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Factors Controlling the Germination Date of Winter AnnualsAuthor(s): E. I. NewmanReviewed work(s):Source: Journal of Ecology, Vol. 51, No. 3 (Nov., 1963), pp. 625-638Published by: British Ecological SocietyStable URL: http://www.jstor.org/stable/2257751 .

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625

FACTORS CONTROLLING THE GERMINATION DATE OF

WINTER ANNUALSBY E. I. NEWMAN

Botany chool,Cambridge*

INTRODUCTION

A winter nnual is a plant whichgerminatesn theautumn, asses the winter n thevegetativetate, nd flowers,ets eed and dies n thefollowingpring r summer. hustheseeds are normallyiberated n late spring r summer, ut theydo not germinate

until he autumn.Thispaper s concernedwith hereasonsfor hisfailure o germinatein the summer, nd with he factors ontrollingheprecisedate of germinationn theautumn.

Severalworkers ave conductedaboratoryxperimentsn the germinationf winterannualspecies, hough ftenwithout nyparticularnterestn thegerminationn naturalconditions.n mostspecies nvestigatedhere s a change n ability o germinates theseed ages: after period of storage heseeds give a higher otal germinationnd/orfaster ate hando fresheedsunder hesameconditions.n most pecies his hange ngermination bility s related to physiological hanges in the seed (after-ripening),but n someLeguminosaet s due to the softeningf a hard seed coat. In some species

thegerminationbility t somegerminationemperatureshangesmore han at others,so that s a result fafter-ripeningchange always rise)occurs n theoptimum nd/orthemaximumemperatureorgermination.his willbe referredo as change n tempera-ture esponsewith torage.t canbe illustrated ythegerminationn thedarkofBromustectorumdata fromHulbert1955).The total germination f fresh eed at 100C was53%, at20? C 1%; after weeks' torage ermination as about 95% at either empera-ture, utfastert20?C.

Amongthespecieswhichhavebeenstudied, ive orts f changewith torage an bedistinguished.

1. Inthefollowingpecies erminationffresh eeds s ow becauseofhard eedcoats:

Trifoliumlomeratum,. procumbens= T. campestre), . reflexum,. resupinatum,.subterraneumToole & Hollowell1939); T. hirtum,. incarnatumWilliams& Elliott1960); Lupinusdigitatus,Medicago tribuloidesQuinlivan1961). Some of these speciesalso show fter-ripeninghanges seesection below,where heconclusions n Trifoliumspp. arebased on experimentsith carifiedrnaturally oftened eed). For several fthese hard-seededpecies t has beenshown hatsofteningfthetesta s hastenedbyhigh onstant emperatures,ndevenmoreby daily lternationsfhigh nd owtemper-atureLoftusHills1942;Williams& Elliott 960;Quinlivan 961).

2. Thefollowingpecies how changentemperatureesponsewith torage: epidiumlasiocarpumBarton1936); Trifoliumlomeratum,. reflexum,. resupinatumToole &

Hollowell 1939); T. subterraneumLoftus Hills 1942, 1944); Bromus brizaeformis,B. japonicus,B. tectorum= Anisantha ectorum)Hulbert1955); Arabidopsis haliana.Drabamuralis, rophila ernaRatcliffe957).

* Presentddress: epartmentfBotany, niversityollege fWales,Aberystwyth.



626 Germinationf winternnuals

3. In the following pecies storageproduces an increaseof germination bilityofapproximately he same amount in all germination emperatures: aucus pusillus,Plantagofastigiata,Streptanthusrizonicus Barton 1936). Any hard-seeded pecies

(section1) which o not show fter-ripeningould also be placed n thisgroup.4. The following pecies how an increase f germinationbilitywith torage, ut tis notknownwhether here s a change n temperatureesponse, inceonly one tem-peraturewas used: Holosteum mbellatum,hlaspi erfoliatumEbner 1924); Arenariaserpyllifolia, ardaminehirsuta,Hornungia etraea, Saxifragatridactylites, eronicaarvensisRatcliffe957,1961).

5. One of the winter nnual races of Arabidopsis haliana tudiedbyLaibach (1951)showednochangengerminationbilitywith torage; wo other acesdidshow change,as didtherace studied y Ratcliffesee section ).

These results uggest hree xplanations orgerminationccurringnthe autumn, utnot n theprevious pring r summer.

