Chapter 19: MOA Part 4 – Photosynthesis Inhibitors (Photosystems)
19.8 Summary
The majority of available herbicides interact with a plant cell in a manner that causes damage from the energy in sunlight. Inhibitors of electron transfer from Photosystem II block the electron transfer chain that moves electrons from water (which is oxidized to form O2) to NADP+ (which is reduced to form NADPH), leading to uncontrolled energy from the sun and starving the cell of the chemical energy normally produced by photosynthesis. Finally, some herbicides act by diverting high-energy electrons from Photosystem I to generate damaging superoxide and other free radicals. Although each of these classes of herbicides has a distinct site of action, each interferes with the plant’s ability to handle the high energy present in sunlight safely.
Key Takeaways
Key takeaways from this chapter inlcude:
- In plants, the transfer of electrons from Photosystem II to Photosystem I is essential for the production of ATP.
- The D1 protein reduces plastoquinone (PQ) in the thylakoid membrane in a chloroplast.
- Inhibiting the D1 protein compromises a plant’s ability to synthesize ATP and NADPH, which then makes the plant unable to fix CO2
- Photosystem II Inhibitors include Mode of Action Groups 5, 6, and 7.
- Two sites referred to as Site A and Site B are the herbicide binding locations to the D1 protein. Because of this, small changes to the molecular structure of a Photosystem II herbicides may cause differential sensitivity in different species of plants.
- Photosystem II herbicide damage symptoms include yellowing of leaves, followed by necrosis. Older leaves typically exhibit more damage than younger leaves.
- A number of weed species have been found to be resistant to Photosystem II Inhibitors. 26, 1, and 9 weed species have been reported resistant to Groups 5, 6, and 7 herbicides respectively.
- Photosystem I Inhibitors interact with ferredoxin, competing with NADP+ as an electron acceptor. Once the herbicide accepts an electron it rapidly transfer the electron to oxygen creating a highly reactive superoxide. This initiates a series of free radical reactions, which cause extensive cellular damage.
- Photosystem I Inhibitors are contact herbicides meaning only plant tissue coming in direct contact with herbicide will be controlled. Untouched plant tissue will survive, potentially leading to regrowth of a weed.
- Photosystem I Inhibitor damage symptoms include rapid chlorosis and desiccation of treated plant parts.
- 5 weeds species (in the US and globally) have been reported to exhibit resistance to Photosystem I Inhibitors.