Photoperiodism refers to the response of plants to the duration and timing of light and dark periods. It influences processes like flowering, dormancy, and tuber formation. There are five classes of plants based on their photoperiodic response - short day plants that flower in short days, long day plants that flower in long days, and day neutral plants that are insensitive to day length. The critical day length is the minimum or maximum day length required to induce flowering. Studies using grafting techniques provided evidence for a mobile signal called florigen that is produced in leaves in response to photoperiod and transported to the shoot tip to induce flowering. The phytochrome photoreceptor and circadian clock are involved in the time

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3. Contents
 Introduction
 Discovery
 5 photoperiodic classes of plants
 Diff b/w SDP and LDP
 Critical day length
 Night break phenomenon
 Mechanism of photoperiodism
 Florigen
 Grafting experiment
 Florigen concept

4. Contents
 Circadian clock and photoperiodic
timekeeping
 Coincidence model
 Phytochrome - primary photoreceptor in
photoperiodism
 Signficance of photoperiodism

5.  Photoperiodism is the response of plants to
duration and timings of light and dark
periods.
 Plant in order to flower require a particular
day length or light period called photoperiod
and response of plants to photoperiod in
terms of flowering is called photoperiodism
 It influences plant development such as leaf
fall , dormancy and tuber formation but its
major effect is on control of flowering.

6.  It is first discovered by W.W. Garner and H.A.
Allard (1920)
 They observed that tobacco mutant ie.
“Maryland mammoth” flowered at different
times at different places.
 After controlling other factors like nutrition,
temperature , etc, they reported that it was the
length of the day which affected flowering .
 They gave the term photoperiod to favourable
day length for each plant.

7. Five photoperiodic classes
of plants
 Short day plants (Glycine max )
 Long day plants (Beta vulgaris)
 Short –long day plants (Triticum vulgare)
 Long – short day plants (Bryophyllum)
 Day neutral plants (Tomato)

8. Short day plants
 Short day plants flower when day length is shorter
than a certain “critical period” .
 Under photoperiod longer than a critical point, these
plants will not flower.
 These plants require long uninterrupted dark period
hence called long night plants.
 Examples –Cannabis sativum , Nicotiana
tobacum , Glycine max , Xanthium .

11. long day plants
 Long day plants flower when day length is
longer than a certain “critical period”.
 Under photoperiods shorter than a critical
point, these plants will not flower.
 These plants are also called short night
plants.
 Examples –Beta vulgaris , Plantago ,
Spinacea oleracea.

14.  Short –long day plants are actually long day
plants but in their early life they need short
day conditions.
 They will flower only when if a certain no of
short days are followed by certain no of long
days.
 Examples – Trifolium repens, Canterbury
bells , Echeveria ,Triticum vulgare, Secale
cereale.

16. Long –short day plants
 Long –short day plants are short day plants
but require long day condition initially.
 These plants flower when long photoperiods
are followed by short photoperiods.
 Examples –Bryophyllum, Cestrum
nocturnum, Kalanchoe.

18. Day neutral plants/Indeterminate plants
 These plants flower in any photoperiod .
 They are insensitive to day length.
 Examples –tomato, Phaseolus vulgaris,
Castilleja chromosa.

20. SHORT DAY PLANT
 Plant flower when
photoperiod is less than
critical day length
 Interruption during light
period with darkness does not
inhibiting flowering
 Flowering is inhibiting if long
dark period is inhibited by
flash of light
 Long,uninterupted,continu
ous dark period is critical for
flowering
 Examples-Xanthium,coffee,
tobacco, Glycine
LONG DAY PLANT
 Plant flower when
photoperiod is more than
critical day length
 It inhibiting flowering
 Flowering does not inhibited.
 Light period is critical for
flowering .
 Examples-Beta
vulgaris,spinach,Plantago

21. Short day plants Long day plants
 Flowering does not occur
under alternating cycles of
short day and short light
periods.
 SDP flower in autumn- spring
periods
 Supply of GA does not induce
flowering under non inductive
photoperiods.
 ABA promotes flowering in
some cases.
 Flowering occurs under
alternating cycles of short day
followed by still shorter dark
periods .
 LDP flower in late spring –
summer periods
 Supply of GA induces flowering
in many cases under non-
inductive photoperiods .
 ABA does not promote flowering.
DIFFERENCE BETWEEN SDP AND LDP

24. Critical day length
 Critical day length is that length of
photoperiod above or below which flowering
occurs.
 Example –Xanthium is SDP and its CDL is 15
hrs ie., below 15 hrs ,flowering will take place.
 Hyocyamus niger is LDP and its CDL is 11 hrs
ie., above 11 hrs , flowering will take place.

25. Night break phenomenon(associated
with SDP)
 Plants with critical night length, a short flash of
light in middle of night could make the plant
behave as if it has been exposed to long day.
 As a result , flowering will not occur in SDP.

26. Mechanism of photoperiodism
• Leaf is the site of perception of the
photoperiodic signal.
• The floral stimulus is transported in the
phloem.
• Grafting studies have provided evidence for a
transmissible floral stimulus

27. Leaf is the site of perception
of the photoperiodic signal
 The photoperiodic stimulus in both LDP and 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 as photoperiodic induction.
 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.