1. In some speciesfresh eed gives ittle r no germination nderany temperaturecondition;butby autumn, wingto after-ripeningnd/or oftening fthe seed coat,germinationan occur na widerange f emperatures.

2. In some pecies resh eedgives erminationnly ttemperatureselow hose ccur-ringnthefieldn atespringnd summer. s after-ripeningroceeds, heoptimum ndmaximum emperaturesorgerminationise,until hey verlap hose occurringn thefield.

3. In specieswhere here s no change ngermination ith torage, hefield empera-turesn summer re above themaximumroptimumor ermination.he ower emper-atures n autumn llow germination.his explanations offeredyWent and his co-workers or hecontrol fgerminationfwinter-germinatingnnuals n the CaliforniandesertWent 1948, 1949; Went & Westergaard 949; Juhren,Went & Phillips 1956).Theydidnot nvestigatehangengerminationbilitywith torage.

Explanations and 3 mustbe somewhat entativen mostcases, since fewworkersgive emperatureecords rom henaturalhabitats. oole & Hollowell 1939) providetableofdailymaximum oil temperatures,utgiveno reasonwhy he maximumem-

peraturehouldbe moresignificanthantheminimumr themean. Resultspresentedlatern this aper uggest hat, or ome pecies tanyrate, heminimumnd mean re nfactmore mportanthan the maximum.Ratcliffe1957, 1961) and Went and his co-workers ive graphsof maximum nd minimumir temperatures. heyare theonlyworkers ho have been nterestedn theprecise ate ofgermination,nd theonly nestogive nydata ongerminationn natural onditions.

Ratcliffeonsiders oil moisture o be ofprimarymportancen controlling ermina-tiondate.However,hisattemptso show direct elationshipetween erminationndsoilmoisturen the field re largelynconclusive. ield observations yWentand hisco-workershow hatnthedesert, here ain sspasmodic,he recise ateofgermination

inwinter epends n when herain howers ome.Theamount frainoccurring uringparticular hower nfluenceshe amount of germinationthis is confirmedy Tevis1958).There s evidence hat or achspecies here s a certainminimumainfallequiredbeforenygerminationilloccur.



E. I. NEWMAN 627

GERMINATION OF AIRA PRAECOX AND TEESDALIA NUDICAULIS

The remainderfthispaper s concernedwith hegerminationftwospeciesof winter

annual,Airapraecox . and TeesdalianudicaulisL.) R. Br. tdescribesheir erminationin controlledemperatures,nd also ina natural abitat n Thetford eath,west uffolk.I haveshownNewman1961)that nboththese pecies smallvariationngerminationdate can markedlyffecthesubsequent ize and seed production fthe mature lant.The primaryimof thisresearch,herefore, as to discoverhow muchthe mean ger-mination ate varies rom ear oyear n natural onditions,nd which actorsnfluencetheprecise ateofgerminationnanyyear.

This paperconcentratesn the factors uggested ypreviousworkto be important,namelydormancy, emperature equirementsorgermination,hange n temperatureresponsewith geing, nd soil moisture equirements.therfactorswillbe mentionedbriefly,ncludingifferencesn germinationetween arknessnd ight.

In thehabitatwhere ield bservations eremade,the soil is a coarse,flintyandoflowwater-holdingapacity,ubject o rapidfluctuationsf moisture ontent, speciallyin thetopfewmillimetres,heremostof the seedsof Aira and Teesdaliaprobably ie.It seemed mpossible,with he time nd equipmentvailable, o makea recordof themoisturen thissurfaceayerwhichwouldbe sufficientlyomplete nd accurate o beuseful. hereforehe pproachhas beentostudy erminationnconditions fcontinuousmoisturenknown emperatureegimes,ndby comparisonwithfield erminationndfield emperatureso deducethe relativemportance ftemperature,oil moisturendchangesntheresponse f he eed. t has to be assumed hat hese re theonly ignificantfactorswhich ary s theyearproceeds.

AIRA PRAECOX

Detailsof technique

Ripefruitingeads were ollectedn 1958and 1959from he amearea,about 30 x 30m,on FoxholeHeath,westSuffolk. heywere torednenvelopes,whichwerekept nthelaboratory temperaturessually17-20?C), exceptfromNovember1958 to April1959,when heywerekept n a glasshousetemperaturesbout6-10?C inwinter,isingto 10-20?C inApril).

