Lecture 12: Herbicides Flashcards
what is the #1 most used pesticide in the world?
herbicides (45%)
why do we primarily use herbicides?
saves a LOT of money from crop losses
losses prevented by crop protection -> $415 billion
T/F: The number of undernourished people has been
steadily declining
true!! we protect more crops with herbicides, lose less, can feed more people
Many of the
pesticides used in
Manitoba are
________
herbicides
how do pesticides GENERALLY work? why don’t they usually harm humans?
target plant-specific processes, less harmful to humans
T/F: some herbicides can persist
in the environment and still
cause unintended effects to
wildlife
true!!
why do concentrations of pesticides change in different areas of the Red River?
depends where they’re primarily used, location is crucial for distribution!
____: any compound that can kill or severely injure plants
Herbicide
______: applied to soil before a crop is seeded
_______: applied to soil before unwanted plant appears
_______: applied to soil or foliage after germination of crop or the
unwanted plant (weed)
Preplanting
Preemergent
Postemergent
_______ herbicide: applied to soil or above-ground parts and are
absorbed then circulated to distant tissue (e.g. moved from root to
shoot or shoot to root)
Translocated
whats the difference between selective and non-selective herbicides?
Selective allows us to target weeds without damaging crops
T/F: most herbicides are non-selective
true! must be careful with our use, because they will kill a wide range of plants
how do we classify herbicides?
based on their mechanism of action
T/F: Herbicides have relatively low acute toxicity
true, EXCEPT paraquat
where are the absorption sites for herbicides in humans?
dermal or GI tract
what are the symptoms of absorption of herbicides in humans? (GI and dermal)
Dermal absorption – irritation, contact dermatitis
GI tract absorption – vomiting, burning of the mouth,
abdominal pain, hypotension, myotonia (stiffness of muscles)
Parkinson’s disease results from the death of dopamine-generating cells in
the substantia nigra, a region of the midbrain.
Epidemiology studies say ________ exposure increases risk of Parkinson’s
1.6x.
Most likely candidates includes _______, which give a 3.2x increase risk in
farmers who have used it often, and organochlorines like DDT.
Other factors are genetic, life history (brain injuries such as concussion)
pesticide
paraquat
where are the sites of herbicide absorption in the plant?
leaves, stem, shoots, roots
what is the difference between apoplastic and symplastic pathways?
apoplastic- through cell walls
symplastic- through cytoplasm
T/F: herbicides disrupt one or more vital
process to kill a weed
true!
photosynthesis
synthesis of AA, fats, pigments
maintenance of membrane integrity
growth and differentiation
mitosis in plant meristems
uptake and translocation of ions/molecules
these are all examples of…
some vital metabolic plant processes that herbicides can target
what factors affect herbicide activity? (species and genetics obvs matter)
temperature
soil pH
soil moisture
______ affects herbicide activity:
* Young plants more susceptible in cool conditions
* Extreme high temperature causes the plant to have to deal
with multiple stressors
Temperature
_____ affects herbicide activity
* Influences the availability and persistence of certain
herbicides in the soil
Soil pH
_____ affects herbicide activity
* Influences herbicide adsorption to soils. When dry,
herbicide adheres to soil particles and is unavailable for
plant uptake
Soil moisture
in what soil moisture conditions would weed control be more effective?
wet! water outcompetes herbicide for attachment to soil (plant absorbs more herbicide)
______: Applied to foliage (sometimes soil) and can
move through plant in different ways
foliar applied herbicides
______:
(downward
movement)
Symplastically
translocated
_______:
(No significant movement)
Contact herbicide
______:
(upward
movement)
Apoplastically
translocated
synthetic auxins are…
symplastically translocated herbicides
________:
Mode of Action: Mimics auxin (IAA: indole-3-acetic acid) to deregulate
growth. Auxin is a plant hormone that regulates growth including cell
enlargement, division and plant development.
Symptoms:
- Bending and twisting of leaves and stems immediately after application
- Delayed symptoms include root formation on dicot stems, misshapen
leaves, stems, and flowers, and abnormal roots
- Veins appear to be joined together
synthetic auxins
T/F: Injury to non-target vegetation is a major problem when using synthetic auxins
true
amino acid inhibitors are….
symplastically translocated herbicides
______:
Mode of action: disrupt enzymes involved in amino acid production
targets ALS, EPSP, Glutamine
amino acid inhibitors
_________: required for synthesizing branched-
chain amino acids
Symptoms: growth stops within hours, chlorosis, necrosis, then
death. Symptoms appear first in new growth
ALS (Acetolactate Synthase)
________: involved in aromatic amino acid production
Symptoms: systemic translocation to meristems, stunted growth,
chlorosis. Symptoms emerge over days to weeks
EPSP Synthase
________: involved in ammonia assimilation and
glutamine synthesis.
Symptoms: rapid necrosis (within days), leaf scorching, reduced
photosynthetic activity
Glutamine Synthetase
glyphosate is a…
EPSP synthase inhibitor
_______ is a general herbicide (i.e., is not selective)
and one of the most used worldwide.
Monsanto has a patent on a mutant version of EPSP
synthase that _______ will not inhibit. They’ve
genetically engineered this version of the gene into crop
plants to make them ‘roundup ready’.
