Pollination, Types, Agents, Pollen Pistil Interaction

Pollination: Male and female gametes in flowering plants develop in the pollen grain and embryo sac. Because both types of gametes are immotile, they must be brought together before fertilisation can occur, which is accomplished through a process known as pollination. Pollination is the transfer of pollen grains shed from the anther to the stigma of the pistil.

Flowering plants have evolved many adaptations to ensure effective pollination. Pollination is classified into three types based on the pollen source. This article delves into the important topic of pollination in Class 12, specifically sexual reproduction in flowering plants , including pollen-pistil interactions and techniques such as artificial hybridisation.

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What is Pollination?

Pollination is the transfer of pollen from a plant's male reproductive organs to its female reproductive organs, which are usually the stigma of a flower. Pollen, which contains the genetic information required for plant reproduction, can be transferred within the same flower (self-pollination) or between flowers of the same species (cross pollination). This process is necessary for the production of seeds, which are required for the survival of plant species. Pollen can be spread through a variety of means, including water, wind, and animal pollinators like bats, insects, and birds.

Types of Pollination

Pollination is necessary for all flowering plants to reproduce. Pollination is classified into two types: self-pollination and cross-pollination. Self-pollination occurs when a plant has both male and female reproductive organs and pollinates its own flowers or plants. Cross-pollination occurs when an external agent transfers pollen from the male to the female reproductive organs, either between different plants or between flowers of the same plant. Cross-pollination increases genetic diversity because the offspring inherit traits from two different parent plants in the same species.

Pre Fertilisation - Structures And Events

Self Pollination

When pollen moves from the anthers of a flower to the stigma of the same flower, it's called self-pollination. This pollination is common in plants with both male and female parts in the same flower.

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Self-pollinating plants rely less on external factors for pollination, often using wind or small insects. The anthers and stigma are typically similar in length to aid pollen transfer in these flowers. Self-pollination can be of two types:

1. Autogamy: Pollen is transferred from the anthers to the stigma of the same flower.
2. Geitonogamy: Pollen is transferred from the anthers of one flower to the stigma of another flower on the same plant.

Sexual Reproduction in Flowering Plants MCQ

Advantages of self-pollination:

• Maintains genetic purity.
• Less reliance on external factors for pollination.
• Eliminates recessive characters.

Disadvantages of self-pollination:

• Lack of genetic diversity.
• Reduced vigor and vitality.
• Decreased immunity to diseases.

Pollen Grain

Cross Pollination

Cross pollination occurs when pollen is transferred from one flower's anther to another flower's stigma, usually between genetically different flowers. Birds, animals, water, wind, or insects are all examples of external pollinators. Depending on the agent, cross pollination can be classified as follows.

Hydrophilous Flowers: Pollinated by water, with small, inconspicuous flowers designed to hold pollen in place.

Zoophilous Flowers: Pollinated by animals, with pollen designed to stick to animal bodies for transfer.

Anemophilous Flowers: Pollinated by wind, with light, non-sticky pollen grains easily carried by the wind.

Entomophilic Flowers: Pollinated by insects, often with attractive features like bright petals and fragrance to attract insects. Pollen grains may be spiny or have extensions to stick to insects' bodies.

Ornithophilous Flowers: Pollinated by birds, a rare form of pollination.

Advantages of Cross Pollination

• Introduces new genes, improving genetic diversity.
• Enhances resistance to diseases and environmental changes.
• Produces seeds with high vigor and vitality.
• Eliminates recessive characters through genetic recombination.
• Essential for reproduction in unisexual plants.

Female Gametophyte

Disadvantages of Cross Pollination

• High wastage of pollen grains.
• Risk of losing desirable traits and gaining undesirable ones through genetic recombination.

Agents of Pollination

Pollination is an important step in the reproduction of flowering plants. The transfer of pollen grains from the anther (male part) to the stigma (female part) of the same or another flower results in fertilisation.  Plants use a variety of pollination agents, both biotic (living) and abiotic (non-living).

• Biotic agents include animals such as bees, butterflies, birds, and bats.
• Abiotic agents include wind and water.

Pollination by Wind (Anemophily)

Wind pollination is more common in plants that use abiotic agents for pollination.

Characteristics of wind-pollinated plants:

• Pollen grains are light and non-sticky, allowing them to be carried by wind currents.
• Flowers often have well-exposed stamens for easy dispersal of pollen.
• Stigma is large and often feathery to trap airborne pollen grains.
• Examples include grasses and plants like corn, where tassels act as stigmas to trap pollen grains.

Pollination by Water (Hydrophily)

Pollination by water is rare in flowering plants and is limited to certain genera, mostly monocots. Plants like Vallisneria, Hydrilla, and marine sea-grasses use water for pollination. Water is a regular means of transporting male gametes in lower plants like bryophytes and pteridophytes.

Adaptations for Water Pollination

The following are the plants that use water as an agent for pollination:

• Vallisneria: Female flowers reach the water surface via long stalks, and male flowers release pollen grains onto the water surface.
• Sea grasses : Female flowers remain submerged, and pollen grains are released inside the water, carried passively by currents.

Pollination by Animals

Animals serve as pollinators for the vast majority of flowering plants.  Common animal pollinators include bees, butterflies, flies, beetles, birds, and bats. Insects, particularly bees, are the primary biotic pollinators.

