photoperiodism its discovery,significance,classifications,mechanism,critical day length,quality of light, night break phenomenon,phytochrome.florigen,floering genes, circadian rhythm

1. BY:
SRUTISUDHA MOHANTY
ENVIRONMENTAL SCIENCE
FIRST SEMESTER

2. PHOTOPERIODISM
With afew exceptions, virtually all organisms(plant
& animals) need exposureto light for agiven
number of hoursper day for avariety of growth and
reproductivefunctions. Thisday length iscalled
PHOTOPERIOD & thephenomenon iscalled
PHOTOPERIODISM.
Photoperiodism isatransducer(amechanism that
suppliessignalsfrom theenvironment).
'PHOTOPERIODISM' -derived from 2 Greek words
i.e., 'photos' (light) & periods(length of time).

3. PHOTOPERIODISM IN PLANTS
Flowewring plant comprises of 2 phases i.e,vegetative &
reproductive.
Under an appropriate photoperiod,plant switches from
vegetative to reproductive phase,responding to synthesis
of flowering hormones & its subsequent translocation to
buds.(Bartholomew,1977)
The effect of daily duration of light hours(& dark periods)
on the growth & development of plants,especially flowering
is called PHOTOPERIODISM.

4. It has significant role in
bud dormancy
Control of vegetative trait
Tuberization in plants
Bulb formation
Simultaneous leaf fall in deciduous tree
Dark carbon fixation in CAM plants

5. DISCOVERY
The concept of photoperiodism was given by
W.W. Garner & H.A. Allard of U.S Department
of Agriculture,studied flowering in Maryland
mammoth variety of Tobacco plant in 1920.
M.M. Variety was a single gene mutant tobacco
that didn't flower in the spring or summer,like
wild type.
It only flowers when bring into the green house
in the winter.

6. CONTROL OF FLOWERING
Photoperiod mechanism in the
leaves
Change in day lenth
Flower buds
flowering
Florigen hormone

7. Classification of responses
SDP:-flowers when day length is shorter than critical
day length. e.g. -soya bin, chrysanthemum
LDP:-flowers when day length is longer than critical
day length.e.g-oat,radish,spinach
LSDP:-flopwer after a sequence of long days followed
by short days.e.g-jasmine,bryophyllum
SLDP:-flower after a sequence of short days followed
by long days.e.g-winter rye
DNP:-are insensitive to day length.flowering is
controlled by endogenously.e.g-balsam,maize

9. Critical Day Length
Critical day length is the photoperiod required to induce flowering.
It varies from species to species.
e.g-Xanthium(SDP) requires a critical day length of 15.5hrs(15.5
light/8.5 dark).if the plant gets less than 8.5hrs of dark it fails to
flower.
Critical photoperiod mustn't be exceeded in short day plants &
should always be exceeded in long day plants.
There is no relation with total day length.
A single photoperiodic cycle which induces flowering-Inductive cycle
& its effect is called
PHOTOPERIODICPHOTOPERIODIC
INDUCTIONINDUCTION.

11. Quality of light

The wavelength of light used is important.
COLOR WAVE
LENGTH
SHORT DAY LONG DAY
FAR RED
LIGHT
>700 nm STIMULATES REVERSES
RED LIGHT 670-680nm INHIBITS STIMULATES

12. THE NIGHT BREAK
PHENOMENON
(for long day plants)
For plantswith acritical
night length,ashort flash
of light in themiddleof
thenight would makethe
plant behaveasif it had
been exposed to along
day.

13. Photoperiodic pigment- PHYTOCHROME
EXPOSURES FLOWERING
R INHIBITS
R+Fr PROMOTES
R+Fr+R INHIBITS
R+Fr+R+Fr PROMOTES

14.  The above observations indicate presence of some
pigment in the leaf which must be photoreversible.
 Several expt.s indicate that light is absorbed by a
photoreversible pigment -PHYTOCHROME
 This is a bluish biliprotein and exists in 2 interconvrtible
form- Pr & Pfr
 Pr form of phytochrome absorbs Red light & converts into
Pfr form.
 The Pfr form of phytochrome absorbs far red light &
converts to Pr form under continuous darkness.

15. Perception of photoperiodic stimulus
Site of percepton of photoperiodic stimulus-
mature leaf (Knott,1934)
Pigments-
phytochromes,cryptochromes,phototropins
All photoreceptors consist of proteins bound to
light absorbing pigments i.e,chromophores.
The spectral sensitivity of each photoreceptor
depends on its chromophores ability to absorb
different wavelengths.
In Japanese morning glory & pigweed
(Chenopodium rubrum), even the cotyledons
can perceive stimulus.

17. Flowering stimulus :Florigen
The hormone which induces flowering-florigen
Produced in leaves & apeces and conveyed to
meristem.
M.K. Chailakhyan(1953) gave the concept of
'florigen' {Greek word,meaning-flower maker}
It hasn't yet been identified or extracted;hence
hypothetical.
It is only synthesized in favourable
photoperiodic conditions.

18. Flowering hormones & Gibberellins

Gibberellic acid promotes flowering in long day
plants.

Thus some scientists suggested that flowering
hormone is somehow related with GA.

According to Chailakhyan,photoperiodic
flowering involves to steps.

First one is associated with gibberellins and
second one a flowering factor-Anthesins

He stated that florigen consists of both
gibberellins and anthesins.

19. Flowering genesFlowering genes
Genetically,2 classes of genes
regulate floral development –
a)floral meristem identity
b)floral organ identity
Transition from shoot
vegitative meristem to floral
meristem-- floral meristem
identity genes
This group of genes include-
LFY,AP,CAL
The transition is controlled
by endogenous as well as
environmental signals.

20. e.g- In Arabidopsis , flowering is controlled by 4
pathways.Photoperiodic,vernalization, autonomous,gibberellins.
Photoperiodic pathway is regulated by CONSTANS(CO) genes.
PHOTOPERIODIC
PATHWAY
CO
gene
Floral pathway
integrators
VEGETATIVE REPRODUCTIVE

21. Significance
The yield of tubers,corms,bulbs & rhizomes can
be increased.
Vegetative crops like raddish,carrot ,sugarcane
can be made to remain vegetative for longer
periods.
Annuals may be grown twice or thrice.
Winter dormancy & autumnal fall can be
prevented by increasing light hours.

22. CIRCADIAN RHYTHM

The behaviour of animals & plants follow a
diurnal-nocturnal pattern of apprx. 24Hrs.this is
called a circadian rhythm.

'Circa' means about, 'dien' means day

Rhythmicity is controlled by genetically as well
as exogenous factors.

Physiological measurements of factors such as
body temperature,oxygen consumption,blood
sugar level exhibit variations with the daily
cycles.