19.2 Introduction to Herbicides Affecting Photosystems

The previous chapter described how Photosynthesis Inhibiting herbicides interfere directly with the plant’s process of photosynthesizing and safely harnessing sunlight, resulting in uncontrolled cellular damage, as well as a disruption in photosynthesis that further weakens the plant.   Since these herbicides inhibit photosynthesis, they only start causing negative effects on plants once plants emerge and light exposure occurs.

As stated in the previous chapter, there are three Herbicide Mode of Actions and nine Sites of Action that negatively affect plants in this manner:

• Photosynthesis Inhibitors
  • Group 5. Photosystem II Inhibitors – Binding Site Different from 6 or 7 (HRAC Group C1)
  • Group 6. Photosystem II Inhibitors – Binding Site Different from 5 or 7  (HRAC Group C3)
  • Group 7. Photosystem II Inhibitors – Binding Site Different from 5 or 6 (HRAC Group C2)

• Pigment Inhibitors
  • Group 11. Inhibition of carotenoid biosynthesis (unknown target) (HRAC Group F3)
  • Group 12. Inhibition of carotenoid biosynthesis at the phytoene desaturase step (PDS) (HRAC Group F1)
  • Group 13. DOXP Synthase Inhibitors (HRAC Group F4)
  • Group 27. 4-HPPD Inhibitors (4-HPPD) (HRAC Group F2)

• Cell Membrane Disruptors
  • Group 14. PPO Inhibitors (PPO)  (HRAC Group E)
  • Group 22. Photosystem I Electron Diverter (PS1) (HRAC Group D)

This chapter will focus on herbicides that disrupt Photosystem I (Group 22 / HRAC D) and Photosystem II (Groups 5, 6, and 7 / HRAC C1-3).