POLLINATION: PROCESS, TYPES AND IMPORTANCE FOR THE ENVIRONMENT - BIOLOGY - 2025

The pollination is the process of transferring pollen from the "male" part to the female part of the flower. Its purpose is the fertilization of the ovule (contained in the female gametophyte) by pollen grains, which represent the male gametophyte.

Since pollination implies the approach or contact between the sex cells of plants of the same species (although it can also be of the same plant, self-pollination), the sexual reproduction of plants depends considerably on this process.

Bee pollinating a flower (Source: Myriams Zilles from pixabay.com)

In plants with seeds, pollination is only the step prior to fertilization, which is the process where the genetic material of two individuals mixes to produce the seed that will give rise to a new plant, probably with characteristics shared between both parents.

The reproductive organ of angiosperms (flowering plants) that is responsible for the production and protection of sex cells (of the female and male gametophytes) is the flower, and that is where pollination occurs.

There are several types of pollination and some of these differ with respect to the pollinator, which can be biotic (an animal) or abiotic (wind, water), on which the different plant species are completely dependent.

Biotic pollination greatly depends on the characteristics of the flower, since usually animals are attracted by some special attribute, either to feed, take refuge, reproduce, etc.

Process

Pollination is the transfer of pollen grains from the male part of one flower to the female part of another (or from the same, when it comes to self-pollination) and depends on external agents known as pollinators.

Pollination (Source: Mabel Amber, on pixabay.com)

This is one of the fundamental processes for the production of fruits and seeds in vegetables, that is, it is an essential part of the sexual reproduction of plants.

However, to understand in some detail what this process is about, it is necessary to have a basic notion of what a flower looks like.

- Anatomy of a flower

A typical angiosperm flower is a fairly complex structure, especially if one considers that a large number of species have flowers where female and male gametophytes exist at the same time.

The flowers, generally, are produced in the apical meristems of the stem (aerial part of the plants) and, depending on the species, these can be male, female or bisexual.

Scheme of the anatomy of a mature angiosperm flower (Source: LadyofHats via Wikimedia Commons) The portion of the stem that joins the flower with the rest of the plant is known as the peduncle, in whose upper portion is the receptacle, the structure responsible for the support of the parts of the flower (sepals, petals, stamens and carpels).

The sepals and the petals exert functions in the protection of the cocoons and in the visual attraction of some pollinators, respectively; while the stamens and carpels are the gametophytes where the sex cells are produced.

Male gametophyte

Stamens are long filaments that end in anthers, which are the "sacs" where pollen grains are produced. The set of stamens of a flower is known as androecium, which means "the house of man" and, in general, it exceeds the sepals and petals in height.

Female gametophyte

The carpels contain the ovules. These form what is known as a "pistil" and consist of a stigma, a style and an ovary. This part of the flower is called gynoecium, which means "the woman's house."

The pistils are shaped similar to that of a bowling pin. The upper part corresponds to the stigma and is a flattened structure whose sticky surface allows the adhesion of pollen grains.

The style is the middle portion of the pistil and is the one that connects the stigma with the ovary; this can be long or short. Finally, the ovary is the site where one or more ovules are found and is the most dilated part of the pistil. The ovary can become part or all of the fruit.

- How does pollination occur?

When a pollen grain reaches the stigma, it "germinates", producing a long structure known as a pollen tube. The pollen tube grows downward through the style, that is, it grows in the direction of the ovary.

Many recognition and signaling mechanisms are involved in the directionality of growth of the pollen tube towards the ovary and, as in the case of many animals, not all pollen tubes that germinate and grow in the same way reach the ovary and progress towards fertilization.

When the pollen tube penetrates the female gametophyte (the ovary), the sperm cell contained in the pollen grain fertilizes the egg cell. Shortly after, thanks to the fertilization process and once the nuclei of both cells have fused, the zygote is produced.

This zygote, as it develops in the embryo, is what will later make up the seed, which is the most important dispersal organ of plants with sexual reproduction.

