Vernalizationand its importance in plants.ppt

the role of vernalization in flowering is very much significant and this ppt gives a breif gist on the vernalization process and floweriing.

1. VERNALIZATION

2. Dry seeds do not respond to the cold treatment.
Without the cold treatment, plants that require vernalization show delayed flowering or
remain vegetative.
In many cases these plants grow as rosettes with no elongation of the stem.
Due to vernalization the vegetative period of the plant is cut short resulting in an early
flowering.
Some of the characteristics of the cold requirement for flowering, includes the range and
duration of the inductive temperatures, the sites of perception, the relationship to
photoperiodism, and a possible molecular mechanism.
Vernalization
•Vernalization is the process whereby flowering is promoted by a cold treatment given to a
fully hydrated seed (i.e., a seed that has imbibed water) or to a growing plant.

3. History of vernalization
Klippart,1857- first noticed the low temperature requirement for flowering while
working with winter wheat and spring wheat.
Lysenko,1938-used the term vernalization for a low temperature promotion of flowering
in plants.
Chourad ,1960- defined vernalization as “acquisition or acceleration of the ability to
flower by a chilling treatment”.
Vernalization appears to take place primarily in the shoot apical meristem.
Localized cooling causes flowering when only the stem apex is chilled, and this effect
appears to be largely independent of the temperature experienced by the rest of the plant.

4. Conditions necessary for vernalization
1.Age of the plant
•The age of the plant is an important
factor in determining the responsiveness
of the plant to the cold stimulus and it
differs in different species.
•In cereals like winter wheat, the
vernalization is effective only if the
germinating seeds have received cold
temperature treatment for sufficient
time.
•While in the case of biennial variety of
henbane (Hyoscyamus niger), the plant
will respond to the cold treatment, only
if they are at rosette stage and
completed at least 10 days of growth.

5. 2. Appropriate low temperature and duration of the exposure
•Most suitable temperature for vernalizing the plants ranges between 1-6°C.
•The effectiveness of low temperature treatment decreases from 0 to 4°C.
•Low temperature at about -6°C is completely ineffective.
•Similarly at high temperatures from 7°C onwards, the response of the plants is
decreased.
•Temperature of about 12-14°C is almost ineffective in vernalizing the plant.
•Besides an appropriate low temperature, a suitable duration of the cold
treatment is essential for vernalization.
•Depending upon the degree of temperature and in different species this period
may vary, but usually the duration of the chilling treatment is about one and half
months or more.
3. Oxygen
•The vernalization is an aerobic process and requires metabolic energy. In the
absence of O2, cold treatment becomes completely ineffective.
4. Water
•Sufficient amount of water is also essential for vernalization. Vernalization of the
dry seed is not possible.

6. Mechanism of vernalization
Two theories explains the mechenism of vernalization
1.Phasic development theory
2.Hormonal theories
1.Phasic development theory
Proposed by Lysenko in 1934.
According to this theory there is a series of phases in the development of a plant.
Each phase is stimulated by an environmental factor such as temperature,light,etc.
Commencement of one phase will take place only after the completion of the proceeding
phase.
There are two phases
1.Thermophase
2.Photophase
Thermophase depends on temperature.vernalization accelerates thermophase.
Thermophase should be followed by photophase which requires light.

7. a) Thermostage:
• It relies on temperature.
• Vernalization quickens or accelerates thermostat.
•Thermostage is the vegetative stage which requires low heat (0-14°C,
reasonable dampness and air circulation (aeration).
•This phase is of variable length and relies on the nature of plants and condition.
•Winter wheat finishes their life cycle most quickly when given a brief day and
low temperature in thermostat.
b) Photostage
•The high temperature is required.
•In this stage, vernalin helps in the production of florigen.
•Winter wheat finishes their life cycle most quickly when given an extended day
and higher temperature during the photo stage.

8. 2. Hormonal theories
Harmonal theory proposed by Melcher (1939).
He proposed that chilling treatment induces the
formation of a new floral hormone called vernalin.
This hormone is transmitted to other parts of the
plant.
He graphted a vernalized plant with an unvernalized
plant.
The unvernalized plant also initiates flowering.
The hormone, vernalin diffuses from the vernalized
plant to the unvernalized plant and induces flowering.
According to
 
Purvis 
(1961),
formation of a substance A
from its precursor, is converted into B after chilling.
The substance B is unstable. At suitable temperature B
is converted into stable compound D called
 
Vernalin.
Vernalin is converted to F (Florigen).
Florigen induces flower formation.
At high temperature B is converted to C and
devernalization occurs.

9. Devernalization
•The positive effect of the low temperature treatment on the vernalization of the plant
can be counteracted by subsequent high temperature. This is called devernalization.
•The devernalized plant can again be vernalized by subsequent low temperature
treatment.
Vernalization and Gibberellins
•The gibberellins are known to replace the low temperature requirement in certain
biennial plants such as henbane.
•The plant normally remains vegetative and retains its rosette habit during the first
growing season and after passing through the winter period flowers in the next season.
•The gibberellins cause such plants to flower even during the first year.

10. Epigenetic Changes in Gene Expression
Vernalization May Involve Epigenetic Changes in Gene Expression.
Changes in gene expression that are stable even after the signal that induced the change
(in this case cold) is removed are known as epigenetic regulation.
The involvement of epigenetic regulation in the vernalization process has been confirmed
in the LDP Arabidopsis.
A gene that acts as a repressor of flowering has been identified: FLOWERING LOCUS C
(FLC).
FLC is highly expressed in nonvernalized shoot apical meristems (Michaels and Amasino
2000).
After vernalization, this gene is epigenetically switched off by an unknown mechanism
for the remainder of the plant’s life cycle.

11. FIGURE: (Left) Vernalization blocks the expression of the gene FLOWERING LOCUS C
(FLC) in cold-requiring winter annual ecotypes of Arabidopsis. (Right) A winter annual with
an FLC mutation exhibits early flowering without cold treatment. (Photo courtesy of R.
Amasino.)

12. Due to vernalization the vegetative period of the plant is cut short resulting in an early
flowering.
Vernalization increases the resistance of plants to fungal diseases.
It increases the cold resistance of plants.
In the biennials, vernalization induces early flowering and early fruit setting.
Flowering can be induced by graphting and this feature is used in horticulture.
It also helps in crop improvement.
Practical applications of vernalization