PLSC 753 - Herbicide Fate and Action
Herbicides have been used to somewhat magically remove weeds
from cropland, pastures, and other areas to improve the economics or aesthetics of the remaining plants and landscape.
The structure of an herbicide determines the effectiveness and mode of action. The structure also affects chemical properties, such as solubility, pKa, and log Kow, that influence degradation and movement in the environment and plants.
Herbicide action is based on disruption of normal plant processes, and herbicides are grouped according to how they affect plants, or their mode of action. Knowledge of the normal function of plants is the foundation for discussions of herbicides modes of action. (Image from www.calbiochem.com)
Specific symptoms of herbicide action often can be used to identify the herbicide mode of action and, therefore, can be used to discern the difference amond intended herbicide application, unintended herbicide exposure, and environmental effects.
For example, club root is a symptom of pendimethalin activity because this herbicide prevents cell division resulting in swollen root tips. (Image from Pest. Manag. Sci. 61:1052-1059)
The ability of desired plants to survive treatment with herbicide is a function of several factors. The most critical factor, in most cases, is the natural line of defenses within the plant. Plants have several metabolic pathways, such as glutathione conjugation, to deal with toxins and prevent damage to the cell and cell reactions. (Left image from Biol. Chem. 278:23930-23935) 
But even with these detoxification pathways, injury can occur to otherwise tolerant plants as a result of misapplication or environmental influence. Genetic transformation of some crops has introduced target site resistance as an additional safety mechanism. The issues of herbicide resistance and resistance evolution are important factors to consider when discussing weed control programs. Understanding the differences in herbicide modes of action is fundamental to this discussion.
More information about course requirements is available if you select PLSC 753.