Aspects of Clock Resetting in Flowering of Xanthium'. Herbert D. Papenfuss2 and Frank B. Salisbury3. Department of Botany and Plant Pathology, Colorado ...
Plant Physiol. (1967) 42, 1562-1568
Aspects of Clock Resetting in Flowering of Xanthium' Herbert D. Papenfuss2 and Frank B. Salisbury3 Department of Botany and Plant Pathology, Colorado State University, Ft. Collins, Coloradd 80521 Receivcd June 19., 1967. Sunmmary. Flowering ii induced in Xanthuiumz struinarium by a single dark period exceeding about 8.3 hours in length (the critical night). To study the mechanism which measures this dark period, plants were placed in growth chambers for about 2 days under constant 'light and temperature. given a phasing dark iperiod terminated by an intervening light period (1 min to several hlrs in duration), and ifinalbly a test dark period long enough normally to induce flowering. In some experinments, light interruptions during the test dark -pe'riod were given to establish the t-me of maximum sensitivity. If the phasing dark period was less than 5 hours long, its termination by a light flash only- broadened the subsequent t.me of maximum sensitivity to a light flash, but the critical, ni-ght Nas delayed. In causing the delay, the end of the intervening light period was acting like the dusk signal which initiated time measurement at the beginning of the phasing -dark period. If the phasing dark period vas 6 hours or longer, time of maximum sensitivity during the stubsequent test dark period was shilfted by as much as 10 to 14 hours. In this case the liglht terminating the phasing dark period acted as a rephaser or a dawn signal. Folloxving a 7.5-hour phasing dark period, interven;ng light periods of 1 minute to 5 hours did not shift the subsequent time of maximum sensitivity, but witlh intervening Ilght periods longer than 5 hours, termination of the light acts clearly like a dusk signal. The cd.'ock appears to be suspended during intervening light periods longer than 5 to 15 hours. It is restarted by a dusk s.gnal. There is an anomaly vith intervening light Deriods of 10 to 13 hours, following which time of maximum sensitivity is actually less than the usual 8 hours after cldusk. Ability of the clock in Xa-nthilu to be rephased, suspended, restarted, or delayed, depeniding alxvays upon conditions of the experiment, is characteristic of an oscillating t'ilmer and miiay confer upon this plant its ability to respon'd to a single inductive cycle. it is suggested that phytochrcmlie mnay influence only the phase of the clock and not other aspects of flowering such as s;ynthesis of flowering hormone.
Certain slhort-day p'ants such as Xaithium stri-i-
warittrn, Pliarbitis nil, and Lcminla pcrpitisilla exhibit a photoperiod:c riesponse to a s.ngle c! c'e; that is, they wvill flowver after exposure to a single dark period longer thani the critical night. It has, tlherefore, seemed reasonable to assumiie that the c!ock which mea.sures this dark iperiodlhad somiie similarities to an hourglass mechanismi. For examiiple. a substrate might decay over a period of tiime-, and(l h-hen a certain level had beeni reached, otlher proce^ses might 'be initiated which eventually led to floxwering. This idea would apply equallyXwll to p'ants rtqCuiringg more tlhan 1 cycle. The daily light period imight reintiate 1 Part of this material wN is submitted 1)-w Herbert D. Papenfuss in partial fulfillment of the reciltirements for the doctoral degree. Work supportcd by NSF Grants number GB 2271 an(' GB 5363. ' Present address: l)e)artiellt of Biology, Boise College, Boise. Idaho. 3 Present address: Plalnt Scienice Departmient. U'tah State University. Logan,ULTtajh 84321.
