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Dormancy in Plants
The Mechanism of Action
of the Dormancy-
Breaking Agent DORMEX ®
Dr. R. J. Youngman
SKW Trostberg AG
"MG PGR/Dr.Le/Dormex08.doc"
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The aim of this report is to provide a general description of the phenomenon of
dormancy in plants and in particular, the possibilities of breaking dormancy through
the use of DORMEX. In this context, the mechanism by which DORMEX exerts its
effect will be discussed.
1. What is Dormancy?
Dormancy is known to occur in the vast majority of plants at some stage during
their life-cycle and is typified as a period when no growth takes place. The
phenomenon has been extensively studied in seeds and buds. The main function
of dormancy appears to be to ensure that the plant is able to survive periods of
adverse environmental conditions, such as low temperature and reduced water
availability. During dormancy, the biosynthetic capabilities of the plant are shut
down and the dormant organs become partly dehydrated.
It is well known that many dormant plants also show a requirement for a period of
low temperature before they are released from the constraints of dormancy and
are able to resume normal growth. Moreover, many plant species requiring low
temperature to break dormancy subsequently need high temperatures and much
sun to achieve optimal growth. Typical examples of such plants are apple, peach,
cherry, citrus fruits and grapes.
In order to achieve best possible growth, it is thus generally necessary to make a
compromise with regard to the differing physiological requirements of the plant.
Thus, plant species are often cultivated where normal growth conditions are good,
at the risk of suboptimal conditions for the breaking of dormancy. The
consequence of such a practice is that a total breaking of dormancy may not be
attained and/or that subsequent growth is non-uniform. The effect of this
suboptimal initial growth may be carried through all growth stages, so that the
yield at harvest is substantially reduced.
The breaking of bud dormancy can be induced by various treatments such as
removal of the bud scales or by the application of hot water, but inevitably these
practices are unsuitable for commercial applications. Chemicals such as ethanol,
potassium nitrate, thiourea and mineral oils have also been used with varying
degrees of success. The problem of overcoming dormancy has been greatly
improved by the introduction of DORMEX, a special formulation of hydrogen
cyanamide, which leads to high and very uniform bud break.
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Photosynthesis
Protein synthesis
Respiration
Hormones
1. How is dormancy induced?
DORMANCY IN PLANTS
It is generally recognised that dormancy is induced by the onset of low
temperatures and shorter day-length associated with late autumn. The metabolic
rate of the plant decreases to such an extent, that the residual metabolism is just
sufficient for the plant to survive. Growth ceases for the period of dormancy.
During dormancy, the level of endogenous plant growth inhibitors is maintained at
a high level in the dormant organs. It is still unclear whether the increase in the
concentration of such substances is a prerequisite for the induction and/or the
continuation of the dormant stage.
2. How is dormancy terminated?
DORMANT STATE
DORMANCY BREAK
PLANT GROWTH
DORMEX
Cell increase
Cell differentiation
Cell division
BIORJY 90-004
MG PGR/Dr. Le "BIORJY90.ppt"
Dormancy allows the plant to survive periods of unfavourable growth conditions
and thus it is generally a prerequisite that an improvement in the environmental
conditions must occur before a natural breaking of dormancy can be initiated. For
dormant buds, temperature increases coupled with longer day-length appear to be
of critical importance.
The release of the plant from dormancy is accompanied by a synthesis of new
compounds essential for new growth and also by a decrease in the levels of
endogenous inhibitors.
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3. How does DORMEX induce the breaking of dormancy?
DORMEX has proved to be a particularly effective agent for breaking dormancy,
particularly in woody plants such as grapes. As mentioned, the period of
dormancy is accompanied with high levels of natural growth inhibitors and hence,
cessation of growth. It would thus seem logical that treatments capable of leading
to a breaking of dormancy might achieve their effect by reducing the concentration
of these growth inhibitors. With the removal of these substances, it might be
expected that normal growth could be resumed.
The main natural plant growth inhibitor believed to be involved with dormancy is
the hormone, abscisic acid. Studies have been instigated to investigate the
relationship between abscisic acid levels in grape buds following DORMEX
application and release from dormancy. Somewhat surprisingly, it was found that
there was no correlation between abscisic levels in the plant and DORMEX
induced release from dormancy.
The enzyme catalase catalyses the breakdown of hydrogen peroxide to water and
oxygen. It possesses a particularly important role in the plant, as many enzyme
reactions produce hydrogen peroxide as a by-product of metabolism, which, if not
detoxified, could have serious deleterious effects on the plant. It has been known
for many years that hydrogen cyanamide inhibits the action of catalase. However,
since the application of DORMEX at the recommended rates does not lead to
phytotoxicity, it must be assumed that the plant possesses an alternative
mechanism to detoxify hydrogen peroxide. It is conceivable that the activation of
this second pathway is related in some way to the mechanism of dormancy
breaking by DORMEX.
Extensive studies in the research laboratories at SKW have shown that when
DORMEX inhibits the action of catalase, the plant subsequently detoxifies
hydrogen peroxide via a sequence of reactions which are eventually coupled to
the oxidative pentose phosphate pathway. It could be shown that the presence of
DORMEX stimulates the reaction between hydrogen peroxide and ascorbate,
which in turn leads to an increase in the rate of turnover of the pentose phosphate
pathway. The consequences of this are two-fold: by stimulating the oxidation of
ascorbate by hydrogen peroxide, DORMEX ensures that although it inhibits the
catalase-dependent breakdown of hydrogen peroxide, another pathway is
stimulated, thus allowing its detoxification. Due to the concomitant activity
increase in the pentose phosphate pathway, a range of essential substances such
as lipids, RNA, DNA and pentose sugars are produced at higher rates. These
compounds are fundamental for new growth, as would be required during the
release of buds from dormancy.
These studies demonstrated that DORMEX exerts an effect on the plant whereby
the synthesis of compounds essential for new growth is markedly stimulated.
Nevertheless, it is likely that compounds exerting an effect on a physiological
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H 2O
+
H 2O 2
process such as dormancy also influence the hormone status of the plant. Our
investigations have shown that DORMEX affects the cytokinin physiology via
certain mediator compounds. It could also be demonstrated that these mediators
per se led to a further stimulation of the pentose phosphate pathway.
1. Summary
Influence of DORMEX on PP pathway
O
1 / 2
2
catalase
DORMEX
H 2O
DHA
ASC GSSG
GSH
Mode of action
Starch
Breaking of
dormancy
RNA/DNA
synthesis
NADPH
NADP G-6-P
6-P-Gluconat
Glucose
MG PGR 1996
Pentosephosphatepathway
Gehsc31
The fact that a breaking of dormancy can be achieved by chemical substances
which are completely unrelated tends to indicate that a general, unifying
mechanisms for induced release from dormancy probably does not exist.
The investigations carried out to determine the mechanism by which DORMEX
exerts its effect permit the following conclusions to be drawn:
1. The inhibition of catalase by DORMEX leads to a stimulation of the pentose
phosphate pathway and thus to an increase in the availability of new "building
blocks" for new growth.
2. The interaction between DORMEX and the cytokinin metabolism of the plant
allows DORMEX to exert a direct regulatory influence on the physiology of the
plant.
It is likely that the action of DORMEX depends on both these areas of interaction
of DORMEX with plant metabolic processes.
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