Introduction 

Pollination involves the transport of pollen grains from the anther to the stigma or pistil of the plant. The pistil has four parts: the stigma, which is the portion of the ovary that contains the ovule, and the style, which is part of the stigma. Its primary function is pollination, with fertilisation following it.

A pollen tube is formed when a suitable pollen grain falls on the stigma and germinates, resulting in the pollen grain becoming pollen. Both cross-pollination and self-pollination may occur. Self-pollination is encouraged by the fact that most plants are bisexual or hermaphrodites.

Definition of Pollen-Pistil interaction

Pollen-pistil interaction is described as the sequence of events from the moment pollen is deposited on the stigma till the time when the pollen tube enters the ovule. The first stage or the first step in pollen-pistil contact is pollination, which involves moving pollen grains from the anther to the stigma of the same flower or from one bloom to another.

The pollen-pistil interaction takes place in the following steps:

• The initial occurrence of pollen-pistil contact involves real pollen landing on a suitable pistil

• Pollen germination involves the creation of pollen tubes, which allow pollen grains to release their contents

• Pollen tube develops into the ovary via the style of the pistil

• Male gametes enter into the ovule and subsequently into the synergid

Structure of Pistil 

The pistil, which is positioned in the centre of the flower, is the female reproductive part of the flower. The primary components are the swelling base, ovary (with prospective seeds or ovules), ovary stalk or style, pollen receptive tip, and stigma, which is differently shaped and frequently sticky. The number of pistils in flowers varies greatly depending on the blooming in species. Each pistil is made of one to many leaf-like structures, and its primary role is to protect the ovules.

Functions of Pistil

The pistil’s principal purpose is to create an ovule. The following are some of the various roles of the pistil:

• In flowers, the pistil serves as the flower’s female reproductive organ

• It aids in pollen reception as well as in fertilisation

• The pistil is also engaged in the pollen grain germination process

• It also aids in the transport of pollen grains during pollination

• The stigma, which has a sticky tip and plays an important function in pollen grain germination, is made of the pistil

Factors Encouraging Cross-pollination

Self-pollination is frequent and more prevalent in hermaphrodite flowers, but a succession of self-pollination has a negative effect and leads to inbreeding depression. This leads to homozygous genes as well. As a result, plants have evolved to encourage cross-pollination. Outbreeding is the term for this. 

Cross-pollination is encouraged by the following factors:

• Unisexual flower: Cross-pollination is the sole option if a flower is unisexual, meaning it has just one sex, either female or male

• Non-Synchronization: Pollination success is dependent on timing. Pollen release and stigma receptivity should occur at the same time. Pollen develops and discharges before the stigma opens, resulting in pollen vitality loss or vice versa. Even though the bloom is hermaphrodite, this hinders self-pollination

• Self-sterility and structural barriers are examples of incompatibility inside a flower (or plant). Self-sterility indicates that even if pollination occurs, fertilisation is impossible owing to pollen growth failure. Structural barriers are the height difference between gynoecium and androecium and other features that prevent the stigma from receiving pollen. These are the genetic mechanisms, in a sense

Classification of pollen-pistil interaction 

Pollen-pistil interaction involves different steps that start from the time of pollen deposition on the stigma of the flower to the time of entry through the pollen tube into the ovule. Thus, the classifications related to it revolve around the stage of the interaction. 

Through this process, it is easier for the pistil to find the right pollen suitable for one species or diverse species. The pistil recognises if the pollen is compatible or incompatible. After that, the pollens undergo specific chemical levels, like sucrose, inositol, and boron, for the interaction. 

In the context of incompatibility during the pollen-pistil interaction, these classifications are common.

• Interspecific incompatibility – Here, the pollen of one plant species could not germinate on another plant species after landing on the stigma. Example, in interspecific pollination  of Brassica rapa  stigmas with brassica oleracea pollen grains , pollen tubes cannot penetrate stigma tissues
• Intraspecific incompatibility – This type of incompatibility refers to the inability of the pollen to germinate after falling on the stigma of another plant of the same species. 

Example Solanum aethiopicum shows intraspecific incompatibility

• Self-incompatibility – Another form of incompatibility occurs when the pollen of one plant does not suit its own stigma and cannot germinate. Example Potatoes  show self incompatibility due to lack of anther development and pollen production

The pollen can enter and then hydrate the stigma (absorbing the stigma-based water) and notice the pollen tube. The tube follows the carpal structure and grows towards the micropyle. The male gamete can travel through it into the egg cell. 

Here, another classification worth noting, regarding the pollen-pistil reaction, is the route of pollen tube entry into the embryo sac. 

• Porogamy – The pollen tube moves through the plant micropyle. One example is lily
• Mesogamy – Here, the pollen tube pierces through the plant’s integuments. Take the Cucurbita plant
• Chalazagamy – The pollen tube accesses the embryo sac via chalaza. The Casuarina plant is an example

Steps of Pollen-Pistil Interaction

Pollination does not always entail fertilisation though. The pistil must identify the pollen grain of the same species for pollination to take place. Getting pollen and pistils to interact is not a simple task. Because of this, you need to know how pollen and the pistil interaction occur to ensure effective fertilisation. Listed below are the stages at which pollen and pistils interact;

• Pollen grains are released from the male plant

• The female plant’s pistil receives the pollen grain

• Pollen is accepted if it is compatible with the pistil, but the ovary rejects incompatible pollen

• If a female plant accepts the pollen grain, it produces pollen tubes and discharges its contents and chemicals

• Style directs the pollen tubes towards the ovary

• At this point, the male gamete synergises with the female gamete in the ovary

This is the whole process of pollen-pistil interaction classification that is necessary for fertilisation to take place successfully.

Conclusion

Cross-pollination is encouraged by using outbreeding devices, which are plants’ characteristics or equipment that hinder self-pollination. As a consequence of ongoing self-pollination, it avoids the loss of genetic diversity or inbreeding depression. Dicliny, dichogamy, pollen prepotency, self-incompatibility, herkogamy, and heterostyly are some of the outbreeding devices.

Pollen–pistil interaction refers to the sequence of events that takes place between the time when pollen is deposited on the stigma and the time when pollen tubes enter the ovule. There are several areas where pollen tubes may be promoted or stifled, making it a constantly changing process.