the iprocess (iinvert the lhourglass). Indeed, such a suggestion lhas been formulated iby Borthwvick and Hendricks (1,6). They proposed that the governing reaction was the dark conversion of iphvtochromne froml one form (PFR') to the other (P1). In the erdogenous control of leaf movemiients and other circadian rhythms, several lworkers including Bunning, (3) and Pittenidrighl and Bruce (9), have suggested an oscillating timer. Bunning (2) has favored the idea that this same clook may also control timing in iphotoperiodism, and Hamner (4) has chamiipioned this idea. Thus a plant that exhibits both leaf mi-ovenments and photoperiodism might use the samel clock for both functions. Such a mechanismii i.ght seem plausible in plants that need a cvcling series of proper photoperiods before induction occurs, but in plants that are induced after only 1 short day, the evidence 'for an oscillator has not 'been apparent. In fact Moore, Reid, and Hamner (8) found no evidence ifor a rhythm in flowering of Xanthiuni, using the experiments which have indik-ated suclh a
1562
PAPENFUSS AND SALISBURY-CLOCK RESETTING IN XANTIIIUM16
rhythm in floNvering- of other plants, such aS 24-hour cycl-es in flowering with longer and longer dark periods. They suggest that the rhylthm damn.ps out in 1 cycle. If an oscill'ator which daamps out in 1 cycle were a part of the timing processes of Xanitlhml, an .nvestigator niglht expect to observe certaini characteristics besides the rhvthmuic flowxering with increasing night length soutght by Moore, Reid, and Hamner (8). Surely a variability in sensitivity to light interruptions during the daily cycle slhould be apparent (13,15). Muclh is also knowni about the characteristics of clock re.etting in the circalian rhythmiis (2). anld some of the2se characteristic.s mgliht al-so appear. The experillmellts reported in this and(l an earlier paiper (13) show that Xanithimiiii strumar imi does. exhibit an apparent cycle of scnsitivitv t- light. Furthermore, clock resetting may h,e induc-( by a light interruptioni given at the proper tinmes dulrinrZ an inductive dark period. At one tine, such a light interruption tends to rephaze the clock. In otlher circumstances, it may {lelay the clock. Followving prolonged light anld probably dark periodls thle cloc:k may become suspended (another wvay of saying tihe rhlythm damps out after 1 cvc'e), ald it is restarted againi by a dartk pferiod. The mechaniismii of action of a lighlt interruption of an otherwise inductive dark period hals ilong puzzled workers in this field. Experiments discussed in thsz paper suiggest that onlv the clock is affected.
Materials and Methods The methods u-ed in these experiments have been described in some detail by Salisbury (11, 12). Briefly, cocklebur plants of the Chc . d rain of Xanthium strumar-iu L.4 were germinated in flats. transplanted to plastic pots, and mainitainied vegetative by exposure to long days (daylighlt extenided to albout 20 hrs bv incandescent liglht). Thcey were ifert:lized with ammonium sulfate every 2 to 3 wveeks after transplanting. Wlhen at least 60 days old, the plants generally responded well to iphotoperiodic inducltion. For the sake of uniformity, plants Awere used oni which the third leaf from the top (12) \\-a from 6.9 to 8.5 ciii heug. Leaves I eloxw this oni wvere removed.
In all of the experiments, the plants were placed in growth chambers (10), and temperatures were miaintamied between 21 + 1° (for the dark period) and 25 ± 10 (for the light 'period). The experiment was begun by placing the plants in conlitnuous light (fluorescent and iincandescent at about 20,000 lux) for 2 days and theni subjecting them to various lig,hlt and
4 This is the straiin of cocklebur long referre(i to a; Xanithium pen (n)sylvanicunt Wallr. In viewv of the careful work of L6ve and Danisereatu (7), wve hlve decided to adopt the more generatl termii. (X. strumnarium incltudes all pre,viotis "species" except .X. spinosum.)
