Plant Phivsiol. (1968) 43, 1786-1792

Second Positive Phototropism in the Avena ColeoptileMarylee Everett' 2 and Kenneth V. Thimann

Division of Natural Sciences, University of California, Santa Cruz, California

Received June 7, 1968.

Abstract. A method has been developed whereby the second positive phototropism can beobserved separately from the first positive and negative phototropic responses w-hioh alsooocur in oat coleoptiles. Although the second positive phototropic response has often beenreferred to as the base response, photoreception for it is shown to occur mainly in the apical3 mm of the coleoptile. The Bunsen-Roscoe reciprocity law, so typical of first positivephototropism, does not apply to the second positive responses, and the amount of curvatureincrea.ses linearly with the duration of the stimulus. However, although this linear propor-tionality between stimulus duration and response is the major factor determining response atall intensities tested, the inten,sity of the stimulus does influence the Tesponse somewhat.The action spectrum for the response shows no activity above 510 nm and 'has peaks at 375and 450 nm. In all but one particular it closely resembles the aotion spectrum ifor the firstpositive phototropism, and it is conoluded that the same, or similar, pigments may well bethe photoreceptors for both types of response. The identity of this blue light absorbingpigment is not known.

De-spite the availability of action spectra for thefirst positive phototropism in oats (15, 17). thephotoreceptor for this response has not been defi-nitely characterized (9,17). Furthermore, the earlxexperiments on phototropism demonstrated that inaddition to first positive phototropic curvatures, oatcoleoptiles also exhibit negative and second positivetypes of response (1, 4). From a study of th-e dose-response curves for phototropism in Avena and oftheoretical models for these data, Zimmerman andBriggs (19, 20) suggested that Avena coleoptilespossess 3 different photoreceptor systems for the 3types of phototropic response-first positive, nega-tive, and second positive. If this be the case, thenthe photoreceptive pigment for the second positivetype of curvature would he di'fferent from that forthe first positive phototropism and might be identi-fied from an action spectrum for the secolnd positiveresponse. For this reason, and in order to charac-terize the little-known second positive response moreclearly, the present experiments on second positivep)hototropism in coleoptiles of Are(nMa wer-e under-taken.

Although the second positive phototropic re-sponse, which occurs with large stimulus energiesor long stimulus durations, has not beeli extensivelystudied, there are strong indications that the B3unsen-Roscoe reciprocity law, which holds for first positivephototropism, is not applicable to it (2, 5. 17). Thislaw states that the response depends only on the total

'Supported by a pre-doctoral fellowship from theNSF.

' Present address: Biological Laboratories, HarvardUTniversity, Cambridge, Massachusetts.

energy of the light dose and is not influenced by theintensity, or duration of the stimtulus. Thimann andCurry (17) suggested that curvatures of the secondpositive phototropic type are determined mainly bythe duration of the stimulus, and Bri.ggs (2) lpro-vided strong evidence for this hypothesis by showingthat second positive responses of oat coleoptiles tostimuli of equal energy, delivered with various in-tensities of light, increased with increasin'g stimulusduration. Furthermore, when Zimmerman andBriggs ((20) subtracted the responses predicted bytheir theor-etical models for first positive and nega-tive l)hototropism from their phototropic dose-re-sponse curves, the remaining responses, assume(d tobe of the second positive type, increased linearlyxvith stimulus duration and were not affected by a10-fol(d vtariation in intensity. Unfortunately theoverlap of first positive responces bv second positiveotnes when low intensity light is used (1,17, 19)has prevented a direct study of the characteristicsof the second positive phototropic response. Forthis reason it seemiied essential to develolp a methodof i' olating seconid positive curv7atures, and thuts tostuidv them alone. In this paper such a method isdlescril)ed and used to elucidate both some generalproperties of the system and its action spectruni.

