herbicide modes of action lectures 15, 16. herbicide mode of action contact types rapid development...
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Herbicide Modes of Action
Lectures 15, 16
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Herbicide Mode of Action
Contact Types
• Rapid development of symptoms
• Short distance transport
Chronic Types
• Relatively slow development of symptoms
• Systemic
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Herbicide Mode of ActionCell Growth Disruption
• Mimic naturally occurring plant hormones & interferes with nucleic acid synthesis, causes massive cell proliferation
• Soil and foliar activity
• Dicots are more sensitive
• Symptoms from foliar treatment apparent in 1-7 days
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Cell Growth Disruption
• Phenoxycarboxylic Acids
• Benzoic acid
• Pyridines
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Herbicide Mode of ActionCell Growth Disruption
• Dicot symptoms– dark green color
– strap shaped leaves
– leaf cupping
– callous stem tissue
– twisted stems
– stem brittleness
– stem splitting
• Monocot symptoms– onion-leafing
– brittle stalks
– fused brace roots
– curved stems
– malformed seed heads
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Disruption of MitosisRoot Meristematic
Inhibitors• Dinitroanilines
– Treflan
– Prowl
– Pre-M
Shoot Meristematic Inhibitors
• Amides• Benzenedicarboxylate• Benzamide• Substituted Pyridine• Phosphorothioate• Nitrile
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Herbicide Mode of ActionRoot Meristematic Inhibitors
• Monocot symptoms– stunting
– lack of secondary roots
– reddish colored shoots
– symptoms appear in 7-10 days
• Dicot symptoms– stunting– lack of secondary roots– swollen / cracked
hypocotyls– stem brittleness– symptoms
• root 7-10 days• stem 6-8 weeks
DinitroanilinesDinitroanilines
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Pigment InhibitorsInhibition of Caretonoids
• Triazole
• Pyridazinone
• Isoxazolidinone
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Herbicide Mode of ActionPigment Inhibitors
• Inhibit production of plant pigments, disrupts carotene development
• Root uptake
• Symptoms appear as bleaching on new leaves
• Symptoms appear within 10 days of treatment
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Enzyme Pathway InhibitorsProtoporphyrinogen Oxidase (PPO) Inhibitors
• Diphenyl ethers
• N-Phenylphthalimide
• Oxadiazole
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PPO (Protoporphyrinogen Oxidase)
• Enzymatic pathway is found in chloroplasts and mitochondria and blocks photosynthesis in the presence of light. Results in oxidative breakdown of cell constituents
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Photosynthetic Inhibitors
Photosystem I (PS I)• Bipyridiniums
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Herbicide Mode of ActionPhotosynthetic Inhibitors PS I
• MOA-produce superoxides which destroy plant cells
• Cell destruction is rapid and apparent in a few hours
• Contact herbicides
• Symptoms: chlorosis, necrosis
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Photosynthetic Inhibitors
Photosystem II--site A
• Triazines• Triazinones
Photosystem II--site B
• Phenylureas• Uracils• Nitriles• Benzothiadiazoles• Amides• Phenyl-pyridazine• Phenylcarbamate
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Herbicide Mode of ActionPhotosynthetic Inhibitors PS II
• Bind with a pigment protein in thylakoid membrane of chloroplasts
• Absorbed by roots & foliage, translocate• Symptoms: Foliar uptake
– interveinal chlorosis of treated leaves
– symptoms appear in a few days
• Symptoms: Root uptake– interveinal chlorosis, beginning with older leaves
– symptoms appear in 1 to 3 weeks
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Enzyme Pathway Inhibitors
Acetyl CoA Carboxylase (ACCase) Inhibitors• Aryloxyphenoxypropionates• Cyclohexanediones
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Herbicide Mode of ActionEnzyme Pathway Inhibitors
ACCase (Acetyl CoA Carboxylase )Inhibit synthesis of lipids in grasses
• Site of action is the enzyme acetyl CoA carboxylase, located in stroma of plastids
• Grasses do not have this enzyme, tolerant species do
• Foliar treatment only
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Enzyme Pathway Inhibitors
Acetolactate Synthase (ALS) Inhibitors• Sulfonylureas• Imidazolinones• Sulfonamides• Pyrimidinylthiobenzoate
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Herbicide Mode of ActionEnzyme Pathway Inhibitors
ALS (Acetolactate Synthase)
• Inhibition of acetolactate synthase enzyme
• Catalyzes the first step in biosynthesis of valine, leucine and isoleucine
• Root and foliar uptake
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Enzyme Pathway Inhibitors
EPSP Synthase Inhibitors• Glycines
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Herbicide Mode of ActionEnzyme Pathway Inhibitors
EPSP Synthase Inhibitors5-enolpyruvylshikimate-3phosphate
• Blocking this enzymatic step prevents formation of phenylalanine, tryptophane, tyrosine
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Enzyme Pathway Inhibitors
Glutamine Synthetase Inhibitor• Glycines
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Herbicide Mode of ActionEnzyme Pathway Inhibitors
Glutamine Synthetase Inhibitor
• Inhibits the enzyme glutamine synthetase. ammonia accumulates and causes photosynthesis to stop and disruption of chloroplasts
• Symptoms: chlorosis
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Enzyme Pathway Inhibitors
Others• Methanearsonates• Benzofurans