Principles of Plant Genetics and Breeding

Zygote
influenced by the duration of the plant growth cycle.
Angiosperms (flowering plants) may be classified into
four categories based on the duration of their growth
cycle as follows (Figure 4.2).
Annuals
Annual plants (or annuals) complete their life cycle in
one growing season. Examples of such plants include
corn, wheat, and sorghum. Annuals may be further
categorized into winter annuals or summer annuals.
Winter annuals (e.g., wheat) utilize parts of two seasons.
They are planted in fall and undergo a critical physiological
inductive change called vernalization that is required
for flowering and fruiting in spring. In cultivation, certain
non-annuals (e.g., cotton) are produced as though they
were annuals.
Biennials
A biennial completes its life cycle in two growing seasons.
In the first season, it produces basal roots and
leaves; then it grows a stem, produces flowers and fruits,
and dies in the second season. The plant usually requires
a special environmental condition or treatment (e.g.,
vernalization) to be induced to enter the reproductive
phase. For example, sugar beet grows vegetatively in the
first season. In winter, it becomes vernalized and starts
reproductive growth in spring.
PLANT REPRODUCTIVE SYSTEMS 57
Multicellular
sporophyte
generation
Diploid (2n)
Fertilization Meiosis
Gametes
Haploid (n)
Multicellular
gametophyte
generation
Spores
Key activities:
– seed development
– germination
– seedling establishment
– early plant growth
– flowering
Key activities:
– pollen development
– pollen shedding
– pollen germination
and tube development
Figure 4.1 Schematic representation of the alternation of generations in flowering plants. The sporophyte generation is
diploid, and often the more conspicuous phase of the plant life cycle. The gametophyte is haploid.
Perennials
Perennials are plants that have the ability to repeat their
life cycles indefinitely by circumventing the death stage.
They may be herbaceous, as in species with underground
vegetative structures called rhizomes (e.g.,
indiangrass) or aboveground structures called stolons
(e.g., buffalograss). They may also be woody as in
shrubs, vines (grape), and trees (orange).
Monocarps
Monocarps are characterized by repeated, long vegetative
cycles that may go on for many years without entering
the reproductive phase. Once flowering occurs, the plant
dies. Common examples are bromeliads. The top part
dies, so that new plants arise from the root system of the
old plant.
It should be pointed out that certain plants that may
be natural biennials or perennials are cultivated by producers
as annuals. For example, sugar beet, a biennial, is
commercially produced as an annual for its roots. For
breeding purposes it is allowed to bolt to produce
flowers for crossing, and subsequently to produce seed.
Structure of the flower
Genetic manipulation of flowering plants by conventional
tools is accomplished by the technique of crossing,