Mode of Action of the Growth Regulator Herbicides

Since the synthesis of 2,4-D, a number of other synthetic auxins have become commercially available. Although these products are referred to as growth regulators or phytohormones (previously known as plant hormones), they really represent only one group of growth regulators, the auxins. Auxins can be divided into six major groups; indole acids, naphthalene acids, phenoxy carboxylic acids, benzoic acids, picolinic acid derivatives (also called pyridine carboxylic acids) and the quinoline carboxylic acids. The first group contains the natural product IAA, and does not contain any herbicides. IAA is highly unstable in plants and metabolizes too fast to be an effective synthetic herbicide. The naphthalene acids (NAA) are used in research but are not commercially available as herbicides. The other groups contain many well known herbicides; phenoxy carboxylic acids (2,4-D, 2,4,5-T, 2,4-DB, dichlorprop, MCPA, MCPB, mecoprop), benzoic acids (dicamba, chloramben), picolinic acids (aminopyralid, clopyralid, picloram, triclopyr), quinoline carboxylic acids (quinclorac), and one yet to be named family (aminocyclopyrachlor). These compounds are often called auxinic herbicides. Quinclorac has also been shown to have growth regulator activity on broadleaf species, although it is not typically considered to be an auxinic herbicide on grasses.