Cell Membrane Disruptors
Symptomology
Development Summary
Site of Action Summary
Injury Symptoms
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Herbicide Families
Herbicides
Symptomology Development Summary
This diverse group of herbicides causes rapid destruction of cell membranes and thus leakage of cell contents. These herbicides are generally contact killers and do not translocate through the plant in the presence of light.Several herbicide families are commonly grouped into this mode of action including the bipyridiliums and diphenylethers. Herbicides in these families are active in various biochemical pathways in plants, but cause similar symptomology. Symptoms generally include rapid necrosis of leaf tissue, speckling of leaf tissue, and bronzing of soybean leaves.
Once inside the chloroplast
paraquat (a di-cation)
accepts electrons from the electron transport chain of Photosystem I (PS I)
to become a free radical (a mono-cation). Auto-oxidation of molecular oxygen
(O2) by the free radical results in a superoxide (O2-)
that is enzymatically converted to hydrogen peroxide (H2O2).Superoxide
and hydrogen peroxide can yield hydroxyl (OH•) free radicals,
the most potent biological oxidants known, in a reaction catalyzed by transition
metals (iron and/or copper).
Under natural conditions superoxide may exist in plants, but is quickly converted
to hydrogen peroxide, which quickly becomes oxidized to form water. However,
due to the rapid conversion of superoxide to hydroxyl free radicals and the
destructive nature of the components, natural cell devices are overwhelmed,
cell membranes are broken down and leakage of cell contents into intercellular
spaces occurs. This is what causes the “water-soaking” appearance.
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These herbicides are transported
in the xylem and therefore initial symptoms show up on the oldest
growth. Initial symptoms include yellowing on the leaf margin
and between the veins. Yellowing of the tissue is followed by necrosis (death).
These herbicides do not affect germination. Therefore, the symptoms will occur
only after the first leaves emerge (atrazine and oil post-emergence).
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