Photochemistry of parathion on plant surfaces: Relationship between photodecomposition and iodine number of the plant cuticle

The photochemical behavior of parathion was investigated in several model solutions and on enzymatically isolated plant cuticles. Degradation rate and photoproduct distribution were compared. The main degradation pathway was the photoreduction of the nitro group leading to the nitroso compound and the condensation products azo- azoxy- and 2-hydroxy-azoparathion. Only small amounts of the oxidation product paraoxon and the cleavage product nitrophenol could be detected on plant surfaces. Since we observed olefins to accelerate parathion degradation, the results led to the assumption of a high content of olefinic compounds in plant cuticles. Therefore, a rapid method for the determination of the iodine number of isolated cuticles was developed and the fruit cuticles of grape, apple, paprika and tomato were examined. The content of unsaturated compounds differed largely within the investigated plants with iodine numbers from 25 up to 111, which strongly correlated with accelerated degradation of parathion on plant cuticles.