Agriculture Reference
In-Depth Information
(Turnbull and Hanke, 1985; Suttle, 2002). The
growth inhibitors that promote dormancy may
also be inhibiting the effect of cytokinins during
the early phase of rest (Hemberg, 1985).
Another growth-stimulating substance,
gibberellin (GA), is thought to have a role in
regulating either tuber dormancy and/or sprout
growth immediately after dormancy break (Sut-
tle, 2004b). GA is found at low concentrations in
dormant tubers, but increases during early
sprout growth. It has been shown that endogen-
ous GA levels do not begin to increase until after
the end of dormancy, suggesting GA is not a
regulator of dormancy per se, but is important
for regulating sprout growth immediately after
dormancy break (Suttle, 2004b). Seed potatoes
are sometimes treated with GA to accelerate
sprout growth, and it is known that the concen-
tration needed to accelerate tuber sprouting in
the early stages of dormancy is greater than dur-
ing the latter stages. Seed certification agencies
commonly treat seed potatoes with GA to pro-
mote early sprouting for disease assessments in
the off season. Much remains to be learned
about the synthesis of plant growth regulators
and the significance of the interaction between
different plant growth regulators during the rest
phase of tuber dormancy.
The second phase of tuber dormancy, some-
times referred to as the quiescent phase, depends
on the environment surrounding a tuber. In this
case, sprouting is prevented by factors outside
the tuber. Temperature is critical. Tuber sprout-
ing is greatly suppressed below 4°C. This is one
of the reasons why potatoes are often stored at
cool temperatures. Generally, the warmer the
storage temperature, the sooner the tubers will
sprout. For instance, Russet Burbank tubers stored
at 8.9°C were found to sprout approximately 135 days
after harvest, compared to 155 days at 7.2°C and
175 days at 5.6°C (Brandt
et al
., 2003).
Seed potatoes for commercial production
are usually stored for
6-
7 months at
3-
4°C in
temperate zones and are later warmed for
7-
10 days
before handling, to promote sprouting and min-
imize bruising. As a general rule, seed tubers
warmed (>7°C) prior to planting emerge faster
from the soil than those not warmed. Most potato
varieties will likely tolerate storage temperat-
ures down to 2°C; however, storage below 2°C
is not recommended (Burton
et al
., 1992). After
planting, low soil temperatures may hinder
sprout growth. In general, it is recommended that
soil temperatures exceed 7°C before planting.
The length of the dormant period varies
among cultivars; therefore, some cultivars may
require unique approaches to sprout manage-
ment. For example, cultivars with longer dor-
mancy may require a longer period of warming
before planting as seed. Short-dormancy culti-
vars being stored for consumption may require
earlier applications of chemical sprout inhibi-
tors. When tubers were stored at 5.6°C in Idaho,
USA, cultivar dormancy ranged widely; 200 days
for Summit Russet, 175 days for Russet Burbank,
145 days for Umatilla Russet, and 100 days for
Alturas (Brandt
et al
., 2003, 2004, 2006).
5.3
Physiological Age of Tubers
The physiological age of tubers has a significant
impact on dormancy and sprouting. Physio-
logical age accounts for time and environmental
effects. In-season stress and development, plant
maturity at harvest, postharvest stress and stor-
age temperature, humidity, and time all influence
the physiological age of a tuber. Physiological
age is an important consideration for seed pota-
toes and is often characterized as the sprouting
potential of seed. Sprouts of seed tubers with ad-
vanced physiological age typically emerge faster
than those from younger seed. Apical domin-
ance, the inhibition of axillary buds by the dom-
inant, actively growing apical bud (Michener, 1942),
decreases with advancing physiological age,
which results in more stems per plant, increased
tuber set, and shifts in tuber size distribution to-
ward smaller tubers. In fact, the degree of apical
dominance from seed tubers is diagnostic of their
physiological age. According to Krijthe (1962),
the sequence of sprouting from physiologically
young to physiologically old tubers is: (i) single
sprout stage; (ii) multiple sprout stage; (iii) mul-
tiple and branching sprout stage; and (iv) small
tuber formation stage.
Generally, any stresses encountered during
growth, handling, and/or storage can accelerate
the aging process in tubers, though research has
demonstrated that growing conditions have a
smaller effect than postharvest conditions (Knowles
and Knowles, 2006). Physiologically old tubers
often have higher rates of respiration (Kumar
and Knowles, 1996a,b; Zabrouskov
et al
., 2002).
