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Plant Growth Hormones: Growth Promotors and
Inhibitors
Syed Shamshad Mehdi Naqvi
Nuclear Institute of Agriculture, Tando Jam, Pakistan
501I. INTRODUCTION
Since the dawn of agriculture, one of principal aims of human beings has been the control and promotion
of plant growth to satisfy human needs. These two important aspects of people’s work with plants in the
struggle to increase production are by no means synonymous. Humans soon realized that lush green growth
does not always produce the best crop in the form of fruit and seeds and hence were forced to evolve such
well-known cultural methods as pruning, balance manuring, and mineral fertilizers to regulate the nature
and luxuriance of plant growth. This control of the plant development pattern—this adjustment of balance
between root and shoot and between leaf and flowering—was until about seven decades ago mediated by
appropriate and very large empirical combinations of what may be called dietary and surgery. More than
60 years ago proof was given of what Julius Sachs [1] in 1880 postulated: that endogenous substances reg-
ulate the growth of various plant organs. In 1926, Went [2], in Holland, provided convincing evidence of
a diffusible substance (auxin) from oat (Avena sativa) seedlings that promoted growth of these seedlings.
At the same time, Kurosawa [3], in Japan, discovered another substance (gibberellin) from cell-free fun-
gus (Gibberella fujikuroi) filterate that promoted growth of rice (Oryza sativa) seedlings. But it was not
until 1955 that Skoog and his associates [4] discovered kinetin in an autoclaved sample of herring sperm
DNA, which was active in what Wiesner in 1892 called cell division factor [5].
In both scientific and popular literature, these chemicals in plant and crop physiology have come to
be known as plant growth regulators. Starting, as do many great scientific advances, in a few unobtrusive
laboratories as purely academic observations, the volume of research on these substances by both plant
physiologists and chemists has swelled to enormous proportions.
The naturally occurring (endogenous) growth substances are commonly known as plant hormones,
while the synthetic ones are called growth regulators. A plant hormone(synonym: phytohormone) is an
organic compound synthesized in one part of a plant and translocated to another part, where in very low
concentrations it causes a physiological response. Plant hormones are identified as promotors (auxin, gib-
berellin, and cytokinin), inhibitors (abscisic acid, xanthoxin, and violaxanthin), ethylene, and other hy-
pothetical growth substances (florigen, death hormone, etc.). They usually exist in plants and crops at a
concentration lower than 1 M; above this they are generally considered supraoptimal [6].
The mechanism(s) by which hormones trigger a response is still far from well understood. Specific
receptors have been proposed, but no proof for their function in mediating hormone action has been given.