There is, however, considerable evidence that gene expression is controlled by these hormones, but how
it is done biochemically is largely unknown [7]. These hormones are found in all actively growing plant
parts; young leaves and apical buds are particularly high in auxin, whereas young roots are high in gib-
berellins and cytokinins. Fruits and seeds are generally rich in all growth hormones. Therefore, these hor-
mones are ubiquitous in plants and crops and are generally not species specific.
In general, a deficiency of hormone must be created experimentally (as by removing young leaves
or using a hormone-deficient mutant) to show that adding a hormone has an effect. In this respect, the
Mitscherlich law of diminishing returns [8] can be modified as follows: the increase in plant response pro-
duced by a unit increment of a deficient (limiting) hormone is proportional to the decrement of that hor-
mone from the maximum.
To deal in depth and breadth with the entire field within our space limitations here is not possible.
Therefore, we aim in this chapter to discuss advances in our understanding that are relevant to plant and
crop management.
II. AUXINS
Auxinis a Greek word derived from auxein, which means “to increase.” It is a generic term for chemicals
that typically stimulate cell elongation, but auxins also influence a wide range of growth and development
response. The existence of growth-regulating chemicals that control plant growth, and the interrelations
between their parts, was the outcome of experiments on root and shoot responses to external stimuli.
Ciesielski, working with roots, and Charles and Francis Darwin, working with shoots, observed that in
both organs the tip controlled the growth rate of the immediate growing axes as well as the regions lo-
cated some distance away [9]. The Darwins performed simple experiments on the photoresponse of ca-
nary grass (Phalaris canariensis) and oat coleoptiles. When the tip was unilaterally illuminated, a strong
positive curvature (growth toward light) along the growing axes resulted. If the tip was shielded by an
opaque cap and only the lower part was exposed unilaterally, curvature generally did not result. From
these and other empirical experiments, the Darwins concluded that “when seedlings are freely exposed to
a lateral light, some influence is transmitted from the upper to the lower part, causing the latter to bend.”
Boysen-Jensen [10], working with Avenacoleoptiles, concluded that “the transmission of the irritation is
of a material nature produced by concentration changes in the coleoptile tip.” Paál [11] corroborated his
findings by demonstrating that “the stem tip is the seat of the growth regulating center. In it, a substance
(or mixture) is formed and internally secreted, and this substance equally distributes on all sides, moving
downwards through the living tissue.” But it remained for Went [2] to make the definitive discovery of
auxin and to determine it quantitatively by Avenacurvature bioassay (Figure 1). The chemical isolation
and characterization were, however, done by Kogl et al. [12]. The details of the development of the con-
cept and discovery of indoleacetic acid (IAA), as outlined above, are described in two classical works
[10,13].
It was not until 1946 that a good chemical identification of IAA was made in a higher plant [14]. IAA
has come to be recognized as perhaps the only true auxin of plants and crops. This auxin has also been
isolated from culture filterates of bacteria, fungi, and yeast plasmolysate, but its role in these organisms
is less clear.
Besides IAA, plants contain three other compounds that are structurally similar and elicit many of
the same responses as that of IAA: 4-chloroindoleacetic acid (CIIAA), phenylacetic acid (PAA), and in-
dolebutyric acid (IBA). However, their physiological significance and transport properties remain ob-
scure at present. Engvild [15] has advanced the idea of death hormones and suggested CIIAA as one of
them. Four additional compounds—indoleacetaldehyde (IAALD), indoleacetamide (IAM), indoleace-
tonitrile (IAN), and indole ethanol—are also found in a range of plants, but they are readily converted to
IAA in vivo. The enzymes aldehyde dehydrogenase and indoleacetamide hydrolase, which catalyze the
conversion of IAALD and IAM, respectively, to IAA, are active in plant tissues in which workers have
detected IAALD or IAM. Similarly, IAN, found in the Cruciferae and Gramineae families, is also ac-
companied by the enzyme nitrilase, which is involved in the conversion of IAN to IAA. These similar cir-
cumstances, in a range of plants, indicate that IAA is the true active free auxin in plants. Furthermore, free
auxin forms are probably the most immediately utilizable by plants in their growth processes.
502 NAQVI