Farmers can spray Roundup on these GMO crops and
kill ‘weeds’ in the field, but not the crop
glyphosate
is glyphosate dangerous to humans?
no, EPSP synthase not present in mammals, therefore
glyphosate not very toxic to humans (acute oral LD50 in
the rat is 5,600 mg/kg)
Oddly, an additive to the Roundup formula
(polyethoxylated tallowamine, POEA), is much more toxic
than glyphosate itself
why is glyphosate not really fair to use?
not every framer has access to GMO crops, so when a neighbouring farm sprays RoundUp everywhere, it kills their natural crops too
is glyphosate a carcinogen according to EPA?
no, yet they have thousands of lawsuits for this claim
though WHO classified it as probably carcinogenic
Chlorophyll/Carotenoid Pigment Inhibitors are…
symplastically translocated herbicides
________:
Mode of action: interferes with carotenoid biosynthesis, which
are essential for shielding chlorophyll from photodynamic
damage caused by light exposure. Chlorophyll is then
degraded which disrupts photosynthesis. Common target
enzymes are phytoene desaturase and phydroxyphenyl
pyruvate dioxygenase.
Applied preemergence or postemergence
Symptoms: Vivid white new growth, sometimes tinged with
pink or purple. New growth initially appears normal but lacks
green and yellow pigments.
Chlorophyll/Carotenoid Pigment Inhibitors
what are the two target enzymes of chlorophyll pigment inhibitors
phytoene desaturase
phydroxyphenyl pyruvate dioxygenase
Lipid Biosynthesis Inhibitors are…
symplastically translocated herbicides
_______-:
Mode of action: inhibits acetyl-CoA carboxylase
which is involved in fatty acid biosynthesis.
Symptoms: discoloration and disintegration of
meristematic tissue. Leaves yellow, redden, and wilt.
Grass seedlings may not emerge from the soil.
Used for selective removal of most grass species from
any non-grass crop
Lipid Biosynthesis Inhibitors
Photosystem I (PSI) Electron
Diverter
(Cell Membrane Destroyers) are…
contact herbicides
________-:
Mode of Action: accept electrons from
photosynthetic transport chain and NADPH to
form an herbicide radical that reduces other
molecules to form ROS. Destroys membrane
lipids and chlorophyll.
Non-selective and very rapid kill.
Symptoms: Severe injury 1–2 hours after
application that looks like water-soaked areas
which later turn yellow or brown (necrosis)
Photosystem I (PSI) Electron
Diverter
(Cell Membrane Destroyers)
______: General non-selective herbicide for broad
leaf and grass weeds.
* The most acutely toxic herbicide (rat LD50
100 mg kg-1).
* Accumulates in lung and kidney cells
regardless of route of exposure.
* Like its action in plants, it generates
reactive oxygen species (ROS) in
vertebrates
The ROS then damage membranes leading pulmonary edema and
fibrosis. People that survive the acute phase, can still die from loss of lung
function
paraquat
paraquat is a…
Photosystem I (PSI) Electron
Diverter
(Cell Membrane Destroyers)
Photosynthetic Inhibitors are…
apoplastically translocated herbicides
_________:
Mode of action: binds proteins on
photosynthetic transport chain to block
electron transport. Creates ROS and
stops photosynthesis.
Symptoms: Chlorosis between leaf
veins and along the margins followed by
necrosis of the tissue. Develop from
bottom to top on plant shoots (older
leaves will show most injury followed by
newer leaves with least injury).
Photosynthetic Inhibitors
where does necrosis occur from photosynthetic inhibitors?
begins around the edge of leaf, moves toward center
______: Applied to soil so that herbicide taken up by
roots or shoot of emerging plant
soil applied herbicides
microtubule assembly inhibitors are…
soil applied herbicides
_________:
Mode of action: inhibit cell division by interfering with the formation of
microtubules. Meristematic regions (the growing points of stems and roots)
mostly affected.
Symptoms: short and swollen coleoptiles, swollen hypocotyls, nutrient
deficiency or drought symptoms due to poorly developed root system
Microtubule Assembly Inhibitors
(Cell division inhibitors)
how can we reduce herbicide resistance?
use sparingly
use more than 1 type! (rotate)
are there environmental impacts of herbicides?
yes, but they vary
Per- and polyfluoroalkyl substances (PFAS) are also known as…
forever chemicals
______: a group of over 4,700 related organic compounds
that have a fluorinated carbon chain structure
PFAS
PFAs are synthetic chemicals with high
chemical and thermal stability that can
repel _____ AND ______
water and oils
T/F: PFAS are highly persistent in the
environment and can accumulate in the
body over time
true
Human exposure to ______ can
affect the liver and metabolism, the
nervous and immune systems as well as
the birth weight of infants
PFOA and PFOS
what is the difference between PFAs and POPs
PFAs aren’t in the “dirty dozen”
however some might be found in the Stockholm convention
are forever chemicals banned in Canada?
only some of them
why do concentrations of PFAs increase as we age?
because they bioaccumulate over time! find the highest conc. at the oldest ages measured
why do Nunavik women have so much more PFAs bioaccumulating in them? compared to women of the general public
their diet of fish, suggests that PFAs biomagnify in the environment, therefore bioaccumulating in humans eating these top predator fish
John Giesy was one of the first people to
document PFAS in the environment, why is he controversial?
he handled most of the papers published, has a net worth of over 20 million… was being paid by 3M while saying he was an independent editor
prevented “bad” papers from being published i.e. papers that made 3M look bad
generally just not a stand-up academic unfortunately
