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Top 3 Modes of Action
- better understanding of how herbicides perform
- improve herbicide performance
- Diagnose herbicide injury
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6 Plant Processes and Functions
- photosynthesis (food)
- pigments (energy/light capture)
- respiration (energy)
- amino acids (protein/growth)
- lipids (cell membranes)
- mitosis (cell division)
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Why do we use herbicides?
- Reduce impact of invasive species
- secure the presence of targeted species
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Two classes of herbicides
- Selective-controls one species of plant without affecting the growth of another species (2 4-D)
- Nonselective- controls plants regardless of species (Roundup)
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Sites of Absorptions and Translocation
- foliar contact
- root contact
- xylem
- xylem & phloem
- phloem
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What is Mode of Action?
sequence of events from absorption of the herbicide into the plant until the plant dies
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2 most common modes of action
- plant growth regulators
- amino acid biosynthesis inhibitors
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2nd most common modes of action
- photosynthesis inhibitors
- cell membrane disrupters
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plant growth regulators
- synthetic auxins (regulate plant growth)
- affect several plant processes like cell division, cell enlargement, protein synthesis and respiration
- act by upsetting the normal hormonal blance in plants
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herbicide uptake in plant growth regulators
- foliar contact
- root contact
- translocated in both xylem & phloem
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plant growth regulators selectively kill
- broadleaf plants
- injury may occur in grasses
- (2 4-D)
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plant growth regulators symptoms
- on newly developed leaves
- abnormal growth causing twisting stems
- stems swell cuz of rapid cell division
- leaves exhibit cupping, crinkling, strapping or drawstring affect
- symptoms on grasses include leaf rolling, crinkling, brace root fusion and malformation
- flower sterillity and missing grain in crops
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plant growth regulators concerns
- herbicide resistance
- drift and injury to nontarget plants
- groundwater contamination
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amino acid biosynthesis inhibitors
- prevent synthesis of certain amino acids produced by plants
- excellent foliar and root absorption
- broad weed spectrum
- translocates to shoot and root new growth in both xylem and phloem
- (Roundup)
- plants stop growing shortly after application
- plant death may be slow (10+ days)
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Symptoms of amino acid biosynthesis inhibitors
- chlorosis of leaves (turn yellow)
- stunted growth
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important category of amino acid biosynthesis inhibitors
EPSP inhibitors
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glyphosphate
- tightly absorbed and inactive in soil
- phloem translocated
- inhibits EPSP enzyme responsible for production of aromatic amino acids
- very nontoxic
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photosynthesis inhibitors
- control annual or perennial grasses or broadleaves
- shut down the photosynthesis process
- slow starvation of the plant
- plant experiences more rapid death due to production of secondary toxic substances
- injury symptoms: yellowing of leaf tissue followed by death of the tissue
- (Spike 20P)
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photosynthesis inhibitors symptoms
- occur after coteyledons and first leaves emerge (Dont prevent seedlings from germinating or emerging)
- yellowing (chlorosis) of leaf tissue followed by death (necrosis) of tissue
- older and larger leaves affected first
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cell membrane disruptors
- postemergence contact herbicides
- little soil activity
- activated by exposure to sunlight to form oxygen compounds such as hydrogen peroxide
- oxygen compounds destroy plant tissue by rupturing plant cell membranes
- perennial weeds usually regrow cuz there is no herbicide movement to underground root or shoot systems
- controls weeds in 24-48 hours
- broad-spectrum and non-selective control of grasses, broadleaf weeds and sedges
- no residual effect
- gramoxone max
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symptoms of cell membrane disruptors
rapid necrosis (cell death)
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herbicide resistance is not due to
- sprayer skips or plugged nozzles
- weather problems that cause poor control
- plants that are naturally tolerant to herbicide
- genetic changes caused by herbicide
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herbicide resistance
the ability of a plant to survive and reproduce after treatment with a dose of herbicide that would normally kill a plant
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