Herbicide Modes of Action

Lectures 15, 16

Herbicide Mode of Action

Contact Types

• Rapid development of symptoms

• Short distance transport

Chronic Types

• Relatively slow development of symptoms

• Systemic

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

Cell Growth Disruption

• Phenoxycarboxylic Acids

• Benzoic acid

• Pyridines

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

Disruption of MitosisRoot Meristematic

Inhibitors• Dinitroanilines

– Treflan

– Prowl

– Pre-M

Shoot Meristematic Inhibitors

• Amides• Benzenedicarboxylate• Benzamide• Substituted Pyridine• Phosphorothioate• Nitrile

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

Pigment InhibitorsInhibition of Caretonoids

• Triazole

• Pyridazinone

• Isoxazolidinone

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

Enzyme Pathway InhibitorsProtoporphyrinogen Oxidase (PPO) Inhibitors

• Diphenyl ethers

• N-Phenylphthalimide

• Oxadiazole

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

Photosynthetic Inhibitors

Photosystem I (PS I)• Bipyridiniums

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

Photosynthetic Inhibitors

Photosystem II--site A

• Triazines• Triazinones

Photosystem II--site B

• Phenylureas• Uracils• Nitriles• Benzothiadiazoles• Amides• Phenyl-pyridazine• Phenylcarbamate

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

Enzyme Pathway Inhibitors

Acetyl CoA Carboxylase (ACCase) Inhibitors• Aryloxyphenoxypropionates• Cyclohexanediones

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

Enzyme Pathway Inhibitors

Acetolactate Synthase (ALS) Inhibitors• Sulfonylureas• Imidazolinones• Sulfonamides• Pyrimidinylthiobenzoate

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

Enzyme Pathway Inhibitors

EPSP Synthase Inhibitors• Glycines

Herbicide Mode of ActionEnzyme Pathway Inhibitors

EPSP Synthase Inhibitors5-enolpyruvylshikimate-3phosphate

• Blocking this enzymatic step prevents formation of phenylalanine, tryptophane, tyrosine

Enzyme Pathway Inhibitors

Glutamine Synthetase Inhibitor• Glycines

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

Enzyme Pathway Inhibitors

Others• Methanearsonates• Benzofurans