How Herbicides

Work Matt Raymond

SCST Superworkshop

February 11, 2016

Mode of Action vs. Site of Action

Mode of Action (MOA)

Describes the process within the plant that is inhibited by

the herbicide. Another way to think of it is “how” the

herbicide works. For example, amino acid inhibitors are a

mode of action that disrupts the formation of amino acids.

Site of Action (SOA)

Describes the physical location within the plant where the

herbicide binds. This can also be thought of as “where”

the herbicide works. For example, within the amino acid

synthesis inhibitors mode of action, there are two different

site-of-action groups. These are ALS inhibitors and the EPSP

synthase inhibitors. Herbicides within these two sites of

action inhibit different enzymes within the plant.

Small arrows show herbicide mobile in system from site of

uptake

Large arrows indicate placement

of herbicide

Dots indicate a site of uptake without

herbicide translocation

These numbers refer to the Site of Action

(SOA) Group as referred to in the

Herbicide Classification Guide.

Amino Acid Synthesis Inhibitor

ALS Inhibitors – Blocks the normal function of

the acetolactate synthase(ALS) enzyme,

inhibiting plant metabolism.

Symptoms

• Roots may be unbranched and

stubby resulting in purple stems and

leaves

• Stunting

• Leaves turn yellow from outer edge,

veins turn light yellow to dark purple

Examples

• STS soybeans

• Clearfield (IMI) tolerant crops

Comments

• Residual and drift injury are

concerns

Amino Acid Synthesis Inhibitor

ALS Inhibitors – Blocks the normal function of

the acetolactate synthase(ALS) enzyme,

inhibiting plant metabolism.

Amino Acid Synthesis Inhibitor EPSP Synthase Inhibitor - Interferes with the EPSP(5-enolpyruvly-shikimate-3-phosphate) enzyme, disrupts synthesis of essential amino acids.

Symptoms

• Plants turn light green to yellow

• Stunting

• Necrosis

Examples

• Glyphosate tolerant crops

Comments

• Drift injury is a concern with spray

applications

• Systemic, moves readily throughout

plant

Amino Acid Synthesis Inhibitor EPSP Synthase Inhibitor - Interferes with the EPSP(5-enolpyruvly-shikimate-3-phosphate) enzyme, disrupts synthesis of essential amino acids.

Growth Regulators T1R1 Auxin Receptors – Has activity that is similar to endogenous auxin. Affects growth in the newest tissues by affecting protein synthesis and normal cell division

Symptoms

• Stunted, malformed seedlings

• New growth twisted or malformed

• Leaf rolling in grass plants

Examples

• Dicamba & 2,4-D tolerant crops

Comments

• Drift injury is a concern since some

chemicals are highly volatile.

• Equipment contamination

Nitrogen Metabolism Inhibitor

Symptoms

• Yellow and necrotic plant tissue

• Stunted, necrotic, seedlings in

substrate imbibition testing

Examples

• Glufosinate tolerant crops

Comments

• Primarily a contact herbicide, limited

translocation from foliar application

Glutamine Synthase Inhibitor – Inhibits the glutamate synthetase enzyme. Causes ammonia to build up in plant which destroys cells and inhibits photosystem I & II reactions. Ammonia also reduces pH gradient which can uncouple photophosphorylation.

Nitrogen Metabolism Inhibitor Glutamine Synthase Inhibitor – Inhibits the glutamate synthetase enzyme. Causes ammonia to build up in plant which destroys cells and inhibits photosystem I & II reactions. Ammonia also reduces pH gradient which can uncouple photophosphorylation.

Pigment Inhibitors

Symptoms

• Bleaching on new growth

Examples

• Isoxaflutole

Comments

• No commercially available tolerant

crops at this time. Estimated release is

mid decade pending regulatory

approval

HPPD Inhibitor – Inhibits a key step in

plastoquinone biosynthesis which indirectly

inhibits production of carotenoids.

Pigment Inhibitors

HPPD Inhibitor – Inhibits a key step in

plastoquinone biosynthesis which indirectly

inhibits production of carotenoids.

Photosynthesis Inhibitors Photosystem II Inhibitors – Disrupts processes and products needed for transport of chemical energy. Causes cell membrane disintegration. Different binding than SOA groups 6 & 7.

Symptoms

• Yellowing & necrosis

• Stunting

Examples

• Metribuzin

Comments

• Some soybeans are extremely

sensitive to carry over

• Traditional trait testing of +/- not done,

instead rated on a tolerance scale

Photosynthesis Inhibitors Photosystem II Inhibitors – Disrupts processes and products needed for transport of chemical energy. Causes cell membrane disintegration. Different binding than SOA groups 6 & 7.

Cell Membrane Disrupters

Symptoms

• Yellow & necrotic spots on leaf

margins

Examples

• Sulfentrazone

Comments

• Some soybeans are extremely

sensitive to carry over

• Traditional trait testing of +/- not done,

instead rated on a tolerance scale

PPO Inhibitors – Inhibits protoporphyrinogen oxidase, an enzyme involved in chlorophyll and heme synthesis. Causes a chain reaction that leads to cellular leakage and death.

References &

Acknowledgements

• Printed Materials

• United Soybean Board, (2014) Herbicide

Classification Guide, Take Action,

Herbicide-Resisitance Management.

• Philip L. Nixon and Mark Mohr, University of

Illinois Extension. (2002) Illinois Pesticide

Applicator Training Manual – Demonstration

and Research, Urbana, IL University of Illinois

at Urbana-Champaign

References &

Acknowledgements

• Websites

• Bayer CropScience. Balance GT. Retrieved from http://www.balancegtsoybeans.com/

• Dow AgroSciences. Enlist Weed Control Systems. Retrieved from http://www.enlist.com/en

• Monsanto. Roundup Ready Xtend Crop System. Retrieved from http://www.monsanto.com/products/pages/roundup-ready-xtend-crop-system.aspx

References &

Acknowledgements

• People

• Aaron Hager, University of Illinois Extension

Specialist -Weed Science. Urbana, IL

hager@illinois.edu

• Photos

• Matt Raymond, Illinois Crop Improvement

Association, Inc. Champaign, IL

mraymond@ilcrop.com

The End

Questions?