28. The floral stimulus is transported
in the phloem
 The leaf –derived photoperiodic floral stimulus is
translocated via the phloem to the shoot apical
meristem ,where it promotes floral evocation.
 Floral evocation- the transfer of floral stimulus
(florigen) from leaf to apical meristem brings about
several biochemical and cytological changes in
meristem and it is transformed in to a floral bud.
 Treatments that blocks phloem translocation, such
as girdling or localized heat –killing block flowering
by preventing the movement of floral stimulus out of
the leaf.

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

30. Grafting experiment of
florigen existence
 Chailakhyan removed the leaves from the upper half
of Chrysanthemum (SDP) but left the leaves on
lower half untouched.
 He then exposed the lower half to short days- the
plants did not flower.
 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 concept instead of florigen .

31. Florigen concept
 After the discovery of gibberellins , Chailakhyan
modified his florigen concept .
 He proposed that florigen is 2 hormones rather than
1, a GA and a hypothetical hormone he called
anthesin .
 Florigen = GA + Anthesins (perhaps steroids).
 He suggested that LDP could produce anthesin
under any day length but GA only under long days.
 He also suggested that SDP produce GA under any
day length but anthesin only under short days.
 he also suggested that day neutral plants could
produce both under any day length.

32.  A plant could flower only when both GA and
anthesin were present .
 But again anthesin is just as hypothetical as
florigen has been.
 Antiflorigen has found in some LDPs ex-
Nicotiana sylvestris.
 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 .

34. The circadian clock and
photoperiodic timekeeping
 In the 1930, E .Bunning proposed a theory
(clock hypothesis ) involving the same clock
for circadian (circa + dian = approx 24 hr)
rhythm and for the time measurement in
photoperiodism.
 Acc to the theory , the metabolism of plants
oscillates between 2 separate phases , the
photophile (light loving ) and skotophile
(dark loving) phase.

35.  When the plant is in photophile phase,
exposure to light increases its various
processes including flowering.
 In scotophile phase ,light inhibits flowering
and other plant processes.
 The photoperiodism is executed by an
endogenous free running oscillation between
these 2 phases at a regular interval of approx
12 hrs.

37.  In previous slide ,we come to know that in
1936, Erwin Binning proposed that the
control of flowering by photoperiodism is
achieved by an oscillation of phases with
different sensitivities to light.
 This proposal has evolved in to the
coincidence model (Bunning 1960) in which
the circadian oscillator controls the timing of
light –sensitive and light insensitive phases.

38. Phytochrome is the primary
photoreceptor in photoperiodism
 Phytochrome is a bright blue chromoprotein made
up of 2parts –a chromophore or light absorbing
moiety and a protein called the apoprotein .
 It exist in to interconvertible forms referred by Pr
(P660) which has a light absorption peak in red light
(660 micrometer) and Pfr (P730), which has a light
absorption in far red light ( 730 micrometer).

39.  If P 660 is exposed either to sunlight or to red
light it changes to P730.
 If P730 is then exposed to far- red light ,it
changes quickly back to P660. this change
also occurs slowly in darkness.
 Pr Pfr

40.  A flash of red light during the dark period
induces flowering in an LDP, and the effect is
reversed by a flash of far –red light .This
response indicates the involvement of the
pytochrome .
 In SDPs ,a flash of red light prevents
flowering , and the effect is reversed by a
flash of far –red light.

41. IN THE SHORT-DAY PLANT
PFR PR builds up
Darkness (slow)
Far red light (fast)
Short-day
plants
FLORIGEN
Activated
FLOWERING

42. IN THE LONG-DAY PLANT
PFR builds up PR
Sunlight
Red light
Long-day
plants
FLOWERING
FLORIGEN
Activated

43. Summary
Sunlight
Red light
Darkness (slow)
Far red light (fast)
PFR builds up
Long-day
plants
FLOWERING
FLORIGEN
Activated
PR builds up
Short-day
plants
FLORIGEN
Activated
FLOWERING
© 2010 Paul Billiet ODWS

45. SIGNIFICANCE OF
PHOTOPERIODISM
 Photoperiodism determines the season in
which a particular plant shall come to flower.
SDP develop flowers in autumn-spring period ex-
Dahlia while LDP produce flowers in summer ex-
Amaranthus .
 Knowledge of photoperiodic effect is useful in
keeping some plants in vegetative growth to obtain
higher yield of tubers, rhizomes or keep the plant
in reproductive stage to yield more flowers and
fruits.
 A plant can be made to flower throughout the
year under green house conditions if a
favourable photoperiod is being provided to it.
This is used by commercial growers to meet the
demands of market for prized vegetables and

46.  The phenomenon helped the plant breeders in
effecting cross –breeding or hybridisation
experiments in plants which normally develop
flowers in different seasons.
 Photoperiodic response of plants is under the
control of genes. Manipulation of genes can
enable the plants to flower in different seasons.
ex-NBRI develop varieties of Chrysanthemum
which flower in different months of the year.
 knowledge of Photoperiodism is highly useful in
laying out gardens , orchards and planning
crop pattern of the area.

47. Thank you
From Prerna jain
MSC botany honours
Registation no -11615509
section-1628