Physiological, Biochemical and Molecular Effects of Sulfur Dioxide

Damage of leaves due to air pollutants such as sulfur dioxide is mediated through the production of reactive oxygen species. The site of action of sulfur dioxide is the chloroplast and deleterious effects on foliar tissue depend on light and photosynthetic electron transport. Protection may be afforded, in part, by components of the antioxidant (photo)scavenging cycle. Relative resistance to sulfur dioxide and cross-resistance to other oxidative stresses which originate in the chloroplast have been correlated, in many cases, with elevated levels of various antioxidant proteins and/or substrates. Recent studies utilizing differentially sensitive cultivars, antioxidant enzyme analyses which differentiate between specific isoforms at the gene and protein levels, and plants genetically engineered to alter the expression of specific antioxidant isozymes, have provided new insights into the mechanisms of resistance to sulfur dioxide and other stresses. These data suggest that complex regulatory mechanisms function at both the gene and protein level to coordinate antioxidant responses and that a critical role is played by organellar localization and inter-compartment coordination. An involvement of a strong developmental component in resistance is indicated.