Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

POSTHARVEST FACTORS AFFECTING POTATO QUALITY AND STORABILITY 403
eye using a luciferase reporter gene. Similarly, lipoxygenase expression increased both during
sprouting and tuberization (Ronning et al., 2003). Interestingly, when transgenic plants
expressing inorganic pyrophosphatase gene with tuber-specific promoter, tubers sprouted
6–7 weeks earlier than control plants (Farré et al., 2001). Authors hypothesized that increased
mobilization of starch to sucrose led to an accelerated sprouting phenotype. However,
Hajirezaei and Sonnewald (1999) reported that tubers from transgenic plants overexpressing
pyrophosphatase never sprouted. The reason suggested by the authors was a
complete shutdown of glycolysis due to the inhibition of pyrophosphate-dependent phosphofructokinase.
During the sprouting process, the mother tuber supplies energy to the growing sprout
by mobilization of starch reserves. Enzymes that are involved in starch mobilization are
upregulated. At the time of sprouting, amylase enzyme activity involved in starch break down
increases near the tuber eye tissue (Bailey et al., 1978; Biemelt et al., 2000). However, Davies
and Viola, 1988 reported a decrease in total amylase activity around the time of sprouting.
Thus, Biemelt et al. (2000) concluded that there is no clear-cut evidence for an increase
in starch-degrading enzymes around the time of tuber sprouting. Similar gene expression
patterns are also observed in the case of starch synthesis enzymes during the sprouting
process, contrary to the expectations (Claassens; Verhees, 2002; Ronning et al., 2003). The
starch biosynthetic enzyme (AGPase) and starch-degrading enzyme (amylase) are active
during both the sprouting and tuberization processes (Vreugdenhil, 2004). Vreugdenhil
(2004) suggested that the biochemical machinery for starch synthesis and breakdown is
present during all stages of development of the tuber, including sprouting, and is coordinately
up- or downregulated based on flux and metabolite concentration.
19.5.2 Physiological aging influences sprouting
Potatoes have a set period of dormancy before they sprout depending on the cultivar. The
precise timing for sprout initiation however depends on the age of the tuber. The physiological
age is different from chronological age. Physiological age has a greater impact on
sprouting and the number of stems per tuber, and it is the primary factor that determines
viability of tubers used for seed. Physiological age is a cumulative effect of biochemical
changes taking place within a tuber (Bohl et al., 1995). Factors that influence physiological
aging are growing conditions, storage conditions, and wounding. Growing conditions
such as low moisture, high temperatures, fertilizer, frost damage, and disease pressure may
all cause stress on the potato. Wounding and bruising during harvest and the cutting of
seed tubers before planting increases the respiration rate, which results the tuber stress and
causes aging of the tuber. Respiration levels remain low at low-temperature storages. Any
fluctuations from an ideal storage temperature may also rapidly age the tubers. However,
the major aging of seed tubers occurs during storage.
The physiological age of seed can be determined by leaving the tubers at room temperature
in dark conditions and by assaying the number of days required to sprout and number
of sprouts per tuber (van der Zaag and van Loon, 1987). There is no biochemical marker
available to determine physiological age. Knowles et al. (2003) suggested that 2-methyl
butanol can be used as a marker but needs to be tested further for its potential use. Heat
accumulation model measures aging by counting number of degree days or the number of
the heat units tubers might have exposed (Knowles and Botar, 1991; Jenkins et al., 1993).