Principles of Plant Genetics and Breeding. George Acquaah

They are produced from specialized diploid cells called microspore mother cells in anthers and megaspore mother cells in the ovary (Figure 5.4). Microspores derived from the mother cells are haploid cells each dividing by mitosis to produce an immature male gametophyte (pollen grain). Most pollen is shed in the two‐cell stage, even though sometimes, as in grasses, one of the cells later divides again to produce two sperm cells. In the ovule, four megaspores are similarly produced by meiosis. The nucleus of the functional megaspore divides three times by mitosis to produce eight nuclei, one of which eventually becomes the egg. The female gametophyte is the seven‐celled, eight‐nucleate structure. This structure is also called the embryo sac. Two free nuclei remain in the sac. These are called polar nuclei because they originate from opposite ends of the embryo sac.

Figure 5.4 Gametogenesis in plants results in the production of pollen and egg cells. Pollen is transported by agents to the stigma of the female flower, from which it travels to the egg cell to unite with it.

      5.4.7 Pollination and fertilization

Pollination is the transfer of pollen grains from the anther to the stigma of a flower. This transfer is achieved through a vector or pollination agent. The common pollination vectors are wind, insect, mammals, and birds. Flowers have certain features that suit the various pollination mechanisms (Table 5.1). Insect‐pollinated flowers tend to be showy and exude strong fragrances. 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.

Table 5.1 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; 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, flowers produce copious, thick nectar
Birds	Bright colors (red, yellow); odorless; thick copious nectar

      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.

5.5 What is autogamy?

      Self‐pollination or autogamy occurs in a wide variety of plant species – vegetables (lettuce, tomatoes, snap beans, endive), legumes (soybean, peas, lima beans), and grasses (barley, wheat, oats). Certain natural mechanisms promote or ensure self‐pollination, specifically cleistogamy and chasmogamy, while other mechanisms prevent self‐pollination (e.g. self‐incompatibility, male sterility).

      5.5.1 Mechanisms that promote autogamy

      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 self‐pollination. 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 5.2.

Table 5.2 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	Letuca sativa
Oat	Avena sativa
Pea	Pisum sativum
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