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New Post has been published on https://ramneetkaur.com/vernalization/

VERNALIZATION

Vernalization

Vernalization is the effect of cold temperature on flowering.

It prevents precocious reproductive development late in the growing season and enables the plant to have sufficient time to reach maturity.

These plants remain vegetative during the warm season, receive low temperature during winter, grow further & then bear flower & fruits.

Lysenko 1st reported the effect of cold temperature on flowering in the wheat seedling.

Seen in winter varieties of annual plants (wheat, barley & rye), some biennials (cabbage, sugarbeet, carrot) & perennial plants (Chrysanthemum).

The annual winter varieties plants also have spring varieties. Spring varieties are planted in spring and they flower & fruit in the same season.

Winter varieties plants are sown in autumn & they form seedling in winter. These seedlings receive the cold treatment & resume growth in spring. They then flower and fruit in the summer.

Low temperature required is from 0o to 5o.

Vernalization is perceived by the meristems, shoot tip, root apex, developing leaves etc.

The stimulus is transferred to all parts of the plant preparing it to flower.

Vernalin (hypothetical hormone) is produced that prepares the plant for flowering. It itself cannot induce flowering.

Devernalization occurs when a vernalized plant is exposed to high-temperature.

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New Post has been published on https://ramneetkaur.com/photoperiodism/

PHOTOPERIODISM

Photoperiodism

Photoperiodism is the effect of light on flowering.

Garner & Allard 1st reported the effect of light on flowering in Maryland Mammoth (variety of tobacco) & Glycine max.

On the basis of photoperiodic requirements, plants are divided into 3 types:

Short day plants: in these plants, the dark period is very important for flowering. The dark period should exceed a critical length. Light has no effect on flowering. E.g., rice, sugarcane, potato, tobacco, soybean, Chrysanthemum.

Long day plants: in these plants, the light period is very important for flowering. The light period should exceed a critical length. Darkness has an inhibitory effect on flowering. E.g., wheat, oat, spinach, radish, barley, lettuce.

Day-neutral plants: these plants blossom throughout the year. Length of light or dark period has no effect. E.g., tomato, cucumber, sunflower, cotton, maize, pepper.

 Leaves receive the photoperiodic stimulus. The minimum number of appropriate photoperiods required for induction is 1 to 25.

Effect of light on flowering is due to the presence of a photoreceptive pigment called Phytochrome. Discovered by Hedrick & Borthwick.

Phytochrome occurs in 2 forms that are inter-convertible. PR & PFR form.

  PR form absorbs red light (during the day) & changes into PFR,

While the PFR absorbs far-red light (during night) & changes back to PR.

PFR is the physiologically active form.

In short day plant, PR form causes flowering.

In long day plant, PFR form causes flowering.

A flowering hormone called Florigen (hypothetical) is produced in the leaves that are transferred to the shoot apices which causes flowering.

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[simplifiedbiology-blog]

New Post has been published on https://ramneetkaur.com/ethylene/

Ethylene

Ethylene:

Ethylene is a volatile gaseous compound that stimulates transverse growth but retards longitudinal growth.

Cousins confirmed its presence in ripened oranges, that hastened the ripening of stored unripe bananas.

Formed in all parts of the plant i.e., roots, leaves, flowers, seeds.

Maximum synthesis occurs in ripening fruits & tissues undergoing senescence.

Synthesized from Methionine.

Excess of auxin induces synthesis of ethylene.

Biologically active at low concentration.

Physiological effects of ethylene:

Causes horizontal growth of seedling, swelling of axis & apical hook formation in dicot seedling.

Horizontal growth of seedling

Climacteric is increase in the rate of respiration in ripening fruits due to the production of ethylene. Loosening fruits. To prevent fruits from ripening oxygen must be eliminated from the environment in which the fruits are stored.

Promotes senescence & abscission of leaves, flowers & fruits.

Can break dormancy of buds, seeds & storage organs. E.g., peanut & sprouting of potato tubers.

Causes elongation of internodes of deepwater rice plants, helping the upper part to remain above the water level.

It promotes root growth & root hair formation thus increasing the surface area for absorption.

Interferes with geotropic responses of seedlings.

Promotes flowering in pineapple & mango. Helps in synchronizing fruit set.

  Ethylene is widely used in agriculture as it effects a range of physiological processes.

Source of ethylene in agriculture is Ethephon, as it is readily absorbed in aqueous solution, transported to the various parts of the plant & releases ethylene slowly.

It hastens fruit ripening in tomato, apple, banana.

Accelerates abscission of excess flowers & fruits in cotton, cherry & walnut.

Promotes the formation of female flowers in Cucumber.

Rhizome, corm, tubers, seeds & storage organs can be made to sprout.

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New Post has been published on https://ramneetkaur.com/cytokinin/

CYTOKININ

Cytokinin:

Skoog & co-workers showed that callus formation occurred when internodal segments of tobacco are cultured. In the nutrient medium in addition to auxin, they provided extracts of yeast, vascular tissues, coconut milk or DNA.

Miller first discovered it from autoclaved Herring sperm DNA. He named it as kinetin.

Letham first obtained natural cytokinin from maize grain & coconut milk, it is referred as Zeatin.

Site of synthesis: Major source is roots & regions where rapid cell division occurs like meristems, root tip, buds, young leaves & developing fruits.