In the area ofThetford eathwhere ield bservations eremade, Cladonia ylvatica

matcovers bout 400% ofthearea, and is interspersedy smallpatchesof Dicranumscopariumnd bareground.Higher lant over s sparse; t consists fscatteredussocksofFestucaovina, nd open growth fAgrostis anina,A. tenuis nd Rumextenuifolius,together ithAirapraecoxand Teesdalianudicaulis. lthoughAira occurs nthe ichenandmosscover, t s muchmore bundantn the barepatches.Sinceseedlings rowingonbareground re more asily ecorded,ll thefield atapresented ere refrom lotson bare ground.To obtainareas of sufficientize for theplots,areas dominated yCladonia ylvatica ere leared f llvegetation.n these reasnumerous lots10x 25cmweremarked ut. n 1958and 1959 eedswere ownon theplots n lateAugust. n 1960no seedswere own,butseeds fellon theplotsfrom lantsgrownnthem heprevious

year thishad beenpreventedn 1959).Each yearnaturallyccurring iraplants n thevicinity ereremoved efore hey etseed, n order o preventheir eed, whichmightdiffereneticallyrom hat own,from alling n theplots. The seed' of Aira s in facta one-seeded ruiturroundedy emma ndpalea.)



E. I. NEWMAN 629

progress f germinationn a particular emperatureegime.Recordingwas normallycontinued ntil t least2 weekshadelapsedwithno furtherermination,ut n order osimplifyhegraphs hisfinalperiodwithout hangehas been omitted.Whereno ger-

mination ccurred,bservations erecontinued or t least6 weeks,before heexperi-mentwasabandoned.The graphs howdifferencesn the final otalsof germinationelated o seed age,

temperaturend light r darkness.The importance f these willbe consideredater.At themomentt s the ratesofgerminationhichprimarilyoncern s. These willbediscussedwithreferenceo Fig. 2, consideringirst he 10-week-oldeed (b, started 2August).At 50 C there s great niformityetweenhe seeds:all thegerminationccursduring weeks.At 10? C, however, here s greatvariation etween heseeds: a fewgerminate ithin 4 daysfrom hestart, thersn laterweeks, nd germinations still

100

80 1 1958z

0~

AUG. SEPT. OCT. NOV. DEC. JAN FEB

6IG. 1. Aira germinationn fieldplots.

continuingn the astperiodofrecording.t willbe conveniento givenamesto thesetwotypes fbehaviour. hegerminationonfined o a short eriod, iving steep lopeon thegraphe.g. at5?C),will e called brupt ermination;hegerminationivinggentlelopee.g. at10?C) will ecalled rolongedermination.

Salisbury 1929) distinguishedhree ypesofgermination:imultaneous,ontinuousand discontinuous. bruptgermination,s just defined, orresponds airly losely oSalisbury's imultaneous ermination,nd prolongedgerminationo his continuous.However, brupt nd prolonged re not considered s absoluteterms:therecan be

differentegreesof abruptness nd prolongedness,nd intermediatesetween hemoccur.

On thegraph ust consideredb, 10-week-oldeed)thegerminationt 5?C is abrupt;at 6-15?C it snearly brupt; t 10,15 and 16-25?C it s prolonged. n thenextgraph(c, 16-week-oldeed)thegerminationt 5?C is virtuallyhe sameas before.However,germinationt IO' C has become brupt,ndat15?C nearlybrupt. 0? C,atwhich herewasformerlyo germination,owgivesprolonged ermination. ith eed 59-65weeksold, d) 5, 10, 15,20 and6-15?C all give brupt ermination,251 givesprolonged,nd16-25FC givesntermediate.

Thus the seeds showa progressivehange n temperatureesponsewith ncreasing

length fstorage.Ast peri nly owtemperaturesiveabruptgermination,igher em-peratures iveprolonged ermination,ndhighertillnone at all.As after-ripeningro-ceeds,higherndhigheremperaturesive brupt ermination.ncethegerminationn agiven emperatureecomes ullybrupt, o furtherhange nrateoccurs.



