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1. PHOTOPERIODISM

2. PHOTOPERIODISM
➢"The developmental responses of plants to the relative lengths of light and
dark periods is called Photoperiodism".
➢ Photoperiodism is the ability of plants to measure the day length.
Many physiological events such as:
Initiation flowering
Seed germination
Asexual rep.
Seed dormancy
5
These events are triggered by
photoperiods of a certain length
this specific photoperiod length
is referred as critical day length
or critical photoperiod. The
length of the critical day length
varies between species.
Formation of storage
organs

3. ➢ Long day plant (LDPs)
These plants flowers only when theday length is longer than the critical day length (qualitative
LDPs) or their flowering is accelerated by longer day length (quantitative LDPs).
➢ Short day plant (SDPS)
These are plants which flowers onlywhen the day length is less than critical day length
(qualitative SDPs) or their flowering is accelerated only by shorter day length (quantitative SDPs).
➢ DAY NETRAL PLANTS (DNPS)
Some plants species show no response to day length they are described as day neutral plants.
DNPs are insensitive to day length and flower under any photoperiodic conditions. They are
under autonomous regulation.
Plants can be classified into 3 main categories in response to critical photoperiod
on flowering:

4. Long-short day plants (LSDPS)
Flower only after a sequence of long days followed by short days. For example:
Bryophyllum, Cestrum nocturnum (Night-blooming jasmine) etc.
Short-long day plants (SLDPs)
Flower only after a sequence of short days followed by long days. For example:
Campinula medium (Canterbury bells), Trifolium repens (white clover) etc.
BESIDES ABOVE CATEGORIES, THERE ARE FEW OTHER CATEGORIES SUCH AS
"Photoperiodism was discovered by W. W. Garner & H. A. Allard in Nicotiana
tabacum var. Maryland Mammoth (Tobacco)".

5. ➢In Short Day Plants, flowering is inhibited if a flash of light is given in dark conditions.
➢Long Day Plants can flower even under short day condition, if a flash of light is given in
the dark period.
➢In both LDPs & SDPS a night break was found to be mosteffective when given near
the middle of dark period.
➢Critical day length is the "limit" of the photoperiod below which SDPS flowers and
above which LDPs flowers.
NIGHT BREAK PHENOMENON & CRITICAL DAY LENGTH
The flowering effect of dark period in Xanthium (SDP) can be nullified by
interrupting the dark period with a light break "Dark interruption or Night break".

6. • The experimental studies on Photoperiodism showed that the plants measure
neither the relative length of day and night nor the length of the photoperiod
they measure the length of the dark period. The critical role of the dark period
was confirmed by interrupting the dark period with brief light exposure.
• Karl Hammer and James Bonner studied the effect of photoperiod in Xanthium
(Cocklebur) a short day plant.
• They found that interruption of the day with darkness had little or no effect but
interruption of night with light inhibited flowering in Cocklebur. The light
interruption of the dark period is called night break. Thus it is also possible to
speak the critical night, the night length that must exceed for flowering in SDPs.
Plants monitor critical day length by measuring the length of
dark period.

7. ➢Therefore it was concluded that short day plants flower when a critical
duration of darkness is exceeded.
➢The duration of darkness is also important in LDPs.
➢Interruption of night with flash of light promotes flowering in short
photoperiod conditions. Night break treatment of longer duration promotes
flowering in LDPs.
➢ Long Day Plants flower when the length of the dark period is less than a critical
night length.
➢A light break during long night promotes flowering of LDPs & inhibits the
flowering of SDPS.

8. flower when night length exceeds a critical dark
period. Interruption of the dark period by a brief
light treatment (a night break) prevents flowering.
Short- Day Plants Short- Day Plants
flower if the night length is shorter than critical
dark period. In long-day plants, shortening the
night with a night break induces flowering.
Critical
Night
Length

9. • The photoperiodic stimulus in both LDPs and SDPS is perceived by the
leaves.
• Plants measure photoperiod using phytochrome.
• Phytochrome acts as a primary photoreceptor for photoperiodic stimulus.
• In response to photoperiod the leaf transmit a signal (floral stimulus)
through phloem to Shoot Apical Meristem that regulates the transition to
flowering at the shoot apex called photoperiodic induction.
PHYTOCHROME IS THE PRIMARY
PHOTORECEPTOR IN PHOTOPERIODISM

10. FAR RED
RED
FAR RED
RED
FAR RED
RED
FAR RED
RED
FAR RED
FLOWERING
FLOWERING
NO FLOWERING
NO FLOWERING
EFFECT OF RED, FAR RED AND BLUE LIGHT ON FLOWERING
IN SHORT DAY PLANTS (SDPs):
➢ In SDPs a flash of Red light during the dark period (night break) prevents flowering and flash
of Far red light promotes flowering.
➢ If Red and Far red light are given in sequence then the radiation used last in the sequence
will determine the response. This demonstrates the role of phytochrome in control of
flowering in SDPS.
CASE-A

11. RED
FAR RED
RED
FAR RED
RED
FAR RED
RED
FAR RED
RED
FLOWERING
FLOWERING
NO FLOWERING
NO FLOWERING
CASE-B
IN LONG DAY PLANTS (LDPs):
➢In LDPs a flash of Far red light during the dark period (night break) prevents flowering and
flash of Red light promotes flowering.
➢If Red and Far red light are given in sequence then the radiation used last in the sequence
will determine the response.
➢In some LDPs such as Arabidopsis blue light promote flowering suggesting the participation
of a blue light photoreceptor Cryptochrome in control of flowering in addition to
Phytochrome.

12. R
FR FR
R R
R
R
R
FR
FR SDPs, a flash of Red light prevents
flowering & flash of Far red light
promotes flowering during dark
period.
In LDPs, a flash of Red light during
the dark period induces flowering
& the effect is reversed by a flash
of Far red light.
Critical
Night
Length
24
20
16
12
8
4
0
H
O
U
R
S

13. Pr Pfr
RED LIGHT
FAR RED LIGHT
Flowering stimulus
Activates
Flowering stimulus
Activates
Flowering
Short
day
plant
Long
day
plant
Dark
➢Red light promotes flowering in LDPs.
➢Far-Red light inhibits flowering in LDPs.
➢In LDPs, Pfr form of phytochrome
promotes flowering.
➢In LDPs, Pr form of phytochrome
inhibits flowering
➢ Far Red light promotes flowering in
SDPS.
➢ Red light inhibits flowering in SDPs.
➢ In SDPs, Pr form of phytochrome
promotes flowering.
➢ In SDPS, Pfr form of phytochrome
inhibits flowering.