1 563
dark periods depending uponi the type of experiment. Th,e last period was always a dark period wlhich was long enough normally to iniduce flowering. Thle plants iwN-ere dissected 9 days later and clasiified according to the arbitrary floral sitages of Salisbury (12). Certain terms need to be defined: [liasinc(i Dark Pcriod. This is the first dark veriod following the 2-day continuous liglht period. A liglht lerio,l, at least long enoughI to saturate the phytochrome system (several seconds in our system), terminates this phasing dark period. In many of our experiments the 'phasing dark period w-as 7.5 hours long, but it can be any desired length. Initerveninig Liglit P.eriod. This is the light whiceh terminates tlhe phas ng- dark period, and it may ke anx lengtlh from very brief in duration (in -our experiments, usually 1 min) to quite long ( sometMies 16 hrs or longer). Test Dark Perio(l. This .-lark period, foliAwing the intervening light iperiod. nmay vary in length but :s usually long enotghli nol-rmiall- to induce flowern,g (loniger than 8.5 hrs). Tiuic of Maxini;iniii Scnisitivity to Liglit. In some experinmeints, light interruptionis are gyven (tistually at 1- or 2-hr intervals) duiring the test dark perod. The approximate hour at which lighlt is Imlost inhibitory, or stated conversely, the hiouir at wvhilh the flowvering process is most sensitive to a light interrutption, is considered to be the timne of maximum sensitivity to light. This is often shortened to the time of maximum sensitivity. Real Tic. This is the amllounit of elapsed tim1e counting from the beginning of the phas:ng dark period. it continues through the phasing dcark Der:od, inter-ening light period and the test (lark period. Time after b)egnning of the intervening light period or tilne after beginning of the test dark period may also le convenient.
Results
and Discussion The Basic ExperMicidt. In tlis exiperimelt, thle plants were given various phasing (lark periods ranging in length from 1 to 8 hours. Each plhasing dark period was followed by a 12-hour test dark ner ocl. The curve shown in figure 1 indicates that inhNb:ton due to termination of the phasing dark period Nvas rather suddenly more pronounced for phasng dark periods longer than 5 hours. Figure 2 shows phasing dark periods ranging ifromii 1 to 16 hours long. The sharp drop between 0 and 1 hour wvas not ev.idenit iin the first experiment or in any others (not shown), and the drop following the 5-hour phasing dark period is somewlhat less evident in the figure. Inhibition was clearly m,ore pronotunced for phasing dark periods between 6 and 9 hours in length tlhan for other phasing dark periods. Phasinlg dark periods longer than 9 hours undou'btedly allow synthesis of flow%ering hormone before the intervening light period, so that any effect of light on time measurement is masked.
1564
PLANT PTIYSIOLOGY
rup)tioln exp. r ilnenlt,
ais Indicated
1b)
the control
inie
(essentially a zero-hotur" phaMilflg- da rk perio(l There is a broadening of time of miiaximillnum senI.s tivitx 7.0 6.0
0~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 0
---0
0~~~~
5.0
w
trol becati e it shows a point of maximumni sensitivitx which i,s delaved 10 ilotirs compare(l to the otll,!timiies of mnaximutmlli sensitivity. TI 1>. alonlg xxitlh previously reported exiperiments 13 ) (ldmoinstrates
0-
4
()
folloxxving the 2- and 4-hour phasing dal-k periods. especially the 4-hour phasing dark period. The 6-lhour phasing d(ark period differs frouti these ani the coIn-
4.0
that brief light periods which terminate pliahsinl< dark periods of 6 to 8 hours (luration. hax e a marked effect
-J
4
-30
on1 timiie mea-ziireiment.
-J
2.0 1.0
+
+
4 +
4
4
41
-
0
FI(.. 1.
0 7.0 1\2 Hur Hours6
5 7 8 4 6 2 3 1 HOURS OF PHASING DARK PERIOD Effect of 1-miniiiute light interruiptionis ter-
i-inating phasing
dark
periods of various
lenigtlhs
otrl*
when
followed by 16-hour test dark periods. Temperature 230. Bars above the figtire represent the plan of the . 0 experiniienit: a 16-hour (larlk period as conitrol; a 24.0 hour phasing dark period followed by a 1-miniute inter\vening light period anid 16 hours of dark; and so 03.0~~~~~~~~~~ on for other treatmenlts. Bars are draw-ni to scale but l 3;.0 Q AELTM 6.0 ~ ~ A HUR LN not the scale of the ahscissa.
