Transcript Weed 3: Weed establishment

AGR 3102
Principles of Weed Science
Herbicide
Muhammad Saiful Ahmad Hamdani
Unit 7 – Topics Covered
Herbicide Activities in Plants:
• Absorption
• Translocation
• MOA and Injury Symptom
Herbicide Absorption
• Herbicides need to enter plant cell/tissue to cause
phytotoxicity.
• Several entry sites – upon types of herbicide and
weed spp.
• To enter/penetrate, must be absorbed first.
• Absorption: entry of herbicide into the plant. Often
called “uptake”.
• How herbicides enter plant?
– Aboveground organs: stem, flower, buds, foliage
– Belowground organs: seeds, shoots, stem, roots
Herbicide Absorption cont…
1.
Foliar-applied Herbicide Absorption
•
Leaves: most important entry site; main target for postemergence chemical weed control.
•
Entry via: channels through cell wall, trichomes,
stomata, and cracks in cuticle.
•
Fate of herbicides applied to plants via foliage:
A. The herbicide may volatilize and lost to the
atmosphere.
B. The herbicide may be washed off the leaf surface.
C. The herbicide may remain on the outer
surface and dry up / thicken.
D. The herbicide may be photo-degraded
(broken down by sunlight).
E. The herbicide may penetrate the cuticle, but
remain absorbed in the lipid components of
the cuticle.
F. It may absorb and penetrate the cuticle and then
enter the apoplast or symplast, where it may be
subject to translocation and metabolism.
Biotic factors affecting foliar herbicide absorption:
1.
2.
3.
4.
5.
Cuticle thickness (waxiness of leaf surface): the thicker
the slower absorption.
Presence of trichomes: the more trichomes the less
herbicide retention on the leaf surface. But can be site
of absorption too.
Damage to leaf surface (cracks in cuticle): the more
cracks the more absorption.
Physiological state of plant: good condition, high
absorption.
Age of leaf or plant: the younger the higher absorption,
but not too young…
Non-biotic factors affecting foliar herbicide absorption:
1.
2.
3.
4.
5.
6.
7.
8.
Light intensity: higher intensity = higher p/synthesis = higher
absorption
Temperature: higher tempt = faster drying time = lower
absorption. Too low = plant less active = low absorption.
Humidity: high humidity = stomates open, cuticle not so thick
(hydrated by the humidity) = high absorption
Precipitation/rain: depending on herbicides’ activity. Systemic
= more affected (6h to absorb); contact = less affected (30
min to absorb)
Wind: too windy = high herbicide drift = less deposition = less
absorption
Herbicide formulation: ester/amine/salt formulation better than
pure acid formulation
pH: low pH = high solubility in lipid = high absorption
Adjuvants: help a lot!!!
• The 3 barriers to herbicide
absorption:
- Cuticle
- Cell wall
- Plasmalemma (cell
membrane)
Cuticle:
• Main barrier to absorption
• Function: prevent dehydration of the leaf
• Content: wax, cutin and pectin (hydrophobic/lipophilic)
and cellulose (hydrophilic/lipophobic)
• Herbicide movement via diffusion
• Thus: only oil-soluble herbicides can easily penetrate the
cuticle layers.
• How about water-soluble herbicides:
Surface Active Agent – reduce water tension
Cell wall:
• Function: provide rigidity to the cell and the plant
• Content: cellulose with water filled interspaces hydrophilic in nature
• Herbicide movement via diffusion
Plasmalemma:
• Function: containment. It holds things in the cell. It is
comprised of lipids/fats – lipophilic
• All herbicides must enter the cell to be active, therefore,
it must cross the cell membrane/plasma membrane.
• Major movement via diffusion
Herbicide Absorption cont…
2. Soil-applied Herbicide Absorption
•
•
Herbicides with soil activity
Underground parts need to have contact with
herbicides via:
a. interception - the underground plant parts
intercept herbicide molecules in the soil
b. mass flow - herbicide moves to the
underground parts of the plant in the soil due to
transpiration
c. diffusion - the herbicide moves along a
concentration gradient to plant
Sites of herbicide absorption in soil:
• seed absorption
• root absorption - major mode for
broadleaves
• shoot absorption - major mode for grasses
Absorption complete when
herbicide released into
cytoplasm in the cell. Next is
herbicidal action or move
symplastically or apoplastically
or both of the herbicides
throughout the plant
Herbicide Translocation
• Herbicides must be translocated to the site
of action within the plant to give toxic.
• Translocation: movement of herbicide from
site of entry to other locations within plant.
• Translocation pathways:
apoplastic
symplastic
both
Herbicide Translocation cont…
• Apoplast is the non-living, continuous network of cell
walls, intercellular spaces, and xylem tissue that
transport water and mineral nutrients from roots to
shoots.
• Apoplastic translocation: translocation via the apoplast.
• Xylem mobile herbicides move through the apoplast.
• Apoplastically mobile herbicides follow the same
pathway and move in the same direction as the net
movement of water within the plant.
• Transpiration is the driving force.
• Soil-applied herbicides are primarily apoplastically
mobile.
• Apoplast movement would be acropetal in the plant.
