PLANT HORMONES
Auxins
Auxins are involved in a variety of processes including stem growth, root formation, inhibition of lateral
bud development, fruit and leaf abscission, fruit development and activating cambium cells. Of all the
hormones, it is the auxins which have the greatest effect on root formation in cuttings.
The three most commonly used auxins are:
Indole-3-acetic acid (IAA) occurs naturally, but is less effective than IBA or NAA in promoting root
growth.
Indolebutyric acid (IBA), an artificial auxin, is the most widely used rooting hormone. It is used on
its own or in combination with NAA.
Napthaleneacetic acid (NAA), an artificial auxin, is used on its own or in combination with IBA.
Auxins can be applied to plants as a powder, gel or a liquid. Cuttings are frequently treated with
hormones which encourage root development. The strength of hormone used will vary depending on
such factors as the type of cutting being struck, the variety of plant and the application method.
Powder
The advantages of using powder are that it is easy to apply, it comes in specific concentrations,
and the user doesn't need much expertise. The major disadvantage is that the powder is only
available in a limited range of concentrations.
Liquid
Liquid hormones offer greater flexibility because the concentration can be varied easily by adding
an appropriate liquid. The length of time whish the cutting is dipped (and exposed to the
hormone) can also be varied. For example, basal parts of cuttings can be soaked for 24 hours in
a dilute solution (eg: 100 ppm), or dipped in a concentrated solution (500-10,000 ppm) for about
5 seconds.
Gel
Gel remains on the cutting well. Strength can be adjusted to some degree. Other ingredients
are sometimes included in the gel (eg: vitamins).
Cytokinins
These are hormones which help with cell differentiation. Certain cytokinins applied to some plants at
specific rates can help with root initiation, although more generally, cytokinins are found to inhibit root
development. Cytokins are used in tissue culture to stimulate leafy growth.
Giberellins
At high concentrations gibberellins inhibit root formation. Chemicals such as "Alar" and "Arrest" are
commercial preparations which work by interfering with the effect of gibberellins. Gibberellins are also
sometimes used as a means of overcoming seed germination inhibitors.
Paclobutryzol (eg: "bonzi")
This chemical is absorbed by stems, leaves and roots. It moves to just below growing tips and inhibits
gibberellin production, causing a reduction in vegetative growth and stimulation of flowering in some
species. Sold as a 4 gram per litre suspension under the brand name "Bonzi". Its use may increase
chlorophyll production, causing darker coloured foliage.
Daminozide (eg: "alar")
This hormone slows down and reduces cell expansion, causing shorter internodes. It also reduces
dominance of the apical tip, causing more branching. It is mainly absorbed through leaves, therefore it is
best applied under slow drying conditions. The use of Alar has been banned in some countries due to
health concerns.
Chlormequat (eg: "cycocel")
An anti-gibberellin which reduces internode spaces on sensitive plants.
Absciscic acid (ABA)
This naturally occurring compound plays an important role in inhibiting germination of many types of
seeds, particularly those with immature embryos. ABA applications have been used to inhibit the
germination of non-dormant seeds, and to offset the effects of gibberellic acid applications.
The natural Plant Growth Hormones and their main synthetic counterparts useful in nursery production:
GROUP
Auxins
NATURAL
SUBSTANCES
Indoleacetic acid
(IAA)*
SYNTHETIC
COUNTERPARTS
Indolebutyric acid (IBA)
Naphthalene acetic acid
(NAA)
USES
Control of flower and fruit
drop.
Rooting of cuttings.
Control of suckering.
2,4-Dichlorophenoxyacetic acid (2,4-D)
Gibberellins
Gibberellic acid (GA3)
and over 70 other
forms
Cytokinins
Zeatin, etc
Abscisins
Abscisic acid
Ethylene
Ethylene*
Article by B. Balaba
Kinetin.
Benzyladenine (BA) =
Benzylaminopurine (BAP)
PBA
Ethephon is a synthetic
product which releases
ethylene gas inside the
plant
Release of dormancy in
seeds and buds.
Increased stem length.
Promotion of flowering in
some "long day plants".
Release of dormancy in
seeds and buds.
No significant practical
applications.
Shortening and thickening
of stems. Possible
increase in branching.
Flowering in bromeliaeds.
Release of dormancy in
seeds and buds.