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Physiological Mechanisms of Herbicide Actions
Francisco F. de la Rosa
University of Seville, Seville, Spain
773I. GENERAL STATEMENT
The goal of this work was to study different types of herbicides as well as their diverse mechanisms of
action. A short generalization on the selectivity of herbicides, forms of application, and resistance mech-
anisms will be given, followed by a specific study of each type of herbicide.
Herbicides are substances used to combat the development of weeds. For a herbicide to be effective,
it has to have a series of characteristics:
- Adequate contact with plants
- Absorbed with facility
- Movement within the plant, arriving at its action site without being deactivated
- Reach levels with sufficient toxicity at the action site.
Finding the suitable moment at which to apply each herbicide is also important. For example, herbi-
cides that break the cellular membrane, such as acifluorfen or paraquat, have to be applied when the plants
are forming leaf materials. Inhibitors of seed germination, such as trifluralin or alachlor, need to be ap-
plied in soil before germination begins.
A. Herbicide Selectivity
Herbicides must have potent biological activity against a broad spectrum of weeds and at the same time
be nontoxic to crop plants. Plants that quickly degrade or deactivate a herbicide can escape the toxic ef-
fects of this product. For example, corn is tolerant to herbicides derived from triazines because it quickly
deactivates the herbicides, joining them to several products of its metabolism. In a similar way, soybean
deactivates metribuzin upon conjugating it with sugar molecules.
On some occasions, a crop plant can be damaged by a herbicide to which it is normally tolerant. This
is due to environmental stress, such as extreme temperatures, high relative humidity, or a strong hail-
storm. All these situations can affect the ability of plants to prevent entry or to deactivate herbicides;
cyanazine, for example, to which corn is naturally tolerant, causes substantial damages to corn plants
when atmospheric conditions are cold and rainy.