Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

POSTHARVEST FACTORS AFFECTING POTATO QUALITY AND STORABILITY 401
is believed that the activity of AGPase in the starch synthesis is inhibited by this feedback
system (Sweetlove et al., 1999).
19.4.5 Uridine-5-diphosphoglucose pyrophorylase
Uridine-5-diphosphoglucose pyrophorylase (UGPase) catalyzes the first step in the formation
of sucrose. Hill et al. (1996) showed the correlation between the amount of UDP-glucose
and sucrose levels during the process of CIS. Antisense constructs using UGPase showed
a decrease in sucrose levels in potato tubers during cold storage (Spychalla et al., 1994;
Borovkov et al., 1996). This is a committed step in the CIS process, and UDP-glucose
limits the formation of sucrose in tubers (Sowokinos et al., 1997, 2000). By analyzing the
number of susceptible and resistant CIS clones, Sowokinos et al. (1997) found allelic polymorphism
at the UGPase locus. Two cDNA clones for UGPase differing in the BamHI site
have been cloned (Spychalla et al., 1994). The UGPase allele UgpB showed certain isozyme
of UGPase involvement in CIS process. Also, small differences in the cDNA sequence of
UGPases in potato have previously been identified and explained as resulting from allelic
polymorphism(Sowokinos et al., 1997).
Sowokinos (2001) suggested that cultivars that are less prone to CIS might have a
higher rate of respiration in cold storage compared to those that are prone to CIS. Barichell
et al. (1991) showed that the cold-resistant potato clone ND860-2 has a higher respiratory
rate when compared to the cold-susceptible cultivar Norchip in storage. When tubers were
subjected to low temperatures, the respiration rate in the tubers declines after the initial
burst (Isherwood, 1973). Sherman and Ewing (1983) attributed this initial respiratory burst
to cytochrome mediated and alternative oxidase-mediated pathways. At cold temperatures,
the cell walls lose their fluid character, which results in leakage from cellular membranes.
The plant uncoupling mitochondrial protein (PUMP) is strongly induced when tubers are
exposed to cold temperatures. The PUMP is also believed to reduce oxidative stress at
low temperatures (Nantes et al., 1999). Gounaris and Sowokinos (1992) isolated and tested
mitochondria from tubers that were resistant and susceptible to CIS. Tubers that accumulated
higher amounts of reducing sugar tend to have mitochondria with lower-buoyant density
when compared with resistant cultivars. It is speculated that this phenomenon is due to
alterations in the permeability of the inner mitochondrial membrane.
CIS in tubers can be reversed by reconditioning tubers at elevated storage temperatures
(Pritchard and Adam, 1994; Edwards et al., 2002). During the reconditioning period, the
reducing sugar (glucose and fructose) concentration decreases in tubers and 80% of the
reducing sugars are converted back to starch. The remaining 20% are lost through respiration.
Response to reconditioning is a cultivar-dependent process, and some cultivars
respond better than others. The other option suggested by Pritchard and Adam (1992) is the
preconditioning of tubers at 15 ◦ C to limit the increase in reducing sugars during subsequent
storage. Storage at temperatures of 20 ◦ C or above also leads to an increase in sugar levels
due to increase in respiration and other biological processes (Linnemann et al., 1985).
Conditions such as elevated temperature or low temperature can cause sugar accumulation
in tubers. The sugar content rose slightly at 16 ◦ C, whereas reduction in sugar content was
observed at 7 ◦ C. Potato tubers need to be stored at an ideal temperature range, specific for
individual cultivars where equilibrium is achieved and the net production of free sugars is
at its minimum (Kumar et al., 2004).