Principles of Plant Genetics and Breeding

364 CHAPTER 19
to four-dwarfs. It is also known that the manifestation of
the dwarf trait is influenced by modifying genes such
that cultivars with an identical set of dwarfing genes vary
in plant stature.
Plants that have dwarfing genes are unable to respond
to gibberellin or gibberellic acid, the plant hormone that
promotes stem elongation. Using transposon tagging,
scientists have cloned a gene, GAI, in Arabidospsis that
is responsible for dwarfism in that species. DNA sequence
analysis of the gene followed by bioinformatics
has revealed that other species have equivalent genes.
This indicates the possibility of using rDNA technology
to transfer the GAI gene into other species.
Breeding determinacy
Another way in which plant form is manipulated is
through determinacy.
Nature, types, and economic importance
Plant growth form may be indeterminate or determinate.
Some species such as corn and wheat are determinate
in growth form, whereas soybean has both determinate
and indeterminate types. In indeterminate cultivars,
new leaves continue to be initiated even after flowering
has begun. Flowering occurs in axillary racemes. On the
other hand, determinate cultivars or species do not initiate
new leaves after flowering has begun. Also, flowering
occurs in both axillary and terminal racemes.
Generally, determinate plant types tend to have stiffer
and shorter stems (called bush types). In soybean, both
determinate and indeterminate commercial cultivars are
used in production. Determinate cultivars are used in
areas with long growing seasons (i.e., southern latitudes),
while indeterminate cultivars are grown in areas with
short growing season (i.e., northern latitudes).
Genetics and germplasm resources
Determinacy in soybean is conditioned by a recessive
gene, designated dt1 . This gene has been used to breed
determinate cultivars for the northern growing regions.
Semideterminacy is conditioned by the Dt2 allele, while
the S allele conditions short plant internodes.
Photoperiod response
Plants exhibit environmentally determined developmental
switches from the initiation of leaves (vegetative
phase) to flowering (reproductive phase). The two developmental
switches that plant breeders pay attention to
are photoperiod and vernalization. The key environmental
factors are temperature and day length. In some
plants, flowering is not promoted by temperature and
day length but occurs regardless of the conditions (called
facultative plants), whereas in others, flowering will not
occur without the appropriate temperature–day length
combination (called obligate plants). Photoperiodism is
a photomorphogenic responsive to day length (actually,
plants track or measure the duration of the dark period
rather than the duration of day light). Three categories
of responses are known.
1 Long-day (short-night) plants. These plants require
a light period longer than a certain critical length in
order to flower. They will flower under continuous
light. Cool season species (e.g., wheat, barley, alfalfa,
sugar beet) are examples of long-day plants.
2 Short-day (long-night) plants. These plants will
not flower under continuous light, requiring a
photoperiod of less than a certain critical value in
a 24-hour daily cycle. Examples include corn, rice,
soybean, peanut, and sugarcane.
3 Day-neutral (photoperiod-insensitive) plants.
Photoperiod-insensitive plants will flower regardless
of duration of day length. This trait is very desirable,
enabling producers to grow the crops in a broad range
of latitudes. Examples include tomato, cucumber,
cotton, and sunflower.
Plant breeders need plants to flower at the appropriate
time for hybridization in a breeding program, and
also to influence the cultivars they develop for different
growing areas. Photoperiod influences the duration of
the vegetative phase versus reproductive phase and
hence crop yield at different latitudes. Soybean, for
example, has 13 recognized adaptation zones, ranging
from 000 in the northern latitudes to IX in the southern
latitudes. Day length increases as one goes north in
summer in the northern hemisphere. Consequently,
a cultivar developed for the southern latitudes may not
be as productive in the northern latitudes where reproductive
growth is not initiated until the fall season when
day length is short.
Vernalization is the process by which floral induction
in some plants is promoted by exposing plants to chilling
temperatures for a certain period of time. Plant
breeders of crops such as wheat either sow in the fall so
the plant goes through a natural vernalization in the
winter, or they place trays of seedlings in a cold chamber
for the same purpose, prior to transplanting.