Herbicide Mode of Action: Why herbicides kill plants and not you. Brett Bultemeier & Greg MacDonald University of Florida

Transcription

1 Herbicide Mode of Action: Why herbicides kill plants and not you. Brett Bultemeier & Greg MacDonald University of Florida

2 Chemical - Herbicides over 150 active ingredients registered close to 4000 trade names herbicides account for 55% of pesticide use

3 Herbicide History Carthage B.C. (salted the ground) Copper SO4, sulfuric acid, salts - late 1800s petroleum oils, Stoddard s Solvent s 2,4,6-dinitrophenol ,4-D -1944

4 Herbicides Herbicide is a chemical compound, generally organic, that is used to control plants Chemistry of the herbicide dictates how it works in the plant how it behaves in the environment how it can be stored, mixed and applied

5 How herbicides work Controlled/selective plant poisoning applied to soil (root uptake) and/or leaves contact or systemic selective vs. non-selective Each herbicide has unique properties

6 Herbicide Selectivity placement - keep it away from desirable plant uptake - apply so it doesn t get in and move metabolism - some plants breakdown herbicides but target site is different

7 Mechanisms of Tolerance X Herbicide X not absorbed Sequestered in vacuole X does not binds to enzyme Y Y1 X metabolized to Z

8 Formulations water soluble, oil soluble, salts, esters clay suspension, pellets, granules, wettable powders, emusifiable concentrates the goal of the formulation is to: store, mix/spray in water, get it in the plant, and minimize off-target damage

9 Persistence/Environmental Fate physical processes volatility - hot and windy leaching - sandy soils, high rain erosion - high rain in short time

10 Persistence/Environmental Fate chemical processes adsorption - sandy soils, variable ph photodecomposition - high light hydrolysis

11 Persistence/Environmental Fate biological processes microbial - high temp., humidity detoxification

12 W.H. Freeman and Sinauer Associates

13 Plant Processes 1. photosynthesis 2. amino acids and proteins 3. fatty acid synthesis 4. growth inhibition (not covered here)* 5. cell membranes 6. pigment synthesis 7. growth regulation

14 Mode vs. Mechanism mode-of-action -- symptoms that occur after herbicide application leading to plant death mechanism-of-action -- actual biochemical site of herbicide activity generally enzyme or co-factor in some cases actual site is unknown

15 Photosynthesis process where the plant uses light energy to convert CO 2 and H 2 O to sugars, release O 2 Light reactions chlorophyll absorbs light light transferred as energy through a chain forms compounds that become sugar

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17 1. Photosynthesis block the flow of energy from chlorophyll to the intermediates plant cannot make sugars starves chlorophyll continues to absorb light and this excess energy forms toxic intermediates generally soil active, root uptake and movement through water stream to leaves

18 Atrazine, Diuron used in a variety of cropping systems for broadleaf and some grass control mostly soil applied, but some foliar activity root uptake, translocation via water stream, little movement from foliar applications potent inhibitors of photosynthesis

19 Photosynthesis Symptoms

20 Photosynthesis Symptoms

21 2. Amino Acid Inhibitors essential building blocks for plant growth and function unlike animals, plants make their own amino acids are the primary components of proteins and nucleic acids Herbicide examples: Glyphosate, ALS compounds

22 Amino Acid Inhibitors generally target a specific enzyme blocks a vital step in the formation of amino acids-- proteins, enzymes aromatic amino acids branched chain amino acids dependent on plant growth for activity better growth - better control, slow death systemic herbicides, some have soil activity

23 How do they work? Block a specific enzyme

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26 Soil Activity!!

