Photoperiodism is the phenomenon of physiological changes that occur in plants in response to relative length of day and night (i.e. photoperiod). The response of the plants to the photoperiod, expressed in the form of flowering is also called as photoperiodism. The phenomenon of photoperiodism was first discovered by Garner and Allard (1920).Depending upon the duration of photoperiod, the plants are classified into three categories. 1. Short day plants (SDP) 2. Long day plants (LDP) 3. Day neutral plants (DNP) Vernalization is the induction of a plant's flowering process by exposure to the prolonged cold of winter, or by an artificial equivalent. After vernalization, plants have acquired the ability to flower, but they may require additional seasonal cues or weeks of growth before they will actually flower.The vernalization requirement ensures that plants do not flower in the fall when the environmental conditions are unfavorable for reproduction. The strength of the vernalization requirement can vary within plant species.

1. PHOTOPERIODISM AND
VERNALIZATION

2. C
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 Photoperiodism-
Types
phytochrome
Significance
 Vernalisation-
Introduction
Site
requirement
mechanism
significance

3. PH
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4. I
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 Photoperiodism is the phenomenon of physiological changes
that occur in plants in response to relative length of day and
night (i.e. photoperiod).
 The response of the plants to the photoperiod, expressed
in the form of flowering is also called as photoperiodism.
 The phenomenon of photoperiodism was first
discovered by Garner and Allard (1920).

5. T
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 Depending upon the
duration of photoperiod,
the plants are classified
into three categories.
 1. Short day plants (SDP)
 2. Long day plants (LDP)
 3. Day neutral plants
(DNP)

6. 1.S
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 These plants require a relatively short day light period (usually
8-10 hours) and a continuous dark period of about 14-16
hours for subsequent flowering.
 These plants are also known as long-night plants.
 E.g. Rice, coffee, soybean, tobacco and chrysanthemum
 In short day plants, the dark period is critical and must be
continuous. If this dark period is interrupted with a brief
exposure of red light (660-665 nm wavelength), the short day
plant will not flower.

7. 2.L
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 These plants require longer day light period (usually 14-16
hours) in a 24 hours cycle for subsequent flowering.
 These plants are also called as short night plants.
 E.g. Wheat, radish, cabbage, sugar beet and spinach.
 In long day plants, light period is critical.
 A brief exposure of red light in the dark period or the
prolongation of light period stimulates flowering in long day
plants.

8. PA
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9. 3.D
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These plants flower in all photoperiod ranging
from 5 hours to 24 hours continuous exposure.
E.g. Tomato, cotton, sunflower, cucumber, peas
and certain varieties of tobacco.

10. I
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i. Long short day plants
 These are short day plants but must be exposed
to long days during early periods of growth for
subsequent flowering. E.g. Bryophyllum.
ii. Short –long day plants
 These are long day plants but must be exposed
to short day during early periods of growth for
subsequent flowering. E.g. certain varieties of
wheat and rye.

11. Leaf i s t hesi t e of percept i on of the
photoperiodic signal
 The photoperiodic stimulus in both LDPand SDP is perceived by
the leaves.
 Leaf transmits a signal that regulates the transition to
flowering at the shoot apex.
 The photoperiod - regulated processes that occur in the leaves
resulting in the transmission of a floral stimulus to the shoot
apex are collectively asphotoperiodicinduction.
 Extensive grafting studies have shown that the induced leaf is
the source of a mobile floral stimulus that is transported to the
shoot apical meristem.

12. The name Florigen was proposed by
Chailakhyan in 1936, for the unknown
chemical stimulus which could act asafloral
inducer.
The significant evidence of the existence of
such a substance comes from interspecific
grafting experiment.

13. Grafting experiment of
florigen existence
 Chailakhyan removed the leaves from the upper half
of Chrysanthemum (SDP) but left the leaves on
lower halfuntouched.
 He then exposed the lower half to short days- the
plants did notflower.
 He concluded that day length does not have an
effect directly on the buds but causes the leaves to
manufacture a hormone that moves from the leaves
to the buds and induce flowering.
 This hypothetical hormone has been named florigen
and it has never been isolated and therefore we now
talk of florigen conceptinstead of florigen .

14. Fl ori gen concept
 After the discoveryof gibberellins , Chailakhyan
modified hisflorigen concept.
 He proposed that florigen is 2 hormones rather than
1, a GA and a hypothetical hormone he called
anthesin .
 Florigen =GA +Anthesins (perhapssteroids).
 He suggested that LDP could produce anthesin
under anyday length butGAonly under long days.
 He also suggested that SDP produce GA under any
daylength but anthesin only under short days.
 he also suggested that day neutral plants could
produceboth under any day length.

