Photoperiodism
Photoperiodism
Photoperiodism:
• It is mechanism in plants in relation to light. Both plants
and animals are able to detect length of day and night
and respond to it.
Definitions:
• i) It is the response of plant to relative length of day and
night i.e. light and dark period.
• ii) It is the response of plant to the timing of light and
darkness
• iii) The response of plant to the length of day which
enable and organism to adapt to seasonal changes in its
environment.
First experimental study
• Garner and Allard (1920) grew Maryland mammoth
variety of tobacco (Nicotiana tabaccum) and Biloxi
cultivar of soybean (Glycine max)
• Both the plants were grown in high light intensity or light
period, they did not flower. In summer, during long day
length, these two species were growth in field and both
the varieties did not flower but some of the plants grown
in pots were transferred in a hut near the field (shade or
darkness). The plants of field did not flower but the
plants placed in hut started flowering at the same time.
Both the plants were short day plants because both
required short photoperiod or short day length. It reveals
from the experiment that plants definitely possess
response to light period.
Conclusion
• Garner and Allard concluded that the
length of the day controlled flowering. This
conclusion was supported by the fact that
mutant could be kept in vegetative state
during the winter months by merely
lengthening the days with artificial light.
• Garner and Allard termed the response of
maryland mammoth and biloxi to day
length as “photoperiodism”.
Classification of plants according to
photoperiodic reactions
• Photoperiodic response
• Any response by a plant to the duration
and order of sequence of light and dark
periods may be called as photoperiodic
response
• On the basis of photoperiodic response
plants are of following types
• 1. Long day plants (LDPs)
• These plants flower when the day length is
higher than the critical day length
• There are two categories
• a) Qualitative LDPs
• b) Quantitative LDPs
• a) Qualitative LDPs
• The plants which specifically require long days
for flowering. If no long day, they will not flower
e.g.
• Beta vulgaris (sugar beet)
• Spinacea oleracea (spinach)
• Raphanus sativus (Raddish)
• Hyocyamus niger (Ajwain)
• Hordeum vulgare (barley)
• b) Quantitative LDPs
• Those plants in which flowering is promoted by long
days e.g.
• Riccinus communis (castor)
• Triticum aestivum
• Pisum sativum (Pea)
• 2. Short day plants (SDPs)
• These plants flower when the day length is less than the
critical day length
• A) Qualitative SDPs
• Those plants that specifically require short day for
flowering e.g.
• Xanthium
• Chrysanthemum
• Nicotiana tabaccum
• Zea mays
• Glycine max
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B) Quantitative SDPs
Those plants in which flowering is promoted by short days e.g.
Helianthus annuus (sunflower)
Saccharum officinarum (sugarcane)
Gossypium hirsutum (cotton)
3. Day neutral plants (DNPs)
Those plants which do not respond to photoperiods e.g.
Oryza sativa (rice)
Cucumber
Zea mays (some varieties)
Tomato
4. Short long day plants (SLDPs)
These plants require short days followed by long days for flowering.
They flower in late spring or early summer e.g.
Trifolium alexendrinum
T. pratense (Red clover)
Some varieties of wheat
These plants not flower when kept in continuous short or long day
periods
• 5. Long short day plants (LSDPs)
• These plants require long days followed by short days.
They flower in late summer or autumn e.g.
• Bryophyllum (pather chut)
• Cestrum nocturnum
• Certrum diurnum
• 6. Intermediate day plants (IDPs)
• Plants in which flowering neither occur in short days nor
long days e.g.
• Chenopodium album (bathoo)
• Saccharum spontaneum (species of sugarcane i.e.
surkunda kai)
• Coleus hybrida
• 7. Amphiphotoperiodic plants
• Plants in which flowering is inhibited by intermediate day
length e.g.
• Media elegans (tarweed)
• Setaria verticillata
Critical day length or photoperiod
• The light period which is essentially required to
induce flowering in plants known as critical
photoperiod e.g.
