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 di fferential response to ozone (O-3) in terms of visible symptoms, when expos ed to O-3 (60 nl l(-1) 5 h day(-1), 7 and 15 days). The photosynthetic abil ity 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 con cluded that differential effects on electron transport capacity were respon sible for the observed reduction in photosynthesis. The reduction of photos ynthetic rate in Eridano was due mainly to a reduced mesophyll activity, as evidenced by the increase in intercellular CO2 concentration and the minim al changes in stomatal conductance. In contrast, in I-214. stomatal effects were primarily responsible, although effects on the mesophyll cannot be ex cluded. Data obtained indicate that the effects observed at the mesophyll l evel may be attributed to indirect effects caused by membrane disorders.