Pollination is the process in which pollen grains are transferred from the male reproductive organs of a flower to the female reproductive organs, leading to fertilisation and the production of seeds and fruits.
Different kinds of pollination:
● Autogamy: Autogamy refers to the self-pollination of a flower, where pollen from the anthers is transferred to the stigma of the same flower. This can occur in different ways:
○ Cleistogamy: Cleistogamy is a form of autogamy where flowers remain closed, preventing access to external pollinators. The flowers self-pollinate within the closed structure, ensuring fertilisation. Some examples of plants that employ cleistogamy are certain species of violets and peanuts.
○ Chasmogamy: Chasmogamy is the opposite of cleistogamy. In chasmogamous flowers, the reproductive organs are exposed, and the flower is open, allowing for cross-pollination by various agents such as insects, wind, or animals.
Some plants such as Viola (common pansy), Oxalis, and Commelina produce both types of flowers i.e. cleistogamous and chasmogamous flowers.
● Geitonogamy: Geitonogamy occurs when pollen is transferred from the anthers of one flower to the stigma of another flower on the same plant. This type of pollination allows for genetic exchange within the same individual and can occur in both self-compatible and self-incompatible plants.
● Xenogamy: Xenogamy refers to cross-pollination between different plants of the same species. It involves the transfer of pollen from the anthers of one plant to the stigma of another plant. This type of pollination promotes genetic diversity, as it introduces new combinations of genetic material from different individuals.
Agents of pollination
Pollination can occur through both biotic (living organisms) and abiotic (non-living factors) agents.
1.Biotic Agents of Pollination:
● Insects: Entomophily
Insects, such as bees, butterflies, moths, flies, and beetles, are attracted to flowers by their colours, fragrances, and the presence of nectar. Pollen grains adapted for insect pollination are usually sticky or spiky.
● Birds: Ornithophily
Certain bird species, like hummingbirds and sunbirds, act as pollinators. They are attracted to brightly coloured, tubular-shaped flowers that provide ample nectar. Pollen grains adapted for bird pollination are often large and sticky.
● Bats: Chiropterophily
Bats are important pollinators, particularly in tropical regions. They are attracted to night-blooming flowers that emit strong fragrances and produce copious amounts of nectar. Pollen grains adapted for bat pollination are often sticky and large.
● Other Animals: Zoophily
Other animals, such as small mammals, rodents, and primates, can also act as pollinators for certain plant species. They inadvertently carry pollen while feeding on floral resources.
Animal pollination involves various agents such as bees, butterflies, flies, beetles, wasps, ants, moths, birds, and bats.
Insects, particularly bees, are the dominant biotic pollinating agents.
Animal-pollinated flowers are often large, colourful, fragrant, and rich in nectar.
Flowers attract animals through colour, fragrance, and rewards like nectar and pollen grains.
Animal-pollinated flowers have sticky pollen grains, which adhere to the animal's body.
Animals transfer pollen to the stigma while collecting rewards, leading to pollination.
Some animal-pollinated flowers provide safe places for egg-laying, ensuring a mutualistic relationship.
Examples include the Amorphophallus flower and its association with a species of moth (Yucca plant and moth relationship).
The moth deposits its eggs in the locule of the ovary and the flower, in turn, gets pollinated by the moth.
2.Abiotic Agents of Pollination:
● Wind: Anemophily
Wind pollination is prevalent in many plant species. Wind-pollinated flowers are usually small, inconspicuous, and produce large quantities of lightweight, non sticky pollen grains that are designed for efficient dispersal by air.
They possess exposed stamens and large, feathery stigma to trap airborne pollen grains. Wind-pollinated flowers often have a single ovule in each ovary and are packed into inflorescences.
Examples of wind-pollinated plants include grasses and corn.
● Water: Hydrophily
Some aquatic plants rely on water for pollination. They release their pollen grains directly into the water, where currents carry them to the stigmas of female flowers.
Water pollination is more common in lower plant groups such as algae, bryophytes, and pteridophytes.
Examples of water-pollinated plants include Vallisneria, Hydrilla, and marine sea-grasses like Zostera.
Not all aquatic plants use water for pollination; some are pollinated by insects or wind.
In water-pollinated plants like Vallisneria, female flowers reach the water surface while male flowers release pollen grains onto the water.
● Majority of flowering plants produce hermaphrodite flowers, leading to the potential for self-pollination.
● Self-pollination can result in inbreeding depression, which negatively impacts the offspring's genetic diversity.
● Flowering plants have developed various mechanisms to discourage self-pollination and promote cross-pollination.
● In some species, there is a temporal separation between pollen release and stigma receptivity.
○ Pollen may be released before the stigma becomes receptive or vice versa, preventing self-pollination.
● In other species, anther and stigma are positioned at different locations within the flower.