• Hyocyamus niger which is long day plant requires
photoperiod greater than 11 hours
• Xanthium is a short day plant and requires
photoperiod less than 15.5 hours
• Long day Hyocyamus plant will start flowering only
if critical photoperiod is 11 or greater than 11hours
• Short day plant Xanthium will start flowering when
light period will be less than 15.5 hours
• If light period is 14 hours then both plants will flower
Photoinductive cycle
• Photoperiod induction means that plant need to
experience favorable photoperiod for few days then they
will flower even if they are placed in unfavorable day
length
• Favorable photoperiod cause the change of state in the
leaves which is called photoperiodic induction
• For both SDP and LDPs, definite number of such cycles
are required for flowering to occurs and this is know as
photoinductive cycle
• Number of photoinductive cycles apply to a plant is very
important because they control the number of flowers
and their maturation
Continuo---• For example, Xanthium a short day plant, when
we apply one short day cycle its flower will take
64 days to mature i.e. late flowering and late
maturation
• In continues short day condition, it will take 13
days to mature
• Short day plant cocklebur grown under long
days
one cycle of short day and long
night
return to long days
• Flower after 64 days
Mechanism of photoperiodism
• This process is involved following steps i.e. general
principles
• 1. Perception: by leaves
• 2. Transmission: Stimulus from leaf to shoot apex
• 3. Evocation: occurance of flowering at apex
• There are two important aspects
• 1. phytochrome is a photo or light receptor because e it
receives light
• It is a chemical or pigment that receives light
• 2. stimulus: some scientists say that it’s a florigen
produced in leaves and transmitted to stem apex and
causes flowering (it is hypothetical substance)
Vernalization
• It is a process in which flowering is promoted by
a cold treatment given to fully hydrated seeds or
growing plants
• Plants that require cold treatment
• Plants that require vernalization show delayed
flowering or even remain vegetative or show
rosette like growth if they are not exposed to
cold treatment during germination
• Range of vernalization temperature
• The vernalization temperature is just below 0 to
10 C but the most effective and precise temp
range is 1-7 °C depending on species
Continuo ---• The effect of cold treatment increased with
increasing duration of cold treatment until
the response is saturated
• Response usually requires several weeks
of exposure to low temp but precise
duration varies from species to species
• Vernalization appears to take place
primarily in shoot apical meristem
• If stem apex is chilled, flowering will occur
Vernalization may involve epigenic
changes in gene expression
• Epigenic regulation: changes in gene expression that are
stable even after the removal of signal that induced changes.
• Winter-annual ecotypes of Arabidopsis that require long days
and vernalization for flowering, a gene (FLC) that acts as
repressor of flowering has been identified (FLC)
• FLOWERING LOCUS (FLC): highly expressed in
nonvernalized shoot apical meristems (Michaels and
Amasino, 2000).
• After vernalization this gene is epigenetically switched off by
an unknown mechanism in rest of life cycle, permitting
flowering to occur in response to long days.
• In the next generation, however, the gene is switched on
again.
Phytochromes
• Phytochrome is a pigment that absorbs
light (red and far-red) which is effective in
causing photomorphogenesis in plants. It
is ubiquitous chromoprotein which plays a
significant role in almost every stage of
plant development
• Photomorphogenesis: the control of
morphogenesis or development by light is
called photomorphogenesis.
Discovery
• Garner and Allard (1920) work led to the discovery of the
pigments responsible for the absorption of light. Most
researvh leading to phytochrome detection and isolation
was accomplished in Maryland (USA) between 19451960.
• I) H. A. Borthwick (1972): He summarized the history of
discovery of phytochrome.
• II) Briggs (1976)
• III) S. B. Hendricks (1970): Briefly described the
discovery of phytochromes
Continuo ---• The initial observations were on lettuce (salad plants) seed.
The germination of lettuce seeds is stimulated by red light
and inhibited by far-red light. After many years, further
research shows that 100% germination in seeds occurs
that received red light as final treatment. However,
germination was strongly inhibited by far-red treatment
(Flint, 1936). On the basis of these observations, there
were two possibilities, one is “there are two pigments, one
pigment is red-light absorbing and other is far-red light and
two pigments act antagonistically or there might be a single
pigment that can exist in two interconvertable forms, a red
light absorbing and a far-red light absorbing form
(Borthwick, 1952). Buttler et al. (1959) demonstrated
phytochrome properties in vitro from plant extracts.
Types of Phytochromes
• In plants, phytochromes are of 5 different
types
• Phytochrome A
• Phytochrome B
• Phytochrome C
• Phytochrome D
• Phytochrome E
Physical properties of
phytochrome
• Absorption spectrum: Different types of
pigments absorb the different properties of
visible spectrum of radiant energy and %age
of each wavelength of light absorbed by the
pigment is known as absorption spectrum.
