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1. REPRODUCTION IN LOWER & HIGHER PLANTS

-CREATED BY-

PROF. DESHMUKH A. B.

ASST. TEACHER

SEC. & HIGHER SEC. SCHOOL, SHENDI (BHANDARDARA DAM), TAL. – AKOLE, DIST. - AHMEDNAGAR

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1.7 Outbreeding devices (contrivances):

  • Many plants have mechanisms that discourage or prevent self pollination.
  • To promote cross pollination and increase genetic diversity, plants have evolved a wide variety of sexual strategies.
    • Genetic diversity is an essential factor for evolution by natural selection.
  • Continued self pollination results in the inbreeding depression.

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  • Thus plants have developed many devices to encourage cross pollination.
  • The examples of outbreeding devices are as follows:
    • Unisexuality,
    • Dichogamy,
    • Prepotency,
    • Heterostyly,
    • Herkogamy,
    • Self-incompatibility, etc.

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1.7 Outbreeding devices - Unisexuality :

  • In this case, the plant bears either male or female flowers.
    • It is also called as dioecism.
  • As flowers are unisexual, self pollination is not possible.
  • Plants may be monoecious,

e.g. Maize or

dioecious,

e.g. Mulberry, Papaya

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Unisexual Flowers

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Male Flowers of Maize

Anthers

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Female flowers of Maize

Flowers

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Male inflorescence of White Mulberry

Anther

Filament

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Female inflorescence of White Mulberry

Stigma

Inflorescence

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Female flower of Papaya

Male flower of Papaya

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1.7 Outbreeding devices - Dichogamy :

  • In this device, anthers and stigmas mature at different times in a bisexual flower so as to prevent self pollination.
  • It can be further divided into two types:

1. Protandry :

    • In this type, androecium matures earlier than the gynoecium,

e.g. in the disc florets of sunflower.

2. Protogyny :

    • In this type, gynoecium matures earlier

than the androecium,

e.g. Gloriosa.

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Protandry

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Gloriosa flower

Carpel

Stamen

Protogyny

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1.7 Outbreeding devices - Prepotency :

  • Pollen grains of other flowers germinate rapidly over the stigma than the pollen grains from the same flower.

e.g. Apple.

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1.7 Outbreeding devices - Heterostyly (heteromorphy):

  • In some plants like Primula (Primrose), there are two or three forms/ 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 pollinate stigmas produced at the same level.

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Primula

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Heterostyly and Heteroanthy

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1.7 Outbreeding devices - 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).

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Herkogamy

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1.7 Outbreeding devices - 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.

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1.8 Pollen - Pistil Interaction :

  • It is the interaction of pollen grains with sporophytic tissue (stigma).
  • It begins with pollination and ends with fertilization.
  • 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.
  • Pollination
    • does not guarantee the transfer of right type of pollen,
    • often wrong type also land on stigma.

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Right type

Pollen

Wrong type

Pollen

Stigma

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  • The pistil has the ability to recognize and accept the right or compatible pollen of the same species.
  • Thus, wrong type of pollen is discarded by pistil.
  • Compatibility and incompatibility of the pollen-pistil is determined by special proteins.
  • This process involves pollen recognition followed by promotion or inhibition of pollen grmination.

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Protein of Right type Pollen

Protein of Wrong type Pollen

Pistil Protein

Protein interacted

No Protein interaction

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  • The stigmatic surface of flower refuse other wrong type or incompatible pollen grains.
  • A physiological mechanism operates to ensure that only intraspecific pollen germinate successfully.
  • The compatible pollen absorbs water and nutrients from the surface of stigma, germinates and produces pollen tube.
  • Its growth through the style is determined by specific chemicals.

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Germination of Right type Pollen

Germination of Wrong type Pollen is Prohibited

Pollen Tube

Specific Chemical molecules allows entry of Pollen tube through Style

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  • The stigmatic surface provides the essential prerequisites for a successful germination, which are absent in the pollen.
  • The pollen tube is finally pushed through the ovule and reaches the embryo sac.
  • The tip of the pollen tube enters in one of the synergids and then ruptures to release the contents.
  • Due to pollen pistil interaction, intense competition develops even in the compatible pollen grains (gametes).

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  • It also plays important role in sexual reproduction and seed formation.
  • Pollen grain can also be induced to germinate in a synthetic medium.
  • Sucrose induces pollen germination and tube growth in vitro.
  • Addition of boric acid facilitates and accelerates pollen germination.

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Artificial hybridization :

  • It is one of the major approaches used in the crop improvement.
  • Only the desired pollen grains are hand pollinated and used for fertilization.
  • This is accomplished through emasculation and bagging procedure.

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Emasculation

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Bagging

Emasculated flower is covered with a polythene bag. It avoids contamination.

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Tagging:-

A tag is tied properly to the plant. Circular or rectangular tags may be used. A tag bears following information –

  • Cross combination

(name of varieties crossed)

  • Date of emasculation
  • Date of pollination
  • Numbers of birds pollinated
  • Signature of the breeder