Testing phaeophytinization as an index of ozone stress in trees

Abstract

Ground-level ozone pollution is a menace for vegetation in the northern hemisphere, limiting photosynthetic pigments and suppressing photosynthesis in trees and other types of plants. Phaeophytinization is the process of converting chlorophylls into phaeophytins, for example by acidification. Ozone is a highly oxidizing molecule and well known to degrade chlorophylls; however, the effect of ozone on phaeophytinization in leaves of higher plants is largely unknown. To reveal ozone effect on phaeophytinization and evaluate the potential of phaeophytinization as an index of ozone stress in trees, the absorbance at the optical density of 665 nm was measured before (OD665) and after (OD665a) acidification in three independent experiments with nearly 30 conditions of ozone exposure. Both current ambient and elevated ozone widely affected phaeophytinization, as indicated by decreases or increases in the phaeophytinization quotient OD665/OD665a. These effects were commonly moderate to large in magnitude and practically significant, and occurred even in ozone-asymptomatic leaves. It emerges that the ozone effect on phaeophytinization is bimodal, likely depending on the intensity of ozone stress. These results indicate a promising feature of OD665/OD665a as a thorough index of ozone stress in the future, but further studies are needed to reveal the underlying biochemical mechanisms of the bimodal effect on phaeophytinization.