Herbicides Phytoxicity - Adam Oliver Brownadamoliverbrown.com/wp-content/uploads/2014/02/7... ·...

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BIO 4101: Pesticides and the Environment BIO 4101: Pesticides and the Environment 1

Herbicides


Phytoxicity   Must be able to

inhibit a vital process so plants cannot grow or survive

  Because weeds grow among target plants, selectivity is important


Mode of Entry

  1) Foliar Penetration – Main protection of leaf is

cuticle (lipophilic) – Secondary protection is

cell wall made of cellulose (hydrophilic)

– Therefore, foliar herbicides must be both aqueous and lipoidal

BIO 4101: Pesticides and the Environment BIO 4101: Pesticides and the Environment

Foliar Penetration

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Polar entry route Non-polar

entry route Cuticular wax

Cutin

Cellulose Pectin Plasmodesmata Plasma membrane Cytoplasm

Cuticle

Cell wall

Protoplasm Fig. 24


Effect of Surfactants

5 BIO 4101: Pesticides and the Environment BIO 4101: Pesticides and the Environment

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  2) Root uptake – Herbicides

applied to soil can also penetrate seeds

–  If to be taken up by roots, must be able to pass endodermis (lignin or suberin coated ring of cells)

Mode of Entry

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Mode of Entry   3) Stem uptake – Little use for herbaceous weeds,

good for woody plants – Water-soluble solution must be

injected or mecanically penetrate bark

– Oil-based sprays may penetrate bark but must be applied in high concentrations (enter through pores in bark)


Translocation within plant   Symplast: total mass of living cells in a plant   Apoplast: the non-living cell wall continuum

that surrounds the symplast.


Translocation Within Plants   1) Symplastic movement: – protoplasm of plants is more or less

continuous because of plasmodesmata and connectedness of transport cells (e.g. sieve-tube)


Translocation Within Plants

  2) Apoplastic movement: –  Includes cell walls,

intercellular spaces and xylem elements – Permeable to water and dissolved solutes – Rate of water transport is great (>1 gallon/day

for sunflower)   Limitation: xylem moves upwards,

therefore not good for foliar pesticides


Toxic Effects on Plants   Complicated because a single chemical

may have different effects at different doses and/or different sites of action

  Some herbicides stimulate growth in one area while inhibiting in another – Called Epinastic effects – Cause stem twisting and leaf

curling – Plant usually dies from choking

of vascular tissues


Toxic Effects on Plants   1) Contact Toxicity

–  Kill quickly –  Translocation often not possible in

time –  Usually act by destroying cell

membranes by breaking bonds between membrane proteins and plasma membrane

–  Causes necrosis in all exposed tissues in a few days

–  Because translocation limited, plants may regrow from roots and buds

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  2) Mitotic Inhibitors –  Inhibit growth by blocking

some stage of mitosis –  Plant growth occurs

principally in meristematic regions (root/shoot tips, young leaves, buds, vascular cambium)

–  Often applied to soil to act on roots or germinating seedlings

Toxic Effects on Plants



  3) Photosynthetic Inhibitors – Majority of herbicides – Because affects

photosynthesis, often of little toxicity to animals


Rotenone blocks

Glucose G-6-P

Triose Phosphate 1,3-GDP

Phosphoglycerate Pyruvate Acetate

NAD ADP ATP

NADH

Citrate Cis-aconitate Isocitrate

Ketoglutarate Succinate Fumarate Malate

Oxaloacetate

NAD

NADH NADH

Cytoplasm

ADP+Pi

ATP

Complex I

Complex II

Complex III

Complex IV

Matrix

H+in

e -

e -

e -

Succinate

Com

plex V

GLY

CO

LYSI

S

H+out

As(3) blocks

As(3) blocks

As(5) uncouples

Nitrophenol uncouples

Carboxin blocks

Fluoro-acetate blocks

CN and phosphine block

As(5)

uncouples

Strobilins block

Cyt c

CoQ

Krebs cycle

O2

2H2O

15

 4

) Res

pira

tion

Inhi

bito

rs a

nd

unco

uple

rs

Fig. 39



  3 basic processes of respiration –  Glycolysis, Krebs Cycle, Electron Transport Chain

  Inhibitors: reduce 02 uptake and phosphorylation (ATP production)

