concentration (+CO2, 72 Pa), increased O-3 concentration (+O-3, 2 x ambient O-3 with seasonal AOT40 up to 28 p.p.m. h) and in combination (+CO2 + O-3). Thirty-seven photosynthetic parameters were measured in the laboratory immediately after excising leaves using a computer-operated routine of gas exchange and optical measurements. In control leaves the photosynthetic parameters were close to the values widely used in a model (Farquhar, von Caemmerer and Berry, Planta 149, 78-90, 1980). The distribution of chlorophyll between photosystem II and photosystem I, intrinsic quantum yield of electron transport, uncoupled turnover rate of Cyt b(6)f, Rubisco specificity and K-m (CO2) were not influenced by treatments. Net photosynthetic rate responded to +CO2 with a mean increase of 17% in both clones. Dry weight of leaves increased, whereas protein, especially Rubisco content and the related photosynthetic parameters decreased. Averaged over 3 years, eight and 17 mechanistically independent parameters were significantly influenced by the elevated CO2 in clones K1659 and V5952, respectively. The elevated O-3 caused a significant decrease in the average photosynthetic rate of clone V5952, but not of clone K1659. The treatment caused changes in one parameter of clone K1659 and in 11 parameters of clone V5952. Results of the combined treatment indicated that +O-3 had less effect in the presence of +CO2 than alone. Interestingly, changes in the same photosynthetic parameters were observed in chamberless grown trees of clone V5952 as under +O-3 treatment in chambers, but this was not observed for clone K1659. These results suggest that during chronic fumigation, at concentrations below the threshold of visible leaf injuries, ozone influenced the photosynthetic parameters as a general stress factor, in a similar manner to weather conditions that were more stressful outside the chambers. According to this hypothesis, the sensitivity of a species or a clone to ozone is expected to depend on the growth conditions: the plant is less sensitive to ozone if the conditions are close to optimal and it is more sensitive to ozone under conditions of stress.