REPRODUCTION in ANGIOSPERMS - POLLINATION

WHAT IS POLLINATION?

Transfer of pollen grains from steman/anther to the stigma of a appropriate flower.

Types of pollination

 1. autogamy 2. Geitonogamy 3. Xenogamy

1. Autogamy- 

• It is rare in opening flowers

Requirements

A. Flower must be functionally bisexual

B. Synchrony in pollen release and stigma receptivity

C. Anther and stigma should lie close to each other

In some plants, two types of flowers are present- 

1. Chasmogamous- normal flowers.

2. Cleistogamous- these flowers do not open like normal flowers.

So, autogamy is the only option. 

They fulfill all requirements of autogamy.

Eg. Oxalis commolia.

Advantages-

This process does not require external agent like insect.

Disadvantages-

Since the pollen is transferred to the same plant, less new combinations of chromosomes are formed. 

2. Geitonogamy- 

Requirement- plant must be bisexual, even if flowers are unisexual.

Advantages-

Pollination can occur even if any part ( male or female) gets damaged in a flower.

Disadvantage-

1. Pollen grains are not transferred to different plant, so less new combinations of chromosomes.

3. Xenogamy-

It is pure cross pollination.

Advantages- new combinations of chromosomes are formed so new characteristics develop in plants.

Disadvantage- it depends on external agents for pollination

Agents of pollination

1. Biotic agents- majority of plants use biotic agent for pollination. 

• These are the most common agents of pollination. 
• Particularly bees are responsible for majority of pollination.
• Other insects- butterflies, flies, moths, beetles, ants, wasps etc
• Other animals- bats, shrews etc.

Flower characteristics-

• Majority of insect pollinated flowers are large, colourful, fragrant and rich in nectar-- to attract insects.
• And if flowers are small - they are present as inflorescence - to attract insects.
• Flowers pollinated by flies and beetles secrete foul odour to attract flies and beetles.
• Plants also provide some rewards to pollinating agents eg. Nectar, pollen grains.
• Other rewards- providing safe places for to lay eggs, eg. Amorphophallus (tallest flower, 6ft tall flower).
• Another eg.- mutualism between a moth and the plant yucca- they both cannot complete their life cycle without each other. Moth deposit eggs in the locule of ovary, and flower gets pollinated by moth. The larvae of moth also uses seed for their nutrition.

2. ABIOTIC AGENTS-

• Water and wind.
• Due to uncertainty in abiotic pollination, flowers pollinated by water and wind produces large number of pollen grains as compared to the ovules available for pollination.

A. Wind pollination-

• More common in abiotic pollination.
• Pollens grains should be- light, non sticky - so they can be easily carried away in wind.
• Stamens - well exposed 
• Stigma- feathery - so pollen grains can be easily trapped.
• Flowers usually have single ovule
• Flowers arranged as inflorescence
• Very common in grass family, eg rice wheat etc

B. Water pollination-

• Rare
• Only occur in approx 30 genera, mostly monocots.
• Eg. Vallisneria, Hydrilla ( fresh water); Zootera ( marine sea grass).
• In Vallisneria, flowers are present at surface, and pollen grains are released and carried by water currents to female flower.
• In submeged plants, pollen grains are ribbon like, and carried passively by water to stigma.
• Pollens are covered with mucilagenous sheath, which protect them from getting wet.

Note- 

• Not all aquatic plant use water pollination.
• Most aquatic plants are pollinated by insects or wind.
• Eg. Water lily, water hyacinth etc.
• Here, flowers are present above water surface.

OUTBREEDING DEVICES

• Any factor that prevents self pollination and promote cross pollination.

Why? Because

• Majority of plants are bisexual.
• And continuous self pollination -- inbreeding depression.

Devices

• Pollen release and stigma receptivity is not synchronized.
• Anther and stigma are placed at a distance.
• Self incompatibility.
• Production of unisexual flowers.

POLLEN PISTIL INTERACTION

• From pollen deposition on stigma to pollen tube entry in ovule.
• The pistil has the ability to recognize the right pollen (same species, compatible, viable, same plant {self incompatible} ).
• Wrong pollen- pistil rejects pollen -- prevents germination and growth of pollen tube.
• Right pollen-- pistil accepts the pollen -- post pollination events --- ultimately fertilization.
• Chemical exchange between pollen and pistil-- Acceptance or rejection

Note-

• More knowledge about these chemicals help plant breeders to to get desired hybrid plant, even with incompatible pollen grains.

Example-

• Our need- hybrid with plant B

• Then we will use pollens of plant B, with chemical of pollen gran A.

• Compatible pollen -- germination

• From one of the germ pore -- pollen tube

• Content of pollen grain -- pollen tube

• Pollen tube -- grows through the tissue of style -- reaches to ovary

• In majority of the plants, pollen tube enters in the ovule, from micropylar end.

• Filliform apparatus guides pollen tube to enter into one of the synergid.

ARTIFICIAL HYBRIDISATION-

We produce hybrid plant with desired qualities. 

Requirement-

• Cross pollination
• Pollen grain from the desired plant only, land on the stigma.

Technique-

Now we want hybrid of A and B

1.Problem- both plant produce bisexual flowers)

Male A.      Female B

Remove anthers from flower bud B - Emasculation

After this, B is covered with a bag -- to prevent pollination by undesired plant -- Bagging

When stigma of B matures -- pollen from A are sprinkled on B -- rebagging

2. Unisexual flowers- 

 No need for emasculation