Herbicides kill plants by disrupting and mimicking plant hormones and by blocking photosynthesis. To appreciate how herbicides affect photosynthesis, it is necessary to understand the chemistry of two essential processes of photosynthesis, called photosystem I and photosystem II. Herbicides work by blocking one of these two processes.
Photosystem I: Process
Photosynthesis takes place in light-absorbing pigments of cells that produce chlorophyll. This chemical action generates ATP (adenosine triphosphate), an enzyme that produces energy for plant metabolism.
Photosystem II: Process
Organic compounds called quinones trap electrons needed by plants for chemical reactions that produce chlorophyll. To make up for the deficit, plants use clusters of manganese ions to extract electrons for water. An ion is an atom or molecule that has an unequal number of electrons and protons, giving it a positive or negative electrical charge.
Photosystem I Inhibitors
These herbicides steal electrons from their usual route to producing ATP and discharge them onto oxygen. This produces excessive oxidation that plants cannot tolerate, and they die.
Photosystem II Inhibitors
These herbicides reduce the extraction of electrons from water by clusters of manganese and deposit them onto chlorophyll molecules. As the electrons accumulate on the chlorophyll molecules, they cause rates of oxidation that plant cells cannot tolerate, and the plant dies.
Photosynthesis Suppressing Herbicides
Herbicides containing the active ingredients bentazon and bromoxynil are photosystem I inhibitors. They kill annual weeds with broad leaves on contact. Herbicides containing paraquat and diquat are also photosystem I inhibitors. They are nonselective, meaning that they kill all plants on contact.
Triazine herbicides, photosystem II inhibitors, contain the active ingredients atrazine, metribuzin or simazine and are used to control grass and broadleaf weeds. They travel through the plant's vascular system when roots transport them upward from the soil. Phenylureas herbicides and also photosystem II inhibitors; they contain the active ingredients fluometuron, linuron or tebuthiuron and are transported through plant vascular system. They are used to control some broadleaf weeds and annual grasses.