photoperiodismandvernalization-210620142806.pdf

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1. Photoperiodismand
Vernalization
PRP

3. PHOTOPERIODISM
•Photoperiodismisthephenomenonofphysiologicalchanges
thatoccurinplantsinresponsetorelativelengthofdayand
night(i.e.photoperiod).
•Plantinordertoflowerrequireaparticulardaylengthorlight
periodcalledphotoperiodandresponseofplantsto
photoperiodintermsoffloweringiscalledphotoperiodism.
•Itinfluencesplantdevelopmentsuchasleaffall,dormancy
andtuberformationbutitsmajoreffectisoncontrolof
flowering.

4. Discovery of Photoperiodism
•ItisfirstdiscoveredbyW.W.GarnerandH.A.Allard(1920)
•Theyobservedthat“Marylandmammoth”varietyoftobacco
floweredatdifferenttimesatdifferentplaces.
•Aftercontrollingotherfactorslikenutrition,temperatureetc,they
reportedthatitwastherelativelengthofthedaywhichaffected
flowering.
•Theygavethetermphotoperiodtofavourabledaylengthforeach
plant.

5. CLASSIFICATION OF PLANTS BASED
ON PHOTOPERIODIC RESPONSES
GarnerandAllardclassifiedtheplantsinto5types
basedontheirphotoperiodresponseofflowering:
1.Short day plants (SDP)
2.Long day plants (LDP)
3.Day neutral plants (DNP)
4.Short -long-day plants (SLDP)
5.Long-short-day plants (LSDP)

6. 1.Shortdayplants
•Theseplantsrequirearelativelyshortdaylightperiod(usually8-10hours)anda
continuousdarkperiodofabout14-16hoursforsubsequentflowering.
•Inshortdayplants,thedarkperiodiscriticalandmustbecontinuous.Ifthisdark
periodisinterruptedwithabriefexposureofredlight(660-665nmwavelength),the
shortdayplantwillnotflower.
•Ontheotherhand,abriefperiodofdarknessduringdaytimehadnoeffecton
flowering.
•Becauseoftheimportanceofdarkperiodinflowering,thesearealsocalledasLong
NightPlants.
•E.g.Rice,Coffee,Soybean,Tobacco,Chrysanthemum,XanthiumandMorningglory
(Ipomoeapurpurea)
2.Longdayplants
•Theseplantsrequirelongerdaylightperiod(usually14-16hours)ina24hourscycle
forsubsequentflowering.
•Theseplantsarealsocalledasshortnightplants.
•Inlongdayplants,lightperiodiscriticalandabriefexposureofredlightinthedark
periodortheprolongationoflightperiodstimulatesfloweringinlongdayplants
•E.g.Barley(winterbarley),Wheat,Henbane(HyoscyamusNiger),Radish,Cabbage,
Sugarbeet,Spinach,carrot,lettuceandonion

8. 3.Dayneutralplants
•IndeterminateorDNPflowersreadilyoverawiderangeofdaylengthfromrelativelySDL
tocontinuousillumination.
•E.g.Tomato,Chilly,Potato,Cotton,Sunflower,Cucumber,Peas,BalsamandMaize
4.Short-longdayplants
•Thesearelongdayplantsbutmustbeexposedtoshortdayduringearlyperiodsofgrowth
forsubsequentflowering.
•Theseplantsrequirefirstshortphotoperiodandthenlongphotoperiodforflowering.
•E.g.Winterrye,Candytuft
5.Long-shortdayplants
•Theseareshortdayplantsbutmustbeexposedtolongdaysduringearlyperiodsof
growthforsubsequentflowering.
•Theseplantsrequirefirstlongdaysandthenshortphotoperiodforflowering.
•TheseplantsdonotflowerwhenexposedtoeitherSDorLDandneedbothLDandSDfor
itsflowering.
•E.g.Bryophllum,CestrumNocturnum(NightbloomingJasmine)

9. Differences between short day and long day
plants
Short day plant
1.Plants flower when
photoperiodislessthanthe
criticaldaylength.
2.Interruptionduringlightperiod
withdarknessdoesnotinhibit
flowering.
3.Floweringisinhibitedifthelong
darkperiodisinterruptedbya
flashoflight.
4.Long continuous and
uninterrupteddarkperiodis
criticalforflowering.
5.Examples-Xanthium,coffee,
tobacco,Glycinemaxetc.
Long day plant
1.Plants flower when
photoperiodismorethanthe
criticaldaylength.
2.Interruptionduringlightperiod
with darknessinhibit
flowering.
3.Floweringoccursifthedark
periodisinterruptedbyaflash
oflight.
4.Darkperiodisnotcriticalfor
flowering.
5.Examples-Sugarbeet,Spinach,
carrot,lettuce