7Tinc ]j' .IaxinniiiiJI .SYositiV ity Folo-winlg 2-, 4-, antd 6-Hour Phlasing Dark Pcriods. Figure 3 shows the restilts of light interruptions given duiring the test (lark lperiods whiclh followed 2-, 4-, anid 6-hour phasing dark periods. T'lhe curves showv that the time Of maximum sensitivity i-emiiainied essentially the same followinig 2- anid 4-hour p)hasingcdark perio(ds, namely, at the 8-hotur time exp)ecte(l for the usual lighlt inter-
7.0-
6.01 w I~-
0~~~~~~
-J
0~~
4
3.00 x -1
2.0$ I
@0
\,O/
1.0-
I .4 4 6 10 12 8 14 16 HOURS OF PHASING DARK PERIOD FIG. 2. Efffect of a 1-miniute liglht interrultion terminating phasing dark periods of various lengths when followed by 16-lhoUr dark )eriod. Temperature 230. I
O
6
In figulre 4. time of maxim,numi sensitivity durin)long test dark period following a 2-hour phasing darkl period (as in fig 3) is comi-pared with restult. when the test dark iperiod is simiply terminated li returnin- plants to light. T1'hat is, critical dark period.
0
4
0
4
a
5.0-
cn4.0-
U.L
8 10 I2th 14 6 820 22 24 HOURS IN REAL TIME F'IG. .3. Effvcts of a light 1ttiaterrl)nton giveni a5 v-ariouis timieS duiring lonig test dark periods whichi fol lowedI phasing (kitrk periods of variouis lenigths. Temii perature 210. Arrows above the hars at the top mndi0
catc times w-heni light interruptionls \x-ere giveni cid correspond to the data poinits. Of course, a giveni svt of plants receivinig a given test dlark periodl receivcd only onie light interruiption durinig the test lark periodl and(l followilng the interruptions at 0. 2, 4. ;nd 6 hours.
v
.
.
.
vau 2.00 4 Hours A
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2
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I
I
.
another indicationi of time meastiremnent (9, 11), determiined follow ing the 2-lhotir plas.ng dark period and comilpared w-ithi timne of imia.ximiiuim sensitivitv. Results of these 2 treatments are againi compared wvith the 'zero-lhour' "phasing dark period. 'rhe light perio(d after 2 hours of darkness does affect the critical nigt.t causing it to shift b\ about an hour (not thie 'exl)exted" 2 hrs), and time of llmaximiuni sensitivit\ is again broadened. In several experiments not sho\w, the delay in the critical niglht was alw,,vaxvs evident and sometimies nearlV equaled the length of the phasilng (lark period (tip to 4 Irs
PAPENFUSS AND SALISBURY-CLOCK RESETTING IN XANTHIUM +1
1 +4 '4
4
4'
1565
hours required before maximum sensitivity is reached were a phase in which light is at least innocuous, then a skeleton photoperiod comisting of 2 brief intervals of light corresponding to the 'beginning and the end of the real photoperiod (intervening ,ight period) should have the same effect upon time of maximum sensitivity as the continuous-light real photoperiod.
