BY:SRUTISUDHA MOHANTYENVIRONMENTAL SCIENCEFIRST SEMESTER

PHOTOPERIODISMWith a few exceptions, virtually all organisms(plant & animals) need exposure to light for a given number of hours per day for a variety of growth and reproductive functions. This day length is called PHOTOPERIOD & the phenomenon is called PHOTOPERIODISM. Photoperiodism is a transducer(a mechanism that supplies signals from the environment).'PHOTOPERIODISM' -derived from 2 Greek words i.e., 'photos' (light) & periods(length of time).

PHOTOPERIODISM IN PLANTSFlowewring 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.

It has significant role inbud dormancyControl of vegetative traitTuberization in plantsBulb formationSimultaneous leaf fall in deciduous treeDark carbon fixation in CAM plants

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.

CONTROL OF FLOWERING

Photoperiod mechanism in the leaves

Change in day lenth

Flower buds

flowering

Florigen hormone

Classification of responsesSDP:-flowers when day length is shorter than critical day length. e.g. -soya bin, chrysanthemumLDP:-flowers when day length is longer than critical day length.e.g-oat,radish,spinachLSDP:-flopwer after a sequence of long days followed by short days.e.g-jasmine,bryophyllumSLDP:-flower after a sequence of short days followed by long days.e.g-winter ryeDNP:-are insensitive to day length.flowering is controlled by endogenously.e.g-balsam,maize

Critical Day LengthCritical 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.

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

THE NIGHT BREAK PHENOMENON

(for long day plants)For plants with a critical night length,a short flash of light in the middle of the night would make the plant behave as if it had been exposed to a long day.

Photoperiodic pigment- PHYTOCHROME

EXPOSURES FLOWERING

R INHIBITS

R+Fr PROMOTES

R+Fr+R INHIBITS

R+Fr+R+Fr PROMOTES

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.

Perception of photoperiodic stimulusSite of percepton of photoperiodic stimulus- mature leaf (Knott,1934)Pigments-phytochromes,cryptochromes,phototropinsAll 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.

Flowering stimulus :Florigen

The hormone which induces flowering-florigenProduced 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.

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.

Flowering genesFlowering genesGenetically,2 classes of genes regulate floral development –

a)floral meristem identity b)floral organ identityTransition from shoot vegitative meristem to floral meristem-- floral meristem identity genesThis group of genes include-LFY,AP,CALThe transition is controlled by endogenous as well as environmental signals.

e.g- In Arabidopsis , flowering is controlled by 4 pathways.Photoperiodic,vernalization, autonomous,gibberellins.

Photoperiodic pathway is regulated by CONSTANS(CO) genes.

PHOTOPERIODICPATHWAY

COgene

Floral pathwayintegrators

VEGETATIVE REPRODUCTIVE

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.

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.