Herbicide Mode of Herbicide Mode of ActionAction

Greg MacDonaldGreg MacDonaldUniversity of FloridaUniversity of Florida

Chemical Chemical -- HerbicidesHerbicides•• over 150 active ingredients registeredover 150 active ingredients registered•• over 4000 trade namesover 4000 trade names•• herbicides account for 55% of herbicides account for 55% of

pesticide usepesticide use

HerbicidesHerbicides•• Herbicide is a chemical compound, Herbicide is a chemical compound,

generally organic, that is used to control generally organic, that is used to control plants. plants.

•• Chemistry of the herbicide dictatesChemistry of the herbicide dictates --–– how it works in the planthow it works in the plant–– how it behaves in the environmenthow it behaves in the environment

How herbicides workHow herbicides work•• Controlled/selective plant poisoningControlled/selective plant poisoning

–– applied to soil (root uptake) and/or applied to soil (root uptake) and/or leavesleaves

–– contact or systemiccontact or systemic–– selective vs. nonselective vs. non--selectiveselective

Herbicide SelectivityHerbicide Selectivity•• placement placement -- keep it away from keep it away from

desirable plantdesirable plant•• uptake uptake -- apply so it doesnapply so it doesn’’t get in and t get in and

movemove•• metabolism metabolism -- some plants breakdown some plants breakdown

herbicides butherbicides but……. . ……•• target site is differenttarget site is different

Sequestered in vacuoleSequestered in vacuoleX does not X does not binds to binds to enzyme Yenzyme Y

Herbicide X Herbicide X not absorbednot absorbed

XX

X metabolized to ZX metabolized to Z

Y1Y1

Mechanisms of ToleranceMechanisms of Tolerance

FormulationsFormulations•• water soluble, oil soluble, salts, esterswater soluble, oil soluble, salts, esters•• clay suspension, pellets, granules, clay suspension, pellets, granules,

wettable powders, emusifiable wettable powders, emusifiable concentratesconcentrates

•• the goal of the formulation is tothe goal of the formulation is to: store, : store, mix/spray in water, get it in the plant, and mix/spray in water, get it in the plant, and minimize offminimize off--target damagetarget damage

W.H. Freeman and Sinauer Associates

Plant ProcessesPlant Processes1.1. photosynthesisphotosynthesis2.2. amino acids and proteins amino acids and proteins 3.3. fatty acid synthesisfatty acid synthesis4.4. growth inhibition (not covered here)*growth inhibition (not covered here)*5.5. cell membranescell membranes6.6. pigment synthesispigment synthesis7.7. growth regulationgrowth regulation

Mode vs. MechanismMode vs. Mechanism•• modemode--ofof--actionaction ---- symptoms that occur symptoms that occur

after herbicide application leading to plant after herbicide application leading to plant deathdeath

•• mechanismmechanism--ofof--actionaction ---- actual actual biochemical site of herbicide activitybiochemical site of herbicide activity–– generally enzyme or cogenerally enzyme or co--factorfactor–– in some cases actual in some cases actual ‘‘sitesite’’ is unknownis unknown

PhotosynthesisPhotosynthesis•• process where the plant uses light energy process where the plant uses light energy

to convert COto convert CO22 and Hand H22O to sugars, release O to sugars, release OO22

•• Light reactionsLight reactions–– chlorophyll absorbs lightchlorophyll absorbs light–– passes energy down a biochemical chainpasses energy down a biochemical chain–– form intermediates to drive sugar formationform intermediates to drive sugar formation

xxxx

Herbicides Herbicides -- PhotosynthesisPhotosynthesis•• block the flow of energy from chlorophyll to block the flow of energy from chlorophyll to

the intermediatesthe intermediates–– plant cannot make sugars plant cannot make sugars -- starvesstarves–– chlorophyll continues to absorb light and this chlorophyll continues to absorb light and this