10. Photoperiodic Induction
•Theinfluenceofthelengthofdayandnightontheinitiationoffloweringis
calledphotoperiodicinductionorphotoinduction.
•Plantsmayrequireoneormoreinductivecycleforflowering.An
appropriatephotoperiodin24hourscycleconstitutesoneinductivecycle.
•Ifaplantwhichhasreceivedsufficientinductivecycleissubsequently
placedunderunfavourablephotoperiod,itwillstillflower.
•Floweringwillalsooccurifaplantreceivesinductivecyclesafterintervalsof
unfavourablephotoperiods(i.e.discontinuousinductivecycle).This
persistenceofphotoperiodicaftereffectiscalledasphotoperiodic
induction.
•Anincreaseinthenumberofinductivecyclesresultsinearlyfloweringof
theplant.Forinstance,xanthium(ashortdayplant)requiresonlyone
inductivecycleandnormallyflowersafterabout64days.Itcanbemadeto
flowerevenafter13daysifithasreceived4-8inductivecycle.Insuchcase
numberofflowersisalsoincreased.

11. Critical day length
•Marylandmammothtobaccoandxanthiumareshortdayplants,but
theMarylandmammothtobaccoisinducedtoflowerwhenthe
photoperiodisshorterthan12hourswhereas,xanthiumisinducedto
flowerwhenthephotoperiodisshorterthan15.5hours.
•Thephotoperiodrequiredtoinducefloweringisreferredtoasthe
criticaldaylength.Hence,thecriticaldaylengthforMaryland
mammothtobaccoandxanthiumare12and15.5hoursrespectively.A
shortdayplantisonethatflowersonphotoperiodsshorterthanthe
criticaldaylength.
•Longdayplants,ontheotherland,areinducedtofloweron
photoperiodslongerthancriticaldaylength.Forexample,thecritical
daylengthforHyoscyamusnigeris11hoursanditisinducedtoflower
onphotoperiodslongerthan11hours.

12. Perception of photoperiodic stimulus and
presence of a floral hormone
•Photoperiodicstimulusisperceivedbytheleavesandafloralhormoneisproducedinthe
leaveswhicharethentranslocatedtotheapicaltip(apicalmeristem),subsequentlycausing
initiationoffloralprimordia.
•Photoperiodicstimulusperceivedbytheleavescanbeshownbyasimpleexperimenton
chrysanthemum,ashortdayplant.Chailakhyan(1936)dividedchrysanthemumplantsinto
fourgroupsandvariedtheregimeoflightanddarkcyclesinallthefourgroupsbyusinglight
proofcases.Thefourgroupsofplantsareasfollows:
I.GroupA:Entireplantcontinuouslyreceivedlongdaytreatment
II.GroupB:Lowerleafportionreceivedshortdaytreatmentwhileupperdefoliatedportion
receivedlongdaytreatment
III.GroupC:Lowerleafyportionreceivedlongdaytreatmentwhileupperdefoliatedportion
receivedshortdaytreatment
IV.GroupD:Entireplantcontinuouslyreceivedshortdaytreatment
Heobservedthatfloweringoccurredinthoseplantswheretheleavesreceivedshortday
treatment(GroupB&D)butitfailedinthoseplantswheretheleavesreceivedlongday
treatment(GroupA&C).Onthebasisofaboveobservationheconcludedthatshortday
stimulusisperceivedbytheleaves.

13. Flowering stimulus: Florigen
•Thefloweringstimulusproducedinthephotoinducedleavesis
translocatedtotheshootapiceswhereflowerformationisinitiated.
•Floweringstimulusissimilarinlongdayplantsandshortdayplants.This
canbeprovedbyagraftingexperimentandcanbetranslocatedfromone
planttoanother.
•Ifaleaffromaphotoinducedplantisremovedandthengraftedona
non-photoinducedplant,thenthisplantflowers.Apparentlyachemical
substanceisproducedduringphotoinductivecycle,whichistransmitted
duringgraftingtononinducedplantsandinducesflowering.
•Chailakhyan(1937)calledthisflowerinducingchemicalsubstanceas
Florigen(floweringhormone).
•Florigenmaybeamacromolecule-itispossiblethatflorigenisanRNAor
proteinmoleculethatistranslocatedfromtheleaftotheapicalmeristem
viathephloem.