○ This arrangement ensures that pollen cannot come into contact with the stigma of the same flower, thereby preventing self-pollination.
● Self-incompatibility is a genetic mechanism that prevents self-pollen from fertilising ovules.
○ It inhibits pollen germination of pollen tube growth in the pistil, ensuring cross-pollination.
● Unisexual flowers, found in some plant species like castor and maize (monoecious plants), have separate male and female flowers on the same plant.
○ This arrangement prevents self-pollination but allows for geitonogamy (pollen transfer between flowers of the same plant).
● Dioecious plants, such as papaya, have male and female flowers on separate plants.
○ Each plant is either male or female, completely avoiding both autogamy and geitonogamy.
Pollen pistil interaction
● Pollination doesn't guarantee the transfer of compatible pollen (from the same species) to the stigma.
● Pollen from other species or self-incompatible pollen can also land on the stigma.
● The pistil has the ability to recognize and differentiate between compatible and incompatible pollen.
● If the pollen is compatible, the pistil accepts it and facilitates post-pollination events leading to fertilisation.
● If the pollen is incompatible, the pistil rejects it by preventing pollen germination of pollen tube growth.
● The recognition and response of the pistil to pollen involve a continuous dialogue mediated by chemical components.
● Recent research has identified some of the pollen and pistil components involved in this interaction.
● Following compatible pollination, the pollen grain germinates on the stigma, producing a pollen tube.
● The pollen tube grows through the stigma, style, and reaches the ovary.
● In some plants, pollen grains are shed as two-celled, and the generative cell divides to form the male gametes during pollen tube growth.
● In three-celled pollen, the pollen tube carries the two male gametes from the beginning.
● The pollen tube enters the ovule through the micropyle and then enters one of the synergids.
● The filiform apparatus present at the micropylar part of the synergids guides the entry of the pollen tube.
● The series of events from pollen deposition on the stigma to the entry of pollen tubes into the ovule is known as pollen-pistil interaction.
● Understanding pollen-pistil interaction can assist plant breeders in manipulating incompatible pollinations to obtain desired hybrids.
○ Artificial hybridization is an important method in crop improvement to create superior varieties by crossing different species and sometimes even genera.
○ Emasculation is the removal of anthers from the flower bud of the female parent before they release pollen.
○ Emasculation is necessary when the female parent has bisexual flowers.
○ Bagging involves covering the emasculated flowers with a suitable bag, often made of butter paper, to protect the stigma from unwanted pollen contamination.
○ Bagging prevents the stigma from being exposed to pollen other than the desired pollen grains.
○ In the case of female parents with unisexual flowers, emasculation is not required.
○ Instead, the flower buds of the female parent are bagged before they open.
○ When the stigma of the bagged flower becomes receptive, pollination is performed using the desired pollen.
○ After pollination, the flowers are rebagged to prevent further contamination.
○ The fruits are then allowed to develop.
○ In addition to emasculation and bagging, another technique used in artificial hybridization is tagging.
○ Tagging involves the identification and labeling of the flowers or plants involved in the crossing experiment.
○ Tags or labels are attached to the flowers or branches to keep track of specific crosses or parental lines.
○ Tags usually contain information such as the parent plants' names, cross details, and date of pollination.
○ Tagging helps breeders keep records and easily identify and differentiate the different crosses.
○ It ensures that the desired crosses are properly documented and monitored throughout the breeding process.
○ Tagging is particularly useful when conducting multiple hybridization experiments simultaneously.
Double fertilisation is a unique reproductive process that occurs in flowering plants (angiosperms) and involves the fusion of two sperm cells with two different cells of the female gametophyte (embryo sac) within the ovule.
● Pollen Grain Germination: A mature pollen grain, containing two sperm cells, lands on the stigma and forms a pollen tube.
● Pollen Tube Growth: The pollen tube grows down through the style, guided by chemical signals and the guidance of the filiform apparatus. It enters the ovule through the micropyle.
● Entry into the Embryo Sac: The pollen tube enters the embryo sac, which contains two synergids, a central cell, and three antipodal cells.
● Sperm Cell Delivery: The pollen tube releases the two sperm cells into the embryo sac.
● Syngamy (Fertilisation 1): One sperm cell fuses with the egg cell (located at the base of the embryo sac), resulting in the formation of a zygote.
● Triple Fusion (Fertilisation 2): The other sperm cell fuses with the two polar nuclei (located at the centre of the embryo sac), forming a triploid (3n) structure called the primary endosperm nucleus (PEN).
● Development of Embryo and Endosperm: The zygote develops into an embryo, which will give rise to the new plant. The central cell after triple fusion becomes the primary endosperm cell (PEC) and undergoes multiple rounds of division to form the endosperm, a tissue that provides nourishment to the developing embryo.