‘‘excessexcess’’ energy forms toxic intermediates energy forms toxic intermediates •• generally soil active, root uptake and generally soil active, root uptake and

movement through water stream to leavesmovement through water stream to leaves

Diuron, Spike, Pramitol, Diuron, Spike, Pramitol, Bromacil, VelparBromacil, Velpar

•• used in a variety of cropping systems for used in a variety of cropping systems for broadleaf and some grass controlbroadleaf and some grass control

•• mostly soil applied, but some foliar activitymostly soil applied, but some foliar activity•• root uptake, translocation via water root uptake, translocation via water

stream, little movement from foliar stream, little movement from foliar applicationsapplications

•• potent inhibitors of photosynthesispotent inhibitors of photosynthesis

Photosynthesis Photosynthesis –– SymptomsSymptoms

Amino acids and ProteinsAmino acids and Proteins•• essential building blocks for plant growth essential building blocks for plant growth

and functionand function•• unlike animals, plants make their ownunlike animals, plants make their own•• amino acids are the primary components amino acids are the primary components

of proteins and nucleic acidsof proteins and nucleic acids•• proteins are generally storage proteins or proteins are generally storage proteins or

enzymesenzymes

Herbicides Herbicides -- Amino AcidsAmino Acids

•• generally target a specific enzyme generally target a specific enzyme –– blocks a vital step in the formation of amino blocks a vital step in the formation of amino

acidsacids---- proteins, enzymesproteins, enzymes……–– aromatic amino acidsaromatic amino acids–– branched chain amino acidsbranched chain amino acids

•• dependent on plant growth for activitydependent on plant growth for activity–– better growth better growth -- better control, slow deathbetter control, slow death

•• systemic herbicides, some have soil systemic herbicides, some have soil activityactivity

Arsenal/Habitat, Arsenal/Habitat, Plateau, Plateau, Clearcast, Oust, EscortClearcast, Oust, Escort

•• crops & vegetation managementcrops & vegetation management•• foliar & soil activity, used PPI, PRE or foliar & soil activity, used PPI, PRE or

POST (soil activity can cause persistence)POST (soil activity can cause persistence)•• grasses, BLgrasses, BL’’s and good nutsedge activitys and good nutsedge activity•• accumulates in meristematic regionsaccumulates in meristematic regions•• inhibit the synthesis of the amino acids inhibit the synthesis of the amino acids

valine, leucine, and isoleucinevaline, leucine, and isoleucine

Soil Activity!!Soil Activity!!

Glyphosate Glyphosate -- RoundupRoundup……etc.etc.•• broadspectrum postemergence weed broadspectrum postemergence weed

controlcontrol•• glyphosate labeled in multitude of areasglyphosate labeled in multitude of areas•• extensively translocated throughout the extensively translocated throughout the

plant, extremely stable in plantplant, extremely stable in plant•• blocks synthesis of aromatic amino acidsblocks synthesis of aromatic amino acids

glyphosateglyphosate

Entireleaf morninggloryEntireleaf morningglory

Glyphosate injuryGlyphosate injuryon pecanson pecans

Fatty Acid SynthesisFatty Acid Synthesis

•• Fatty acids are the primary building Fatty acids are the primary building components of cell membranes and many components of cell membranes and many intercellular membranes intercellular membranes

•• Inhibition of fatty acids results in stunted Inhibition of fatty acids results in stunted growth and eventually death due to lack of growth and eventually death due to lack of membranes for cellular integritymembranes for cellular integrity

Fusilade, SelectFusilade, Select•• registered in several crops, noncroplandregistered in several crops, noncropland•• grass activity only, postemergence grass activity only, postemergence •• translocated to growing portionstranslocated to growing portions•• inhibits fatty acid synthesis, slow deathinhibits fatty acid synthesis, slow death

Select/FusiladeSelect/Fusilade

Cell MembranesCell Membranes•• maintain cell integritymaintain cell integrity

–– keep things in & outkeep things in & out•• generate electrochemical gradientsgenerate electrochemical gradients

–– allows for energy productionallows for energy production•• maintain cell structuremaintain cell structure

