Agriculture Reference
In-Depth Information
are seldom used, because they are genotypically
variable and give rise to plants with traits that may
be completely different from those of the parent
plants (Burton, 1989; Allen
et al
., 1992). More-
over, production from TPS often requires several
extra months of initial growth in a greenhouse
or seedbed, followed by labor-intensive trans-
plantation into the field. The International
Potato Center (CIP), however, has been making
efforts to improve TPS by selecting parental lines
that produce more uniform progeny with good
agronomic characters, especially for use in trop-
ical regions or in developing countries where
seed potato storage and transport is not practical
(Golmirzaie
et al
., 1994).
Plant breeders use TPS as a means of devel-
oping new potato cultivars. Potato cultivars are
clonal selections with specific characteristics
that are fixed only when propagated with vege-
tative tissue. Major characteristics important to
commercial production include yield, tuber shape,
size, solids content and quality, skin texture and
color, and storability. It is important to under-
stand tuber physiology and morphology when
tubers are used for propagation because they
can impact how the plant will develop.
to play critical roles in the control of tuber dor-
mancy and subsequent sprout growth. It is prob-
ably a balance of these inhibiting and promoting
substances that controls dormancy and sprouting.
Evidence indicates that ABA, a growth-
inhibiting substance, is largely responsible for
the induction and maintenance of dormancy
in tubers. It has an analogous role in inhibit-
ing the germination of true seeds (Karssen and
Van Loon, 1992). ABA levels in tuber tissues are
highest in dormant tubers and decline as dor-
mancy weakens (Coleman and King, 1984;
Suttle, 1995). However, there does not appear to
be a threshold concentration of endogenous ABA
below which dormancy will cease and sprouting
occurs (Coleman and King, 1984; Suttle, 1995;
Classens and Vreugdenhil, 2000). Furthermore,
it has been demonstrated that application of an
ABA synthesis inhibitor to dormant tubers re-
sults in premature sprouting, while subsequent
application of ABA restores dormancy (Suttle
and Hulstrand, 1994).
Ethylene, another growth inhibitor, prob-
ably has a role in inducing tuber dormancy,
though it is not thought to maintain dormancy
(Suttle, 2004a). Treatment with an ethylene in-
hibitor was shown to hasten sprouting in devel-
oping microtubers when applied shortly after
tuberization, but later application of the inhibi-
tor had no effect on dormancy (Suttle, 1998).
Ethylene levels in tubers vary throughout the life
of a tuber; ethylene levels are highest at harvest
and decline in storage (Cvikrova
et al
., 1994), but
ethylene production increases again as tubers
begin to sprout (Poapst
et al
., 1968; Suttle, 2003).
The increase in ethylene as sprouting begins
suggests that ethylene may also play a role in the
termination of dormancy, or may be involved
with events occurring during early sprout growth
(Suttle, 2004a).
Dormancy break is associated with cytoki-
nins, plant growth regulators known to stimu-
late cell division. Cytokinin levels in dormant
tubers are relatively low, and an increase in the
cytokinin in tubers has been shown to immedi-
ately precede the onset of sprouting. Moreover,
the application of cytokinin to dormant tubers
has been shown to hasten sprouting, though
sensitivity to cytokinins appears to be time de-
pendent; for example, tubers are insensitive to
cytokinins applied shortly after harvest, but be-
come increasingly sensitive to them in storage
5.2
Seed Tuber Dormancy
Potato tubers are usually dormant at harvest
and will not sprout even under optimum condi-
tions for growth. Dictated largely by genetic
cues, weeks or months must pass before tuber
dormancy is broken and sprout growth can begin.
This is thought to be a natural adaptation that
prevents potatoes from sprouting in the winter
months, when growing conditions are adverse
in the temperate zones where they originated.
The first phase of tuber dormancy is referred to
as “rest” and is regulated by internal physiological
processes. During this period, it is thought that
either the genome of dormant buds is repressed
(Tuan and Bonner, 1964) or the tuber is unable to
supply metabolites for bud growth (Cutter, 1992).
Following the rest period, tubers may continue
to be dormant due to external factors, particu-
larly low temperatures (Hemberg, 1985; Burton
et al
., 1992).
A number of endogenous plant growth
regulators, especially abscisic acid (ABA), ethyl-
ene, cytokinin, and gibberellin, have been shown