Characteristics of Insect Pollinated Flowers

• Insect-pollinated flowers are large, colorful, and fragrant, with abundant nectar.
• Flowers pollinated by flies and beetles may secrete foul odors to attract these animals.
• Pollen grains in insect-pollinated flowers are sticky to adhere to the bodies of insects.
• Flowers provide rewards like nectar and pollen grains to sustain animal visits.
• Animals that visit flowers come in contact with anthers and stigma, facilitating pollination.

Pollination is a complex process involving various agents and adaptations in plants. Understanding pollination mechanisms is crucial for the conservation and propagation of plant species.

Outbreeding Devices

Hermaphrodite flowers can undergo self-pollination, a process where pollen from the same flower fertilizes its ovules. However, continued self-pollination can lead to a phenomenon known as inbreeding depression. Inbreeding depression reduces the biological fitness of offspring produced by closely related individuals, leading to lower survival rates and fertility.

To avoid the negative effects of self-pollination, flowering plants have evolved various mechanisms to encourage cross-pollination, where pollen is transferred between different flowers. These mechanisms, known as outbreeding devices, include:

1. Dicliny (Unisexuality) Unisexual Flowers

Monoecious: These plants have both male and female flowers on the same plant, such as castor and maize. While this arrangement prevents self-pollination (autogamy), it does not prevent pollination between flowers of the same plant (geitonogamy).

Dioecious: In some species, male and female flowers are present on separate plants, meaning each plant is either male or female (dioecy). This condition prevents both autogamy and geitonogamy.

2. Dichogamy

In many bisexual flowers, the stamens (male reproductive organs) and carpels (female reproductive organs) do not mature at the same time. This temporal separation ensures that the pollen is released before the stigma (part of the carpel that receives pollen) becomes receptive, or vice versa, preventing autogamy.

3. Heterostyly

In certain species, the anthers (part of the stamen that produces pollen) and stigma are positioned at different heights within the flower, preventing self-pollination by ensuring that pollen cannot reach the stigma of the same flower.

4. Self-incompatibility

This genetic mechanism prevents self-pollen (either from the same flower or other flowers of the same plant) from fertilizing the ovules. It does so by inhibiting pollen germination or pollen tube growth in the pistil (female reproductive organ).

Pollen Pistil Interaction

Pollen-pistil interaction is a crucial process in plant reproduction, encompassing all events from the deposition of pollen on the stigma (pollen-receptive part of the pistil) to the entry of the pollen tube into the ovule (structure containing the female gametophyte). This dynamic interaction involves the recognition of compatible pollen by the pistil, followed by either acceptance or rejection of the pollen.

The ability of the pistil to recognize pollen as compatible or incompatible is due to a continuous dialogue between the chemical components of the pollen and those of the pistil. If the pollen is compatible, the pistil promotes post-pollination events that lead to fertilisation. In contrast, if the pollen is incompatible, the pistil rejects it by preventing pollen germination or pollen tube growth.

After compatible pollination, the pollen grain germinates on the stigma, producing a pollen tube that grows through the tissues of the stigma and style (elongated part of the pistil) and reaches the ovary. In some plants, male gametes are formed during the growth of the pollen tube in the stigma, while in others, the pollen tube carries the male gametes from the beginning.

Once the pollen tube reaches the ovule, it enters through the micropyle (small opening in the ovule). Then it penetrates one of the synergids (cells in the ovule) through the filiform apparatus. The tip of the pollen tube bursts, releasing its contents, including the male gametes, inside the synergids. This leads to fertilisation and the development of the male gametophyte, ultimately resulting in seed formation.

Overall, pollen-pistil interaction is a dynamic process that can be manipulated by plant breeders to achieve desired hybrids, even in cases of incompatible pollinations.

Artificial Hybridisation

Artificial hybridisation is a method used in crop improvement programs to control pollination and achieve desired traits in offspring. It involves several steps:

1. Emasculation: This involves the removal of the anthers from the flower bud before they release pollen. This is done to prevent self-pollination and ensure that only the desired pollen is used for pollination. Emasculation is typically performed on bisexual flowers.
2. Bagging: After emasculation, the female flowers or emasculated flowers are covered with a bag made of material like butter paper. This protects the stigma from unwanted pollen, ensuring that only the desired pollen grains are used for pollination.
3. Pollination: Once the stigma of the bagged flower is receptive, mature pollen grains collected from the anthers of the male parent are dusted onto the stigma. This ensures fertilisation with the desired pollen.
4. Rebagging: After pollination, the flowers are rebagged to prevent unwantd pollination. This step ensures the developed seeds or fruits result from the desired cross-pollination.

Double Fertilisation

Double fertilisation is a unique process that occurs in angiosperms, where two fertilisation events take place during the same reproductive cycle, leading to the formation of two different structures:

1. Syngamy or True Fertilisation: One male gametes fuse with the egg cell to form a diploid zygote. This process is known as syngamy or true fertilisation, resulting in embryo formation.
2. Triple Fusion: The second male gamete fuses with two polar nuclei in the central cell to form a triploid primary endosperm nucleus (PEN). This process is called triple fusion, resulting in endosperm formation, a nutritive tissue for the developing embryo.

Syngamy + Triple Fusion = Double Fertilisation

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