In addition to the sperm cell that achieves fertilization of the egg cell, another sperm cell contained in the same pollen grain fuses with two or more nuclei derived from the female gametophyte; this process is known as double fertilization.

The aforementioned fusion forms a "polyploid endospermic nucleus", which will be responsible for producing the endosperm (the food material) from which the embryo will nourish itself within the seed during its development and during germination.

Types

Pollination can be classified as "self-pollination" and "cross-pollination" depending on where the pollen grains come from, or as "biotic" and "abiotic" of who is transporting the pollen grains (the pollinating agent).

- Self pollination

There are species of plants that have female and male flowers on the same stem, but there are also those that have bisexual flowers, that is, they present, at the same time and in the same flower, both male and female gametophytes (androecium and gynoecium).).

Some authors consider that the pollination that occurs between unisexual flowers of the same plant is an "interfloral pollination", while that which occurs between the reproductive structures of the same flower is an "intrafloral pollination".

Although it allows the multiplication of the individuals that reproduce, self-pollination implies that the sex cells that merge are genetically identical, so that the plants that will emerge from the resulting seeds will be a kind of “clones” of the parent plants.

- Cross pollination

Contrary to the self-pollination process, cross-pollination involves the exchange of pollen between the flowers (unisexual or bisexual) of different plants. In other words, this process involves the transfer of a grain of pollen from the anther of one flower to the stigma of another, on a different plant.

Since the genetic material that is exchanged during cross-pollination derives from genetically different parents, the seeds that will be produced once the fertilization process is complete will give rise to different plants, genetically and phenotypically speaking.

- Biotic and abiotic pollination

Depending on the vector that mediates the transfer of a pollen grain from the anthers of one flower to the stigma of another (or the same), pollination can be classified as biotic and abiotic

Biotic pollination

This type of pollination is perhaps the most representative and important of all. It has to do with the participation of an animal, usually an insect, in the transfer of pollen grains from one place to another.

Although more than 50% of pollination is carried out by many insects and arthropods of various species, vertebrate animals such as birds and bats play an important role in this process.

Biotic pollination can promote both cross-pollination and self-pollination, and plants can be specific or generalist in terms of the type of animal that pollinates them.

However, pollinators do not participate in the sexual reproduction of plants "ad honorem", since they are attracted to floral structures either by their visible characteristics or by the reward elements they receive (food, shelter, etc..).

The plant-pollinator relationship means an important interaction that shapes the evolution of the floral structure at the same time as that of the animals that pollinate them. For this reason, it is not strange to get flowers specifically adapted to the structures of your visitors.

Abiotic pollination

Abiotic pollination is that which takes place thanks to the participation of "non-living" entities, such as wind and water. The first is known as anemophilic pollination and the second as hydrophilic.

Photograph of a plant and its pollen (Source: pixabay.com)

Plants that are pollinated by water are restricted (as is logical) to aquatic environments and often present flowers with very particular structures, in order to ensure both the release and reception of sex cells.

Importance for the environment

Pollination is a crucial process for the life cycle of many angiosperms. Since without pollination fertilization does not occur and without the latter, seeds are not produced, pollination is not only vital for plants, but also for many of the animals that feed on them.

The process itself is very important for the maintenance of the genetic variability of plant species, which is essential for the emergence of adaptive mechanisms against various environmental factors such as climate change, the presence of pathogens, etc.

It is also an essential process for world agricultural production, this from the anthropocentric point of view.

References

1. Faegri, K., & van der Pijl, L. (1979). The Principles of Pollination Ecology (3rd ed.). Pergamon Press.
2. Heinrich, B., & Raven, PH (1972). Energetics and Pollination Ecology. Science, 176 (4035), 597-602.
3. Nabors, M. (2004). Introduction to Botany (1st ed.). Pearson Education.
4. Picó, F., Rodrigo, A., & Retana, J. (2008). Plant Demography. Population Dynamics, 2811–2817.
5. Solomon, E., Berg, L., & Martin, D. (1999). Biology (5th ed.). Philadelphia, Pennsylvania: Saunders College Publishing.