630 Germinationfwinternnuals

The sametrend ntemperatureesponses shownwithgerminationn darknessFig.3). In both ight nddarknesshepositions complicated ychangesn thefinal otals fgermination, hichare not alwaysrelated o changes n rate. t will also be noticed

thatthere re differencesn germinationetween hetwo lots offresh eed (the 1959and 1962 experiments,ig. 3 (a, b)). Such differencesetween eed populationsmeritfurthernvestigation.

Somewhat imilar hanges n rateof germinationn relation o age and temperaturehave beenfound yHulbert1955) n Bromus rizaeformis,. aponicus ndB. tectorum.

100(a) (b)

80-

60 -

z 40 -

0 -

z 20-_

LU -e

C0 7 0 7 14 21 28 35 42 67 88(9

L (C) (d)

oL80-

60-

40 -~

20

0 7 14 21 28 36 0 7 14 21 28 36DAYS

FIG. 2. Progressurves fAiragerminationn light, omparingarious eedages,andvarious emperatureegimes.a) Started3June,eed0 days ld; (b) 22August,0weeksold; (c) 7 October, 6 weeks ld; (d) 59-65weeks ld. 50 C (o), 100C ([), 15?C (x),

200 C (+), 250 C (A), 6-150 C (A), 16-250 C (A).

Effectfredryingoistenedeeds

In order o understandheecological ignificancefthegerminationehaviour ust

described,t snecessaryoknowwhetherhe seedsrequire ontinuousmoisture orger-mination.An experimentnvestigatinghiswascarried ut in a glasshouse n February1960,usingseed collected heprevious pring. eedswereplacedon dampfilter aperin four ishes, iftyeeds n eachdish.After days he eedswere emoved rom ishes ,3 and4 andplaced ndry, ncovered etri ishes n the aboratory. fter aryingengthsoftimetheseeds werereturnedo dampfilter aper.The seedsin dish 1 weremoistthroughout. ermination as recorded aily.

In every ish thefinalgermination as over 90%. Table 2 showsthe ength ftimefrommoisteningo 500%germination;n thecase oftheredried eedsthis meansthetimefrom he secondmoistening.

It appears hat he seed whichwasdried utgerminatedfter emoisteningomewhatmore uicklyhan he ontinuously oist eed.Thisrequiresonfirmation,nd thematterisbeing nvestigatedurther.t ismore mportanttthemomento note hat11 hdrying(equivalent o one drydayin thefield) s enough o return he seedalmost o thestate




belowthe surface f bare soil on Thetford eath; the centre fthe bulb was at about 5mmdepth.Readingsweretakenapproximately eekly rom une o October1959, ndless frequentlyhroughouthefollowing months.The figures re givenby Newman

(1961),where hey recomparedwith he bsolutemaximumnd minimumir tempera-tures or he sameperiods, ecorded tElveden,ess than mileaway. This comparisonshows hat hemaximumemperaturesn the oilwereup to 15? C above thoseofthe irduring hesummer, ut the differenceeclined uring eptember nd October, nd wassmall from arlyNovember o late March. The minimumemperatures ere about1-4?C highern the oil than n the irthroughout.

The soiltemperatureata are not suitable ordirect se,for everal easons.Weeklyreadings ickout thedaysandnights f extremeonditions, hichwilloccurmainly nclear, dry weather,whenthe soil will be dry, nd the seeds unableto germinate.naddition,n the ummernd autumn f1959,when eadingswerebeing aken, heweatherwas exceptionally arm nd dry.The soiltemperaturesre onlydirectlyelevantwhile

Table 3. Monthlymeansof dailymaximum ndminimumirtemperaturest Elveden ?C)

Maximum Minimum1958 1959 1960 1958 1959 1960

May 16-5 17-0 18-0 70 6-5 8-0June 18-0 21-0 21-0 10-0 9 5 11-0July 20-5 24-0 19-0 12-5 12-0 12-0August 20-5 23-5 20-0 12-5 12-5 11-0September 19-0 21-0 17-0 11-5 8-5 9.5October 14-0 17-0 13-5 7.5 7.5 7.5November 8-5 9 5 10-0 4 0 2-5 5-5

the soil is continuouslymoist.This will only happen, n summer nd early autumn,whenthe weathers predominantlyloudy.Under suchconditions he soil and air willbe at about thesametemperature. uring unny pells, f the soilremainsmoist t thesurface, vaporation f waterwill reduceheating f thesurface ayers.Thereforet isproposed otakethe irtemperaturess approximationsothe oiltemperatures,houghbearingn mind hat he soilmaysometimes ecomeratherwarmer hanthe air in theday-time.Means ofmaximumnd minimumirtemperaturesregivennTable3.