4,~4,4,4,4,4,4,4,4,+4,+4~,4,4 ~
-m
2
4 6 8 10 12 14 16 HOURS IN REAL TIME FIG. 4. Effect of a light break terminating a 2hour phasing dark period upon time of maximum sensitivity (upper) or the critical night (lower) as compared witth time of maximum sensitivity and critical night of controls where no phasing dark period with its terminating light break was given. Temperature 22°. O
Effects of Inttervening Light Period Following 7.5-Hour Phasing Dark Period. Having ascertainedi the effect of phasing dark periods of various lengths, we returned to a study of the 14-hour timie of maximum sensitivity following a 7.5-hour phasing dark period. Our reason for doing this w.%as that 7.5 hours is the approximate time when inhibition by a light interruption is most pronounced, 'providing test dark periods are fairly long (13). Thus the results are often more clear-cut. Intervening light periods of varying lengths were given following the 7.5-hour phasing daifk period. These light periods were then followed by test dark periods which 'were interrupted at 2-hour intervals. This indicated the time of maximum sensitivitv during the test dark period for each intervening light period. Critical night 'had already 'been studied in such an experiment (13). The results were then plotted in terms of real time, as in figure 5 which shows only a few representative results. Times of maximum sensitivity for all experiments are indicated in figure 6. It was evident that the time of maximum sensitivity occurred at the same time for intervening light periods that do not exceed 5 hours. Light duration appears to have no effect u-p to 5 hours. It was thought that if the first 5 hours of the 14
(4)
(12) (8) (16) (20) (24) HOURS IN REAL TIME FIG. 5. Effects of a light interruption given at various times during test dark periods following intervening light periods of various lengths. Phasing dark period was 7.5 hours long. Results are plotted in terms of real time with figures below the abscissa but in parentheses referring to time after beginning of the intervening light period. Typical of several experiments not shown (times of maximum sensitivity shown in fig 6). Temperature 250 light period; 210 dark period. (0)
a
34 4TIME OFMAXIMUM SENSITIVITY: \
A\Time aftrr Beginning
32
*N Test Dark Period
z
/0 //
2z I~~~~~~ ~~~~~~~~~0
~~~~~~~~~~~~
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80 z
~~~z
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22
201 0
H~~~~~~-;°° 16 IS 20z
2F 4 T 6 8T 10 12 14 HOURS OF INTERVENING LIGHT
FIG. 6. Time of maximum sensitivity during the test dark period as a function of the length of the intervening light period, using data of figure 5 and many experiments not shown. Heavy line corresponds with left ordinate and indicates time of maximum sensitivity in terms of real time. Lighter line shows time of maximum sensitivity in terms of time after beginning of the test dark period (right ordinate). Dashed lines are interpolations and extrapolations.
1
1566,
PLANT PHYSIOLOGY 6. I
9
22 (12)
26 30 34 (20) (24) (16) HOURS IN REAL TIME I;i(;. 7. lEffect of 3-lhouil slkcleton lpllotol)erio(l and a 3-hour continuous liglit rcadl photoperiod on time of maximumii sensitivity (Itirilig the test dark period. Temperattire 230. Figures in parentheses belox thie abscissa refer to tin'l after beginniog() of the test d irk period.
(0)
14 (4)
18
(8) a
For this emxeriielnt wxe used a 3-llouri llotoperiod ancd a 3-lhouir skeletoni photoperiod. 'I'lTe results fig 7) indicate that the timie of iax:muin seisftivitv is tlle same ini eaclh case. Liglht is innoc0tlotls (Itirilng a 3-hotur interveniing light perio(l.
ThIle effect of
a
12-hiour- ;keleton plmotoperiodNxvas
also investigate(d. as showxn in figure 8. The results for the skeleton an(d the real photoperiods xvere not the same. Probably the second(I iighlt ilterrupltion acts as a rephaser a
(daxvn ) rather than
12-hour intervening lih(,-t period
a
simutlated enld of
(dusk).
