Herbicides: Mode of Action

Question 1

Lipid synthesis inhibitors

Answer 1

Action: Prevent formation of essential fatty acids vital to cell wall growth and integrity
Injury: Seen on grasses only. New leaf tissue chlorotic or necrotic. Leaves in whorl easily separated from plant

Question 2

Amino Acid Acid Synthesis Inhibitors

Answer 2

Action: Inhibit enzymes, ALS (Amino Lactate Synthase) or EPSP synthase, to prevent amino acid production
Injury: Grasses- stunted with interveinal chlorosis or purpling
Broadleaf- stunted and chlorotic or purple

Question 3

Seedling Root Inhibitors

Answer 3

Action: Stop cell division (microtubual assembly), inhibiting shoot elongation and lateral root elongation
Injury: Highly susceptible weeds do not emerge.
Stunted plants have short, thick lateral roots & deformed shoots
Grass shoots may be red or purple.
Broadleaf plants have swollen/cracked hypocotyls

Question 4

Growth Regulators/ Synthetic Auxins

Answer 4

Action: Disrupt hormone balance and protein synthesis resulting in abnormal growth.
Injury: Corn show onion-leafing, fused brace roots, bent and brittle stalks.
Broadleaf plants show epinasty (twisted & turned), callus tissue, and leaf malformations.

Question 5

Mobile Photosynthesis Inhibitors

Answer 5

Action: Bind to sites within chloroplasts and stop carbohydrate production
Injury: Symptoms occur after leaves emerge. Chlorosis of older leaf margins or tips initially. Interveinal chlorosis in broadleaf plants. Tissue will turn brown and die.

Question 6

Non-Mobile Photosynthesis Inhibitors

Answer 6

Action: Bind to sites within chloroplasts and stop carbohydrate production.
Injury: Postemergence activity only. Leaves turn yellow or bronze, and then turn brown and die. Symptoms like that from cell membrane disrupters.

Question 7

Glutamine Synthetase Inhibitors

Answer 7

Action: Inhibits glutamine synthetase. Accumulation of ammonia destroys cells and directly inhibits PSI and PSII
Injury: Chlorosis and wilting usually within 3-5 days, followed by necrosis in 1-2 weeks

Question 8

Cell Membrane Disruptors

Answer 8

Action: Activated by sun to form oxygen compounds which rupture cell membranes. (Common burn-down)
Injury: With Gramoxone Inteon, a water soaked appearance followed by necrosis. With others, leaves turn yellow, then bronze/brown and die.

Question 9

Seedling Shoot Inhibitors

Answer 9

Action: Affect multiple sites, interfere with fatty acid, lipid, and protein synthesis.
Injury: Highly susceptible weeds do not emerge.
Grasses- leaf out underground, leaves do not unfurl
Broadleaf- leaves crinkle and pucker, shortened mid-vein

Question 10

Pigment Inhibitors

Answer 10

Action: Inhibit 4-HPPB which affects carotenoid (photosynthetic pigments) synthesis.
Injury: Plants appear white or beached, often becoming translucent at the leaf tips.