SEXUAL REPRODUCTION IN FLOWERING PLANTS

 

Outbreeding Devices:

  • Majority of the flowering plants produce hermaphrodite flower and undergo autogamy.
  • Continuous autogamy or self-pollination results in inbreeding depression.
  • Flowering plants have developed many devices to avoid self pollination and to encourage cross-pollination. Such devices are called Outbreeding devices.
    • Pollen released and stigma receptivity is not synchronized.
    • Spatial separation of anthers and stigmas
    • Anther and stigma are placed at different positions.
    • Self incompatibility.
    • Production of unisexual flowers.

Pollen pistil Interaction:

  • All the events – from pollen deposition on the stigma until pollen tubes enter the ovule – are together referred as pollen-pistil interaction.
  • The pistil has the ability to recognize the pollen whether it is compatible or incompatible.
  • If it is right type the stigma allow the pollen to germinate.
  • If it is wrong type the stigma rejects the pollen, preventing germination.
  • The ability of the pistil to recognize the pollen by continuous dialogue mediated by chemical like Boron, Inositol and sucrose level.
  • Following compatible pollination, the pollen grain produce pollen tube through one of the germ pore.
  • Content of the pollen grain move into the pollen tube.
  • Pollen tube grows through the tissues of the stigma and style and reaches the ovary.
  • If the pollen grain is in 2-celled stage the generative cell divides and forms two male gametes inside the pollen tube.
  • If the pollen grain is in 3- cell stage the pollen tube carry two male gametes from the beginning.
  • Pollen tube enters into the ovule through micropyle and then into the embryo sac through synergids guided by filiform apparatus.

Artificial hybridization:

  • One of the major approaches of crop improvement programme.
  • Only desired pollen grain used for pollination.
  • Stigma is protected from contamination (from unwanted pollen grain).
  • Removal of anthers from the flower bud before the anther dehisces is called emasculation.
  • Emasculated flowers covered by bag generally made up of butter paper, to prevent contamination of its stigma with unwanted pollen. This step is called bagging.
  • If the female flower is unisexual there is no need of emasculation.

Double fertilization:

  • After entering one of the synergids, the pollen tube releases two male gametes into the cytoplasm of the synergids.
  • Syngamy: one of the male gamete fused with egg cell, to form a diploid zygote.
  • Two polar nuclei of central cell fused to form a diploid secondary nucleus.
  • Triple fusion: The second male gamete fused with the secondary nucleus to form a triploid primary endosperm nucleus.
  • Since two type of fusion, syngamy and triple fusion take place in the embryo sac the phenomenon is termed as double fertilization.
  • The central cell after triple fusion becomes primary endosperm cell and developed into the endosperm.
  • The zygote developed into an embryo.

POST- FERTILIZATION : STRUCTURE AND EVENTS

Events of endosperm and embryo development, maturation of ovule into seed and ovary into fruit, are collectively termed as post-fertilization events.

Endosperm:

  • Development of endosperm takes place before the embryo development.
  • Primary endosperm cell divides repeatedly to form a triploid endosperm.
  • Cells are filled with reserve food material and are used for the nutrition of the developing embryo.
  • PEN undergoes successive nuclear division to give rise to free nuclei. This is called free-nuclear endosperm.
  • Subsequently cell wall formation takes place and become cellular endosperm.
  • The coconut water is free nuclear endosperm and the white kernel is the cellular endosperm.
  • Endosperm may be consumed completely during embryo developed or it may be consumed during germination of seed.

Embryo:

  • Zygote formed and placed at the micropylar end of the embryo sac.
  • Zygote starts its development only after some amount of endosperm formed.
  • Embryo development takes place in following stages:
    • Proembryo
    • Globular stage
    • Heart shaped
    • Matured embryo.

Dicot embryo:

  • A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons.
  • Embryonal axis above the cotyledon is the epicotyls.
  • Terminal part of the epicotyls is the plumule (gives rise to the shoot).
  • Embryonal axis below the cotyledon is the hypocotyl.
  • The terminal part of the hypocotyl is called the radicle (root tip).
  • The root tip is covered by the root cap.

Monocot embryo:

  • Possesses only one cotyledon
  • In grass family the cotyledon is called scutellum.
  • Scutellum situated towards one side of the embryonal axis.
  • Radicle and the root cap enclosed by a sheath called coleorhiza.
  • The portion of the embryonal axis above level of attachment of scutellum is called epicotyls.
  • Epicotyl has the shoot apex or plumule enclosed by hollow foliar structure called coleoptile.
  • Seed is the final product of the sexual reproduction.
  • Seed consists of seed coat, cotyledon and an embryo axis.
  • Cotyledon stores the reserve food material for development and germination.
  • Matured seed without endosperm called non-albuminous. (Ground nut)
  • A part of the endosperm retained in matured seed is Albuminous.
  • Remainants of nucellus in the matured seed is called perisperm. E.g. black peeper, beet.
  • The wall of the ovary develops into the wall of fruit called pericarp.
  • Fruit developed from the ovary is called true fruit.
  • In apple, strawberry, cashew, the thalamus contributes in the fruit formation is called false fruit.
  • Fruit developed without fertilization is called Parthenocarpic fruits.

APOMIXIS AND POLYEMBRYONY.

  • Apomixis is very common in Asteraceae and grasses.
  • Seeds are produced without fertilization.
  • Apomixis is a type of asexual reproduction which mimics the sexual reproduction.
  • Diploid egg cell is formed without meiosis and develops into seed without fertilization.
  • In Citrusand Mango the nucellar cells starts dividing, protrude into the embryo sac and develop into embryo.
  • Ovule having more than one embryo is termed as polyembryony.
  • Hybrid plants are developed by apomixis to maintain the genetic identity.