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

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

Herbicide Absorption cont… 1.Foliar-applied Herbicide Absorption Leaves: most important entry site; main target for post- emergence 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.Cuticle thickness (waxiness of leaf surface): the thicker the slower absorption. 2.Presence of trichomes: the more trichomes the less herbicide retention on the leaf surface. But can be site of absorption too. 3.Damage to leaf surface (cracks in cuticle): the more cracks the more absorption. 4.Physiological state of plant: good condition, high absorption. 5.Age of leaf or plant: the younger the higher absorption, but not too young…

Non-biotic factors affecting foliar herbicide absorption: 1.Light intensity: higher intensity = higher p/synthesis = higher absorption 2.Temperature: higher tempt = faster drying time = lower absorption. Too low = plant less active = low absorption. 3.Humidity: high humidity = stomates open, cuticle not so thick (hydrated by the humidity) = high absorption 4.Precipitation/rain: depending on herbicides’ activity. Systemic = more affected (6h to absorb); contact = less affected (30 min to absorb) 5.Wind: too windy = high herbicide drift = less deposition = less absorption 6.Herbicide formulation: ester/amine/salt formulation better than pure acid formulation 7.pH: low pH = high solubility in lipid = high absorption 8.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

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 Herbicide Absorption cont…

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

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. Herbicide Translocation cont…

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.

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. Herbicide Translocation cont…

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.