Many plants species respond to ambient ozone (O3) pollution with O3–specific visible foliar injury that can be diagnosed in the field. Visible foliar O3 injury is considered as an important plant–response indicator in forest monitoring  because it is specific of O3 injury and is not caused by other co–occurring factors. 

 

Conifers develop yellowish spots with ill-defined contours (chlorotic mottle), which in the most severe cases can degenerate into necrosis. Broadleaves develop a wide range of symptoms especially on the adaxial surface of the leaf, affecting the interveinal areas.

 

Acute injuries includes bleaching (small unpigmented necrotic spots), flecking (small necrotic areas), stippling (tiny punctuate spots were a few palisade cells are dead or injured, and may be white, black, red or red purple) necrosis. Chronic injury consists in pigmentation (leaves turn red-brown to brown as pigments accumulate), chlorosis (due to chlorophyll breakdown) and premature senescence.