FI(;. 8. Effect of zl 12- hour skeleton plhotoperiod conitrol 12-hour conistant liglht period oni time of maximum sensitivity to liglht initerruptionis durinig the test dark period. Points plotted from the time when the test dark period started. Temperature 260. Figures in parentheses below the abscissa refer to time after beginning of the test dark period. and
I' urthcr finplicatliois of tlie IRs.iht.fs. [rom tlu' experiments reported here (figs 1-4) it is evidelit that the timilIg milechanisimi or c'ock which controls flowering in N. strimiarlin i is only (lelax ed by a li-hlt interruiption during the first 4 houirs ot phiasng darkness, and typically bl an amotiiit less than or at ilmo)4 equial to the )ha.:ng dark period. It is Ver-V strolng-\ affected soon after the fifthi lhouir of p)hasing darkine-.. however. In this respect, the clock laLs the a:iects of a relaxationi or oscillator m-chalnisiii ( 3). Y-By a delav followving phasing (lark perods if 4 o5 hours or less, we mean that the cvcl( i 1not chlan-ecl to a new phia-se, hut that the cuir-r-ent phase s only sh;i ftedl. There i.istill a requir-eiiielit for 7.5 to 8.5 lhouirs of darknet!ss following tlle lighlt flashi before ;nduction occuirs- if critical niglt i-z stmdied. 1\v ll this delay is seeln to l)e partial, hoxever. WXe foutind that the time of miaximiuimii sensitix itv vas shifted frolim the usuial 8 houil-s (approx. ) to ahont 18 to 22 houris (real time) when a hrief li-lit period was a given after about the fifthi h or of phlasing darkiles.s. In teriiis of timiie atter- suich a Fght initerrul)tlon, time of maximumi selnsitivitv com.e-s 10 to 14 lhouirs later insteadl of ani expect cl 8. \t f'rst it wx v thlought that time measurement wx a. beinlg-slox C(l (lown ('13 ), buit the l)resenlt experimetlints Imiake it clear that the change ini time of maximum sensitivitv a real pliase shift aiul lnot just a delay. The brief light perio(l gixven after S or t hours of phasilln larkue..s apiparently repihases the clock from a phase (lurin-lxhich light is iost inhlihitorv to one (luringxhlich lighlt is 'iml1oculous [promotive in som0e expel-ienlts ( 13) These wx-ould he the photophiil and -skotoplil phases- of Bunning ( 3). hut wve foundi little or nlo real promotion during the so-called photophil phase. In thlis rephasiiig. light actted as a dazein siga!(ii, a(lvanc-ing the cycle, so that thie dawn dtate comIes several h1ours earlier. Th1uts thie clock miiust go through1 about a 5-hlotir lighit-innocuotius sate ( miiililum)n, tileni a light i11hilitorv state of about 8.5 hours, before inlduction clml occuIr. If this is a.n ilitegral part (o),f the thining mechilaism in Xan/hilm,i thieni the possibility of anl oscTlat;n- mechanism is likelv. Such1 an1 oscillatioin Wvoultl normally be obscured because cocklebur r e(itlires only 1 suclh oscillation to in(diuce flowering. Therefore, the importalice of the dark period emphlasizcd by Haminiier a(ld Bonner (5) and( most stibse(itiiet authors is (liminishied somiexvhat 1b the likelilhood that both lighit and darkness appear to be equally neces,sarv in floxvering of cocklebiuir as well as ill pllants that requiire a nunilber of cv clev to indilce floxvering. In figtire o, the time of miaxinimimi sensitivitv is p)lotte(l as a fulinction of the lengtlh of the intrl-veni-
a
The term '"phase' in Biinning's termiiniology refer-s
to a portion of a cycle. In mo(lerrn circa(lian rhlthlii utsage, it refers to a poinit on a cycle. \Ve will iise th worct "state" to imp)- a portion of a cy cl.
