Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

402 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
19.5 Sprouting
Regulating dormancy and sprouting is an important aspect of potato crop management
because the tuber also represents the starting material for the next generation of plants (seed
tubers). Potato seed storage management is different from the regular storage. Seed tubers
can be stored at low temperatures unlike tubers meant for processing. Sprouting is one of
the main causes for loss in stored potatoes. Sprouted tubers are not marketable and tend to
lose water by evaporation, which results in loss of weight.
Potato-processing plants require high-quality tubers for year-round operations. To ensure
constant supply from storage, tuber dormancy needs to be extended beyond the winter
months. During an extended storage period, tubers age physiologically, break dormancy,
and sprout. Major factors for deterioration in the processing quality of tubers in storage are
weight loss due to respiration; water and turgor loss due to sprouting; and an increase in
reducing sugar concentration due to starch conversion.
19.5.1 Mechanism of sprouting
The suppression of bud growth in dormant potato tubers is believed to be under hormonal
regulation (Suttle, 1996). The major plant hormones playing an active roles in determining
bud dormancy are gibberellins (Suttle, 1996), cytokinins (Suttle and Banowetz, 2000), abscisic
acid (Suttle, 1995), and indoleacetic acid (Sorce et al., 2000). Before the onset of dormancy,
ABA levels are higher and gradually decrease as dormancy progresses. It is believed
that ABA levels act as a potential signal for breaking dormancy. It has been proposed that
initiation of sprouting occurs when ABA levels fall below a threshold value. So far no such
threshold value for ABA to maintain tuber dormancy has been found (Destefano-Beltrán
et al., 2006a, b). In contrast to earlier reports, Sorce et al. (1996) found that ABA concentration
in sprouting eyes goes up during the release from dormancy. Destefano-Beltrán
et al. (2006a) analyzed the expression pattern of both ABA biosynthetic and degradative
pathway genes during the process of dormancy and sprouting. Destefano-Beltrán et al.
(2006a) hypothesized that ABA levels are maintained in a tuber with respect to its physiological
age by activation of biosynthetic and catabolic pathways. Claassens et al. (2005)
showed that ethanol and other primary alcohols can break dormancy of apical bud tissue,
and this action can be inhibited by alcohol dehydrogenase inhibitor. ABA could reverse the
effect of ethanol, suggesting that ethanol may lower endogenous ABA levels, by promoting
sprouting.
Recently, there are a few reports in the literature comparing gene expression during
sprouting and tuber formation (Claassens, 2002; Verhees, 2002; Ronning et al., 2003).
Ronning et al. (2003) compared potato-EST libraries from various tuber developmental
stages. Some of these genes that are upregulated during the sprouting process are involved
in offering protection against oxidative stress in plants such as putative glutathione transferase
and glutathione-dependent dehydroascorbate reductase. These enzymes are involved
in glutathione metabolism and turnover. However, Rojas-Beltran et al. (2000) found relatively
little change in expression of antioxidant genes during sprouting. Upregulation of
two starch biosynthetic genes (AGPase and granule-bound starch synthase (GBSS)) during
sprouting was found (Claassens; Verhees, 2002; Ronning et al., 2003). Verhees et al. (2002)
showed that expression of cell cycle–related genes and AGPase increased specifically in the