The influence of ambient and elevated ozone concentrations on photosynthesis in Populus nigra

Light-saturated net leaf photosynthesis (A_sat), CO₂ response curves (A/C_i), current photochemical capacity (F_v/F_m) and pigment contents were measured in leaves of Populus nigra (Clone T107) which had been exposed to ozone stress in open-top chambers for the entire growth period. Surprisingly, not only elevated (ao⁺, i.e. ambient air + 50 mm³ m^?3 ozone) but also ambient (aa) ozone concentrations led to a reduction in A_sat, in comparison with leaves exposed to air containing almost no ozone (cf^?, i.e. charcoal filtered ambient air). The very small change in leaf conductance (g₁) indicated that the decrease in A_sat was not due to stomatal limitation. This finding was supported by the fact that, a decrease in carboxylation efficiency (CE) correlated with a loss in A_sat. In comparison to cf^? leaves, aa leaves showed no change in current photochemical capacity (F_v/F_m) throughout the whole experiment. However, a marked decline in F_v/F_m in ao⁺ leaves was observed at a time when A_sat and CE were already decreased by about 45% and 60% respectively. As the chlorophyll b content of leaves is known to correlate with the amount of LHC and PSII centres, it was used to normalize fluorescence parameters in relation to PSII centres present. The normalized values for F_m and F₀ increased with the dosage of ozone in ao⁺ leaves but not in aa leaves, indicating a change of the pigment content of PSII in the former, but not in the latter. These data led to the conclusion that ozone interacts primarily with components of the Calvin cycle, which results in a decrease in A_sat, with subsequent feedback on the current photochemical capacity of PSII centres.