Materials and Methods

Seeds of Aventa sativa var. Victory, obtainedfrom the U.S.D.A. Branch Experiment Station atAberdeen, Idaho, were husked and soaked for 2 hrin distilled water. The soaked seeds were plantedindividually on 1.5 % agar slanted in 1.5 ml vials,which were placed in deep petri dishes. covered tomainltaini a high humidity. The dishes were placed

1796

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1787EVERETT AND THIMANN-SECOND POSITIVE PHOTOTROPISAI

in a growth room maintained at 250 + 1° anidexposed to 22 to 24 hr of red light at a diistance ofabout 50 cim froin a 25 watt ruiby red bulb, wherethe intensity was abouit 1..5 X l 10 ergs Cm 2 sec-1.This treatment comipletely inhibited the growth ofthe mesocotyl. The plants were then placed in alight-tight cabinet in the growuth roomii. At 70 to72 hr from the time of soaking, when the coleoptileswere 2.5 to 3.0 cm tall, they were exposed to 1 to 2hr of the same red light to bring the sensitivitychanges caused by red light to a maximumi (5. 19).Exposure to the stimulus followed this treatment.

The seedlings were selected for straightness andpositioned (in their vials) in racks so that thenarrowN side of the coleoptile would he exposed tothe stimulutis beam. Ten plants at a time were ex-posed to each stimtulus, the entire coleoptile beingillut-minated uniless otherwvise stated. 'Monochromaticlight, of band width 5 nm, was obtained from a1 50 watt xenon laml) and a Bausclh and Lonmbhigh-intensity grating mionochromator. The mono-chromator, located in a part of the growth roomlpartitioned from the rest by a black curtain, wasencased in a light-tight box provided with a shutter.Tntensity wvas usually varied by changing the dis-tance of the plants from the exit slit *of the mono-chromator, but in a few cases neutral density filterswere employed. For stimulus times of less than 1sec the shutter mechanism was used, while for longerstimuli the shutter was operated by hand and timedwith a stop watch. If only the bases of the plantswere to be exposed, aluminum foil caps of the ap-propriate size were placed on the coleoptiles dturingstimula.tioin.

Immediately after exposure to the stimulus, thevials containing the plants were rotated 900 arounda vertical axis and placed in position before a filmholder containing Kodabromide paper. A shadow-graph was taken with phototropically inactive greenlight; the plants were left in position, and a secondshadowgraph was taken 100 min after the time ofthe start of the stimulus. The double shadowgraphswere measured twice with a goniometer: the 2 setsof values for each plant were compared and averaged,and if a discrepancy of more thani 3° occurred inthe 2 measurements of each plant, it was correctedbyr a third measurement. Standard errors for themeans of these measurements were no more than 2°,and rarely more than 1.5°. Each value in the plotteddata rests on measurements of not less than 50plants.

A Reeder vacuum thermocotuple connected to aKeithley microvoltammeter was used to measurelight intensities. Absolute values of intensity weredetermined with an 8-junction bismuth-silver Epplevthermopile which had been calibrated against astandard lamp 'by the manufacturer. Variation ofintensity with distance from the monochromator wasdetermined with an Evans electroselenium micro-photometer used witlh a neutral density filter. Sincethe readings on the photometer indicated that the

intensity of the light decreased as the square of thedistanlce from the monochromator (except muchcloser to the monochromator thanl any plants wereplaced), intensities in the range where this relationwas valid were calculated using this assumption.

Results

Isolation of the Response. In preliminarv ex-periments the dose-response curves for 5 intenisitiesof 475 nmi light wvere determined. The results areplotted in figure 1 and are in agreement wvith thedata of Zimmerman and Briggs 1(19) on the slhapeof the dose-response curves at a series of intensitiesof stimulus light. Negative responses were not ob-served because the stimulus intensities reached witlthe narrow slit width were not sufficiently high.Since the first positive responses depenid only onithe total stimulus energy, the 5 curves coincide inthe region of the lower energy doses. In th-e higherenergy region where the second positive responsesoccur, the curves do not overlap, and the secondpositive curvature is initiated at increasingly lowerenergies as the stimulus intensity decreases. rheexplanation of the onset of second positive curvatureat progressively decreasing energy levels is given bythe theory that the second positive responses dependprimarily on the stimulus dutration, irrespective ofthe total energy contained in the stimulus. Forinstance, the time required to deliver the dose of1000 ergs/cm2 decreases from over 1600 sec at thelowest intensity to 21 sec at the higlhest intensity.With long stimnulus durationis the second positiveresponses overlap those of the first positive, and( thedifficulty of studying responses observed in this wayis clearly illustrated in figure 1.

