Mode of Entry Foliar Penetration - Adam Oliver...
Transcript of Mode of Entry Foliar Penetration - Adam Oliver...
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BIO 4101: Pesticides and the Environment BIO 4101: Pesticides and the Environment 1
Herbicides and Fungicides
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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
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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
BIO 4101: Pesticides and the Environment BIO 4101: Pesticides and the Environment
Effect of Surfactants
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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)
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Translocation within plant ● Symplast: total mass of living cells in a plant ● Apoplast: the non-living cell wall continuum
that surrounds the symplast.
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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)
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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
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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
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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
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Toxic Effects on Plants
● 3) Photosynthetic Inhibitors – Majority of herbicides – Because affects
photosynthesis, often of little toxicity to animals
BIO 4101: Pesticides and the Environment BIO 4101: Pesticides and the Environment
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
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● 4
) Res
pira
tion
Inhi
bito
rs a
nd
unco
uple
rs
Fig. 39
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● 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
Toxic Effects on Plants
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● A) Synthetic auxins (plant growth regulators)
● 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
Categories of Herbicides
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● 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)
Categories of Herbicides
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● 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)
● Agent Orange in Vietnam (1965-1970) has left TCDD contamination 40yrs later – www.hatfieldgroup.com
Categories of Herbicides
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● B) Triazines ● Heterocyclic nitrogen compounds ● R-groups differ but often include chlorine ● 2nd most important herbicides
discovered ● Quite persistent and resist degradation
Categories of Herbicides
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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
Categories of Herbicides
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● 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 – Widely used in corn crops
● 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)
Categories of Herbicides
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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
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● 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
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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
*Also: resistance from genetic engineering (Round-Up ready crops)
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Categories of Herbicides
● 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 26
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● 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)
Categories of Herbicides
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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
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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)
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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…
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Types of Fungicides
● 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…
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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
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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 pesticide dissipates
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Fungicide Resistance
● Potential is high due to extremely high numbers of spores (fecundity) – May spread rapidly
● Often, single base mutation can lead to resistance
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Questions?