EFFECTS OF ISOXAZOLE HERBICIDES ON PROTOPORPHYRINOGEN OXIDASE AND PORPHYRIN PHYSIOLOGY

The biochemical and physiological effects of 10 isoxazoles were invest igated. The amount of protoporphyrin IX caused to accumulate by the co mpounds correlated well with their herbicidal activity. Protoporphyrin ogen oxidase (Protox) was inhibited competitively in the proximity of the catalytic site. However, the Protox I-50 values of the methyl este rs and acid chloride derivatives were lower than expected on the basis of their in vivo herbicidal activity. The results suggest that some t olerance mechanism, other than differential absorption and translocati on, may protect the plants against these compounds. The molecular prop erties of 9 isoxazoles and 2 other well-known inhibitors of different herbicide groups were compared to those of protoporphyrinogen (Protoge n). The most active compounds have similar bulk, electronic, and energ y properties that approximate half of the Protogen molecule. Furthermo re, these compounds have atoms/groups on the ring that generate distin ct negative electrostatic potential fields that may mimic the reactive part of the Protogen molecule.