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

15. Phytochrome
•Phyrochromeisablueproteinaceousnon-photosyntheticpigment.
•Itiscomposedoftwocomponents:aproteinandachromophore-
whichgiveitslightabsorbingproperties.
•Phytochromeexistsintwodifferentformsi.e.,redlightabsorbing
formwhichisdesignatedasPrandfarredlightabsorbingform
whichisdesignatedasPfr.
•Phytochromeisproducedinlightgrowntissues.
•Thesetwoformsofthepigmentarephotochemicallyinter
convertibleanditislocatedwithinplasmamembrane.
•WhenPrformofthepigmentabsorbsredlight(660-665nm),itis
convertedintoPfrformduringdaytime.
•WhenPfrformofthepigmentabsorbsfarredlight(730-735nm),it
isconvertedintoPrform.ThePfrformofpigmentgradually
changesintoPrformindark.

17. Role of phytochromein short and
long day plants
•Itisconsideredthatduringdaytime,thePfrformofthepigment
isaccumulatedintheplantswhichareinhibitorytofloweringin
shortdayplantsbutisstimulatoryinlongdayplants.During
criticaldarkperiodinshortdayplants,thisformgradually
changesintoPrformresultinginflowering.Abriefexposurewith
redlightwillconvertthisformagainintoPfrformthusinhibiting
flowering.
•PlantsmeasuretheratioofPr/Pfr.
•LDPwouldflowerwhenthePr/Pfrratioislow.
•SDPwouldflowerwhenthePr/Pfrratioishigh.

18. A. In short-day plants:
•Shortdayplantsflowerwhenthenightperiodislong.
•Indaylightorredlight,phytochromered(Pr)isconvertedto
phytochromefarred(Pfr).Theconversionactuallyonlyrequiresa
briefexposuretowhiteorredlight.
•Inthedark,PfrisslowlyconvertedbacktoPr.
•Alongnightmeansthatthereisalongtimefortheconversion.
•Undershortdayconditions(longnight),attheendofthenight
periodtheconcentrationofPfrislow.
•InSDP,lowPfrconcentrationisthetriggerforflowering.

19. In the short-day plant
PRbuilds up PFR
Darkness (slow)
Short-day plants
Florigenactivated
Flowering

20. B. In long-day plants:
•Long day plants flower when the night period is short.
•In day light (white or red) the Pr is converted to Pfr.
•During periods when the day light period is long but critically
the dark period is short, Pfrdoes not have long to breakdown
in the dark. There is a higher concentration of Pfr.
•In LDP, high Pfrconcentration is the trigger for flowering.

21. In the long-day plant
Sunlight
Red light
PR PFR builds up
Long-day plants
Florigenactivated
Flowering

22. Theories of Flowering
1.Phytochrometheory
2.Bunning’shypothesis
3.Chailakhyan’shypothesis
1.PhytochromeTheory:
•Phytochromecontrolsfloweringinplants.
•PRpromotesfloweringinSDPsandsuppressesfloweringinLDPs.
•OntheotherhandPFRsuppressesfloweringinSDPsand
promotesfloweringinLDPs.
•InSDP,ifthelongdarkperiodisinterruptedbyredlight,then
sufficientamountofPRisnotformed.Hencedecreaseflowering
inSDPs.
•FloweringinSDPsdependsuponPR/PFRratio.HighPR/PFRratio
stimulatesfloweringinSDPsandinhibitsfloweringinLDPs.

23. 2. Bunning’s hypothesis:
•Bunning(1958)assumedthepresenceofendogenousrhythms
(Oscillator)whichconsistoftwohalfcycles.
•Thefirsthalfcycleoccursindayandiscalledphotophilousphase.
Duringthis,anabolicprocesspredominatesincludingfloweringin
plants.
•Theotherhalfcycleisdark,sensitiveandiscalledskotophilousphase.
Inthis,catabolicprocess(dehydrationofstarch)predominates.
•SDplantshaveacriticaldaylengthof9hours.Thisperiodfallswithin
thephotophilousphase.Lightduringskotophilousphasewillinhibit
photoprocessinitiatedduringphotophase.
•TheL.D.plantshaveacriticaldaylengthof15hoursandsomelight
fallsintheskotophilousphase.UndertheseconditionsinL.D.plants
willflower.
•InS.D.plantsoscillatorispresentclosetoskotophilousphase,whilein
L.D.plantsitisclosetophotophilousphase.