27 Glyphosate - Roundup etc. broadspectrum postemergence weed control glyphosate labeled in multitude of areas extensively translocated throughout the plant, extremely stable in plant blocks synthesis of aromatic amino acids

28 glyphosate Entireleaf morningglory

29 Glyphosate injury on pecans

30 Why is glyphosate not harmful to humans? There is no EPSP enzyme in our bodies The herbicide has nowhere to bind It flushes out in urine

31 3. Fatty Acid Synthesis Fatty acids primary building blocks Cell membranes Intercellular membranes Inhibition stunts growth and reduces membrane integrity causing death Fops and Dims

32 Fatty Acid Synthesis registered in several crops, noncropland grass activity only, postemergence translocated to growing portions inhibits fatty acid synthesis, slow death

33 4. Cell Membranes maintain cell integrity keep things in & out generate electrochemical gradients allows for energy production maintain cell structure turgor pressure maintains plant structure and helps the plant grow

34 Herbicides - Cell Membranes divert normal energy flow to form toxic intermediates interact w/ membrane and cause disruption cause the membrane to become leaky gradients cannot be formed, no energy for cellular functions - endothall generally contact, little movement in plant

35 Diquat non-selective little translocation, contact diverts normal electron flow Radical oxygen formation Disrupts cell membrane causes cell death no soil activity

36 Other examples Carfentrazone and flumioxazin Disrupts processes in chlorophyll production Intermediates in the wrong place Causes radical oxygen production

37 DQ DQH2 O 2 -

38 Symptoms

39 5. Pigment synthesis- bleaching herbicides (fluridone) Blocks synthesis of carotenoids These compound protect chlorophyll Causes the plant to turn white Chlorophyll gets too much sun and can t produce energy Plant starves to death

40 Pigment Injury Symptoms

41 6. Growth Regulators: 2,4-D, Triclopyr hormones control plant growth and developmental changes, always present These herbicides are very similar to these growth hormones They overload the plant with hormone The plant grows itself to death

42 Herbicides - Growth Regulation auxin mimics - cause uncontrolled growth auxin transport inhibitors prevent the normal distribution of auxin indirectly, all herbicides can be regulators Fluridone blocks abscisic acid Glyphosate inhibits auxin regulation soil and foliar applied, systemic throughout

43 2,4-D Cucumber

44 Why do herbicides kill plants but not people? They inhibit a process in the plant that is essential to life Many inhibited processes do not occur in humans Exposure to humans is limited Used at ppm or ppb

45 PPB fluridone 5-10 ppb only 5-10 people in all of China are herbicide

46 Glyphosate toxicity Caffine Nicotine salt glyphosate Advil 8 tablets= 20 mg/kg LD50 Tylenol 8 tablets= 50 mg/kg *The smaller the number, the greater the toxicity

47 Acute Toxicity (LD 50 mg/kg) Fluridone > 10,000 Glyphosate > 5,000 2,4-D 1,000 Diquat 600 Endothall (191) 575 Copper 470 Endothall (K) 280

48 Acute Toxicity (LD 50 mg/kg) Endothall at 280 mg/kg 80 kg person (~180 lbs) needs 22,400 mg (0.05 pounds) Endothall treated at ~11mg per gallon of water Would need to drink 2,036 gallons Death by water!!!

49 Silent Spring Rachel Carson 1962 In response to unrestricted irresponsible actions At the time she was ABSOLUTELY correct

50 Silent Spring Concerns Persistent chlorinated hydrocarbons Large scale applications Unknown effects to food chains Untrained applicators

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57 A new era FIFRA (Federal Insecticide, Fungicide, and Rodenticide Act) 1972 Placed pesticides under authority of EPA. Charged with protecting env. and pub. health. Required registration and labeling. Gave rise to a new families of pesticides.

58 Silent Spring Concerns Persistent chlorinated hydrocarbons Large scale applications Unknown effects to food chains Untrained applicators

59 Conclussions Herbicides work by attacking a specific plant process That specificity can lead to selectivity Plant specificity helps protect people Herbicides kill plants by affecting plant specific processes Humans aren t plants Training and focus on safety have protected people

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62 Don t just blindly follow, pay attention or lead

63 GO GATORS plants.ifas.ufl.edu