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 Action Cell 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 Action Cell 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 Mitosis Root Meristematic Inhibitors Dinitroanilines –Treflan –Prowl –Pre-M Shoot Meristematic Inhibitors Amides Benzenedicarboxylate Benzamide Substituted Pyridine Phosphorothioate Nitrile

Herbicide Mode of Action Root Meristematic Inhibitors Monocot symptoms –stunting –lack of secondary roots –reddish colored shoots –symptoms appear in days Dicot symptoms –stunting –lack of secondary roots –swollen / cracked hypocotyls –stem brittleness –symptoms root 7-10 days stem 6-8 weeks Dinitroanilines Dinitroanilines

Pigment Inhibitors Inhibition of Caretonoids Triazole Pyridazinone Isoxazolidinone

Herbicide Mode of Action Pigment 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 Inhibitors Protoporphyrinogen 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 Action Photosynthetic 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 Action Photosynthetic 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 Action Enzyme 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 Action Enzyme 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 Action Enzyme Pathway Inhibitors EPSP Synthase Inhibitors 5-enolpyruvylshikimate-3phosphate Blocking this enzymatic step prevents formation of phenylalanine, tryptophane, tyrosine

Enzyme Pathway Inhibitors Glutamine Synthetase Inhibitor Glycines

Herbicide Mode of Action Enzyme 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