Sexual reproduction in flowering plants (New syllabus chapter 1 / old syllabus Chapter 2)
Outbreeding devices | Pollen - pistil interaction|reproduction in flowering plants|short notes| NCERT| standard 12| Biology
Note 10
> Majority of flowering plants produce - hermaphrodite flowers (bisexual flower - flowers who have both Androecium and Gynoecium)
> Pollen grains have high chances to come in contact with stigma of same flower - self-pollination
> But continued self-pollination cause inbreeding depression - which may cause genetic defects and other physiological problems
> So, plants have developed devices (techniques) to discourage self-pollination.
---(1) No synchrony:
-------> pollen release and stigma receptivity are not synchronised
-------> pollen is released before stigma becomes receptive or stigma becomes receptive much before pollen is released
-------> prevents autogamy
---(2) positioning:
-------> anther and stigma are at different positions
-------> position is such that pollen cannot come in contact with stigma on same flower
---(3) self-incompatibility:
--------> genetic mechanism
--------> prevent self - pollen ( from same flower or flower of same plant) from fertilizing ovules
--------> it inhibits pollen germination or pollen tube growth in pistil
---(4) Production of unisexual flowers:
--------> both male and female flowers on same plant (monoecious) eg. Castor and maize - this prevents autogamy but not geitanogamy
--------> male and female flowers on different plants (dioecious) eg. Papaya - each plant is unisexual either male or female - this prevents both autogamy and geitanogamy
Pollen-Pistil interaction
> Definition: All events from pollen landing (deposition) on stigma until pollen tube's entry into ovule are called as pollen-pistil interaction
> Pollination does not guarantee transfer of the right type of pollen (compatible pollen of same species as the stigma eg. Pollen of same species of peas falling on stigma of same species of peas)
> Pollen of wrong type can be - from other species or same plant (is self-incompatibility)
> Pistil can recognise the pollen - if pollen is right (compatible) or wrong (incompatible)
> If pollen is of right type → pistil accepts the pollen →post-pollination events (pollen germination on stigma or pollen tube growth in style) take place → fertilisation
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| Pollen grains germinating on the stigma |
> If pollen is of wrong type → pistil rejects the pollen → post-pollination events are prevented
> Pistil has ability to recognise the pollen and then acceptance or rejection →this is due to continuous dialogue (interaction) between pollen grains and pistil
> This dialogue is mediated by chemicals - these chemicals are produced by both pollen and pistil as they come in contact with each other - this has been recently discovered by botanist
> If pollen is compatible with stigma/pistil → compatible pollination
---> pollen grains germinate on stigma
---> pollen tube is produced through one of the germ pores of pollen grain
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| Pollen tubes growing through the style |
---> content of pollen grain move into pollen tube
---> pollen tube grows through the tissues of stigma and style and then reaches ovary
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| enlarged view of an egg apparatus showing entry of pollen tube into a synergid |
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| Discharge of male gametes into a synergid and the movements of the sperms, one into the egg and the other into the central cell |
> male gamete in pollen grain:
---> some plants shed pollen grains at two-celled stage - it has one vegetative cell and one germinative cell - in the growing pollen tube the germinative cell divide and produce 2 male gametes
---> in other plants, pollen grains are shed at 3-celled stage - pollen tube contains 2 male gametes from beginning
> Pollen tube reaches ovary → enters through micropylar end → enters one of the synergids through filliform apparatus
Note: filliform apparatus present at micropylar end of synergid - guide the pollen tube for its entry
Note: pollen germination can be studied under microscope - dust pollen from any flower on a glass slide which has drop of 10% sugar solution → after 15-30 minutes watch in microscope under low power lens → pollen tubes coming out of the pollen grains can be seen
How is pollen-pistil interaction useful?
> Pollen pistil interaction is a dynamic process (process that is constantly changing and progressing)
> It involves pollen recognition followed by promotion or inhibition of pollen
> This knowledge can be used by plant breeders (people who use various techniques to improve crop and develope desired characteristics)
> Using this knowledge, they can manipulate pollen-pistil interaction to get desired hybrids.
> This manipulation can also be done in incompatible pollination.
Artificial hybridization
> Breeder is interested in crop-improvement programs.
> It includes crossing different species and often genera to combine desirable characters to produce commercially 'superior' varieties.
> Artificial hybridization is one of the major approaches of crop-improvement programme.
> 2 requirements for successful artificial hybridization:
---> (1) only the desired pollen grains are used for pollination
---> (2) Stigma is protected from contamination from unwanted pollens
> In artificial hybridization, two types of flowers/plants are used.
---> (1) those flowers/plants whose pollen are used for hybridization - they are called as male parent. Because they contribute male gamete.
---> (2) those flowers/plants whose stigma is used in hybridization - they are called as female parent. Because they contribute female gamete.
> Techniques for artificial hybridization
>> Emasculation:
----> if female parent bears bisexual flowers - autogamy can occur
----> so, before anther dehisce anthers are removed from the flower bud (pair of forceps are used if needed)
>> Bagging:
----> emasculated flowers have to be covered with a bag of suitable size.
----> bag is generally made up of butter paper.
----> this is to prevent stigma contamination with unwanted pollen.
> mature pollen grains from anther of male parent are used - these are the desired pollen grains
> When stigma of bagged flower attains (reaches) receptivity - the desired pollen grains are dusted on stigma
> Flowers are then re-bagged to avoid infection from environments
> Fruits are allowed to develop.
> If female parent produce unisexual flowers - emasculation is not required
> Female flower buds are bagged before flower open up
> When stigma become receptive - pollination is carried out using the desired pollen
> Flower is rebagged and fruits are allowed to develop
Thank you for reading!
Happy learning!
Manish Mevada
Urvi Bhanushali
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