20

w

a:

0)n

w00

Is

to

0

l

0

x

10 20 30 40

LOG (I t), ERGS/CM2FIG. 1. Dose-response curves for phototropism in oat

coleoptiles at 5 different intensities of 475 nm light.*) 48 ergs cm ' sec-1 (80 I in fig 3), A) 24 ergs cm-2sec L (40 I in fig 3), *) 6 ergs cMn2 sec'1 (10 I infig 3), X ) 1.5 ergs cm-2 sec'l (2.5 I in fig 3), Q)0.6 ergs cm-2 sec'1 (I in fig 3).

Copyright (c) 2020 American Society of Plant Biologists. All rights reserved.

1788

There is 1 situation in which the second positiveresponses can be isolated, namely when high inten-sity light is used for the stimulus. Such a situationis illustrated by the responses of the coleoptiles tothe highest intensity of light used in figure 1. Herethere is an "indifferent" responsive region betweenfirst and second positive curvatures, the plants show-ing no response to a stimulus of this energy. Firstpositive responses occur only with s-timulus energieslower than those in this indifferent zone; and becausethe inten.sity being used is high, the stimtullts dura-tions are not long enough to cause significant secondpositive curvatures. It is possible, however, to use

this system to obtain isolated second positive re-sponses by following a high intensity dose with oneof a lower intensity. Thus, if a high intensity lightdose, causing an indifferent response, is given uni-

laterally to the experimental plants and is immedi-ately followed by a second unilateral stimulus ofany intensity, lasting several hundred sec, the plantsnow respond with a positive curvature. Such curva-

tures take place when the sum of the energies of the2 stimuli is greater than the energies that cause

first positive or negative curvatures. Evidence willbe presented below that these curvatures have thecharacteristics of second positive responses as de-scribed above. The initial stimulus used in most ofthe experiments was a dose of 4800 ergs/Cm2 of475 nm light, delivered in 100 sec. The secondstimulus was usually given for 200 see or more.

In the action spectrum experiments, the initial dosewas changed to 5700 ergs/Cm2, delivered in 100 sec,

at 470 nm.

Characteristics of the Response. Authors haveoccasionally used the terminology "tip response" and"base response" in referring to first and secondpositive curvatures, respectively, to suggest thatphotoreception and response for the 2 types of photo-tropism occur in different regions of the coleoptile(18). However, in the experiments of Arisz (1)and of Zimmerman and Briggs (19) only the apical3 mm tips of the coleoptiles were stimulated, butnevertheless second positive responses were observed.Thus, as pointed out previously (17), the term

"base response" is inappropriate for second positivephototropism. In addition, the "base responses"observed with 260 to 330 nm light (7) differ indose-response characteristics from second positivephototropism so that these responses may well repre-

sent a type of phototropic curvature distinct fromthose occurring with wavelengths of liglht above350 nm.

WVith the experimental method used here, a com-

parison of the responses of plants hearing 3 mm

apical caps with those of plants stimulated alongtheir entire length showed that light-reception in thetip of the coleoptile accounts for 75 to 80 % of theresponses observed in these experiments. For in-stance, in 1 experiment capped plants responded toa stimulus consisting of the initial 00 sec highintensity dose followed by 400 sec of 6 ergs cm-2

PHYSIOLOGY

sec-1 at 475 nm with a curvature of 3.7° 0.9°while plants without caps curved 14.50 1.50 tothe same stimulus combination. Only with illumi-nation lasting 1000 see or more did capped plantsgive curvatures of over 10°. Thus, the responses de-scribed in these experiments are primarily tip re-

sponses, and the photoreception and resulting re-

sponse that undoubtedly cani occur in the base ofthe coleoptile during some second positive curvatures(1, 17) play a minior role here.

