Pollination

Introduction

Pollination is the science in which a mature flower undergoes a mechanism where the transfer of pollen grains happens. The grains are shed from the male part- anther and they reach the stigma-female part of a pistil. Pollination involves a lot of mediators and stimulants that can help or participate in this process. But, in olden days, it was only linked to honey bees as there was limited research being done hence the information about the complete science behind the pollination was merely nil. However, modern floriculture has deeply experimented to describe the complete mechanism. This article will discuss in detail about the types, processes, and types of pollination.

1. Introduction

2. Types of pollination

    2.1 Autogamy

   2.2 Geitonogamy

   2.3Xenogamy

 

3. Abiotic pollination

    3.1 Abiotic pollination by wind

   3.2 Abiotic pollination by water

4. Biotic pollination

 

5. Outbreeding Devices

6. Events and Issues related to pollen-pistil interaction

Types of pollination

There are a multitude of classifications of pollination but the one in practice depends upon the source of pollen. Based on the source of where pollination occurs, pollination can be divided into autogamy, geitonogamy, and xenogamy.

  1. Autogamy is otherwise known as self-pollination. It is a process within the same flower, in other words, the transfer of pollen grains occur from the anther of a flower to the stigma of the same flower. When the flower is matured, it`s external parts get opened enabling a clear contact between anthers and the stigma. This requires compatibility between stigma and anther.  The pollen release must coordinate with the stigma receptivity. Depending upon whether the flowers get easily opened or not, there are 2 types of pollinating flowers namely chasmogamous flowers and the cleistogamous flowers. Plants like Viola, Oxalis, and Commelina belongs to chasmogamous as they easily opens up to allow contact between anther and sigma. In the case of cleistogamous flowers, they don’t easily open up to allow contact between anther and stigma. So, in order to pollinate the male and female parts must stay close to each other. However, Cleistogamous flowers produce assured seed-set even in the absence of pollinators. The advantage of self- pollination is that the breeds maintain purity because the chances of diversity is low. The plants do not depend on external factors for pollination and even smaller quantities of pollen grains produce have a good success rate in getting pollinated. Self- pollination will help in eliminating the recessive characters of the breed.

  2.  Geitonogamy is a subtype of autogamy as there is a transfer of pollen grains from the anther of one flower to the stigma of another flower of the same plant but not to the other flower of another plant. This type of pollination produces genetically the same offspring as that of autogamy process.  Although geitonogamy is functionally cross-pollination involving a pollinating agent, genetically it is similar to autogamy since the pollen grains come from the same plant.

  3.   Xenogamy is otherwise known as cross-pollination. In this type, there is a transfer of pollen grains from the anther of one plant`s flower to the stigma of another plant`s anther. This crossing ensures a good variation in the newly formed generations and they are genetically expressive.

 

Agents of Pollination

Pollination needs a medium, different agents play a role in transferring the anther to the stigma. Few of the plants use two media in common, namely the abiotic medium without the involvement of biological living organism (wind and water) and the other one is biotic which includes  (animal) agents to achieve pollination. Most plants use biotic agents as a medium of pollination.

NOTE:

In a normal case, pollen grains contacting the stigma of a flower are influenced by external factors like excess of wind and rain, so, it’s a chance factor in both wind and water pollination. To substitute this loss, plants are fortunate enough to have the ability to produce a maximum amount of pollen.

 

Abiotic pollination

Abiotic pollination by wind

In the case of abiotic pollination,  wind is the most common medium but, wind helps only when the pollen grains are light, non-sticky, allowing them to pass easily on to the next flower. Such a transfer can happen only when there are healthy wind currents. Wind pollinated flowers usually often have a single ovule in each ovary and numerous flowers packed into an inflorescence; a familiar example is the corn cob – the tassels you see are nothing but the stigma and style which wave in the wind to trap pollen grains. Wind-pollination is quite common in grasses.

Abiotic pollination by water

On the contrary, pollination by water is extremely rare and such flowers are called the Hygrophilous flowers. It is seen hardly in about 30 genera, belongs to monocotyledons. Some of the water pollinated plants are common waterweed, pondweed, eel-grass, hydrilla, and coontail. But surprisingly, not all aquatic plants use water for pollination. In a majority of aquatic plants such as water hyacinth and water lily, the flowers emerge above the surface level of water and they get pollinated by insects or wind.  In plants like seagrasses, female flowers remain deeply submerged in water and the pollen grains will be released by the mature flower inside the water, in such cases, the grain is ribbon-like and very long and slender.  These ribbon-like structures get transported passively inside the water; some of them reach the stigma to achieve pollination. In order to protect the pollen grains underwater, the pollen grains are protected from wetting by a mucilaginous covering.  Flowers pollinated by abiotic medium are usually not so colourful or they are dull.

 

Biotic pollination

It is the pollination by biological media like insects, animals, and birds. Most of the pollination happens in this way. Flowers are again classified based on which type of animal, insect or bird is being used for pollination, they are namely zoophilous flowersentomophilic flowers, ornithophilous flowers.

 

Biotic Pollination

Biotic pollination by honey bee- Image from wiki commons

 

Zoophilous flowers use animals like human beingsbats, birds for pollination. These flowers have pollen that can easily stick on to the body of the animal to ensure it is easily carried from one flower to another. Entomophilic flowers are those pollinated by insects and these flowers look generally attractive with bright petals, highly fragrant to attract the insect visitors to them. They often have broader stigmas or anthers to allow the insect to perch on it. Many of the insect-pollinated flowers also secrete nectar which attracts bees, butterflies or other similar insects towards them. The pollen grains in these flowers are often spiny with extensions that help them to stick on to the body of the insects. Ornithophilous flowers are pollinated are those pollinated by birds and these are rare cases

 

Outbreeding Devices

Outbreeding devices help to prevent self-pollination because of continued self-pollination (autogamy) that can depress the species with the least variation. Some of the plants have voluntarily developed techniques to block this self-pollination in order to evolve and produce a diverse range of seeds. 4 techniques are mainly used in this case.

  1. Creating a situation in which there is poor synchronization between the time of pollen release and stigma receptivity so that pollination cannot happen within the plant.

  2. In few species, a wrongly positioned anther and stigma with a varied angle, distance makes pollination impossible.

  3. In some cases, self-incompatibility is being instilled. It is a genetic mechanism to block self-pollination by suppressing the growth of the pollen tube in the pistil.

  4. Another method is by producing unisexual flowers within a plant to a larger extent, this ensures there is no exchange of male and female gametes. It can be seen in plants like castor and maize (monoecious).

 

Events and Issues related to pollen-pistil interaction

Some of the sequential events that happen during pollen-pistil interaction are

  1. The landing of a right and suitable pollen on a compatible pistil.

  2. Germination of pollen and the formation of pollen tube where pollen grains release its contents.

  3. Growth of pollen tube through the style of the pistil towards the ovary.

  4. The entry of male gametes into the ovule and then to synergid.

It is not always true that the right kind of pollen contacts the right stigma. It can be flowers within a plant or flowers of different plants. This will make the incompatibility of pollen matching. Generally, the pistil has the ability to recognize the pollen for its compatibility. Pistil decides whether to accept the pollen or to reject it based on a receptivity mechanism. Only if there is compatible pollination, the pollen grain undergoes proper matching and seeds gets germinated. This interaction where a pistil is capable of recognizing its pollen is the result of long term pollen-pistil interaction and chemicals released by pollen. Knowledge about outbreeding devices and pollen-pistil interaction helped in economic and financial development.

 



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