Auxins phytotoxicity

Indole-3-acetic acid is rather readily oxidized by peroxidases and is, in fact, probably not present in the plant in the free form to any appreciable extent. The nature of the complexing groups is not clear. The inherent instability of the compound in living tissue has made experimental observations difficult, and (the more stable) 1-naphthaleneacetic acid has often been used instead, although it is by no means certain that the biological activities are comparable. One view held is that auxin herbicides are effective either because they do not readily form conjugate systems, or because the conjugate retains the phytotoxic properties. 

The reason for the phytotoxic action of the auxin herbicides is proposed to be an aberrant nucleic acid metabolism induced by a hormonal imbalance in treated tissues." ... 

It is necessary to briefly consider the current view on the mode of action of auxins in order to identify the basis of phytotoxicity of the auxin herbicides. However, in spite of 40 years of extensive research, aided by the use of the auxin herbicides, there is no universal view on the auxin binding sites (receptors) and the events following binding. 

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