Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

404 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
Accumulated heat units, also known as day-degrees and degree-days, are calculated by
taking the average daily temperature from each day and subtracting the growing base temperature
(8 ◦ C) (Allen and Brien, 1986). The heat units for each day are then added over
time to provide accumulated heat units. This requires information on growing and harvest
conditions, handling procedures, and the storage environment (Mac Kerron, 1998).
The physiological age of the potato seed influences performance of the crop (Reust,
1986). Seed produced at low temperature, moisture or fertility stress, disease pressure, and
stored under fluctuating temperatures could result in physiologically old seed, which is
beneficial under short growing season conditions in terms of vigor. Environmental conditions
need to be considered when determining how to manage physiologically young
or old seed (vander Zaag and van Loon, 1987). Physiologically old seed usually sprouts
quicker than young seed. The emergence rate under cool weather conditions is quicker for
physiologically old seed compared to young seed. So if both types of seed are stored at
the same temperature, emergence of younger seed is slower. Early emergence of plants is
desirable when potential problems such as seed piece decay, Rhizoctonia stem canker, and
other soil-based pathogen infection (Bohl et al., 1995). Seed potato producers typically try
for higher yield with a smaller tuber profile, whereas commercial growers have different
tuber size and total yield expectations. Physiologically old seed with early emergence of
sprouts and higher sprout numbers is a better option for seed growers (Asiedu et al., 2003).
The other important aspect concerning physiologically old seed relates to seed spacing. In
order to decrease the number of stems per acre and to maintain plant vigor, physiologically
old seed requires increase seed spacing at planting (Kleinkopf and Barta, 1991).
19.5.3 Sprout-stimulating agents
Potatoes sprout as a result of stimulation from the environment such as light and temperature.
Often seed potatoes are treated with chemicals to hasten the dormancy break for sprout
growth. This is needed in regions where two potato crops are grown each year, when seed
tubers are exported to different climatic regions for planting, and when it is advantageous to
reduce dormancy period for planting. The chemicals commonly used for dormancy break
are ethylene, carbon disulfide (CS2), rindite, gibberellic acid (GA3), and thiourea. Sucrose
content is higher in tubers treated with GA3, CS2, and rindite as compared to thiourea
(Rehman et al., 2003). Breaking dormancy in thiourea-treated tubers is slower due to delay
in starch mobilization compared to other treatments
GA3 is applied by spraying or dipping tubers in a solution. GA3 is applied immediately
after harvest or after seed cutting for better absorption through the wounded areas. This
treatment increases the number of stems resulting in smaller tuber size (Kustiati et al.,
2005). Rindite is applied as a vapor prepared by mixing ethylene chlorohydrin, ethylene
dichloride, and carbon tetrachloride in 7:3:1 ratio. Treatment should be in a closed container
because this is also toxic substance. Tubers are exposed to vapors for 48 h. Decaying has been
reported when treated with rindite without wound healing for 14 days (Kim et al., 1999).
Rehman et al. (2003) compared different treatments for breaking dormancy in microtubers
and found that rindite treatment resulted in the highest sprout ratio in all cultivars tested and
tubers sprouted earlier (10 days) compared to other treatments. Decay has been reported in
tubers treated with thiourea (Rehman et al., 2003). Carbon disulfide was also tested for the
same, but its usage is minimal because but it is toxic.