DESCRIPTION OF ASEXUAL REPRODUCTION IN A TEXAS SUCCULENT SEDUM WRIGHTII (CRASSULACEAE)

2004·3.6 MB·English

by  Dennis A Gravatt

DESCRIPTIONOFASEXUALREPRODUCTIONINA
TEXASSUCCULENTSEDUMWRIGHTII(CRASSULACEAE)
DennisA.Gravatt^ andJosephineTaylor
DepartmentofBiology
StephenF.AustinStateUniversity
SFABox13003,i936NortinStreet
Nacogdocties,Texas75962,U.S.A.
[email protected]
ABSTRACT
ScdumwrighiWA.Gray,awestTexasnativesucculent,isoneolagrowingnumberolspeciesknown
toreproducebyvegetativemeans.DetachedleavesofS.wrighiiiproduceplantletsonthebasalpor-
tionoftheadultleafmassociationwithcallustissuefromwhatappearstobethinned-walledpa-
renchymaandepidermalcellsofthepetiole.Shootprimordiaappearfirst,emergingoverlateralre-
gionsofthecallustissuebyaboutday5,withtherootsemergingseveraldayslater
RESUMEN
ScdumwnghliiA.Gray,plantasuculentanativadeTexas,pertenecealcrecientegrupodeespecies
conocidasc^uesereproducenvegetativairrente.LashojasarrancadasdeS.wrightiiproducenplantulas
enlaporcionbasaldelahojaadulta,enasociacionconcallodeloquepareceserparenquimadeparedes
finasycelulasepiderinicasdelpeciolo.Primeroaparecenlosprimordiosdelvastago,emergiendode
lasregioneslateralesdelcallosobreelquintodia.ylasraicesemergenvariesdiasmastarde.
INTRODUCTION
Survivalofseedsandseedlingsmshallowsoilsofrockoutcrops(Sharitz&
McCormick1973)andonthedesertsoilsurfaceispoor(Jordan&Nobel1981),
resultinginlovv^sexualreproductivesuccessandlimitingtheestablishmentof
manydesertperennialstocertainfavorableyears.Thedevelopmentofyoung
plantletsfromvegetativebudsborneonaparentplant,orondetachedleaves
occursmseveralgroupsofplantsincludingSedum,BryophyllumandCrassula,
eithernormallyoruponwounding(Stoudt1934,1938;Yarbrough1934,1936;
McVeigh1938).Thepropagativecapacityofspeciesofthesegenerahaveinter-
estedbotanists,anditsadvantageshavebeenexploitedbyhorticulturists.
Thedevelopmentofplantletsfromleavesisanefficientmechanismofveg-
etativereproductioninmanyplantfamilies.Thesevegetativeplantletsmaybe
moresuccessfulthanseedlingsbecauseofseveralfactors,includinggreater
waterandcarbonreserves(Holthe&Szarek1985).InanexaminationofSedum
wrightiiA.Gray,Clausen(1975)notedthatafter11weeks,82%ofdetachedleaves
developedplantlets,eachwithasmallrosetteofleavesandwell-developedroots.
Alaterstudy(Gravatt2003)showedthattheadditionalsourceofwaterand
'Correspondence:DennisA.dravatt,e-mail:[email protected]
SIDA21(2):943-950.2004
944 BRIT.ORG/SIDA21(2)
carbohydratesreserves,overanextendedperiodprovidedbythedetachedleaf,
wasanaddedbenefitforthedevelopingpropagule.Byday120ottheexperi-
ment,detachedleavesremainedphysiologicallyactive,whereasthepropagules
maintainedconstantgrowth.Forsucculentssuchas5.wnghtii,droughtsur-
vivalofpropagulesmaybedeterminedbytheamountolwater-storingtissue
inthedetachedleai.
Gravatt(2003)foundthatdetachedleavesoi5.wri<^hliireadilydeveloped
propagulesatthebaseottheleaves.Eachpropaguledevelops2to3rootsanda
smallrosetteot3to5leaves.Bytheendoltheexperimentnearly80%(98%oi
survivingleaves)otthedetachedleaveshaddevelopedpropagules.Themor-
phologicaleventsassociatedwithasexualreproductionin5.wnghtUhavenot
beenpreviouslycharacterized.Thepurposeotthisinvestigationistodescribe
thedevelopmentolyoungpropagulesIrombasalcallustissueotdetached,adult
leavesinthisspeciesusingscanningelectronmicroscopy.