1 5 67
PAPENNFUSS AN-D SALISBURY-CLOCK RESETTING IN XANTHI AtM
light iperiod. Whlen plotted in this wav, the f'rst hours of intervening light shoxv a straigllt line of slope zero (real time), after which the line has a slope of 1 except for an interesting dip from 10 to 13 *hours. The zero slope during the first 5; hours indicates, as mentioned above, that increasing the intervening light period to 5 houors has no effect on timing. The slope of 1 indlicates that the tilme of mlaxinmum sensitivitv remains constant at about 8 hours after the. beginning of the dark period. This is also evideent wheni data are plotted in terms of time after beginnin.g of the test dark period, in w%-hich case a slope of zero indicates no effect of the previous lighlt period on time of maximutmii sensitivitv during the test dark period. The dip between 10 and 13 hours indicates that these intervening light periods result in times to maximum sensitivity less than the usual 8 hours after beginning of the test dark period. Thlis agrees with Salisbury's (13) previous observation that crlticat night is less than 8 hours followving a 12-hour initervening light period. The explanation for this phenomenon remains to be verified. but it is as thouglh the tendency for the cloak to oscillate were so strong that it begins to go into the dark status after about 10 hours of light. By the end of 13 hours of light. the clock is re-entrained to the light state. Forniatio ; of an inhibitor after 12 hours was suggested in the last paper (13), but this does not agree with the present data, namely the 8-hour timne of maximun sensitivity following intervening- light periods of 6 to 9 hours. The broad implication of figutre 6 is that the l)eginning of the int rvening light period following phasing dark periods of 6 hours or longer acts as dawn, rephasing the clock so that a period of maximum sensitivity wvill occur abotut 14 hlours later. This effect remains unchanged as the intervening, liglht periocl is extended to about 5 hours. After this tilme, a return to darkness acts like a true dusk signal. starting anid phasing the clock so that a tmne of maximum sensitivity will occur about 8 hours later. We mnight imagine that the clock has been sisuspended' after 5 hours until it is "started" by a dusk signal. [It is possible that the clock cani also be suspended during long cdatk iperieds. See Mloore, Reid, and Hamner (8).] In this species, we coukl say that the operationi of the clocik can be delayed during the first few lhours of phasing darkness (a dusk efifect) ; it can be rephlased after 5 hours of phasing darkness (a dawn effect) it can be suspended (as witlh intervening light periods exceeding 5 hrs); anid it cani be restarted following suspension (also a dusk effect). These interesting properties of cocklebur (especially suspension and restarting) may confer upon this plant its nearly unique ability to !flower in respon-se to a single dark
period follorwing continuous lig&ht. These properties are illustrated in figure 9. Workers in this field have long wonidered about the mechanism of a light interruption during an other-
CLOCK IN COCKLEBUR
OPERATION OF THE
Normal conditions: (12hours light, 12 hours dark) Light phDse
Down,;
I
t ~~~~~Dorkness
Dark pa
Position allowing florigen synthesis
Suspending and Restarting
Dusk
{ ~~~~~Suspended J r
14 hr
Delaying and Delay Q
Delay;
~~~~~~~~Restarte
s
-12 hrs.-
-
\
8. 3 hrs.-I
Rephasing: Second
Early down
Phase Shift:
dusk
8-Hour Interruption in 24-Hour Dark Period
x ntervenin Period,
Light Seconds to
20 hrs 24 hrs. .
5hrs. No
flowering
Some flowering
FIG. 9. A schematic represenitation of the features of time measurement in flowerinig of Xmithiumil as presented in this paper.
wviss inductive dark period. Did it reset the clock or did it inhliibit flowering only to an extent determined by the phase of the clock? Salisbury (12) decided against resetting because he assumed that resetting could onlV 1man a delay as used in the sense of this paper, and if that is ali that there iwere to it, why should an interruption after 8 hours of a 20-hour dark period be inhibitory? The remaining 12 hours should provide plenty of time for a critical night ancL for synthesis of flowering hormone. The concept of rephasing introdluced in this paper..
however, makes it possible to exiplain most lighlt interruption experimnents on- the basis of effects oni the clock alone. Assume that flowering hormone synthesis delpends only upon phase of the clock, andl that this plhase is only achieve(d 14 hours after a dawn signal or 8.5 hours after a dusk signal. The interruiption after say 4 hours inhibits with fairly short dark periods (e.g., 16 hrs) because time for f;lowering hormone synthesis is shortened by up to 4 lhours. The interrtuptioni at 8 hours requires at least 14 more hours of darkness before the clock will be in the phase allowving flowering hormone synthesis, or a total of at least 22 hours. A light interruption after 8 hours in a 24-hour dark period does allow flowering (13). If the interruption colmes after flowering hormone synthesis has ibeen initiated (after 8.5 hrs), it will always act as a dawn signal, rephasing the clock and thus changing its phase so that flowering hormone synthesis is no locnger allowed.
