Agriculture Reference
In-Depth Information
cytokinin levels increase at the break of dormancy. This
together with findings that potatoes transformed with a
cytokinin biosynthesis gene show early sprouting (Ooms &
Lenton 1985) suggest that cytokinins are involved in
dormancy break.
Abscissic acid, (ABA) is high in dormant tubers and
declines during post-harvest storage. Moreover Suttle and
Hultstrand (1994) showed that Fluridone, an inhibitor of ABA
synthesis, caused premature sprouting in an
in vitro
micro-
tuber system, while addition of ABA suppressed sprouting.
Ethylene can have different effects on sprouting. Rylski
et al
. (1974) showed that short-term ethylene treatment can
prematurely terminate tuber dormancy, while continuous
treatment (at levels of about 5 ppm) results in sprout growth
inhibition. Ethylene is induced by auxins, but despite the
fact that it is now in commercial use, its precise role in
dormancy control is unclear (Suttle 2003a).
The overall picture of dormancy control put forward by
Suttle (2004a) is that both ABA and ethylene are required
for the initiation of tuber dormancy, but only ABA is
needed to maintain the dormant state. Cytokinins are
involved in dormancy break. Thus endogenous cytokinins
levels are relatively low in highly dormant tubers and
tubers are non-responsive to exogenous cytokinins. During
dormancy tubers actively metabolise ABA and cytokinins
to inactive products. As dormancy weakens, tuber ABA
levels decline and tubers become increasingly sensitive to
exogenous cytokinins.
In addition to the clear importance of ethylene for
commercial dormancy control, and GA
3
for promoting
sprouting, the possibility of using synthetic forms of plant
hormones for sprout control is also being considered
(Suttle 2005).
naphthalenes have been tested singly or as mixtures (Lewis
et al
. 1997), and have been shown to control sprouting as
effectively as CIPC. As well as DMN, 2,6 diisopropyl-
naphthalene has also been commercialized, and may even
be more effective than DMN. It is thought that substituted
naphthalenes work through a hormonal effect, and it has
already by noted that their structure is auxin-like. It has
been shown that multiple application of 1, 4 DMN not only
inhibits sprouting of seed tubers but can modify tuber yield
and size distribution to obtain a tubers of smaller average
size which may be desirable for fresh, processing and seed
potato industry (Knowles
et al
. 2005).
Several other compounds with sprout inhibitory activity
at least of the same order as CIPC have been isolated from
potato surface tissues (Filmer and Rhodes 1985). The
chemical diversity of these compounds (1,4- 1,6-dimethyl-
naphthalene, 1,4,6-trimethylnaphthalene and diphenylamine)
suggest that they act by different mechanisms.
Essential oils
S-Carvone is a monoterpene isolated from caraway (
Carum
carvi
) or dill (
Anethum graveolems)
seeds, which is mar-
keted as the sprout suppressant, Talent™ (Kleinkopf
et al
.
2003). This chemical also inhibits the formation of wound
periderm, and must therefore be applied after the curing
process. Extracts from spearmint (
Mentha spicata
) and
peppermint (
Mentha pipenta
) have also been successfully
used. These essential oils are applied as thermal fogging,
cold aerosol application or by forced evaporation. Given
their volatility, continuous application is necessary
(Kleinkopf
et al
. 2003). Biox-ATM, or engenol is a purified
extract from clove (
Sygzygium aromaticum
(L) ) which has
similar efficiacy and acts through damaging the developing
buds (Kleinkopf
et al
. 2003).
Other natural sprout suppressants
Research on dormancy control in potato has been taken in
a number of directions. More than thirty years ago a num-
ber of aromatic hydrocarbon volatiles were isolated from
potato skin (Meigh
et al
. 1973; Lewis
et al
. 1997) many of
which were found to have sprout suppressant effects. One
of these was 1-4 dimethylnaphthalene (1,4-DMN), which
is now used as a commercial sprout suppressant. 1,4-DMN
is naturally present in potatoes at levels between 1 and
10 ppm. It appears that when conditions are optimum for
sprouting 1,4-DMN is metabolised to a low level to
allow sprouting. Commercially, 1,4-DMN has to be applied
repeatedly, as its effect is temporary. Usually four applica-
tions over the storage season are necessary to maintain a
sufficient concentration in the potato to control the
sprouting. (Federal Registry 1995). Several substituted
Hydrogen peroxide
Hydrogen peroxide can act as a sprout inhibitor and is part
of a commercial product marketed as Hydrogen Peroxide
Plus (Prange
et al
. 1997). It damages the meristematic
tissues once dormancy has broken. Interestingly, hydro-
gen peroxide has also been identified as a signal for
dormancy break in other species such as grapevines
(Perez & Lira 2005).
Irradiation
Ionizing radiation is effective at inhibiting sprout develop-
ment. The University of Idaho has also looked at high-energy
electron treatment. The use of irradiation is dependent on its
acceptability to consumers, and at this time it is only used
commercially in Japan (Kleinhopf
et al
. 2003).
