Agriculture Reference
In-Depth Information
Tubers harvested when immature or overma-
ture can contain high levels of reducing sugars
(Driskill
et al
., 2007; Sabba
et al
., 2007), which
can increase in storage and result in processed
products that are excessively dark and bitter
tasting (Kumar
et al
., 2004).
Physical maturity, which is the strengthen-
ing of the periderm during tuber maturation, is
commonly referred to as “skin set”. The periderm
is composed of three cell types: phellem (or cork),
phellogen (or cork cambium), and phelloderm.
The phellem, which is derived from the phello-
gen cells underneath, forms a protective layer on
the outside of the tuber when its cells become
suberized and then die. Suberization involves de-
position of a waxy, fatty material, called suberin,
within the cell wall (Kolattukudy, 1984). This
suberized layer of cells plays an important role in
resistance to infection by bacterial and fungal
pathogens (Lulai and Corsini, 1998) and resist-
ance to water vapor loss (Kolattukudy, 1984).
The phelloderm, the innermost part of the peri-
derm, is also derived from the phellogen and is
there to provide energy and biochemical building
materials for periderm growth. The phellogen, in
the middle, is a single layer of meristematic cells,
with cell walls that are very thin and elastic dur-
ing tuber bulking to allow for rapid cell division
and expansion as tubers increase in size. Imma-
ture phellogen fractures easily, which causes the
phellem to slough off when immature tubers are
abraded during harvest and handling (Lulai and
Freeman, 2001). When tuber growth stops, the
phellogen becomes meristematically inactive
and its cell walls thicken and become rigid (Lulai
and Freeman, 2001). The width of radial cell
walls in the phellogen can more than double in
size between the mid-bulking period and skin set
(Sabba and Bussan, 2012). This makes the tuber
highly resistant to skinning injury. In fact, the
progress of periderm maturation can be moni-
tored easily by digging a sample of tubers and then
assessing how easily the skin “slips” when pres-
sure is applied with a sideways motion of the
thumb. Skin set is accomplished when the skin
no longer slips with applied pressure.
Skin set is influenced greatly by cultivar and
vine condition, and is also affected by soil mois-
ture and temperature. The rate of skin set varies
significantly among cultivars. This is due mainly
to genetic differences, but can also depend on
how cultivars respond to conditions in the field
(Lulai and Orr, 1993). It is generally recognized
that smooth-skinned cultivars set skin more slowly
than russet-skinned varieties, such as Russet Burbank.
Soil moisture and soil temperature affect the rate
of skin set, but cultural practices and growing
conditions often modify their effects in the field
(Lulai and Orr, 1993; Sabba and Bussan, 2012).
In general, a moderately dry soil, with
70-
75%
available soil moisture, is recommended to promote
skin set. Soil temperatures in the range of
21-
24°C
are ideal for skin set, but temperatures below
7°C or above 32°C will delay skin maturation.
Therefore, tubers that encounter cool, wet wea-
ther at the end of the season usually take longer
to mature properly. Periderm maturation usu-
ally begins in the field, but can and does con-
tinue after harvest, depending on temperature
and the humidity conditions in storage (Lulai
and Orr, 1993).
The process of skin set is initiated after
tuber growth ceases, usually at the time the
vines senesce in the field. Therefore, tubers
harvested when the vines are still green and
active are highly susceptible to skinning in-
jury. In some regions, immature tubers are
harvested while the vines are still green, but
these potatoes are usually marketed or pro-
cessed immediately. Depending on the cultivar
and soil conditions, tubers require
10-
21 days
after vines are killed, removed, or have sen-
esced completely for the skin to set sufficiently
to resist skinning damage at harvest. It should
also be noted, however, that delaying harvest
for many weeks can increase problems with
soilborne diseases, like silver scurf and Rhiz-
octonia black scurf, and can result in tubers
that are physiologically overmature.
A fertility management plan that allows
soil nitrogen levels to deplete towards the end of
the growing season encourages natural vine
senescence and time for proper skin set before
harvest. Short periods of freezing or near-freez-
ing temperatures that kill vines (killing frost) a
few weeks before harvest can also promote skin
set. However, it is not always practical to allow
vines to senesce naturally before harvest; there-
fore, many potato producers remove or kill vines
via mechanical defoliation or chemical desicca-
tion. Mechanical defoliation methods include
flaming, pulling, undercutting, rolling, flailing,
and beating vines. Chemical vine desiccation
products include sulfuric acid and non-selective