MATERIALSANDMHTHODS
Sedumwrightiiisaleal-succulentperennialoccurringinthesemi-desertsof
thesouthwesternUSAandMexico.Plantswerecollectedonthelimestoneclifls
overlookingtheDevilsArmoftheAmistadReservoir,ValVerdeCounty,Texas
atanelevationof360minAugust2001.Plantswerepottedinsoilsand:pcrlite:
topsoil(3:3:3)usingplasticpots(10x10cm)andplacedinaplantgrowth
chamberEnvironmentalconditionswereasfollows:photosyntheticphoton
fluxdensity(PPFD)of900|imofm's',25(±1)°Cairtemperature,and1.9(±0.2)
l<Pavaporpressuredeficit(vpd)duringtheday,and20(±1)°Cairtemperature
and1.5(±0.2)kPavpdatnight.Environmentalconditionsweremeasuredus-
inganADC(Hertfordshire,England)ECA4COjandIhOanalyzerAttertrans-
planting,plantswere keptwell-wateredandtertilized biweeklywith 0.5-
strengthPetersProfessionallertilizer(W.R.GraceandCo.,Fogelsvitle,PA,USA)
withtraceelements,15%eachoftotalN,K2PO5,andK2O.
Plantswereallc^wedtogrowuntilmature,asindicatedbythepresenceof
floralbuds.Avoucherspecimen(Gravatts.n.)wasdepositedintheStephenE
AustinStateUniversityHerbarium(ASTC).Leavesweredetachedfromwhole
plants("parentleaves")andwereplacedonmoistvermiculiteinplasticpots
(10x15cm)andsampledondays0,1,2,5,8,10,and14.Thevermiculitewas
keptmoistinthegrowthchamberforthedurationoftheexperiment.Environ-
mentalconditionsinsidethechamberwerethesameasthosedescribedlorthe
wholeplants.
Oneachotthesamplingdaysthebasalthirdof5.wri^htiileaveswereex-
cisedusingarazorblade,tixedina1:1mixtureofglutaraldehyde(5%)and0.1IVI
potassium phosphatebutterandrefrigeratedovernight.Thespecimenswere
rinsedindeionizedwateranddehydratedinagradedethanolseriesto100%
GRAVATTANDTAYLOR,ASEXUALREPRODUCTIONINSEDUMWRIGHTII 945
ethanol(Mims1981).Theleafpieceswerethencriticalpointdriedwithcarbon
dioxideasthetransitionalfluid,mountedonaluminumstubs,andsputter
coatedwithgoldorgold-palladium.Basalendsofleaveswereexaminedusing
aHitachiS-405Ascanningelectronmicroscopeoperatingat15KeV.
RHSUITS
Tbrhoeusl.eaAvdeuslotflSeeadveusmawrrei8ghttoii12armemspilroanlg1,y3a.r5ratong4e.d5,melmlipwtiidcael,oarnodbl1.o5ntgoa2n.d5mglma-
thick.Petiolesaremuchreduced,suchthattheygivetheappearanceoftheleal
beingnearlysessile.Prelimniaryfindingsdeterminedthatleavesof5.wrightii
contamasingleleaftrace.Thepetiolesareverybrittle,sothattheleavesfall
fromthestemwithonlyslightpressureandtakerootfromthebaseoftheleaf,
forminganewplant(Fig.1).
Scanningelectronmicroscopicexaminationofthebasalportionofparent
leavesrevealedthesequenceofeventsassociatedwithpropaguleformation.
Afterdetachmentfromthestem,vasculartissueandthesurroundingparen-
chyma,aswellastheepidermisofthepetiolecanbeseen(Fig.2).Detached
leaveslyingonmoistvermiculiteshowedsignsofwoundtissueformationaf-
ter24hours(Fig.3).
Within48to60hours,acicatrate(scarleftbyawoundorbyabscission)
meristemformsatthebaseoftheleafundertheregionoitheleaftrace.The
cellsofthewoundedsurfacesooncollapse,andthecellsunderneaththesedi-
videinaplaneparalleltothesurface.Asmallprotuberance,visibleundera
binocularmicroscope,appearsneartheregionofthepetioleoltheparentleal
bythesecondorthirddayThisregionisidentiliedasthezoneofshootemer-
genceinFigure3.
Whendetachedleavesaremaintainedonmoistvermiculite,youngshoots
typicallyemergefromthecallustissueafterfivetosevendays(Fig.4).Theear-
liestobservedemergenceofshootswasthreedays,withthelatestrecorded
emergenceattendays.Theshootprimordiumarisesonthelateralsurfaceof
thecallustissue.Theshootapparentlyoriginatesfromperipheraltissueofthe
petiole,atthebaseoftheleaf,notpreviouslycoveredbycallus.Asingleshoot
arisesbyday5,withtwoprimordialleaveseasilyidentifiable(Fig.4).
Theoriginationofrootprimordiawithinthecallustissuefollowsemer-
genceofleafprimordiaandusuallyoccursbytheeighthday(Figs.5&6).Thus,
thedevelopmentofgrowingpointsisasfollows;callustissuetorms;theshoot
appearsbeforetheroot;andtherootsappearassinglerootorinpairs.
