Agriculture Reference
In-Depth Information
eyebrows and axillary buds forming the dor-
mant eye-buds. The eyes are arranged spirally on
the tuber surface and are more concentrated to-
wards the apical end, similar to the main shoot
spiral patterning of leaves and axillary buds. De-
pending on the variety, tuber eye depth may be
elevated, superficial, or deep.
When tuber induction is interrupted due to
changing conditions (heat, drought stress) or
strong competition for assimilates between
tubers, early stage tubers can be “reabsorbed”
(Walworth and Carling, 2002), leaving visible
tuber remnants. Successive switching between
an inductive (tuber formation) and noninduc-
tive status (continuation of apical stolon growth)
can even result in a chain of tubers. The number
of stolons and branches is highly relevant with
respect to the final number of tubers and their
size distribution, as tubers compete for assimi-
lates transported down from the leaves driving
tuber growth (Helder
et al.,
1993). Additional
stolons and tubers may still arise during the
whole growing season; however, normally, the
largest tuber at any stage on the plant will re-
main the largest at the end of the growing sea-
son (Struik
et al.,
1990). Tubers are generally
formed on stolons, but under very strong induct-
ive conditions, may also be sessile, or can even be
formed aboveground from axillary buds (Ewing
and Struik, 1992).
After tuber initiation and initial growth
stages, the tuber enters a bulking phase, in which
it incorporates photoassimilates for growth and
the synthesis of starch and storage proteins. The
control of symplastic and apoplastic transport is
thought to be an important regulatory mechan-
ism with respect to nutrient, hormone, and pho-
toassimilate transport and distribution within
the growing tuber. In non-swelling stolons, phloem
unloading is primarily apoplastic, whereas in
swelling stolons, induction of symplastic unload-
ing allows photoassimilates, mainly sucrose, to be
unloaded into parenchyma cells (Viola
et al.,
2001; Geigenberger, 2003). However, the apical
meristem of the tuber remains symplastically
isolated, maintaining a tightly controlled cellu-
lar domain around the bud enforcing tuber dor-
mancy. Breakage of dormancy is associated with
symplastic reconnection of the bud meristem to
the tuber phloem network (Viola
et al.,
2007).
During the stages of tuber growth and tuber
bulking, and later during storage, the tuber is
protected from desiccation and damage through
the protective layer of the tuber skin. The original
epidermal cell layer of the tuber is replaced early
in development with periderm tissue that is made
up of three cell types: phellem, phellogen, and
phelloderm (Reeve
et al.,
1969). The phellem (or
cork) forms a series of layers at the outermost
level of the periderm and is derived from the
meristematic phellogen layer (or cork cambium)
underneath it. As phellem cells mature, they be-
come suberized, forming a protective layer that is
designated as the “skin”. Gas exchange in the
tuber takes place through lenticels distributed on
the surface of the skin. In terms of gene expres-
sion, very little information exists on genes in-
volved in controlling skin development and the
respiration capacity of the tuber. Tuber dor-
mancy is generally considered to start at the mo-
ment of tuber initiation, once stolon apical
growth is arrested. The loss of tuber dormancy
and the onset of sprout growth is accompanied
by numerous biochemical changes and shifts in
hormone balances (Suttle, 2004; Hartmann
et al.,
2011). From its initiation until the end of its dor-
mancy period, the potato tuber remains a meta-
bolically active organ, reacting to internal and
environmental cues in which balancing the regu-
lation of gene expression and hormone levels
is pivotal.
4.2
Control of Tuber Induction
and Signal Transduction
Day length dependent tuberization
Its equatorial origin makes potato essentially
short-day (SD) adapted for tuberization. Al-
though most modern-day cultivars have lost
their strict need for SD conditions in order to
tuberize, decreased day length (or long nights)
promotes tuber formation, with the degree of
response largely dependent on genotype and
physiological age of the plant (Ewing and Struik,
1992). To study the relationships between
photoperiod and timing of tuberization, researchers
use wild potato genotypes and landraces, such
as
Solanum tuberosum
group Andigenum, which
still require SD conditions to induce tuberiza-
tion. Besides the study of photoperiod control on
tuberization, use of SD-dependent varieties also