I1 ( llR
PLANT PHYSIOLOGY
T,wo kinds of exper-inimnts (lo not readily fit this approach as yet. It is not c'ear why tlhreslhol(ilight sllould inhibit flowering lbuit apparently not intfluence timle mlleasulrem1lent (11). nor wy-lv far-recl light slhould be inihibitory during the fir4st part of Icng- but not slhort dark periods (9a). We lhope to invest4gate these problem-, by the appr-oaclhes descril)ecl lere and based upon the hypothesis thlat the effects of a light Interruption of a dark period mav be interpreted onily as efifects upon time measturement. That is. far-reday influence onl,- the receptive phytochrome (1, ) max clock anrd lnot synthesis of flowering hormone.
Literatu-re Cited 1. BORTHWICK, H. A. AND S. B. HENDRICKS. 1960. Photoperiodism in plants. Science 132: 1223-28. 2. BUNNING, E. 1960. Circadian rlhyvhins anid the time meeasuremiielnt iii photoperiodism. Col(d Spring Harbor Symp. Quant. Biol. 25: 249-56. 3. BfNNING, E. 1964. The Physiological Clock.
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4. HAMNER, K. C. 1960. Photoperiodismii and circadian rhythms. Cold Spring Harbor Simp. Quant. Biol. 25: 269-77. 3. HAMNER, K. C. AND J. BONNER. 1938. Photoperiodism in relation to hormnones as factors in floral initiation and devrelopment. Botan. Gaz. 100:
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periodism in plants. Cold Spring Harbor Symip Quant. Biol. 25: 245-48. L6VE, D. AND P. DANSEREAU. 1959. BiosYstematic studies oil Xaothiiotl : taxonomiiic appraisal and ecological statuis. Cain. J. Botany 37: 173208. MOORE, P. H., H. B. REID, AND K. C. HAMINER. 1967. Flowerinig rCesponise of Xaoitlhlioiii peiisyl7t'OOiCllIi} to long dcark perio(ds. Plant Physiol. 42: 503-09. PITTENDRIGII, C. S. AN-D V. G. BRUCE. 1957. Anl oscillator miiodel for biological clocks. In: Rhythimllic and Synthietic Processes in Growthi. D. Rudnick, ccl. Priincetotn University Press. Princcton. p 75-109. REID, H. B., P. H. MOORE, AND K. C. HAMNER. 1967. Control of floweriig of Xanthliuoii penici by red and far-red light. Planlt Ph,-slvz'oo icn siol. 42: 532-40. SALISBURY, F. B. 1964. A special-purpose coIntrolled environimilenit Unlit. Botan. Gaz. 125: 23741. SALISBURY, F. B). 1963. Biological timiniig and lhormonie synthesis in flowering of Xauithiiiii. Planta 59: 518-34. SALISBURY, F. B. 1963. Tlle Flowering Process. Perg-aimon Press. Ncw York. SALISBURY, F. B. 1965. Time miieasuremenit anid the light period in flowering. Planta 66: 1-26. SALISBURY, F. B. 1965. The iniitiationi of flowering. Endeavouir 24: 74-80. TAKIMIOTO, A. AN-D K. C. HAMNIER. 1964. Effect of temperature and preconditioning on phiotoperiodic response of Pliar-bitis iiil. Planit Physiol. 39: 1024-30.