Severalrootprimordiamaybeorganizedwithinagivencallustissue(Fig
6),sothatbyday14theparentleafmayhavethreetofouradventitiousroots.
Thedevelopmentofroothairscanbeseenonleaveskeptonmoistvermiculite
byaboutday14(Fig.7).
946 BRIT.ORG/SI0A21(2)
Fia.I.StereomicroscopeimageofSerfumwr/p/if//leafbasewith14-day-oldpropagule(-30x)
DISCUSSION
Inanearlierstudy,Gravatt(2003)(oundrharafter11weeks,82%ofdetached
leavesdevelopedplantlets,eachwithasmallrosetteol leavesandwell-devel-
opedroots.Furthermore,detachedleavessurvivedfor120dwithahighrateof
successlorpropaguleformation,89%ofleavesfromthestartoltheexperiment
(Gravatt2003).
Scanningelectronmicroscopicexaminationolthebasalportionolparent
leavesrevealedasequenceoleventsassociatedwithpropaguleformationcom-
parabletothatreportedinCrassulamullicuva(McVeigh 1938),Scdumslahlii
(Yarbrough1936),andGraptopcUdum(Green&rBrooks1978).Thebasipetal
meristematicactivityappearstolinkthepropagulewiththevascularsystem,
asfoundinpreviousstudies(McVeigh1938;Yarbrough1936;Green&rBrooks
1978).Thus,thepropaguleislinkedwiththeparentlealforfoodandwaterre-
serves(seeFig.4),allowingsurvivaloftheyoungplant(Grewatt2003).
Theoriginationol rootprimordiawithinthecallustissuefollowsemer-
genceollealprimordiaandusual!yoccursbytheeighthday(Figs.5&r6).These
findingsareincontrasttothoseloundbyYarbrough(1936),w^hoobserveda
dillercntorderoldevelopmentlorrootsandshoots.Inhisstudy,detachedleaves
GRAVATTANDTAYLOR,ASEXUALREPRODUCTIONINSEDUMWRIGHTII 947
Figs.2-3.ScanningelectronmicrographsofSerfumiv/'/g/if/7propagulesondesignatedsamplingday5.Fig.2.Leafbase
taken1dayafterleafremovalshowingpetioleepidermisandvasculartissue,Fig.3.Callustissueformedovervascular
bundleonleafbasetakenonday2.(PE=petioleepidermis;V=vasculartissue;C=callustissue;ZSE=zoneofshoot
emergence).
948 BRIT.ORG/SIDA21(2)
:Jr.
Fics.4-5.Scanningelectronmicrographsof5erf(;mH'r/g/)f/7propagule5ondesignatedsamplingdays.Fig.4.Emerging
shoottakenonday5showingleavesandareaofrootemergence,Fig.5.Emergingrootsandleavestakenonday8with
remainsofcallustissue.(PL=primordialleaf;ZRE-zoneofrootemergence).
GRAVATTANDTAYLOR,ASEXUALREPRODUCTIONINSEDUMWRI6HTII 949
-*1aia*pii
i:^'^
pr^
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^^•^'* 1^^
Figs.6-7.ScanningelectronmicrographsofSerfumivr/g/if/Vpropagulesondesignatedsamplingdays.Fig.6.Leafpair
andelongatingroottakenonday10.Fig.7.Roothairformation,takenonday14.(C=callustissue;RH=roothairs).
950 BRIT.ORG/SI0A21(2)
of S.s(((/i//i.lefton adrysurface,gaverisetorootslirsrand then shoots.
YarhroughCIQ36)furtherobservedthatinamoistchambcr,asusedinthisstudy,
shootswerenotformedandrootsonlyappearedoccasionally.
Yarbrough(1^)36)concludedthatproductionofrootsandshootsfromleaves
ofSedumstaliUuwhichatthetimeofdetachmentclearlypossessnoresidual
meristem,cfcarlyisanexampleofregenerationthroughtheformationofad-
ventitiousstructures.Hefurtherstatedthattheformationof callustissueby
thethinned-walledparenchymacellsoftheshortpetiofcinvolvesdifferentia-
tion.Inthisstudy,itappearsthatthisisthecaseforScdumvvr/gli(iiaswell,and
compareswellwiththefindingsofYarbrough(1936).Thecontinuedmitotic
activityofthecellssurroundingthecallusmakespossiblethedevelopmentof
twofundamentalgrowingpoints,therootandshootprimordia,andthesubse-
quentdevelopmentolanewplantfromthedetachedleal.
ACKNOWLHDGMHNTS
WewouldliketothankGuyNesom(Bl^lT)andSherwinCadquistCSBBGJlor
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