Control fgerminationnthe ieldThissectionwillconsider hegerminationn eachpartof thesummernd autumn n

the controlledemperatureshichmost losely pproximateothefield emperaturestthat ime.Data fromlternatingemperaturesillbeused where ossible, ince hese remore similar o thefield emperatureegimes; ut sincethedata from lternatingem-peraturesre nsufficient,hosefrom onstant emperaturesillhave to be used as well.It willbe seenfrom igs.2 and3 that he rate n a given lternatingemperaturesuallyliesnear he ate n he onstantemperaturehich qualseither heminimumrthemeanofthe lternation.orexample,nFig.2 b,10-week-oldeed)the6-15?C curve s similarto the50 C, and the16-25?C to the 15?C. (However, hefinaltotalofgerminations

often ighernalternatingemperatureshan n anyconstant emperature.)hus,whenconsidering ates of germination,nd when no data are available from a relevantalternatingemperature,atafrom he constantemperaturesearest he minimumndmeanfield emperaturesillbeused.



E. I. NEWMAN 633

We shallfirst onsiderwhether r not somegermination ouldbe expected t the-temperatureoccurringn thefield.

In June,whenthe seeds are firstiberated, he field emperaturespproximate o a

10-20?C alternation.n June n the controlled onditions he 1962 seed gave somegerminationn thistemperatureegime n the dark,but not in light. t is doubtfulwhetherhe 1959 eed would havegiven ny germinationn thisregime.

In Augustthefield emperaturesre usuallysomewhathigher:15?C constant nd16-25?C alternationre the mostrelevant ontrolled onditions. n both ofthese omegerminationccurred,n darkness nd n ight.

Thusthetemperaturesn the fieldwould allow somegerminationnAugust, ndpro-bably n June. t cannotbe the direct ffect ftemperaturehichprevents erminationduringhe ummer.We mustnow considerwhatrates fgerminationouldbe expectedat the emperaturesccurringn thefield.

In June t 10-20?C alternation ogerminationilloccur n ight.n darkness ermin-ation akesmore han2 weeks ocommence; o it willonlyoccur n thefield fthere smore han weeks fcontinuousmoisture.

In August15?C and 16-25?C giveprolonged erminationn both darkand light.Therates fgerminationn these emperatureshow hat lthough weeks fcontinuousmoisturen thefield ouldgive10 or 20% germination,ven6weeks ouldnotgivemorethan 20 or 30%.

Thusgerminationanonly akeplaceduring une, ulyndAugust f here s continu-ous soil moisture or everalweeks.Although hismustbe an uncommon ccurrence tthis imeofyearon these oils,periods ufficientlyool and wetto allowsomegermin-ation mightbe expected o occur in some summers.However,germinationwill be

limited o a smallpercentage,ince the rates ofgerminationre so slow. ComparisonofFigs.2 and3 shows hat herates ndtotalsn therelevantemperaturesresomewhathighernthedark han n the ight. hegerminationfburied eed sthus avoured, ndit s buried eed whichs more ikely oexperienceontinuousmoisture or henecessarylength ftime.Fig. 1 showsthat n 1960 a smallpercentagef germination id occurduringAugust, nd thiswaspresumablyue to theparticularlyoolweather uringhatmonth.

In August, s we have seen,thetemperatureonditions llow onlyprolongedger-mination.By early Octoberthe temperaturesave fallen, nd the seeds have after-ripened urther.0?C and 6-15?C alternationrenow therelevant egimes.No data

are availableon thegerminationn 6-15?C in October 16-week-oldeed),butwe canassumethat the curvewould lie between he6-15?C curvesfor10-and 59-week-oldseed. Thus we see thatbyOctoberboth10?C and 6-15?C giveabruptgermination,nboth darkand ight, esultingn a highpercentage ermination ithin weeks.