• Action spectrum: It can be defined as the
relative effectiveness of different wavelengths
of light on photomorphogenesis
Phytochrome can interconvert
between Pr and Pfr forms
• In dark or etiolated plants, phytochrome is present in a
red-light absorbing form, referred to as Pr because it is
synthesized in this form. It is then converted to Pfr form
by red light. This far-red absorbing Pfr form again
converted
to
Pr
by
far-red
light.
This
convertion/reconversion property is the most distinctive
property of phytochrome
•
Red-light (660 nm)
• Pr
Pfr
•
Far-red (730 nm)
• The proportion of Pfr form after saturation
by red light is only about 85%, whereas
very small amount of far-red light
absorbed by Pr makes it impossible to
convert Pfr entirely to Pr by broadspectrum far-red light.
• Instead, an equilibrium of 97% Pr and 3%
Pfr is achieved. This equilibrium is termed
as the photostationary state.
• Pfr is physiologically active form of
phytochrome
• Comparison between Pr and Pfr form
Pr
Pfr
First synthesized
Pr is converted into Pfr
Inactive form
Active form
Stable form
Unstable form
Blue color
Olive green
Cis form
COOH
COOH
Trans form
COOH
COOH
Chemical properties of phytochromes
• Phytochrome is a soluble protein with a molecular mass
of about 250 kDa. It is a dimer made up of two
equivalent subunits. Each subunit consists of two
components, one is prosthetic group which is light
absorbing pigment molecule called chromophore and a
polypeptide chain called the Apoprotein. The apoprotein
monomer has a molecular mass of about 125 kDa.
Together, apoprotein and its chromophore make up the
holoprotein. In higher plants the chromophore of
phytochrome is a linear tetrapyrrole termed as
phytochromobilin. The apoprotein and chromophore are
attached through cystein residue.
Synthesis of phytochrome
• Phytochromobilin is synthesized in plastids
and released into the cytosol, where it
binds with phytochrome apoprotein
•
Active – play role in flowering
Red
• Synthesis
Pr
Pfr
Biological
Far-red
response
Dark or thermal
conversion
Distribution among species
• In addition to angiosperms, phytochromes
are also found in gymnosperms,
liverworts, mosses, ferns and some green
algae suggesting it might be found in all
photosynthetic
organisms
except
photosynthetic bacteria. Little is known
about chemical properties of phytochrome
in species other than angiosperms.
Distribution among tissues and
cells
• Phytochromes are present in most organs of plants
including roots. In etiolated seedling the highest
phytochrome levels are usually found in meristematic
regions such as bud and first node. Phytochromes are
most abundant in young, undifferentiated tissues.
• Those cells where mRNA are most abundant,
phytochromes are found. There is a strong correlation
between the abundance of phytochrome and potential
of development in cells which shows that
phytochromes play an important role in controlling
such developmental changes.
Role of phytochromes in
biological processes
• Phytochrome mediated effects can be grouped
into three categories on the basis of their energy
requirements.
• i) Very Low Fluence Responses (VLFRs)
• ii) Low Fluence Responses (LFRs)
• iii) High Irradiance Reactions (HIRs)
• Fluence: Moles of quanta per unit area (mol m-2)
• Fluence rate or Irradiance: The moles of quanta
per unit area per second.
i) Very Low Fluence Responses (VLFRs)
• These are not photo-reversible because these responses
become saturated at very low light levels i.e. that levels
which are below those that cause a measureable
conversion of Pr to Pfr.
ii) Low Fluence Responses (LFRs)
• LFRs include the photoreversible phytochrome responses
such as seed germination and de-etiolation. LFRs convey
information to seeds about its position relative to soil
surface and increase the potential for seedling to become
established in light and initiate photosynthesis before
nutreint reserves of the seedling are exhausted.
iii) High Irradiance Reactions (HIRs)
• These are also not photoreversible. HIRs require prolonged
exposure to high irradiance and are time dependent
•
Morhphogenetic responses
i)
VLFRs: Some phytochrome responses can be initiated
by fluence as low as 0.0001 µmol m-2 (on tenth of the
amount of light emitted from a briefly in a single flash)
and they saturates at about 0.05 µmol m-2 e.g. in darkgrown oat seedlings, red light can stimulate the growth
of coleoptile at such low fluences. Arabidopsis seeds
can be induced to germinate with the red light in the
range of 0.001 to 0.1 µmol m-2.
ii) LFRs: This set of phytochrome responses can not be
initiated until the fulence reaches 1.0 µmol m-2 and they
saturates at 1000 µmol m-2. they include most of the
red/far-red photoreversible responses such as
promotion of lettuce seed germination and regulation of
leaf movement.
