Events in the Germination of Pollen Grains

The process of the landing of pollen on the stigma, pollen tube growth and entry of germ cells in the ovule are the events covered in the germination of pollen grains. Let us study each event in detail.

Pollen Attachment

• The attachment of pollen grains to the stigma depends upon the receptivity of the stigma, and shape and structure of the pollen grains.
• The pollen grains when released from the anther are either dry or wet in nature.
• The wet pollen grains easily land on the stigma, and is mostly a mechanical process.
• The dry pollen grains land on the stigma depending on the layers of the pellicle of the stigma and pollen layers. Once landed, the pollen grains absorb water and then proceed to germinate.

Pollen Tube Growth

• The first event associated with pollen tube growth is the hydration of the intine layer that ruptures the exine.
• The stigmatic surface exudes essential nutrients, such as boron and calcium that are absent in pollen and are required for germination.
• The vegetative cell and the exine components grow into a pollen tube inside the pistil, and build a way for the germ cells to enter the ovule.
• The pollen tube emerges from the germ pore on the pollen grain that has a very thin layer of the exine.
• The entire content of the grain is transferred into the tube.
• The entire cytoplasm is collected at the apex of the tip, and the rest of the tube is filled with a vacuole.
• To restrict the cytoplasm at the tip, callose plugs are formed at regular distances throughout the tube.
• The pollen tube is a three-layered structure with an outer pectin stratum and a middle pectocellulosic stratum.
• The pollen tube grows into the intercellular spaces of the style. Further route of the tube depends on the nature of the style.

Factors Affecting Pollen Tube Growth

1. Carbohydrates: A little sugar in the hydration of the pollen grains while landing on the stigma prevents the pollen from bursting.
2. Boron: Pollen grains are deficient in boron, therefore stigma secretes enough boron so as to not hinder the growth of pollen tubes.
3. Calcium: The presence of calcium ions in the pollen grains promotes the ‘population effect’. Usually, pollen grains have a very small amount of calcium, but when a mass of pollen come together, a large amount of calcium is collected that promotes pollen tube formation.
4. Enzymes: Enzymes such as pectinase and cellulase promote elongation of the tube.
5. Physical Factors: Temperature plays the biggest role in the elongation of the pollen tubes. The optimum temperature range for pollen tube growth is 20-30 ℃.

Entry of Pollen Tube into the Ovule

The pollen tube finally reaches into the ovule. Based on the entry of the pollen tube into the ovary, there are three types of fertilization:

• Porogamy: In this type, the pollen tube enters the ovary through the micropyle. E.g, Lily.
• Chalazogamy: In this type, the pollen tube enters the ovary through the chalazal end. E.g, Casuarina
• Mesogamy: In this type, the pollen tube enters the ovary through the funicle or the integuments. E.g, Cucurbita

Entry of Pollen Tube into the Embryo Sac

• Irrespective of the entry of pollen tubes into the ovary, it enters the embryo sac through the micropyle.
• The filiform apparatus guides the entry of pollen tubes into the cytoplasm of synergids.
• The reproductive cells divide into two male gametes.
• One gamete fuses with the egg in the embryo sac to form a zygote, while the other gamete fuses with two polar nuclei that form a food reserve for the seed called endosperm.
• The double fusion of cells is characteristic of angiosperms, and is termed double fertilization.

There are certain barriers that hinder the entry of pollen into the ovary. Visit BYJU’S Biology to read about the incompatibility barriers and other topics.

You might also be interested in:

• Pollen-Pistil Interaction
• What Is Self Incompatibility?
• The Structure and Functions of Pistil. An Overview
• Why Pollen Grains Are Well Preserved As Fossils?