  Uncouplers: all processes occur but 02 uptake and phosphorylation separated, causing breakdown

Kreb’s Cycle

NADH NAD+ FAD+ FADH2

H+ H+ H+

H+

ADP ATP e-

ADP ATP O2

CO2 H2O

Pyruvic acid Acetyl CoA


  5) Nucleic acid metabolism and protein synthesis disruption – Many ways to have

these effects –  e.g. enter nucleus

and remove histones

–  e.g. could stimulate RNA polymerase in cytoplasm




  6) Others – Pigment synthesis inhibitors – Cell membrane or cellulose

inhibitors – Lipid synthesis inhibitors – Growth hormone disruption

(auxins, gibberellins, cytokinins…)

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 A) Plant Growth Regulators – Very effective at low doses

(0.5-1 lb/acre) – Selectively control

broadleaved weeds in cereal crops

– Grasses can detoxify metabolically

– Still widely used today

Categories of Herbicides


  A) Synthetic auxins   Mode of Action – At low doses, act as IAA mimics

(auxin) & stimulates growth – Plants cannot degrade synthetic

IAA, therefore toxic at higher doses & inhibits growth

– Mobile by symplastic transport – Stimulates growth within stems,

thus choking vascular tissues



  A) Phenoxy Alkanoic Acids (2,4-D)   Not persistent in soil

–  Rapidly degraded by soil microbes to carbon food source

  Non-target effects include desirable plants   LD50 to animals is low (300-1000 mg/kg acute

oral)   Bioaccumulation potential is low

–  Most studies report complete elimination in urine by 24hrs (however, has been linked to endocrine disruption and cancer development by epidemiological studies)



  A) Phenoxy Alkanoic Acids (2,4,5-T)   More persistent in soil than 2,4-D

–  Degradation takes months to years

  LD50 to animals is moderate (400-500mg/kg acute oral)

  Dow Chemical stopped production in mid-1980s   Agent Orange in Vietnam (1965-1970) has left TCDD

contamination 40yrs later –  www.hatfieldgroup.com



Natural and Synthetic Auxins

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N

C H 2 C O 2 H

H N

( C H 2 ) 3 C O 2 H

H N

( C H 2 ) 3 C O 2 H

H

C l C H 2 C O 2 H

C l C l

O C H 2 C O 2 H C l C H 3

O C H 2 C O 2 H

C l C l

O C H C O 2 H C H 3

C l O C H 3 C l

C O 2 H

N C l C O 2 H C l C l

N H 2

N C l C O 2 H

C l N

C H 3 C l C O 2 H

C l

C l O C H 2 C O 2 H C l

Indole acetic acid (IAA) Indole butyric

acid (IBA) 4-chloro indole acetic acid

Naphthalene acetic acid (NAA) 2,4-D MCPA

Dichlorprop 2,4,5-T Dicamba

Picloram Quinclorac Quinmerac


  B) Triazines   Heterocyclic nitrogen compounds   R-groups differ but often include chlorine   2nd most important herbicides

discovered   Quite persistent and resist degradation


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  B) Triazines   Most important is Atrazine

–  Others include simazine, propazine, cynazine etc…

–  Atrazine especially useful in corn   3 applications:

–  Preplant: remove surface vegetation –  Pre-emergence: remove germinating broadleaves –  Post-emergence: weed thinning



Atrazine Site of Action

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C O

C H R

N H

C O

C H N H C H 2 O H

Atrazine

Plastoquinone

N N

C H 2 H

H

N N N

C l

N H

C 2 H 5 C C H 3

H 3 C H H 2 C

D1 protein

O

O

C H 2

C H 2

C H 2 ) 9 C H

C ( H 2 C

C H 3

H

N N N

C l

N H

C 2 H 5 C C H 3

H 3 C H

Fig. 35


  B) Triazines   Mode of action

–  Enters by roots, translocated by xylem –  Ultimate site of action is chloroplasts as

inhibitor of photosystem II (cleavage of water) –  Produces toxic free radicals in light rxn

because of reduced electron transfer   Most grasses have high tolerance due to

ability to detoxify –  Potential for resistance if used repeatedly –  e.g. lamb’s quarter (Chenopodium album)

  LD50 toxicity low for animals (300 [cyanazine] - 5000 [simazine] mg/kg acute oral)



