A method for the numerical assessment of the foliar injury caused by the photochemical oxidant, peroxyacetyl nitrate (PAN), was devised, using three injury indices: fresh weight (FW) loss, decreased photosynthetic pigment content, and increased ion leakage, which can be measured using the same leaves. The injury indices clearly indicated a larger number of PAN-sensitive leaves and a more severe level of injury in the PAN-sensitive variety of Petunia hybrida, White Champion (WHITE), compared to the PAN-tolerant variety, Blue Champion (BLUE). FW and photosynthetic pigment content decreased correlatively in both varieties, but ion leakage increased only in WHITE. Morphological observations revealed that ion leakage started concurrently with the start of plasmolysis-like symptoms at the mesophyll cells of injured WHITE leaves, whereas FW loss corresponded to the shrinkage of cells without loss of their round shape in BLUE leaves. PAN injuries measured by the injury indices were markedly increased in the presence of light, and the morphological changes following PAN exposure were similar to those caused by the superoxide-generating chemical, paraquat. The results suggested that PAN injuries indicated by the three injury indices are all light-dependent, but are caused through several independent mechanisms.