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 per iod. Surprisingly, not only elevated (ao(+), i.e. ambient air + 50 mm( 3) m(-3) ozone) but also ambient (aa) ozone concentrations led to a re duction 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 (g(l)) 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) correla ted with a loss in A(sat). In comparison to cf-leaves, aa leaves showe d 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(+) leave s 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 F-O increased with the dos age 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 c omponents of the Calvin cycle, which results in a decrease in A(sat) w ith subsequent feedback on the current photochemical capacity of PSII centres.