  Resistance is common –  Over 200 biotypes now resistant to a herbicide –  Caused by continuous use of same herbicide or of

same mode of action over several generations –  May occur by mutations to target site of herbicide

or detoxifying ability



Herbicide Resistance   Most common form is

to triazines –  >60 resistant biotypes –  Resistant plants have

single amino acid substitution in the D1 protein, which prevents herbicide from attaching to plastoquinone election acceptor of PSII

–  Caused by a single nucleotide base change


Herbicide Resistance   Created on purpose

through genetic engineering

  Isolation and cloning of bacterial enzymes that detoxify

  Gene sequencing used to insert into plant genome

  e.g. Glyphosate resistant oilseed rape, soya bean

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 C) Glyphosate – Most economically important in the world

because sold with ‘Round-Up Ready’ crops – Absorbed by foliage and readily translocated

symplastically – Non-selective, high activity – Low persistence, degraded by soil microbes

within weeks – Advantage for reduced soil erosion, as

controls weeds without tillage 31


Glyphosate Site of Action

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5-Enolpyruvyl shikimate-3-P synthetase

Phosphoenol pyruvate Erythrose-4-P

Shikimate

Shikimate-3-P

5-Enolpyruvyl shikimate-3-P

Chlorismate

Aromatic Amino Acids

Glyphosate

Fig. 107


 C) Glyphosate   Mode of Action –  Inhibitor of amino acid synthesis –  Plants produce 9/20 essential

amino acids (leucine, isoleucine, histidine, valine, lysine, methionine, theonine, tryptophan and phenylalanine)

–  Chemicals causing breakdown in their production may often be harmless to animals

  LD50 toxicity low in animals (4000mg/kg acute oral)



Fungicides

  Encompasses pesticides that control all types of pathogens –  Bacteria, nematodes, as well as fungus

  Pathogenicity is often cryptic, therefore more difficult to control than weeds or insects

  Employed mostly on vegetable, fruit and nut crops

  Mostly have low mammalian toxicity (LD50 in the thousands mg/kg), low persistence, biodegradable, low solubility (transport)


Fungicide Selection   Chosen based on the following

characteristics (apart from target toxicity) –  Remain active for long time –  Good adhesive properties –  Good spreading properties –  Persistence –  Specificity (not toxic to host plant) –  Active against range of pathogens

  Mode of action varies –  Respiration inhibitors, protein phosphorylation,

enzyme disruption etc…


Types of Fungicides   1) Inorganic

–  Sulfur •  Oldest pesticide •  Used as dust •  Contact toxicity (inhibits electron

transport system of PS) –  Copper

•  1st fungicide (Bordeaux Mixture) •  Mixed with lime because insoluble

in water •  Cells accumulate toxic ions until

poisoned and die •  Often phytotoxic as well

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Types of Fungicides  2) Organic   a) Systemic – Absorbed by the plant and distributed to all

parts – e.g. oxathiins, benzimidazoles, pyrimidines,

organophosphates, triazoles, carbamates…   b) Non-systemic – Effect only at site of application (protection) – E.g. dithiocarbamates, dicarboximides,

dinitriophenols, quinones, antibiotics…


Types of Fungicides   Advantage of systemics: –  Plant continuously protected without

reapplication –  May be translocated to new shoots that grow

after application –  Not subjected to weathering –  No residues (aesthetics) –  Have potential to work on internal plant disease –  Minimal work-related hazards


Types of Fungicides

  Disadvantage of systemics: –  Development of resistance is common

(usually just one mode of action) –  Most fungicides are fungistatic, not actually

fungicidal, therefore organism can recover as chemical dissipates


Fungicide Resistance

  Potential is high due to extremely high numbers of spores (fecundity) – May spread rapidly

  Often, single base mutation can lead to resistance


References   Stephenson and Soloman. 2007. Pesticides and

the Environment. CNTC Press.   McEwen and Stephenson. 1979. The use and

significance of pesticides in the environment. John Wiley and Sons publication.

  Bohmont. 2007. The Standard Pesticide User’s Guide, 7th ed. Pearson Publishing.

  Carlile. 2006. Pesticide Selectivity, Health and the Environment. Cambridge University Press.

  Brown. 1978. The Ecology of Pesticides. John Wiley and Sons.

  Matsumura et al., 1972. Environmental Toxicology of Pesticides. Academic Press.

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