Biochemical effects of sulphur dioxide arise from its unique ability to act as an oxidizing or a reducing agent. Among some of the important metabolic effects are direct interference with photosynthetic CO2 fixation (competitive inhibition of ribulose diphosphate carboxylase by SO3−) and with energy metabolism (inhibition of mitochondrial ATP production by SO3=). Many indirect effects result from formation of sulphites and organic sulphonates with other cell constituents. These compounds can cause inhibition of a variety of metabolic enzyme systems. All these factors are probably instrumental in the gross disruption of chloroplast and mitochondrial ultrastructure. Injurious effects result when sulphur dioxide is taken up in excess of the capacity of the tissue to incorporate sulphur into the normal metabolic activities.
The ubiquitous presence of small amounts of SO, and the subtle and varied nature of its biochemical effects suggest that crop losses to SO, pollution may be more widespread and serious than is generally suspected.