Macroscopic and microscopic symptoms in Abies religiosa exposed to ozone in a forest near Mexico City

Biomarkers to ozone exposure were sought at the macroscopic and microscopic levels to determine the etiology of the injury in Abies religiosa from a forest known to be polluted with ozone. Symptoms in the needles were mainly manifest on the adaxial side and consisted of whitish stippling that turned into larger and reddish mottled lesions, finally joining each other until they covered the surface. The injury progressed from the base to the tip of the leaf which coincided with its pattern of development, this became evident within different types of cells along the palisade parenchyma where the injury was located. Cells classified as being old and injured were prominent even in young needles of trees exposed to ozone, not in those from the control site. These alterations, the early abscission of the leaves and the accelerated deterioration of the trees manifest a premature senescence process which is characteristic of ozone injury.