Thusthegeneral ictures as follows.As the summer roceeds heseeds'response otemperaturehanges, o thatthe maximum emperaturet which bruptgerminationcan occurgradually ises;butup to early eptemberhefield emperaturesemain ighenough o allowonlyprolonged ermination. uringSeptembernd earlyOctober hefall ntemperaturend the continued hangeof the seeds'temperatureesponse llowsprogressively ore bruptgermination;.e. theperiodofcontinuousmoisture equired

for high ercentage erminationecomes rogressivelyhorter. tthis ime heperiodsforwhich he oil does remain ontinuously oist reprobably ecomingonger.And tis in factduring eptembernd October hat most ofthegerminationccurred n thefield n 1958 nd 1960.Hencethe imingfgerminationnthefield anbeattributedo an




interactionfthe hree actors,oilmoisture,emperature,nd thechangingemperatureresponse f he eeds.

In this xplanationhe ffectsf eedage andtemperatureavebeenconsideredmainly

in relation o therateofgerminationatherhan othefinalotalofgermination.twas

shownearlier hatthetotals ofgerminationossible n thetemperaturesccurringnJune nd Augustdo notexplain heusualabsenceofgerminationn thefield nsummer.We must considernowwhetherhanges n totalgerminationould affecthe date ofgerminationn the autumn: t couldbe that certain ercentagef seedsdo notgermin-ateunderfield onditionsn October, ut becomeable to germinateater.ConsideringagainFigs.2 and 3,it willbe seenthat n October n light, , 10 and 15? C all givemorethan90% germination.n darkness hegerminationt constant emperaturesasmuchlower; but6-15? C alternation ouldpresumablyave givennearly 000%germination,since t did so with eed bothnewer nd older 10 and 59 weeksold). Thereforet seemsprobablethatthose field emperatureshichgiveabruptgerminationn October lsogive high otalgermination,o thatno delay of germinations to be expected hroughlowtotals fgermination.

In mostyears, s in 1958 nd 1960, he hree actorsemperature,oilmoisture ndthechanging esponse f the seedwillprobably endto convergeome time n September,allowing apidgerminationobegin.1959 s clearly nusual:although aytemperatureswere xceptionallyigh n that utumn,t was cool enoughby earlyOctoberforrapidgerminationo occur.Yet no germinationccurredn thefield ntilNovember.Thereasonforthis musthave been the exceptional rought f that autumn;between14August nd 16 October nly2 mmofrainfell.Thefirstrolonged ainoccurred n 21October.Even after hisgerminationid not takeplace rapidly, erhapsbecausethe

temperatureserebythenbelowthose t which hefastest erminationccurs.Smallvariationsngerminationate between ears ouldbecausedbyvariationnany

oneofthe hree actorsemperature,oil moisturend rateof after-ripening.utbecauseofthetendencyf all three actorso converge,tseems hat heonly onditionikely ocausea verymarked ifferencengerminationate s drought,uch as occurredn 1959.It is notknownhowmuch herateofafter-ripeninganvary.The effectn it ofcondi-tionsduring hesummers being nvestigated.

TEESDALIA NUDICA ULIS

Thetechniques sedforTeesdaliawerevery imilar o thoseused forAira, nd neednotbe described.

Thegerminationn field lotsonThetford eath n3 years s shown nFig. 4. Itwillbe seenthat herelation etween hegerminationates n the 3 years s similar oAira:i.e. aboutthesame n 1958and 1960,orperhaps lightlyarliern 1960;much ater n1959. n eachyearTeesdaliagerminatedarlier hanAira.

Redryingfmoistenedeesdalia eedshas a similarffecto that nAira.Fig. 5 shows hegerminationf Teesdalia ncontrolledemperaturesn the dark.The

germinationnlights similar. he experimentsereconductedn 1959. t will be seenthat, s withAira,boththeabrupt ndprolongedypes fgerminationccur, ndtheir

relation o temperaturend seedage is much he sameas forAira.There re,however,two important ifferences.irstly,fter-ripeningroceedsmorerapidly n Teesdalia.For instance, yAugust15? C givesfully bruptgermination,hereaswithAira 15? Cdoes notgivefullybrupt erminationven nOctober. econdly, t anytemperaturet



E. I. NEWMAN 635

whichgerminations abrupt,t occursabout twice s fastwithTeesdalia s withAira.Both hesedifferencesill end omakeTeesdaliagerminatearlier hanAira n thefield.

