Agriculture Reference
In-Depth Information
sucrose accumulation in low-invertase tubers was
comparable to that in wild-type tubers. In cultivars
lacking A-II isoenzymes, such as MegaChip, su-
crose accumulation was much greater in the
transgenic lines than in wild type (Wu
et al
., 2011).
A proteinaceous invertase inhibitor has
also been localized to the vacuole in mature po-
tato tubers, and changes in its expression and
activity contribute to differences between culti-
vars in sensitivity to low-temperature storage
(Liu
et al
., 2010, 2011; Brummell
et al
., 2011).
When RNA interference was used to decrease
the activity of sucrose-phosphate phosphatase,
invertase activity was inhibited as well. It was
proposed that sucrose-
6-
phosphate might play a
role in regulating the expression of vacuolar
acid invertase (Chen
et al
., 2008).
It is important to note that the glucose and
fructose formed in the vacuole as a result of in-
vertase activity are not part of the cytosolic pool
of free sugars and sugar phosphates. As such,
they are not immediately available for renewed
starch synthesis or for respiration in mitochon-
dria. It is likely that this partitioning of reducing
sugars into a discrete subcellular location is one
reason that removing them through respiratory
activity is slow or ineffective, such as in the case
of sugar-end defect and senescence sweetening.
Sucrose synthase (SuSy) can also metabol-
ize cytosolic sucrose, and this reaction produces
fructose and UDP-glucose. Sucrose synthase ac-
tivity is very high during potato tuber bulking
and is an important determinant of the ability of
developing tubers to attract assimilates (sink
strength). In mature tubers, however, only a low
level of activity is detectable. Neutral invertase
(NInv) can cleave cytosolic sucrose to glucose
and fructose, but little is known about the activ-
ity or regulation of this enzyme in mature tubers.
Cytosolic glucose and fructose are readily phos-
phorylated by hexokinase (HK) and fructokinase
(FK), respectively, to produce G-
6-
P and F-
6-
P.
Sucrose in the apoplastic space can be cleaved by
cell wall resident acid invertase (AI) (Li
et al
.,
2005), which also affects tuber quality traits.
below established limits for acceptable process-
ing quality.
Sugar management of potatoes occurs dur-
ing three important phases of storage: precondi-
tioning, cooling to set point, and monitoring
during long-term storage. During the precondi-
tioning phase, tubers are stored at relatively
warm temperatures (
12-
13°C) to allow excess
sugars to dissipate through respiration or starch
synthesis and to allow tubers with harvest-
inflicted surface wounds to heal. Successful pre-
conditioning minimizes the adverse effects of
non-optimal irrigation, under- or over-maturity
of tubers at harvest, vine desiccant application,
mechanical disturbance, and damage during
harvest, transport, and loading into storage.
Tubers that are not preconditioned adequately
prior to cooling often exhibit a larger increase in
tuber reducing sugars as temperatures are dropped
compared with conditioned tubers (Pritchard
and Adam, 1992).
Changes in reducing sugar often lag behind
changes in sucrose; therefore, tuber sucrose
levels are typically used to guide management
decisions during preconditioning. Specific tar-
gets for stored tuber sugars vary by cultivar and
are often reported; however, general guidelines
also exist. For chip potatoes, where sugar man-
agement is critical, sucrose should be less than
0.7 mg g
-
1
fresh weight (FW) and glucose should
be less than 0.035 mg g
-
1
FW at the end of pre-
conditioning. For fry processing potatoes, a wider
range of sugars may be acceptable, depending
on cultivar. Sucrose contents of
1-2
mg g
-
1
FW,
stable glucose contents, and acceptable fry color
may be used as a starting point for fry processing
cultivars that have little or no storage data. De-
pending on the growing season and cultivar, it
may take several weeks, or even months, before
tubers are preconditioned properly. The advan-
tages of preconditioning need to be balanced
against the potential disadvantages in terms of in-
creased risk from disease and decreased time
until the onset of senescence sweetening.
The second stage of sugar management
occurs as tubers are cooled to final temperature
set points. During this phase, storage managers
try to prevent the development of CIS by slowly
decreasing the temperature of the storage. Stor-
age temperature is typically reduced between
0.12 and 0.24°C day
-
1
(temperature ramping),
and as such, it may take several weeks to cool
processing and chip potatoes to their holding
Sugar management of stored potatoes
Sugar management is an essential component
of storage management for fry and chip pro-
cessing potatoes. The goal of sugar management
is to keep tuber reducing sugar concentrations
