Principles of Plant Genetics and Breeding

60 CHAPTER 4
Table 4.1 Pollination mechanisms in plants.
Pollination vector Flower characteristics
Wind Tiny flowers (e.g., grasses); dioecious
species
Insects
Bees Bright and showy (blue, yellow); sweet
scent; unique patterns; corolla provides
landing pad for bees
Moths White or pale color for visibility at night
time; strong penetrating odor emitted
after sunset
Beetles White or dull color; large flowers;
solitary or inflorescence
Flies Dull or brownish color
Butterflies Bright colors (often orange, red); nectar
located at base of long slender corolla
tube
Bats Large flower with strong fruity pedicels;
dull or pale colors; strong fruity or musty
scents; thick copious nectar
Birds Bright colors (red, yellow); odorless;
thick copious nectar
through a vector or pollination agent. The common
pollination vectors are wind, insects, mammals, and
birds. Flowers have certain features that suit the various
pollination mechanisms (Table 4.1): insect-pollinated
flowers tend to be showy and exude strong fragrances,
whereas birds are attracted to red and yellow flowers.
When compatible pollen falls on a receptive stigma, a
pollen tube grows down the style to the micropylar end
of the embryo sac, carrying two sperms or male gametes.
The tube penetrates the sac through the micropyle. One
of the sperms unites with the egg cell, a process called
fertilization. The other sperm cell unites with the two
polar nuclei (called triple fusion). The simultaneous
occurrence of two fusion events in the embryo sac is
called double fertilization.
On the basis of pollination mechanisms, plants may
be grouped into two mating systems: self-pollinated
or cross-pollinated. Self-pollinated species accept
pollen primarily from the anthers of the same flower
(autogamy). The flowers, of necessity, must be bisexual.
Cross-pollinated species accept pollen from different
sources. In actuality, species express varying degrees of
cross-pollination, ranging from lack of cross-pollination
to complete cross-pollination.
Table 4.2 Examples of predominantly self-pollinated
species.
Common name Scientific name
Barley Hordeum vulgare
Chickpea Cicer arietinum
Clover Trifolium spp.
Common bean Phaseolus vulgaris
Cotton Gossypium spp.
Cowpea Vigna unguiculata
Eggplant Solanum melongena
Flax Linum usitatissimum
Jute Corchorus espularis
Lettuce Lupinus spp.
Oat Avena sativa
Pea Pisum sativum
Peach Prunus persica
Peanut Arachis hypogeae
Rice Oryza sativa
Sorghum Sorghum bicolor
Soybean Glycine max
Tobacco Nicotiana tabacum
Tomato Lycopersicon esculentum
Wheat Triticum aestivum
Self-pollination
Mechanisms that promote self-pollination
Certain natural mechanisms promote or ensure selfpollination.
Cleistogamy is the condition in which the
flower fails to open. The term is sometimes extended to
mean a condition in which the flower opens only after
it has been pollinated (as occurs in wheat, barley, and
lettuce), a condition called chasmogamy. Some floral
structures such as those found in legumes, favor selfpollination.
Sometimes, the stigma of the flower is closely
surrounded by anthers, making it prone to selfing.
Very few species are completely self-pollinated. The
level of self-pollination is affected by factors including
the nature and amount of insect pollination, air current,
and temperature. In certain species, pollen may become
sterilized when the temperature dips below freezing.
Any flower that opens prior to self-pollination is susceptible
to some cross-pollination. A list of predominantly
self-pollinated species in presented is Table 4.2.
Genetic and breeding implications of self-pollination
Self-pollination is considered the highest degree of
inbreeding a plant can achieve. It promotes homozygosity