Mode of action of herbicides

The Mode of Action of herbicides is important for understanding the management , classification and hierarchy of the herbicides. It also provides an insight into herbicide resistance , which continues to be a problem in sustainable agricultural management .

1. Topic : Mode of Action of Herbicides Presented by Himani M.Sc. Ag (Agril. Chemicals ) 2nd semester G. B. Pant University of Agriculture and Technology, Pantnagar

2. Contents: Herbicide List of common Herbicide Classification of Herbicides MOA Stages Inhibitory MOA Inhibition of Photosynthesis Inhibition of glutamine synthesis Synthetic Auxin References

3. Herbicides Commonly known as Weed Killer. Chemical substances used to control or inhibit growth of unwanted plants (Weed). Selective herbicides control specific weed species, while leaving the desired crop relatively unharmed. (eg. 2,4-D, Pendimethalin, etc.) While Non-selective herbicides formulated to control both broadleaf and grass weeds. (eg. P araquat, Glyphosate, etc.)

4. List of common herbicides 2,4-D Atrazine Pendimethalin (Crossbow) ( AAtrex ) (Stomp)

5. Paraquat (Gramoxane) Dicamba (Banvel) Glyphosate (Round-up)

6. Classification of Herbicides based on MoA Herbicides are classified based upon different aspects such as MOA , site of action , Chemical similarities and selectivity. Herbicides mode of action stages: Contact Absorption Movement Toxicity Death

7. Most common inhibitory mode of action: Inhibition of photosynthesis electron transport Inhibition of enzyme acetolactase synthase (ALS inhibitor) Inhibition of glutamine synthesis

8. Photosynthesis inhibition by herbicides Photosynthesis is a physico-chemical process by which photosynthetic organisms use light energy to drive the synthesis of organic compounds. 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 Photosynthesis inhibitors: These shut down the photosynthesis in susceptible plants by binding to specific sites within the plant’s photosynthetic process . Plant experiences a more rapid death that is believed to be due to the production of secondary toxic substances.

9. Photosynthesis Inhibition Site Inhibition of Photosystem II electron transfer Inhibition of electrons in Photosystem I Electron diverters Families under the mode of action are: Triazines (Atrazine, Metribuzin, Propazine etc.) Phenylurea(Diuron, Isoproturon etc.) Bipyridiliums (Paraquat, Diquat etc.)

10. Photosystem I and Photosystem II Photosystem I (PS I) and photosystem II (PS II) are two multi-subunit membrane-protein complexes involved in photosynthesis. Chlorophyll is the pigment involved in capturing light energy. The main difference between photosystem I and II is that PS I absorbs longer wavelengths of light (>680 nm) whereas PS II absorbs shorter wavelengths of light (<680 nm).

11. Electron Transport in Photosynthesis

12. Site I. Inhibition of Electron transfer in PS II Electron transfer chain is the reduction of plastoquinones by the D1 protein in thylakoid . These group herbicides act as inhibitors of binding (D1 protein with PQ) and stops the production of ATP and NADPH2. It leads to the formation of triplet chlorophyll which reacts with ground state Oxygen to form Singlet Oxygen. Both triplet chlorophyll and singlet oxygen abstract hydrogen from unsaturated lipids and start lipid peroxidation. It leads to the loss of chlorophyll in leaky membranes which allows the cell organnels dry and disintegrate rapidly. Thus herbicide brings about the lethal effect.

13. Site II. Diversion of electron in PS I Diversion include Paraquat , Diquat which are post emergence ,non –selective ,contact and non- residual in nature. When Paraquat interacts with ferrodoxin, competing with NADP+ as an electron acceptor. When it is reduced by an electron , it rapidly transfers the electron to oxygen , forming highly reactive superoxide.

14. Excessives superoxides results in the formation of toxic hydroxyl radical. This toxic form reacts with membrane fatty acid leads to rapid loss of membrane integrity.

15. Diagrammatic representation

16. Glutamine Synthesis Inhibitor Glufosinate is the only commercialized glutamine synthetase herbicide in the United States. It is a broad-spectrum post-emergent herbicide that controls most annual grasses and broadleaves. It is a contact herbicide with limited translocation.

17. The glutamine synthetase herbicides inhibit activity of glutamine synthetas , the enzyme that converts glutamate and ammonia to glutamine. This inhibition results in massive accumulation of ammonia in a plant which destroys cells and directly inhibits photosystem I and photosystem II reactions.

18. Acts as Growth Regulator (Synthetic Auxins ) These herbicides include some of the more effective chemicals for perennial broadleaf weed and bush control. The primary action of these herbicides is likely to affect cell wall plasticity and nucleic acid metabolism. Synthetic auxins also affect protein synthesis, cell division and growth, and stimulate ethylene evolution. Eg . 2,4-D and 2,4,5-trichlorophenoxyacetic acid  (2,4,5-T).

19. Acetolactase synthase (ALS) inhibitors Also known as Amino acid synthesis inhibitors. These herbicides inhibit the action of acetolactase synthase enzyme. ALS catalyzes the first step in the synthesis of the branched-chain amino acids such as leucine,iso leucine and valine . These herbicides are known as AHAS (Acetohydroxy acid synthase) or branch chain amino acid inhibitor. Major group – Triazolopyrimidine ( eg , Penoxsulam) They causes plant wilting and ultimately death.

20. References: Fuerst, E.P., and Norman, M.A. 1991. Interactions of herbicides with photosynthetic electron transport.  Weed Science , 39(3): 458-464. Brown, H. M. 1990 . Mode of action, crop selectivity, and soil relations of the sulfonylurea herbicides.  Pesticide Science , 29(3): 263-281. Kearne, P. C., and Kaufman, D.D. 1975.  Herbicides: chemistry, degradation and mode of action (No. Ed. 2. Volume 1). Handa, S.K. 2017 . Principles of pesticide chemistry. Agribios. Ashton, F.M., and Crafts, A.S. (1973). Mode of action of herbicides.

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