• Foliar-applied herbicides pathway: cell
walls ≡ plasmalemma intercellular spaces
≡ enter xylem (moved upward)
• Soil-applied herbicides pathway: root hair
≡ intercellular spaces ≡ cell walls of
cortical cells ≡ endodermis ≡ diffuse
through casparian strip ≡ enter xylem
(moved upward)
• Casparian strip - impermeable barrier in the root
• Made of waxes
• The function of the casparian strip and leaf cuticle is
the same - to prevent dehydration
• Thus, casparian strip also a main barrier to herbicide
movement across root.
• Herbicides must diffuse through the casparian strip
or go around it which slows down movement.
• An apoplastically mobile herbicide applied to the soil
will enter the roots, move up the stem, and
accumulate in greater quantities in leaves with the
highest transpiration rate.
• An apoplastically mobile herbicide, applied as a
single droplet to a leaf, will move outward toward the
tip and margin of the leaf and will accumulate
around the leaf margin.
Herbicide Translocation cont…
• Symplast is the continuous network of living cells,
intercellular protoplasmic connections (plasmodesmata),
and phloem tissue that transports assimilates (food) from
the site of production (leaves) to the site of use (root and
shoot meristematic areas, developing fruit, and storage
organs such as rhizomes).
• Symplastic translocation: translocation via the symplast.
• Phloem mobile herbicides move through the symplast.
• Symplastically mobile herbicides follow the same
pathway in the plant as movement of assimilates.
• This is often referred to as a "source-to-sink" movement,
with the leaves being the "source" of food production and
apical growing points, buds, roots, and underground
storage organs, such as rhizomes, being the "sinks" or
areas utilizing the food produced by photosynthesis.
• Translocation of herbicides in the symplast is
bidirectional. It can be acropetal or basipetal.
• Symplastically mobile herbicides are almost always
applied post-emergence.
• Symplastic herbicide movement would result in injury
symptom in new growth of terminals.
• A symplastically mobile herbicide applied to a single fully
expanded leaf will move out of that leaf and move both
up and down the stem via the phloem into active areas
such as apical growing points, expanding young leaves,
developing seed or fruit, and root tips.
Herbicide Mode of Action and Injury
Symptoms
1. Plant Growth Regulators (PGR) / Synthetic
Auxins
Injury Symptoms:
• Broadleaf plant leaves become crinkled, strap
shaped, stunted, and malformed; leaf veins
appear parallel rather than netted, and stems
become crooked, twisted, and brittle, with
shortened internodes.
2. Amino Acids Biosynthesis Inhibitors
Injury Symptoms:
• Affected plants stop growth almost immediately
after foliar treatment (2-4) days.
• Established perennials in two to four weeks.
• Plants become straw coloured several days or
weeks after treatment, gradually turn brown, and
die.
3. Fatty Acids (ACCase) Biosynthesis Inhibitors
Injury Symptoms:
• Growing points are killed first, resulting in the
death of the inner leaves.
• Older, outer leaves of seedlings appear healthy
for a few days, but eventually they also wither
and die.
• After several weeks, the growing points begin to
rot, allowing the inner leaves to be pulled out of
the whorl.
• Sensitive grasses commonly turn a purplish
colour before dying.
4. Seedling Growth Inhibitors (Root & Shoot)
Injury Symptoms:
• Seeds will germinate, but they either fail to
emerge or emerge as stunted seedlings
• that have thickened, shortened lower stems,
small leaves, and short roots.
• Seedlings eventually die from lack of moisture
and nutrients because of the restricted root
system.
5. Seedling Growth Inhibitors (Shoot)
Injury Symptom:
• Grass seeds normally do not germinate.
• If they do, young leaves fail to unfold and rolled.
• The leaves of broadleaf plants turn dark green,
become wrinkled, and fail to unfold from the bud.
• The roots become shortened, thickened and
brittle.
6. Photosynthesis (at PS II) Inhibitors
Injury Symptoms:
• Early annual/seasonal seedling growth appears
normal, but shortly after emergence leaves
become mottled, turn yellow to brown, and die.
• For established plants, the oldest leaves turn
yellow on the leaf margins first, the veins remain
green, and eventually the plant turns brown and
dies.
• Perennials starve very slowly because they have
large energy reserves in roots or rhizomes to
live on while photosynthesis is inhibited, but
death is certain.
7. Cell Membrane Disrupters
Injury Symptoms:
• Cellular breakdown from destroyed cell
membranes, allowing cell sap to leak out.
• Effected plants initially have a “water-soaked”
appearance, followed by rapid wilting and
“burning,” or leaf speckling and browning.
• Plant death occurs within a few days.
8. Pigment Inhibitors (Bleaching Agents)
Injury Symptoms:
• Effected plants either do not emerge or emerge
white or bleached and eventually die.
• Older leaf tissue is affected first.
9. Phosphorylated Amino Acid (Nitrogen
Metabolism) Disrupter
Injury Symptoms:
• Injury is similar to that of the cell membrane
disrupter herbicides.
10. Unknown Herbicides
Injury Symptoms:
• Injury is almost similar to that of the cell
membrane disrupter herbicides.