Events in the Germination of Pollen Grains: Definition, Meaning, Formation

Definition Of Germination

The germination of pollen is the process of a pollen grain being deposited on a compatible stigma, hydrating itself, and germinating to a pollen tube that elongates towards the ovule. This process lies at the core of ovule fertilisation, the means via which male-female gametes are brought together.

Pollen germination is a vital step in the life cycle of plant reproduction. This is because the germination of pollen triggers further development through which male germ cells may eventually reach the ovule, thus fertilising and culminating in seeds. In this way, germination allows for genetic variation just like propagation in plant species. This fundamental process is quintessential to both the realm of ecosystems and agriculture.

Structure Of Pollen Grains

Male gametophytes in angiosperms are represented by pollen grains, which deliver the sperm nuclei to the ovule. Generally, a pollen grain consists of a bi-layered wall containing an outer tough layer called the exine and an inner soft layer called the intine.

Overview Of Pollen Grain Anatomy

Exine and Intine Layers

The exine layer can be highly resistant and is usually complex in structure, providing excellent protection to the pollen grain. The intine layer is located below the exine and serves during germination like the manufacture of the pollen tube.

Pollen Tube Formation

When the pollen grain has reached the stigma of an appropriate flowering plant, it rehydrates and germinates to become a pollen tube that grows through the style of the flower to the ovule where the sperm nucleus is deposited and fertilisation occurs.

Conditions For Pollen Germination

The germination of pollen depends upon factors such as temperature, humidity, and nutrient availability in the surroundings. All this should be in optimal conditions; then only it can ensure successful germination followed by fertilisation.

Environmental Factors

The environmental factors are:

Temperature

Temperature is the most critical factor that impacts the viability and germination rate of the pollen.

In general, all the plants have their specific temperature brackets within which the growth of pollen tubes can be done.

Humidity

Mild humidity: It is needed that there should be enough humidity for the grains to hydrate and germinate. Extreme dryness may tend to hinder germination, and over-moist conditions may result in a fungal attack.

Nutrient Availability

The nutrients made available in the stigmatic and style tissues offer essential nutrients for the growth of the pollen tube that will in turn aid in the fertilisation's fruitful success.

Biological Factors

The biological factors are:

Pollen Viability

For the fruitful fertilisation process to occur, the pollen grains have to be fit enough. The quality is due to be high, and genetic and environmental factors may vary.

Compatibility With Stigma

For successful germination, pollen has to be of the same species and compatible with the stigma. In case of incompatibility, the germination of pollen grains can be prevented in most cases either from sticking to the stigma or the germination on it.

Steps In The Germination Of Pollen Grains

If a pollen grain lands on a compatible stigma, it will adhere to the sticky surface by hydrating and will begin the germination on the stigma.

Step 1: Pollen Grain Landing On Stigma

Adhesion to Stigma

The presence of exudates in the pollen grain provides it with enough stickiness to ensure that it adheres strongly to the surface of the stigma so that it does not move from its place and hydrate and germinate.

Hydration of Pollen Grain

The moisture derived by the pollen grain from the stigma undergoes a stimulatory effect for the germination of the pollen grain and the development of the pollen tube.

Step 2: Formation Of The Pollen Tube

Pollen Tube Initiation

After the hydration process, metabolic activity starts developing in the pollen tube. The pollen tube emerges out through the intine layer by digesting the exine and travels down the style.

Role of Enzymes in Pollen Tube Penetration

It has been observed that the pollen tube secretes enzymes that break the polysaccharide network present in the extracellular matrix of the style. This breakage helps the tube to penetrate the style and move down towards the ovule.

Step 3: Pollen Tube Growth Through Style

The pollen tube grows down the style because of the chemical signals in the direction of the ovule.

Mechanism of Pollen Tube Elongation

The pollen tube grows by cell wall loosening and plasma streaming, which guides the pollen tube's movement inside the style tissues.

Navigation of Pollen Tube

Guidance of pollen tube movement — The chemical cues released from the ovule act as chemoattractants that guide pollen tube growth to position the micropyle.

Step 4: Pollen Tube Entry Into Ovule

Once the pollen tube enters the ovule, the tube enters deep by the micropyle, deposits the gametes and double fertilisation occurs.

Entry Through Micropyle

The pollen tube grows through the micropyle—a small pore in the integuments of the ovule—and then digests a pathway through the nucellus to get into the embryo sac.

Double Fertilisation Process

One sperm cell joins with the egg cell forming a diploid zygote and the other with the central cell of the ovule forming the endosperm, which is a nutritionally dense albumen. The endosperm fuels the early development of the embryospore, providing the sustenance for its growth.

Factors Affecting Pollen Tube Growth And Fertilisation

The details are given below:

Genetic Factors

Factors determining the success of the growth and fertilisation of the pollen tube include the genetic makeup of both the pollen and the stigma, as some genetic combinations are more compatible.

Environmental Stressors

Drought, high salinity, and pollution are some of the environmental stressors that may have devastating effects on the viability of the pollen and tube growth and affect the success of fertilisation.

Impact of Pollutants

The germination of pollen and tube development is adversely affected and arrested by air and soil pollutants, and in so doing, even the success of fertilisation and development of the seed is affected.

Climate Change Effects

Climate can alter the temperature and precipitation patterns: these are associated with conditions favourable towards the germination of pollen grains and the successful deposition of pollen on the stigma.

Recommended video on "Events in the Germination of Pollen Grains"