iii) HIRs:
a) Synthesis of anthocyanins in various dicot seedling and
apple skin segments
b) Inhibition of hypocotyl elongation in mustard, lettuce
and petunia seedling
c) Induction of flowering in henbane (Hyoscyamus)
d) Plumular hook opening in mustard
e) Production of ethylene in sorhgum
Ecological role phytochromes
Phytochromes enable plants to adapt to changing light
conditions
Phytochromes regulate the sleep movements of leaves
Floral meristems and floral organ
development
• Floral
meristems:
Floral meristems can be
distinguished from vegetative meristems, even in the
early stages of reproductive development by their larger
size. The transition from vegetative to reproductive
development is marked by an increase in cell division
within the central zone of shoot apical meristem. In
vegetative meristem, the cells of central zone slowly
complete their division cycles. As reproductive
development starts, the increase in size of meristem is
largely as a result of increased division rate of cells
• Most of the genetic and molecular studies have been
done on
• i) Arabidopsis
• ii) Antirrhinum (Snapdragon)
• During vegetative growth of Arabidopsis vegetative
apical meristem produces phytomeres (phytomere
consists of a leaf, the node to which leaf is attached, the
axillary bud and the internode below the node) with very
shot internodes, resulting in the basal rosette of leaves.
• As reproductive development starts, the vegetative
meristem is transformed into an inderminate primary
inflorescence meristem that produces floral meristems
on its flanks. The lateral bud of cauline leaves develop
into secondary inflorescence meristem and their activity
repeats the pattern of development of primary
inflorescence meristem.
Floral organ development
• The four different types of floral organs are intiated as
separate whorls. These organs are i) sepals ii) petals iii)
stamens and iv) carpels. The initiation of innermost
organs, the carpels, consumes all the meristematic cells
in the apical dome.
• In wild type Arabidopsis flower, the whorls are arranges
as follows:
1. The outermost whorl consists of four sepals
2. The second whorl consists of four petals
3. The 3rd whorl consists of six stamens, two shorter and
four larger
4. The 4th whorl is a single complex organ, the gynoecium,
which is composed of an ovary with two fused carpels.
Three classes of genes regulate floral development
• There are three classes of genes that regulate floral
development
• I) Floral organ identity genes
• II) Cadastral genes
• III) Meristem identity genes
• I) Floral organ identity genes: These genes directly control
floral identity. The proteins encoded by these genes are
transcription factors that likely control the expression of other
genes whose products are involved in the formation of floral
organs.
• II) Cadastral genes: These genes act as regulators of the
floral organ identity genes by setting boundaries for their
expression.
• III) Meristem identity genes: These genes are necessary for
the initial induction of the organ identity genes
Floral organ identity genes
• Homeotic genes: First discovered in Drosophila. These
genes regulate the location of body parts in flies.
• The floral organ identity genes were identified through
homeotic mutations that altered the place of floral organs
e.g. Arabidopsis plants with mutations in the APETALA2
(AP2) gene produce flowers with carpels at the place of
sepals and stamens at the position of petals.
Three classes of homeotic genes
control floral organ identity
• Five different genes are known to specify floral organ
identity in Arabidopsis
• APETALA1 (AP1)
• APETALA2 (AP2)
• APETALA3 (AP3)
• PISTILLATA (PI)
• AGAMOUS (AG)
• The homeotic genes fall into three classes/types
depending on three different activities i.e. type A, B and
C activities
• Type A activity: Type A activity is encoded by AP1
and AP2 control organ identity in the first and second
whorls. Loss of type A activity results in the formation of
carpels instead of sepals in first whorl and of stamens
instead of petals in second whorl.
• Type B activity: This activity is encoded by AP3 and
PI control organ determination in the second and 3rd
whorls. Loss of type B activity results in the formation of
sepals instead of petals and carpels instead of stamens.
• Type C activity: This activity is encoded by AG,
controls events in the 3rd and 4th whorls. Loss of type C
activity results in the formation of petals instead of
stamens and sepals instead of carpels.
ABC model
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ABC model was studied in
i) Arabidopsis
ii) Antirrhinum (Snapdragon)
Cells in which only A gene is present or A gene
expresses develop into sepals
Cells in which both A and B genes are expressed
develop into petals
Cells in which both B and C genes are expressed
develop into stamens
Expression of C gene alone turns on the development of
carpel
Genes are present in each cell of the whorl. Their
expression is specific.