Agriculture Reference
In-Depth Information
Conditions that increase the rate of respiration,
even for a short period, can accelerate physio-
logical aging. For example, high temperatures
during the maturation of tubers in the field, or
exposure to high or fluctuating temperatures in
storage, can result in higher basal respiration
rates during storage, which accelerates the
aging process, resulting in physiologically older
tubers at planting (Blauer et al ., 2013b). Tuber
respiration appears to be a pacemaker of aging;
the longer the respiration rate remains high, the
older the seed tubers will be (Blauer et al ., 2013b).
Exposure to low temperatures, below 2.8°C, also
increases respiration and aging of tubers. Factors
other than temperature can increase respiration
and aging, including moisture stress, inadequate
fertility, and disease pressure during the growing
season; bruising at harvest; and harvest of im-
mature or overmature tubers.
Vascular
ring
Apical
(bud)
end
Node (eye)
Cortex
Pith
Periderm
(skin)
Basal
(stem)
end
Perimedullary
tissue
Stolon
Fig. 5.1. The external and internal anatomy of a
potato tuber. (Drawing courtesy of Jeanne Debons,
Bend, Oregon, USA.)
axillary buds are arranged on an aboveground
stem (Cutter, 1992). The meristems within each
eye can sprout to form branch stems from the
tuber. Sprouts that develop below the ground
and in the dark are etiolated and characterized
by extensive elongation of internodes.
Exposure to light induces rapid leaf devel-
opment, resulting in an initial rosette or whorl
of leaves around the apical meristem soon after
emergence. Further growth produces leaves,
branch stems, flowers, roots, stolons, and tubers
( Fig. 5.2 ).
Generally, and depending on tuber physio-
logical age and cultivar, only the primary (apical)
bud in an eye develops to form a sprout, but if it is
removed or injured, the secondary buds will be-
come active. Moreover, apical dominance dic-
tates that eyes on the apical end of whole tubers
sprout before eyes on the basal end. Apical dom-
inance is controlled in part by auxin, a growth-
regulating substance that is produced in the
apical meristem and translocated basipetally
buds to growth of axillary buds (Michener, 1942
Young et al ., 2014). Apical dominance is reduced
as tubers age; therefore, it is possible for multiple
buds in one eye to sprout concurrently, resulting
in multiple stems per eye from older tubers. Ap-
ical dominance is usually attenuated when tubers
are cut into pieces, but there can be a noticeable
difference in the rate of sprout growth from seed
pieces originating from the apical versus the
basal end of a tuber. Sprouts developing from
5.4 Tuber Anatomy and Sprout
Development
According to Cutter (1992), once one under-
stands the anatomy of a potato eye, it becomes
clear that the structure of a potato tuber is es-
sentially an enlarged, modified, belowground
stem. She explains the potato eye is a complete
scale leaf with axillary buds similar to that of
many plant species, containing a primary axil-
lary bud along with second-order axillary buds
that form into the first two leaf primordia. Al-
though technically a stem, the potato tuber
functions as a storage organ for carbohydrates
and nutrients to facilitate its role as a vegetative
propagule to perenniate the plant.
The external morphology of a tuber is simi-
lar to that of an aerial stem, and includes inter-
nodes, nodes, and scale leaves. The tuber surface
is covered by an outer protective tissue called the
native periderm, which includes many small
pores, or lenticels, for gas exchange. Directly
beneath the periderm lies an inner region of
storage parenchyma tissue called the cortex, fol-
lowed by a ring of vascular tissue, and an inner
pith region composed largely of parenchyma
tissue and interxylary phloem ( Fig. 5.1 ).
The nodes (eyes) on tubers contain the axil-
lary meristems (buds), which are subtended by a
scale leaf, as in a leaf axil of an aerial stem. The
eyes are arranged spirally on the tuber, just as
 
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