Transported through the xylem.

Derived from Adenine, it is 6-furfuryl aminopurine.

Physiological effects of cytokinin:

A true cell division factor required for cytokinesis. It can induce cell division in permanent cells along with auxin. It helps in the formation of inter-fascicular cambium.                                                                                                            Used in tissue culture along with auxin.

Can cause cell elongation by increasing permeability of cell membrane & thus increasing water uptake by the cell.

Morphogenesis: given by Skoog & Miller. They showed that when pith of tobacco is cultured in the presence of auxin & cytokinin, callus formation occurred. When the concentration of auxin is increased root started developing & when the concentration of cytokinin is increased shoot & bud formation occurred. It forms plantlets that can grow into new plants.

Promotes the growth of lateral buds thus suppressing apical dominance.

It induces the formation of new leaves, chloroplast in leaves, lateral shoot formation & adventitious shoot formation.

Capable of breaking dormancy of seeds.

It promotes the formation of female flowers.

Delay of senescence (Richmond-Lang effect): it delays senescence of leaves & organs by mobilizing the nutrients, reducing degradation of chlorophyll and protein. Thus preventing leaf fall and yellowing of leaves.

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[simplifiedbiology-blog]

New Post has been published on https://ramneetkaur.com/cell-division-mitosis-meiosis/

Cell Division - Mitosis & Meiosis

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CELL DIVISION: Mitosis & Meiosis

Cell Cycle

Can be divided into 2 stages: INTERPHASE.

G1 Growth phase 1.

S  Synthetic phase.

G2 Growth phase 2.

DIVISIONAL PHASE.

M Mitosis/Meiosis.

C Cytokinesis.

Mnemonic:“Go Sally Go! Make Children!”

  Mitosis

It is an equational division. Occurs in somatic cells.

Prophase, Metaphase, Anaphase, Telophase

Mnemonic: “People Meet And Talk”

Prophase:

Coiling of chromatin occurs, forming thin long threads.

By the end, chromosomes start forming,

Nucleolus & nuclear membrane starts disappearing by the end.

Spindle fiber formation starts.

Centriole in animal cells starts moving towards the poles.

Metaphase:

Nuclear membrane and nucleolus has disappeared,

Spindle fibers have formed, 

2 types of spindle fibers occur chromosomal fibers that are attached to chromosomes at the centromere & continuous fibers that join the 2 poles.

Chromosomes having two chromatids are seen,

Chromosomes align themselves on the equatorial plate due to contraction of spindle fibers.

Amphiastral mitosis occurs in animal cells & anastral mitosis occurs in plant cells.

Anaphase:

Shortest phase.

Centromere splits

Chromosomes start moving towards poles due to contraction of spindle fibers.

Various shapes of chromosomes are seen.

Telophase:

Chromosomes have reached the poles,

Uncoiling of chromosomes occur,

Nucleolus and nuclear membrane reappear,

Spindle fibers disappear.

2 nuclei are formed by the end.

Cytokinesis:

Starts in mid-Anaphase and ends by the end of Telophase dividing the cell into 2 daughter cells.

Occurs by invagination of the cell membrane in animal cells & by cell plate method in plant cells.

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Meiosis

It occurs in 2 stages:

Meiosis I – reductional division:

Prophase I, Metaphase I, Anaphase I, Telophase I.

Prophase I: divided into 5 substages.

Mnemonic: Little Zara in Pink Dress is Dancing.

Leptotene:

Chromatin coils forming thin long threads.

Zygotene:

Further coiling of chromatin occurs.

Pairing of homologous chromosomes occurs due to the mutual attraction between them.

Synapsis is pairing of homologous chromosomes.

Bivalents are seen.

Synaptinemal complex occurs between homologous chromosomes, that helps in precise pairing.

Pachytene:

Each chromosome splits longitudinally to form two chromatids attached at the centromere.

Bivalent changes into tetrad.

Crossing over, i.e., exchange of segments between non-sister chromatids occurs.

Crossing over occurs by the help of recombinase enzyme.

Diplotene:

Homologous chromosomes try to separate.

Chromosomes remain attached at regions where crossing over has occurred.

Chaisma is the regions where crossing over has occurred.

Chromosomes pull themselves apart from the centromere, as a result chaisma starts moving towards ends.

This is Terminalization, which starts in diplotene stage.

Diakinesis:

Terminalization completes forming ring-shaped chromosomes.

Nucleolus & nuclear membrane starts disappearing, spindle fiber formation starts.

Metaphase I:

Nucleolus & nuclear membrane has disappeared, spindle fiber formation is completed.

Chromosomes align on equatorial plate.

Anaphase I:

Homologous chromosomes separate due to contraction of spindle fibers, 

Terminal chaisma opens up & the chromosomes start moving towards poles.

Telophase I:

Chromosomes reach poles and uncoiling starts.

Nucleolus & nuclear membrane reappear, spindle fibers disappear.

Two nuclei one at each pole are formed.

Meiosis II – equational division:

Prophase II, Metaphase II, Anaphase II, Telophase II.

Meiosis II is same as Mitosis.

4 daughters cells each having haploid number of chromosomes are produced.

Also watch:Cell Cycle, Chromosomes.

Also read:

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