Agriculture Reference
In-Depth Information
event between two parents, with progeny seed of
the unique hybridization being termed a “fam-
ily”. Plaisted (1993) reported that seed dried and
stored at room temperature in a cabinet contain-
ing a room dehumidifier germinated following
10
years of storage. Storage in sealed conditions
with a dessicant or in metalized polyester film
can further extend the viability of seed beyond
10
years. True potato seed has an average dor-
mancy of approximately 6 months; soaking for
24
h in 2000 ppm gibberellic acid (GA
3
) can be
used to overcome dormancy in those instances
when germination prior to 6 months is desired
(Lam and Erickson, 1966). For rapid generation
and cycling of true potato seed into seedlings,
Janksy
et al
. (2012) recommended the harvest of
fruits
3
weeks following initial pollination, with
an additional
2
weeks of seed maturation in the
harvested fruit. Freshly extracted seed from the
fruit can then be soaked in GA
3
and planted im-
mediately.
This reciprocity in the exchange of seedling
tubers among breeding programs bolsters the
numbers and diversity of seedling tubers evalu-
ated in potato production regions of the USA,
with obvious benefits in developing new potato
cultivars for the US potato industry. Harvested
seedling tubers are stored at cold temperatures
(
3-
5°C) over the winter months to preserve
tuber dormancy and quality until planting in
the spring. The production of seedling tubers is
an important transition point in preserving the
seedling genotypes generated via sexual repro-
duction. The diversity of genotypes generated
subsequently advance in the breeding program
as clonally propagated breeding clones.
Single-hill selection (1st field
generation)
Seedling tubers harvested from the greenhouse
in the previous year and stored under refriger-
ation are planted in the spring as single-hill selec-
tions. Single hill describes the unique genotype
derived from a planted seedling tuber, repre-
sented by one plant in the first field generation.
A spacing of 91 cm is used between seedling
tubers to prevent the intermixing of tubers
among single-hill clones at harvest, ensuring a
selection is “true-to-type” and is representative
of only one genotype. This is critical in that from
this point forward, a selected breeding clone will
be propagated asexually as it advances through
a breeding program.
Single-hill selections are made based on the
market class the family represents (e.g. process-
ing for fries or chips, specialties with unique skin
or flesh color), with emphasis at this early stage
on an acceptable visual appearance of tubers for
the corresponding market class. Love
et al
. (1997)
assessed
27
tuber traits involving skin, eyes, size,
shape, yield, and defects such as growth cracks
and second growth (knobs) that are commonly
used by breeders in visual selection in the field.
Twenty-four (88%) tuber traits were identified as
having sufficiently high heritabilities and con-
sistency in expression over years to validate their
effectiveness in visual selection. Only three traits
involving the uniformities of skin russeting,
tuber shape, and size were determined to be
sufficiently variable in their expression that the
authors recommended they not be applied in
Greenhouse seedling tubers
Following the generation of true potato seed, the
next step in the breeding process is the germin-
ation of the true potato seed to produce potato
seedlings and their associated tubers. True po-
tato seed representing a family of individuals is
planted in a small tray and allowed to germinate
(
Fig. 16.2e)
. Due to the tetraploid nature of po-
tato and its associated heterozygosity at loci,
each germinated seedling is a unique genotype
that differs from its parents and siblings. Ap-
proximately
2-3
weeks following planting as
true potato seed, potato seedlings begin emer-
ging, with another
2
weeks of growth prior to
their transplanting into individual cells or pots
(size varies among breeding programs), with
subsequent growth into mature plants. Seedling
tubers are harvested (
Fig. 16.2f)
approximately
2.5-3 months following transplanting. The lar-
gest tuber (A-size) of each seedling plant is re-
tained and bulked with the largest tubers of its
siblings in that family. Oftentimes, seedlings will
produce one to two additional tubers (termed
B- and C-size) that are smaller than the A-tubers
retained by the breeding programs. These small-
er seedling tubers are also saved and bulked
as families that historically have been ex-
changed among US public breeding programs.
