ECOPHYSIOLOGICAL AND HISTOCHEMICAL RESPONSES TO OZONE IN TREE SPECIES ARE INFLUENCED BY THE PROVENANCE

It is well known that the responses of trees to air pollutants vary between and within species under strong genetic control, but this phenomenon can be also regulated by plant provenance. Leaf symptoms attributed to ozone have been detected in a growing list of tree species such as ash, commonly planted in urban areas and also important forest tree in Italy. Two-years old seedlings of Fraxinus excelsior represented by two provenances (Piedmont and Tuscany) were exposed to ozone fumigation. Treated plants developed chlorosis and adaxial necrosis on mature fully expanded leaves. This was true for both provenances even if different photosynthetic efficiency was observed. In Piedmont provenance, ozone directly influenced stomatal aperture, because stomata are closed prematurely, inducing a slow CO₂ movement into the leaf. Histochemical markers such as autofluorescence, the presence of cell-wall thickenings and the typical necrotic lesions of palisade tissue were investigated in order to understand whether symptoms induced by ozone allow differential diagnosis of injury. A significant decrease in the photosynthetic pigments (about -30%) was observed at the end of treatment in both provenances.