A. Chemical Nature
In 1882, Nencki and Sieber discovered that indole-3-acetic acid was a constituent of human urine, which
in 1934 was confirmed by Kögl and his coworkers, with the additional information that it was active in
promoting the growth of some plant tissues or organs [16]. Within a year, it was also isolated from yeast
plasmolysate and from the culture filterates of Rhizopus suinus[10,13]. However, its first isolation from
a crop plant [i.e., from immature maize (Zea mays) kernels] was made by Haagen-Smit et al. [14]. It is
now commonly accepted that IAA is perhaps the only endogenous auxin in plants and crops. Interest-
ingly, chemists were aware of IAA long before plant scientists became aware of it. IAA was first synthe-
sized by a German chemist in 1904 [17], but it was not suspected to have biological activity.
There are many purely synthetic compounds that mimic physiological actions similar to that of IAA.
They are chemically diverse but can be classified in five major categories: indole acids, naphthalene acids,
chlorophenoxy acids, benzoic acid, and picolinic acid derivatives. Two compounds belonging to the first
group, indolebutyric acid and indolepropionic acid, are not exclusively synthetic; they have also been
reported to be present in some plant species. The well-known naphthaleneacetic acid and -naphthoxy-
acetic acid belong to the second group. The best known among the chlorophenoxy acid group are 2,4-
dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), and 2-methyl-4-
chlorophenoxyacetic acid (MPCA), which are known to be very powerful defoliants and herbicides when
used in higher concentrations. In the benzoic group, the common synthetic auxins are the 2,3,6- and 2,4,6-
trichlorobenzoic acids and dicamba, which is a powerful herbicide and is effective in some species of
deep-rooted perennials, which are not readily killed by 2,4-D. Among the picolinic acid series, the best
known is picloram (4-amino-3,5,6-trichloropicolinic acid), which is known to be the most powerful se-
PLANT GROWTH HORMONES 503
Figure 1 Diagrammatic representation of the major experiments leading to the discovery and quantification,
by bioassay, of the auxin.