The germinationfTeesdalia n controlledemperaturespproximatingo field em-

peratures illnowbeconsidered.nJunehe ield emperaturespproximateo a 10-20?C100

~~~~~~~z so

z60/ 1959

~40 /0

<20

Z 100 .1 15 1 15 1 3 5 1 15 15 1 15

AUG. SEPT. OCT. NOV. DEC. JAN

FIG. 4. Teesdalia erminationnfield lots.

(a) (b)80 X-X

60~~~~~~~~~~~~~60 p

40

z 20

0 6 36o0 4 7 14 21 28 35 42 74

z10015?C (x), 20?C (+), 25?C (A), 6-15?C (-), 16-(d)Q~~~~~~~~~~~~

ELLJo-(C)0-

60+-A

40-

20 ~ ~ ~ ~ ~ ~ ~ ~~/+ + +~

20

0 4 O7 14 28510 4 7 14 20 28 35 41DAYS

FIG. 5.ProgressurvesfTeesdalia erminationndarkness,omparingarious eed ges,andvarious emperatureegimes.a) Started 0 June,eed4 weeks ld; (b) 22August13 weeksold; (c) 8 October, 9weeks ld; (d) 62-68weeks ld. 50 C (0), 100C ()

150C ( x), 200C (-I), 250C (A), 6-150C (@), 16-250 (A&).

alternation.On 11 June 1962, 2-day-oldseed was put to germinaten 10-20Oalternation,s wellas in 10,15 and200 C constant. herewasno germinationnanyofthese onditions,neither arkness rlight. here ppearsto be a discrepancyetween

this result and the resultsfor4-week-old eed started n June 1959 (Fig. 5a). Thedifference aybe due to the1962 seedbeingfresher. n thebasis of the 1959results,thepossibilityannotbe excluded hat n late June here ouldbe somegerminationtfield emperatures,houghmorethana weekof continuousmoisturewouldprobably




be required. n factthreeTeesdalia eedlingswereseen on Foxhole Heath on 13 July1960, n a depressioncratched ut bya rabbit where he soil wouldperhapshave agreaterhanceofremainingmoist hanon thenormal oil surface).

In August,150C and 16-25?C are the controlled emperatureegimeso consider.

Unfortunatelyhere s a marked ifferenceetweenhegerminationn these woregimes.At 15?C germinations already ully brupt, utat 16-25?C it is stillprolonged,ndthetotal ow. In light 6-25?C gives higher otaland steeper urve, houghnotfullyabrupt.)The interpretations furtheromplicatedy the fact hatwithncreasingengthofstorage fter 3 weeks, herate ofgerminationt 10? C actually ets lower, houghwithout ny change n abruptnessi.e. there s a longer eriodbefore erminationom-mences, utthecurve s still s steep);whereas herate n 6-15? C increases.Boththesechanges re shown n light s well as indarkness.)t is thereforeot clearwhatratesofgerminationre tobe expected n thefield n August,.nor ow theywill change ubse-quently. he mostuseful nformationn this comesfrom he germinationf seeds on

100 O

o80 /zE 60 -

040 -zw

~20

I00 4 7 14

DAYS

FIG. 6. Teesdalia erminationn glasshouse.n) light, m) dark, tarted2 August;o)light,@) dark, tarted October.

filteraper n an unheated lasshousen which he emperatureegimewas fairlyimilartothat f the oil.Theresults, hich re shown nFig. 6, suggesthatfairly apid germi-nation an be expectedn August, hough he otalgerminationndarknessssignificantlylower han hat n ight P<0 001). By October, wing o thefall n temperaturesnd thechanging esponse fthe seed, herateof germinationas increased,ndthetotal nthedarkhas risen ignificantlyP<0-01).

Wecanseefrom his hat hefaster fter-ripeningfTeesdaliaeadsto a differenceromAira n thecontrol f germinationate. Since Teesdalia angivefairlyapidgerminationtoa high otal n ateAugust nthe emperatureshich ommonlyccur,t willrequiremuchshorter eriodof continuousmoisture hanAira for germinationt thattime.And in fact n 1960 some germinationid occurin thefield n August see Fig. 4).Although here s some change n the rateofgerminationetweenAugust ndOctober,the hange s of much esser rder han hat nAira.So the imingfgerminationillbemainly ependent n the ncreasingength,s theautumn rogresses,ftheperiodsofcontinuousoilmoisture;emperaturendafter-ripeningillplaya smaller art.

There s another ifferenceetween eesdalia nd Aira: in August hetotalgermina-

tion ofTeesdalia t most emperaturess significantlyower n thedark han nthe ight.This willtend o favour hegerminationf the seeds ying n thesurface; utthese reless likely o experience ontinuousmoisture hanburiedseeds,so thisreduction fgerminationnthedarkmay end o reduce verall erminationuringhe ummer.



