Principles of Plant Genetics and Breeding

(berries) called potato balls. The underground commercial
part is a modified stem (or tuber) that is borne at
the end of a stolon. The “eyes” on the tuber are actually
rudimentary leaf scars favored by lateral branches. Each
eye contains at least three buds protected by scales.
When potatoes sprout, the sprouts are lateral branches
with several buds. A section across a tuber reveals a
pithy central core with branches leading to each of
the eyes.
Reproductive biology
Floral biology
Potato has a terminal inflorescence consisting of 1–30
(but usually 7–15) flowers, depending on the cultivar.
The five petals give an open flower a star shape. A flower
also has a stigma that protrudes above a cluster of five
large, bright yellow anthers. The corolla color varies
from white to a complex range of blue, red, and purple.
Flowers open, starting with those nearest the base of the
inflorescence and proceeding upwards, at the rate of
about 2–3 each day. At the peak bloom, there are usually
5–10 open flowers. Flowers stay open for only 2–4 days,
and the receptivity of the stigma and duration of pollen
production is about 2 days.
Pollination
Potato is predominantly self-pollinated. The peak time
of pollination is early morning. Pollen can be collected
ahead of the time of use, and kept in a cool dry place
(e.g., in a desiccator for longevity). Pollination is most
successful when temperatures are not high. Some breeders
collect the desirable flowers to be used a day ahead
and lay them out to dry. The pollen is then shaken out
over a sieve. The pollen is collected in tubes for use.
Common breeding methods
Potato has a wide array of wild germplasm that easily
crosses with cultivated types. Hybridization is the principal
procedure for gene transfer. Selection is conducted
in the F 1 , because the parents are also widely used in
modern potato improvement. Protoplast fusion techniques
may be used to fuse monoploid (1x) to form
dihaploids (2x). A cross of 4x × 2x using a particular
accession of S. phureja as the male, is a technique for
generating haploids at a high frequency. Hybridization
BREEDING POTATO 543
can be used to increase the frequency of tetra-allelic loci
and thereby to increase the intra- and interlocus interactions
for increased vigor. The techniques of unilateral
sexual polyploidization (4x × 2x) or bilateral sexual
polyploidization (2x × 2x) may be used in potato breeding.
They are practical procedures because many diploid
potatoes hybridize among themselves or with tetraploid
species.
Genetic engineering procedures have been used to
achieve the development of Bt resistance to Colorado
potato beetle and viral coat protein-based resistance to
several viral disease (e.g., the potato leaf roll virus –
PLRV). The cloning and use of the AGPase gene has
enabled cultivars with high solids to be developed.
Establishing a breeding nursery
Field nursery
Breeders commonly cross potatoes in the greenhouse
because pollination in the field does not yield good seed.
Instead, some breeders produce the pollen source in the
field and cut stems containing large inflorescences for
use in the greenhouse. Parents for pollen sources should
be free from virus infection and should be properly managed,
with protection against insect pests, and fertilizing
and irrigating the plants for healthy flowers to be
produced.
Greenhouse nursery
Potato is a long-day plant. Hence, at least 16 hours of
sunlight (or 20 klux of artificial light) is needed to successfully
grow potato to flowering and maturity. The
greenhouse temperature should be maintained at about
19°C. Seed potato or stem cuttings can be used for
planting. Plants may be raised in a ground bed or in
pots placed on raised benches. Crossing is often done in
the winter. The vegetative growth is controlled by pruning
and staking the plants to make the flowers more
accessible.
Artificial pollination
Materials and equipment
A mechanical vibrator can be used to aid pollen collection,
which is then used to pollinate a large number of
plants. Pollen that is shed is collected into test tubes. A