24. 3. Chilakhyan’shypothesis:
•Thishypothesisassumesthatfloweringhormone–florigenisacomplexof
twotypesofsubstances–gibberellinandanthesins.
•Gibberellinisessentialforgrowthoftheplantstemsandanthesinsare
requiredforflowerformation.
•Accordingtohim,floweringinallannualseedplantsrequirestwophases:
(i)Floralstemformationphase
(ii)Flowerformationphase
•Firstphaseinvolvesincreasedcarbohydratemetabolismandrespiration
withincreasedcontentofGAinleaves.
•Secondphaserequiresintensivenitrogenmetabolism,highercontentof
anthesinsinleavesandnucleicacidmetabolitesinstembuds.
•Longdayconditionsfavourthefirstphasewhileshortdayconditions
favoursecondphase.
•Inlongdayplantsgibberellinsarecritical,whileanthesinsarecriticalin
shortdayplants.

25. Gibberrlicacid and the flowering
response
•Gibberrlicacid(GA)seemstoplayanimportantroleinflowering.It
maybedirectlyinvolvedintheformationofflorigen.
•GAcansubstituteforlongdayrequirementsandalsothecold
treatmentrequirementinseveralspeciesforitsflowering.
•Brian(1958)statedthatGAlikehormoneisproducedintheleaves
duringphotoperiod.
•HighconcentrationofGAlikehormoneleadstosynthesisof
florigeninLDPsandwhenthelevelofthisGAlikehormone
becomeslow,itleadstosynthesisofflorigeninSDPs.

26. SIGNIFICANCE OF PHOTOPERIODISM
Theyieldoftubers,corns,bulbsandrhizomescanbeincreased
substantiallybyincreasingordecreasingthedurationofdayornight.
Cropslikeraddish,carrot,sugarcanecanbemadetoremainvegetativefor
longerperiods.
Annualsmaybegrowntwiceorthriceinayear.
Perennialsmightflowerthroughouttheyear.
Winterdormancy&autumnalfallcanbepreventedbyincreasinglight
hours.
Increaseyield
Hybridizationexperimentscanbehelpfulbecausedifferentvarieties
growingindifferentareaswithdifferentdurationsandfloweringatdifferent
timesaremadetogrowandflowersidebysidebyartificiallycontrolling
theirphotoperiods.
Understandingtheconceptofphotoperiodismhashelpedtochoosephoto
insensitivevarietiesandcultivars.Thesecultivarsarebestsuitedin
intensiveagriculture.

27. Vernalization

28. VERNALIZATION
•Thecoldtreatmentgiventoplantbuds,seedsorseedlingsforpromotingearly
floweringisknownasVernalization.
•Inshort,thechillingtreatmentforinductionofearlyfloweringiscalledas
Vernalization.
•Duetolowtemperaturetreatmenttheperiodofvegetativegrowthoftheplantis
reducedresultingintoearlyflowering.
•ThetermvernalizationwascoinedbyT.D.Lysenkoin1920s.
•Manyplantsgrownintemperateclimatesrequirevernalizationandmust
experienceaperiodoflowwintertemperaturetoinitiateoracceleratethe
floweringprocess.
•Thisensuresthatreproductivedevelopmentandseedproductionoccursinspring
andwinters,ratherthaninautumn.
•Thesuitabletemperatureforvernalizationrangesbetween0to5̊c.
•Athighertemperaturefrom7̊conwardsresponseoftheplantisdecreased.
•Atemperatureofabout12to14̊cismostineffectiveinvernalizingtheplant.

