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.
