6. Disclaimer
• Wide range of background in this room, stop
me with questions anytime
– We will talk about Mode of Action (includes some
herbicide physiology)
– Then get into symptomology and selectivity
• ok presentation for teaching MOA(not
great), since it is geared for contest
– Leaving out elements not important for contest
• Very reduced amount of herbicides discussed
7. Herbicide Mode and Site of Action
• Mode of Action
– the metabolic or physiological process impaired or
inhibited by the herbicide
• HOW the herbicide controls the plant
• Site of Action
– the physical location within the plant where the
herbicide must bind to exert its MOA
• WHERE the herbicide acts within the plant
15. Injury Symptoms of Soil-Applied PSI
• Injury symptoms appear after the food reserves in
the cotyledons have been exhausted
• The injury symptoms will appear first on the oldest
leaves of the plant
• Since these herbicides move in the water conducting
tissue (xylem), injury symptoms
will follow the path of water movement in the plant
Roots
Stem
Leaves
Leaf margins
16. Injury Symptoms of Soil-Applied PSI
• Injury symptoms appear as a yellowing (chlorosis) of
the leaf tissue which, if severe
enough, eventually becomes necrotic (dead) tissue
• Initial chlorosis appears at the leaf margins as the
herbicide follows the same path as water
through the plant
• Advanced chlorosis of the leaf tissue is interveinal
(between the veins)
– Veins remain green
19. Chlorophyll
• Pigments are molecules that absorb light
– Sunlight
Pigments
PS
Food
• Chlorophyll is the plant pigment responsible for the
green color of plants
• The synthesis of chlorophyll has many steps, some
of which can be inhibited by herbicides
• One family of herbicides that block a step in
chlorophyll synthesis is the PPO inhibitors
http://www.youtube.com/watch?v=zaxH4xeMGzM
20. Protoporphyrinogen Oxidase Inhibitors
• These herbicides inhibit a step in the chlorophyll
biosynthesis pathway
– protoporphyrinogen oxidase = PPO inhibitors
• Blockage of this pathway results in the production
of free radicals which damage plant membranes
– some refer to this family as the “membrane disruptors”
– membrane damage results in characteristic leaf burn
• When foliar-applied, these herbicides show limited
movement in the plant
22. Injury Symptoms of PPO Inhibitors
• Soil-applied PPO inhibitors are mobile within
the plant
• Injury often related to multiple factors
– variety, soil pH, tillage, environment
• Symptoms often present on cotyledons and
hypocotyl, older leaves
– reddish lesions
– “wrinkled” true leaves
26. Carotenoid Synthesis Inhibitors (Bleachers)
• Carotenoids are “accessory” plant pigments that
protect chlorophyll during photosynthesis
– dissipate excess energy from excited chlorophyll
molecules
• Chlorophyll is the pigment responsible for the green
color of plants
– chlorophyll reflects the green wavelength of visible light
• Carotenoid synthesis inhibitors block the formation
of carotenoids
– if carotenoids are absent, chlorophyll can be destroyed
during PS and plants can lose their green color
27. Bleachers
• Absorbed through roots and shoots, but
translocated only in the xylem
• Prevents synthesis of pigments that protect
chlorophyl
• Foliage turns white and appears bleached
30. Injury Symptoms of Carotenoid Synthesis
Inhibitors
• Corn foliage is white
– lack of carotenoids leads to destruction of chlorophyll
• Clomazone carryover – corn generally recovers
• Isoxaflutole injury – corn does not always recover
– formulation change from dry to liquid helped mixing
problems
– application problems, hybrid sensitivity issues are not
resolved by formulation changes
35. Light-activated Herbicides
• How bored is everyone/how am I doing on
time
• Reichers explains as multiple SOAs, but kind of
one MOA
• PSI, PSII, PPO, HPPD, Glutamine synthetase
36. Seedling Growth Inhibitors
• These herbicides inhibit the growth and
development of weed seedlings, usually before
the weeds emerge
– do not inhibit seed germination
• These herbicides include the dinitroanilines,
chloroacetamides, and thiocarbamates
• These herbicides are absorbed by the roots and
shoots of weed seedlings
– DNAs and Thiocarbamates - root and shoot
– Chloroacetamides - shoot
37. Cell Reproduction
• Cell division (mitosis) is the process by which cells
reproduce
• During cell division the genetic material
(chromosomes) is replicated and distributed
evenly to the two new daughter cells
• Specialized structures called microtubules function
to distribute the genetic material
equally to the two new cells
39. Injury Symptoms of DNA Herbicides
• Corn seedlings are usually stunted with club-shaped
roots and limited secondary root development
– characteristic symptom of DNA, also corn nematodes
• Foliage often with purple leaf margins, probably
attributable to poor phosphorus uptake
– limited root growth leads to limited nutrient absorption
• Lateral roots of soybeans may be pruned from excessive
rates of DNA herbicides. Hypocotyls may swell (callus)
and crack, causing lodging late in the season
40.
41.
42. Chloroacetamide Herbicides
• These herbicides inhibit primarily shoot
development in susceptible species
– root symptoms not common
• Primary mechanism of plant death not completely
understood
– some speculate these herbicides interfere with cell
division by inhibiting formation of very long chain fatty
acids
• Many formulated with safeners to reduce corn injury
potential
44. Injury Symptoms of Chloroacetamides
• Injury symptoms are most frequent under cool, wet
conditions
– those conditions that slow corn growth
• Corn seedlings may leaf-out underground.
Following emergence, leaves may fail to unfurl
correctly, and the leaves may appear crinkled
– have observed leaf rolling on much taller corn, but is this
more herbicide or environment related?
• Symptoms typically dissipate when temperatures
increase and soils dry
45. Injury Symptoms of Chloroacetamides
• Soybean leaves may also appear crinkled or puckered.
A shortened leaf mid-vein gives the
leaf a heart-shaped or "draw string" appearance
– I think it is hard to differentiate b/n PRE PPO symptomology on
soybeans
51. “Stumble up to a plot”
• We talked a lot about how herbicides work
and their symptomology
• Next step is to put together keys for selectivity
of crops and weeds
• Looking at plots this summer will help
immensely!
• Until then “roll over into a plot”
