Effects of isoxazole herbicides on protoporphyrinogen oxidase and porphyrin physiology

The biochemical and physiological effects of 10 isoxazoles were investigated. The amount of protoporphyrin IX caused to accumulate by the compounds correlated well with their herbicidal activity. Protoporphyrinogen oxidase (Protox) was inhibited competitively in the proximity of the catalytic site. However, the Protox I-50 values of the methyl esters and acid chloride derivatives were lower than expected on the basis of their in vivo herbicidal activity. The results suggest that some tolerance mechanism, other than differential absorption and translocation, may protect the plants against these compounds. The molecular properties of 9 isoxazoles and 2 other well-known inhibitors of different herbicide groups were compared to those of protoporphyrinogen (Protogen). The most active compounds have similar bulk, electronic, and energy properties that approximate half of the Protogen molecule. Furthermore, these compounds have atoms/groups on the ring that generate distinct negative electrostatic potential fields that may mimic the reactive part of the Protogen molecule.