E. I. NEWMAN 637

DISCUSSION

Thispaperhasshown hat n seedsof Airapraecox nd Teesdalianudicaulishe empera-

tureresponse hanges n a characteristic ay as the seed ages, presumablyecause ofafter-ripening.n Aira the timing f germinationn the field s normally ontrolled yan interaction f soil moisture, emperaturend this changing emperatureesponse.In Teesdalia hetemperatureesponse n June nd Julywill make germinationn thesemonthsuncommon, nd nevermorethan a smallpercentage; ut owing o the fasterafter-ripening,he date of the main autumngermination ill be less affectedy tem-perature nd rate of after-ripeninghan n Aira,and willtherefore e relativelymoreaffectedy oil moisture.tmight eexpected rom his hatTeesdaliawould howgreatervariation rom ear oyear n thetiming fgermination;hefield ata arenot dequateto confirmhis.

As mentionedarlier, have shownNewman1961) hat smalldifferencen germina-tiondate can markedlyffect he subsequent ize and seed production f the plant:the ater hegermination,he smaller heplant ndthe ower he seedproduction. hus,otherhings eing qual,natural electionwouldbeexpectedo favour arly ermination.However, heearlier hegermination,he greater he risk hat he eedlings illbekilledby n autumn rought.

When Teesdaliagerminatestsends down a robust ootwhich an penetrate centi-metre r morewhile he above-ground arts consist nly of two cotyledons. ira,ontheotherhand,sends downonlya few hort, ine oots at first. his differencen theamount f rootproductionmaybe related o the smaller ize of theAiraseed. SincetheTeesdalia eedling as a deeperroot, t s probablymoredrought-resistantthough his

has notbeentested xperimentally).f so, selectionmayhave favoured hesomewhatearlier erminationfTeesdalia omparedwithAira.

Thegerminationhich oth pecies an give n August s of nterestn this onnection.These earlyplantshavea high hanceof beingkilledby autumn rought, ut fthey osurvivehey roduce very argenumber f eeds.Thus t maybe a 'worthwhileamble'for hespecies fsome of the seedsgerminaten a cool,moistAugust. t is clearlym-portant hatonly smallpercentagef seedsshouldgerminatenAugust, o that f alltheseearly eedlings ie there re stillplenty f seedsleft o germinatenthe utumn.Airaappears o have a more eliablemechanismhanTeesdaliafor nsuringnly smallpercentage erminationnAugust.

Salisbury1929) states hat ll theviable eedsofRanunculusparvifloruserminatentheSeptember ollowing hedding,within period of about a fortnight. e comments:'This species s a typical xampleof a "winter nnual" with imultaneous ermination,and this atter eature onstitutes definite anger ince almostall the seedlingsmaybe destroyed y the same adversecondition.'However, t could be said forAira andTeesdalia and itmay be trueforotherwinter nnual species s well) thatthey xhibitboth thecontinuous nd simultaneous ypes f germination,nd get theadvantages fboth.

ACKNOWLEDGMENTS

I wishto thankDr A. S. Watt forvaluable advice during he course of thisresearch.Most of theresearchwas performed hile was the holder of a NatureConservancyResearchStudentship; am grateful o the Nature Conservancy orthis support, ndfor heuseofThetford eath NatureReserve.




SUMMARY

Previouswork on thegermination fwinter nnualshas suggested hat the timing f

germinationn variousspecies s controlled ydormancy, hanges n thetemperatureresponse fthe seed with geing, emperature,oil moisture, r somecombination f-these. his paper attempts o accountfor the precise iming f germinationn Airapraecox ndTeesdalianudicaulis. ecordsof germinationn natural onditions regiven,and by comparisonwithgerminationn controlled emperaturest is shown that thegerminationfAira snormallyontrolled yaninteractionf oil moisture,emperatureandthechangingemperatureesponse fthe eed. n Teesdalia, ecauseoffasterhangeintemperatureesponse,oil moisture laysa relatively ore mportant ole.

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(Received 7 November 962)

factors controlling germination

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