–– turgor pressure maintains plant structure and turgor pressure maintains plant structure and helps the plant growhelps the plant grow

Herbicides Herbicides -- Cell MembranesCell Membranes

•• divert normal energy flow to form toxic divert normal energy flow to form toxic intermediatesintermediates–– interact w/ membrane and cause disruptioninteract w/ membrane and cause disruption

•• cause the membrane to become cause the membrane to become ““leakyleaky””–– gradients cannot be formed, no energy for gradients cannot be formed, no energy for

cellular functions cellular functions -- endothall endothall •• generally contact, little movement in plantgenerally contact, little movement in plant

Gramoxone and RewardGramoxone and Reward

•• nonnon--selective postemergence activityselective postemergence activity•• little translocation, contactlittle translocation, contact•• diverts normal electron flow, causes diverts normal electron flow, causes

formation of radical oxygen and cell formation of radical oxygen and cell membrane disruptionmembrane disruption

•• no soil activityno soil activity

DQ DQH2

O2-

GramoxoneGramoxone

Valor, StingrayValor, Stingray•• applied POST for BL weed controlapplied POST for BL weed control•• contact, no translocation in foliar contact, no translocation in foliar

treatments treatments •• xylem movement from xylem movement from soil activitysoil activity•• increase of light absorbing intermediatesincrease of light absorbing intermediates•• formation of highly toxic free radicalsformation of highly toxic free radicals

Pigment SynthesisPigment Synthesis•• absorb light energy for photosynthesisabsorb light energy for photosynthesis•• also protect plant from excess lightalso protect plant from excess light•• types of pigmentstypes of pigments

–– chlorophyllchlorophyll–– carotenoidscarotenoids–– flavenoidsflavenoids–– anthocyaninsanthocyanins

Sonar, SolicamSonar, Solicam•• block the formation of chlorophyllblock the formation of chlorophyll•• block the formation of carotenoidsblock the formation of carotenoids

–– carotenoids accept excess energycarotenoids accept excess energy–– when absent, chlorophyll dissinegrateswhen absent, chlorophyll dissinegrates

•• generally slow death, plant starvesgenerally slow death, plant starves•• soil active, movement through water soil active, movement through water

streamstream

Pigment Injury SymptomsPigment Injury Symptoms

Growth RegulationGrowth Regulation•• hormones control plant growth and hormones control plant growth and

developmental changes, always presentdevelopmental changes, always present•• auxin mimics auxin mimics -- cause uncontrolled growthcause uncontrolled growth

–– auxin transport inhibitorsauxin transport inhibitors–– prevent the normal distribution of auxinprevent the normal distribution of auxin

•• soil and foliar applied, systemic throughoutsoil and foliar applied, systemic throughout

PhenoxysPhenoxys•• 2,42,4--D, 2,4D, 2,4--DB, 2,4DB, 2,4--DP, MCPADP, MCPA•• BL weed control for a variety of crops BL weed control for a variety of crops

(corn, pastures, legumes) and (corn, pastures, legumes) and noncropland noncropland

•• salt, ester and oil formulations salt, ester and oil formulations •• foliar & root uptakefoliar & root uptake-- extensive extensive

translocationtranslocation•• cells undergo rapid uncontrolled division cells undergo rapid uncontrolled division

and elongationand elongation

Triclopyr/Garlon, Transline, Triclopyr/Garlon, Transline, MilestoneMilestone

•• used POST for BL weed control, brush & used POST for BL weed control, brush & woodywoody

•• similar to phenoxys, primarily foliar applied similar to phenoxys, primarily foliar applied but Milestone has soil activitybut Milestone has soil activity

•• foliar & root uptakefoliar & root uptake-- extensive extensive translocationtranslocation

•• cells undergo rapid uncontrolled division cells undergo rapid uncontrolled division and elongationand elongation

TriclopyrTriclopyr

CucumberCucumber

Redroot pigweedRedroot pigweed

QUESTIONS?QUESTIONS?