Sunday, July 26, 2020

Sexual Reproduction in Flowering Plants - Notes | Class 12 | Part 2: Pollination

2. SEXUAL REPRODUCTION IN FLOWERING PLANTS

POLLINATION
It is the transfer of pollen grains from the anther to the stigma of a pistil.

Based on the source of pollen, pollination is 3 types:

a.    Autogamy (self-pollination): It is the transfer of pollen grains from the anther to stigma of the same flower.
In flowers with exposed anthers & stigma, complete autogamy is rare. Autogamy in such flowers requires synchrony in pollen release and stigma receptivity. Also, anthers & stigma should be close to each other.

Plants like Viola (common pansy), Oxalis & Commelina produce 2 types of flowers:
·   Chasmogamous flowers: They are similar to flowers of other species with exposed anthers and stigma.
·   Cleistogamous flowers: They do not open at all.  Anthers & stigma lie close to each other. They are autogamous. When anthers dehisce in the flower buds, pollen grains come in contact with stigma for pollination. Cleistogamous flowers produce assured seed-set even in the absence of pollinators.
Cleistogamy leads to inbreeding depression.  

b.  Geitonogamy: It is the transfer of pollen grains from the anther to the stigma of another flower of the same plant. It is functionally cross-pollination involving a pollinating agent. But it is genetically similar to autogamy since the pollen grains come from the same plant.

c.   Xenogamy: It is the transfer of pollen grains from anther to the stigma of a different plant. It brings genetically different pollen grains to the stigma.

Agents of Pollination

1. Abiotic agents (wind & water)

Pollination by wind (anemophily):

-    More common abiotic agent.
-    Wind pollinated flowers often have a single ovule in each ovary and numerous flowers packed into an inflorescence.
-    E.g. Corncob – the tassels are the stigma and style which wave in the wind to trap pollen grains. Wind-pollination is quite common in grasses.
-    Ways for effective pollination:
o  The flowers produce enormous amount of pollen.
o  Pollen grains are light and non-sticky.
o  They often possess well-exposed stamens (for easy dispersion of pollens into wind currents).
o  Large, feathery stigma to trap air-borne pollen grains.

Pollination by water (hydrophily):

-   It is quite rare. It is limited to about 30 genera, mostly monocotyledons. E.g. Vallisneria & Hydrilla (fresh water), Zostera (marine sea-grasses) etc.
-    But in lower plants, water is a regular mode of transport for the male gametes. Distribution of some bryophytes & pteridophytes is limited because they need water for the transport of male gametes and fertilisation.
-    In Vallisneria, the female flower reaches the surface of water by the long stalk and the male flowers or pollen grains are released on to the surface of water. They are carried by water currents and reach the female flowers.
-    In sea grasses, female flowers remain submerged in water. Pollen grains are long and ribbon like. They are carried inside the water and reach the stigma.
-    The pollen grains of most of the water-pollinated species have a mucilaginous covering to protect from wetting.
-    Not all aquatic plants use hydrophily. In most of aquatic plants (water hyacinth, water lily etc.), the flowers emerge above the level of water for entomophily or anemophily.
-    Wind and water pollinated flowers are not very colourful and do not produce nectar.

2. Biotic agents (animals)

-  Majority of flowering plants use animals as pollinating agents. E.g. Bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds & humming birds) bats, primates (lemurs), arboreal (tree-dwelling) rodents, reptiles (gecko lizard & garden lizard) etc.
-    Pollination by insects (Entomophily), particularly bees is more common.
-    Often flowers of animal pollinated plants are specifically adapted for a particular species of animal.
-    Features of insect-pollinated flowers:
o  Large, colourful, fragrant and rich in nectar. Nectar & pollen grains are the floral rewards for pollination.
o  Small flowers form inflorescence to make them visible.
o  The flowers pollinated by flies and beetles secrete foul odours to attract these animals.
o  The pollen grains are generally sticky.
-    When the animal comes in contact with the anthers and the stigma, its body gets pollen grains. When it comes in contact with the stigma, it results in pollination.
-    Some plants provide safe places as floral reward to lay eggs.
E.g. Amorphophallus (It has the tallest flower of 6 feet).
A moth species and the plant Yucca cannot complete their life cycles without each other. The moth deposits its eggs in the locule of ovary. The flower gets pollinated by moth. The larvae come out of the eggs as seeds start developing.
-    Many insects consume pollen or nectar without bringing about pollination. They are called pollen/nectar robbers.

Outbreeding Devices

Hermaphrodite flowers can undergo self-pollination. Continued self-pollination results in inbreeding depression.
To avoid self-pollination (autogamy) and encourage cross-pollination, there are some devices in plants:
a.    Avoiding synchronization: Here, the pollen is released before the stigma becomes receptive or stigma becomes receptive before the release of pollen.
b.    Arrangement of anther & stigma at different positions.
c.     Self-incompatibility: It is a genetic mechanism to prevent self-pollen (from same flower or other flowers of the same plant) from fertilization by inhibiting pollen germination or pollen tube growth in the pistil.

d.    Production of unisexual flowers: If male & female flowers are present on the same plant (i.e., monoecious, e.g. castor & maize), it prevents autogamy but not geitonogamy. In dioecious plants (e.g. papaya), male and female flowers are present on different plants (dioecy). This prevents both autogamy and geitonogamy.

Pollen-pistil Interaction


-   It is a process in which pistil recognizes compatible or incompatible pollen through the chemical components produced by them.
-    Pistil accepts compatible pollen and promotes post-pollination events.
-    It rejects incompatible pollen by preventing pollen germination or pollen tube growth.
-    Pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores. The contents of pollen grain move into pollen tube. Pollen tube grows through the tissues of stigma and style and reaches the ovary.
-    In plants which shed pollen grains at 2-celled condition (a vegetative cell & a generative cell), the generative cell divides into two male gametes during pollen tube growth.
-    In plants which shed pollen in 3-celled condition, pollen tubes carry 2 male gametes from the beginning.
-    Pollen tube → ovary micropyleovule → enters one of the synergids through filiform apparatus. Filiform apparatus guides the entry of pollen tube.

-    A plant breeder can manipulate pollen-pistil interaction, even in incompatible pollinations, to get desired hybrids.

Artificial hybridisation

It is a crop improvement programme in which desired pollen grains are used for pollination.

Steps:

o  Emasculation: Removal of anthers from the bisexual flower bud of female parent before the anther dehisces.
o  Bagging: Here, emasculated flowers are covered with a bag (butter paper) to prevent contamination of its stigma with unwanted pollen.
o  Pollination: When stigma attains receptivity, pollen grains collected from male parent are dusted on the stigma.
o  Rebagging the flowers. It is allowed to develop the fruits.
For unisexual flowers, there is no need for emasculation. Female flower buds are bagged before the flowers open.
============

👇 Select Your Topic Here 👇

No comments:

Post a Comment