Lecture 12: Herbicides Flashcards by Zoe Phillips

what is the #1 most used pesticide in the world?

herbicides (45%)

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why do we primarily use herbicides?

saves a LOT of money from crop losses
losses prevented by crop protection -> $415 billion

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T/F: The number of undernourished people has been
steadily declining

true!! we protect more crops with herbicides, lose less, can feed more people

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Many of the
pesticides used in
Manitoba are
________

herbicides

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how do pesticides GENERALLY work? why don’t they usually harm humans?

target plant-specific processes, less harmful to humans

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T/F: some herbicides can persist
in the environment and still
cause unintended effects to
wildlife

true!!

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why do concentrations of pesticides change in different areas of the Red River?

depends where they’re primarily used, location is crucial for distribution!

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____: any compound that can kill or severely injure plants

Herbicide

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______: 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

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_______ 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

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whats the difference between selective and non-selective herbicides?

Selective allows us to target weeds without damaging crops

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T/F: most herbicides are non-selective

true! must be careful with our use, because they will kill a wide range of plants

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how do we classify herbicides?

based on their mechanism of action

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T/F: Herbicides have relatively low acute toxicity

true, EXCEPT paraquat

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where are the absorption sites for herbicides in humans?

dermal or GI tract

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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)

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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

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where are the sites of herbicide absorption in the plant?

leaves, stem, shoots, roots

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what is the difference between apoplastic and symplastic pathways?

apoplastic- through cell walls
symplastic- through cytoplasm

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T/F: herbicides disrupt one or more vital
process to kill a weed

true!

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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

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what factors affect herbicide activity? (species and genetics obvs matter)

temperature
soil pH
soil moisture

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______ affects herbicide activity:
* Young plants more susceptible in cool conditions
* Extreme high temperature causes the plant to have to deal
with multiple stressors

Temperature

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_____ affects herbicide activity
* Influences the availability and persistence of certain
herbicides in the soil

Soil pH

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_____ 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