University of California
Herbicide Symptoms

Protoporphyrinogen Oxidase (PPO) Inhibitors

HRAC Group: E

WSSA Group: 14

Protoporphyrinogen oxidase (PPO) is an enzyme in the chloroplast cell that oxidizes protoporphyrinogen IX (PPGIX) to produce protoporphyrin IX (PPIX).  PPIX is important because it is a precursor molecule for both chlorophyll (needed for photosynthesis) and heme (needed for electron transfer chains). Inhibitors of the oxidase enzyme, however, do more than merely block the production of chlorophyll and heme. The inhibition of PPO by inhibitors also results in forming highly reactive molecules that attack and destroy lipids and protein  membranes. When a lipid membrane is destroyed, cell becomes leaky and cell organelles dry and disintegrate rapidly.

PPO Inhibitors have limited translocation in plants and sometimes are referred to as contact herbicides. PPO Inhibitors injure mostly broadleaf plants; however, certain PPO Inhibitors have some activity on grasses. PPO Inhibitors usually burn plant tissues within hours or days of exposure. PPO Inhibitors used in the United States belong to eight different chemistries including diphenylethers, N-phenylphthalimides, oxadiazoles, oxazolidinediones, phenylpyrazoles, pyrimidindiones, thiadiazoles, and triazolinones. These herbicides are used to control weeds in field crops, vegetables, tree fruits and vines, small fruits, nurseries, lawns, and industry.

Injury symptoms: Symptoms can occur within 1 to 2 hours after exposure, appearing first as water-soaked foliage, which is followed by browning (necrosis) of the tissue. Symptoms will appear most quickly with bright, sunny conditions at application. Drift injury will appear as speckling on leaf tissue. The necrotic spots are sometimes surrounded by a reddish colored ring. Injury from soil applications or residues appears as a mottled chlorosis and necrosis.

Chemistry Group and Common Names of PPO Inhibitors
Used in the United States

ChemistryCommon Name
Diphenylether Acifluorfen-sodium
N-phenylphthalimide Flumiclorac
Oxadiazole Oxadiazon
Pyrimidindiones Saflufenacil
Thiadiazole Fluthiacet-methyl
Triazolinone Carfentrazone-ethyl
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