Photosynthesis of two poplar clones under long-term exposure to ozone

The photosynthetic response was studied in two clones (Populus deltoides x maxi-mowiczii Eridano and Populus x euramericana I-214), known for their differential response to ozone (O-3) in terms of visible symptoms, when exposed to O-3 (60 nl l(-1) 5 h day(-1), 7 and 15 days). The photosynthetic ability was tested using gas exchange and chlorophyll fluorescence analysis. O-3 caused a decrease in the CO2 assimilation rate at light saturation level in mature leaves of both clones. Alterations of Chi fluorescence parameters, in particular the F-v/F-m ratio and non-photochemical quenching were also observed. The effects were similar for both clones and it could not be concluded that differential effects on electron transport capacity were responsible for the observed reduction in photosynthesis. The reduction of photosynthetic rate in Eridano was due mainly to a reduced mesophyll activity, as evidenced by the increase in intercellular CO2 concentration and the minimal changes in stomatal conductance. In contrast, in I-214. stomatal effects were primarily responsible, although effects on the mesophyll cannot be excluded. Data obtained indicate that the effects observed at the mesophyll level may be attributed to indirect effects caused by membrane disorders.