Plant Growth Regulators I
Auxins, their Analogues and Inhibitors
HORMONES, GROWTH SUBSTANCES AND GROWTH REGULATORS
Some chemicals occurring naturally within plant
endogenously), have a regulatory, rather
than a nutritional role in growth and development.
These compounds, which are generally active at very
low concentrations, are known as
plant growth substances
Synthetic chemicals with
similar physiological activities to plant growth
substances, or compounds having an ability to modify
plant growth by some other means, for example
polyamines, are usually termed
Some of the natural growth substances
are prepared synthetically or through fermentation
processes and can be purchased from chemical
When these chemicals have been added to
plant tissue culture media, they are termed plant
growth regulators in this book, to indicate the fact
that they have been applied from outside the tissues
There are several recognised classes of plant
Until relatively recently only five
groups were recognised namely:
Auxins and cytokinins are by far the most
important for regulating growth and morphogenesis
in plant tissue and organ cultures; in these classes,
synthetic regulators have been discovered with a
biological activity, which equals or exceeds that of
the equivalent natural growth substances.
gibberellins or abscisic acid are available, but some
natural gibberellins are extracted from cultured fungi
and are available for use as exogenous regulants.
However, several classes of chemicals, which are
highly effective in blocking the synthesis of
gibberellins within the plant, are very effective
They are usually termed anti-
gibberellins (or growth retardants).
can also affect the synthesis of other classes of
hormone or growth regulator such as abscisic acid,
sterols or brassinosteroids.
Exogenous ethylene can be used as a growth
regulant, but being a gas, it is difficult to administer
and to control the available concentration, except in
tightly sealed vessels, this is also true of other
alkynes and alkenes which mimic ethylene action
such as acetylene and propylene.
chemicals have been invented which are capable of
releasing ethylene; effective compounds are taken up
into plants as intact molecules, but then break down
to release ethylene within the tissue of a plant.
of these ethylene-releasing chemicals – ‘ethephon’
(2-chlorethanephosphonic acid) is used as a growth
regulator for tissue cultures.
There are now also
some very specific inhibitors both of ethylene
biosynthesis and of its action.