29. Siteofvernalization
•Apicalmeristem,embryosandactivelydividingcellsarethepotentialsites
ofvernalization.
•Thecommonfeatureforvernalizationisthepresenceofactivelydividing
apicalmeristems.
Perceptionofcoldstimulusandpresenceoffloral
hormone
•Thecoldstimulusisperceivedbytheapicalmeristems.
•Theperceptionofthecoldstimulusresultsintheformationofahormone
Vernalinwhichistransmittedtootherpartsoftheplant.
•Incertaincases,thecoldstimulusmayevenbetransmittedtoanotherplant
acrossagraftunion.Eg.Henbane.Here,ifavernalizedplantisgraftedtoan
non-vernalizedplantthelateralsoshowsflowering.

30. REQUIREMENTS OF VERNALIZATION
I.LowTemperature:Lowtemperaturerequiredforvernalization
isusually0°-5°C.
II.PeriodofLowTemperatureTreatment:Itvariesfromafew
hourstoafewdays.
III.ActivelyDividingCells
IV.Oxygen:Thevernalizationisanaerobicprocessandrequires
metabolicenergy.IntheabsenceofO2,coldtreatment
becomescompletelyineffective.
V.Water:Sufficientamountofwaterisalsoessentialfor
vernalization.Vernalizationofthedryseedisnotpossible.
VI.ProperNourishment

31. Mechanism of Vernalization
Therearetwomaintheoriestoexplainthemechanismofvernalisation.
1.PhasicdevelopmentalTheory
2.HormonalTheory
1.PhasicdevelopmentalTheory
ThistheorywasproposedbyLysenko(1934).
Accordingtothistheory,theprocessofthedevelopmentofanannualseedplantconsistsofa
seriesofphaseswhichmustoccurinsomepredeterminedsequence.
Commencementofanyofthesephaseswilltakeplaceonlywhentheprecedingphasehasbeen
completed.
Thephasesrequiredifferentexternalconditionsforthecompletionsuchaslightand
temperature.
Vernalizationaccelerates:
a)thethermophasei.e.thatphaseofdevelopmentwhichisdependentupontemperature.Thus,
inwinterwheat,lowtemperatureisrequiredforthecompletionoffirstthermophase.
b)Afterthis,thenextphasei.e.,photophasestartswhichisdependentuponlight.

32. 2.Hormonaltheory
•Ithasalreadybeendescribedthatvernalizationprobablyinvolvestheformationofafloral
hormonecalledasvernalin.Basedonthisfact,manyhypotheticalschemeshavebeen
proposedbydifferentworkersfromtimetotime.
•ThefirsthormonaltheoryproposedbyLongandMelchers(1947)isschematicallyshown
below.
coldtreatmentnormaltemp.
A B C Flowering
Precursor Thermolabile Vernalin
compound
Highertemp.
D(doesn’tflower)
•LangandMelcher’s(1947)suggestedthataprecursorAisconvertedintoathermolabile
compoundBduringcoldtreatment.UndernormalconditionsBchangesintoastable
product(CVernalin)whichcausesflowering.ButathighertemperatureBisconvertedinto
Dandfloweringdoesnottakeplaceduetodevernalization.
•Furtherexperimentshaveshownthatvernalinactsinassociationwiththeflowerinducing
stimulus,florigeninordertoinduceflowering.

33. VernalizationandGibberellins
•Thegibberellinsareknowntoreplacethelowtemperaturerequirementin
certainbiennialplants.
•E.g.Henbane.Thisplantnormallyremainsvegetativeandretainsitsrosette
habitduringthefirstgrowingseasonandafterpassingthroughthewinter
period,itflowersinthenextseason.Thegibberellinscausesuchplantsto
flowerevenduringthefirstyear.
DEVERNALIZATION
•Thepositiveeffectofthelowtemperaturetreatmentonthevernaliztaionofthe
plantscanbecounteractedbysubsequenthightemperaturetreatment.This
phenomenoniscalledasdevernalization.
•Thedegreeofdevernalizationdecreasesifthedurationofthecoldtreatment
hasbeenlonger.
•However,thedevernalizedplantcanagainbevernalizedbysubsequentlow
temperaturetreatment.

34. Importance of vernalization
Vernalizationshortensthevegetativeperiodoftheplant.
Itincreasescoldresistanceoftheplants.
Vernalizationincreasestheresistanceofplantstofungaldiseases.
Inbiennials,vernalizationinducesearlyfloweringandearlyfruit
setting.
Anonvernalizedshootapexcanbeinducedtoflowerbygrafting
theplantwithavernalizedplant.
Wintervarietiesofcropplantscanbeconvertedintospring
varietiesbyvernalization.