XII BIOLOGY
CH-1 Reproduction in lower and higher plants
PART-Outbreeding.devices (contrivances)
The examples of outbreeding devicesare as follows
1. Unisexuality :-
Some
plants produce unisexual flowers.
Monoecious
:-
Male
and female flowers are produced separately, but on same plant. It prevent only
atogamy.
e.g.
- Castor, Cucurbita, Maize etc.
Dioecious
:-
Male
and female flowers are produced on different plants. It prevent both autogamy
as well as geitonogamy . e.g. - Papaya, Mulberry etc.
It can be further divided into two types:
a. Protandry:
In this type, androecium
matures earlier than the gynoecium, e.g. in the disc florets of sunflower.
b. Protogyny:
In this type, gynoecium matures
earlier than the androecium, e.g. Gloriosa
3. Prepotency:
Pollen grains of other flowers germinate rapidly over the stigma than the pollen grains from the same flower, e.g. Apple.
4. Heterostyly (Heteromorphy):
In some plants like Primula (Primrose) there are two or three types of flowers in which stigmas and anthers are placed at different levels (heterostyly and heteroanthy).
This prevents the pollens from reaching the stigma and pollinating it.
In heteromorphic flowers, pollen grains produced from anther flower pollinate stigmas produced at the same level.
5. Herkogamy:
It is a mechanical device to prevent self-pollination in a bisexual flower.
In plants, natural physical barrier is present between two sex organs and avoid contact of pollen with stigma of same flower.
e.g. Calotropis-pentangular stigma is positioned above the level of anthers (pollinia).
6 . Self-incompatibility (self-sterility):
This
is a genetic mechanism due to which the germination of pollen on stigma of the
same flower is inhibited. e.g. Tobacco, Thea.
It is the
interaction of pollen grain with stigma. It begins with
pollination and ends with
fertilization .
i)All
the events from the deposition of pollen grain on stigma to the entry of pollen
tube in the ovule (synergid) are referred as pollen - pistil interaction.
ii)
Pollination does not guarantee the transfer of right type of pollen, often
wrong type also lands on stigma.
iii)Stigma
have capacity to recognize the pollen and it allows only right type of pollens
iv)
It is very dynamic process involving pollen recognition followed by inhibition
or promotion of pollen germination.
v)Cross
pollination can transfer right as well as wrong type of pollens on the stigma.
vi)The
right type of pollens i.e. compatible of the same species allow to germinate
and forms pollen tube.
vii)Pollen
tube moves through style, enters the ovary and finally in the ovule.
viii)The
wrong type or incompatible pollen i.e. from other species or self incompatible
pollen does not allow to germinate by stigma.
ix)The
ability of pistil to
recognize the pollen is because of certain chemical components present in
the pollen qrain, which interact with stigma.
x)Pollen
grain can also be induced to germinate in a synthetic medium.
xi)Sucrose
induces pollen germination and tube growth in vitro. Addition of boric acid
facilitates and accelerates pollen germination.
Artificial hybridization:
It is one of the major approaches used in the crop improvement
It is artificial method in which only the desired pollen grains are hand pollinated and used for fertilization.
This is accomplished through emasculation and bagging procedure.
Outbreeding Devices
Self-pollination is
common and more likely to happen in the case of hermaphrodite flowers but a
successive series of self-pollination affects negatively and causes inbreeding
depression. This also results in homozygous genes. Thus plants are adapted to
promote cross-pollination. This is known as outbreeding. Factors which
encourage cross-pollination are as follows:
1.Unisexual flower:
If
a flower is unisexual i.e., containing only one sex either female or male,
cross-pollination is the only choice.
2.Non-Synchronization:
Timing
is important for successful self-pollination. Pollen release and receptivity of
stigma should happen for successful self-pollination. Sometimes, pollen matures
and releases before the stigma is open which leads to loss of pollen vitality
or vice-versa. This prevents self-pollination; even though the flower is
hermaphrodite.
3.Self-incompatibility:
Incompatibility
within a flower (or plant) includes self-sterility; structural barriers.
Self-sterility means even though pollination takes place it can’t proceed to
fertilization due to further pollen growth failure. Structural barriers include
height difference between gynoecium and androecium and some structures which
hinder the stigma from receiving pollen. These are the more or less genetic
mechanism.
Pollen–Pistil
Interaction
All pollinations do
not lead to successful fertilization because for successful fertilization, the
pistil of a flower has to recognize the pollen of the same species. Therefore,
the interaction between pollen grains and the stigma needs to be understood
properly. Once the compatible pollen is recognised and accepted by the pistil,
events for fertilization proceed. The pollen which is incompatible is rejected
by the pistil so that the process of fertilization does not proceed.
Upon reaching the
pistil, the pollen grain releases chemicals which are recognised by the pistil.
This pollen-pistil interaction results in the acceptance of the compatible
pollen.
Pollen-pistil
interaction proceed as follows:
1.
The landing of true
pollen on the compatible pistil.
2.
Germination of pollen
and formation of pollen tube where pollen grains release their contents.
3.
Pollen tube growth
through the style of the pistil towards the ovary.
4. The entry of male gametes into the ovule and then to synergid.