15. Aplant could flower only when both GAand
anthesin were present.
But again anthesin is just ashypothetical as
florigen hasbeen.
Antiflorigen hasfound in some LDPsex-
Nicotianasylvestris.
Florigen may be macromolecule- it is possible
that florigen is an RNA or protein molecule
that is translocated from the leaf to the apical
meristem via the phloem .

16. PHOTORECEPTORS
 All photoreceptors contain an organic non-protein
component known as chromophore that serves as the
primary site of photon absorption (Möglich et al. 2010).
 At present, six classes of photoreceptors are known:
PHYs, cryptochromes (CRYs), light– oxygen–voltage
(LOV) sensors, blue light sensors utilizing flavin adenine
dinucleotide (FAD), rhodopsins, and xanthopsins (Möglich
et al. 2010).

21. SIGNIFICANCE
 hybridization experiments(florigenhormone).
 Physiological preconditioning.
 Increase yield
 Plants remain vegetative for longer period
(radish,carrot,etc.)
 Annuals can be grown twice or thrice a year.
 Prevention of winter dormancy & autumn leaf fall.
 Strawberry-Increased stolon formation through long
days.

22. VE
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N

23. IN
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 It was found by Lysenko (1928), a Russian worker that the
cold requiring annual and biennial plants can be made to
flower in one growing season by providing low temperature
treatment to young plants or moistened seeds.
 He called the effect of this chilling treatment as
vernalization. Vernalization is, therefore, a process of
shortening of the juvenile or vegetative phase and
hastening flowering by a previous cold treatment.
 Some plants require exposure to a low temperature , 0 to
10°C ,for few days to a few weeks for flowering. such an
induction of flowering by a low temperature is called
vernalisation

25. SI
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 The stimulus of
vernalization is
perceived only by the
meristematic
cells(active apical
meristem).
 e.g.- shoot tip,
embryo tips, root
apex, developing
leaves, etc.

26. RE
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 (i) Low Temperature:Low temperature required
for vernalization is usually 0°—5°.
 (ii) Period of Low Temperature Treatment:It varies
from a few hours to a few days.
 (iii) Actively Dividing Cells:
 (iv) Water:
 (v) Aerobic Respiration
 (vi) Proper Nourishment.

27. ME
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 The stimulus received by the actively dividing
cells of shoot or embryo tip travels to all parts of
the plant and prepare it to flower.
 The stimulus has been named as vernalin.
 It can be passed from one plant to another through
grafting in case of Henbane but not in others.

28. Mechanism of Vernalization
Two theories
1. Phasic
Development
Hypothesis
2.Hormonal
Hypothesis.

32. DE
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The positive effect of the low temperature
treatment on the vernaliztaion of the plants
can be counteracted by subsequent high
temperature treatment. This is called
as devernalization.
The degree of devernalization decreases if
the duration of the cold treatment has been
longer.
However, the devernalized plant can
again be vernalized by subsequent low

33. 1.Water and Oxygen
• Seed need to imbibe 10-20 % water, dry seed cannot be vernalized.
• Similarly oxygen is also necessary for vernalization. Oxygen is used during
respiration. A germinating seed need energy. This energy is provided by
respiration. The inhibition of respiration blocks the process of vernalization.
2.Temperature
• The seeds are exposed to low temperature during vernalization.
• Temperature lower than 4°C is not effective. Similarly, temperature above 12°C
does not cause vernalization,the duration of vernalization varies in different
species.
3.Devernalization
The exposing of vernalized seed to high temperature (30 -35°C) , lose the effectof
vernalization called as Devernalization.
FACTORS AFFECTING VERNALIZATION

34. 4.Effect of light
• The seed of a plant can be vernalized in the presence or absence oflight,
• But many plants like carrot require both vernalization and photoperiodic
stimulus. They must be first vernalized. Then are given photoperiodic
stimulus for flowering.
5.Effect of hormones
Many species like carrot and Hyoscyamus have obligate vernalization
requirement. These plants show rapid flowering after vernalization due to GA
synthesis,It causes both bolting and flowering.

35. PRACTICAL APPLICATIONS
• 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 grafting and this feature is used in
horticulture.
• It also helps in crop improvement.

36. C
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37. T
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