The characteristics c,f the reslponse in plantsstimulated with] the 2-dose method described abovewere examined at 1 waxelength, 475 nm. Figure 2

20-

151LUJ

D

D

v) 10U-)

LLi

w

5

3 6 3 8 40 4 2 4 4

LOG (Ixt), ERGS/CM2 AT 475nm

FIG. 2. Phototropic responses of oat coleoptiles stimu-lated with an initial dose of 4800 ergs/cm2 at 475nm [log A! X t) = 3.68], and a second dose deliveredin 5 intensities of 475 nm light: 0) 48 ergs cm-2 sec7lA) 24 ergs cm-2 sec-1, Ol) 6 ergs cm-2 sec-', A) 1.5ergs cm-2 sec-1, 0) 0.6 ergs cm-2 sec-'. Stimuli of 4durations were given at each intensity. The responsesare p!otted against the logarithm of the total energyof the stimulus, Standard errors are indicated.

shows the responses to the seconid doses at 5 initexi-sities, as a function of the logarithm of the totalstimulus energy. All the plants received the initialdose of 4800 ergs/Cm2 [log (I X t) = 3.68], andwere then exposed to a second stimulus, lasting 200to 600 sec, with 475 nm light. When the curvatureis thus plotted against dose, it is evident that thereciprocity law does not apply to these responses;equal stimulus energies do not produce equal curva-tures. At any 1 intensity the response increaseswith increasing dose and therefore with increasingstimulus duration.

The indication that second positive responsesdepend on the stimulus duration is directly tested infigure 3., where the data shown in figure 2 are

Copyright (c) 2020 American Society of Plant Biologists. All rights reserved.

EVERETT AND THIMANN-SECOND POSITIVE PHOTOTROPISM

cr

15~~~ ~ ~~~~~~~~0

I00 200 300 400 500 600 700

STIMULUS TIME IN SECONDS AT 475 nrn

FFIG. 3. Resipoiises of oat coleoptiles plotted againsttlle -stimulus duratioii. T-lic initial 100 sec of all thestimuli gave the same dose of 4800 ergs/cnm2 at 475nmn. The second parts of the stimuli were delivered

CII

xvith TIMLUT5MinNeSECONDSAT475nmlih:)48eg m sc,

*)24 ergs cm- 2 sec.', E1) (i ergs All +sc' ) 1.5ergs cni-2 sec-1, * ) 0.6} ergs Clll-" .sec L.Jines drawxtithrough the pastafol faci iatclesitopiers calculatedbaithe least squares metihod. Standard errors for thesedata are inliated ia feiure 2o

re-plotted agsinst the duration of the stimulus. Thefirst 100i sicc represf7ent the duration of the initialerimulus received y0 sll the plants, brgisng thed nto the zone of indifference. For any 1 intensitr ofthe second dose the response is seen to incre.selinearly with stimulus duration. Howtever, the iTh-tensity does modifv- the slope or X-intercept of theline relating response to stimulus duration, so thatsall the data points do not fall on 1 line. To illus-trate the influence of stimuluain1tensityon secofdpositive ehotutropism, therespisonsee of plants tolinaly wt stimuoludurationhve been plotted againstthe logarithm odfthe interesity of the second stimtlus.The result is shown in figure 4. Resaponse is maxi-mal at ailintensity onf 6 ergs CM secs.higher oandlower intensities causing decreased responses. Thedecrease in response, at high intensities is not causedby heat radiating from the monochromator, sincewater filters placed in front of the monochromatormade no difference in the re Rsponse. It must beemphasized that at any 1 intensity of the seconddose, stimulus duration is the factor determining theamount of curvature, and response increases linearlu-with duration. The modifningeffect (on this lineal,relationship) tof the intensit yof the stiemulushcas iotbeen observed previously, although it is possiblethat the decreas.ed phototropic responsiveness re-ported with very large light doses (I) is a manifes-tation of this phenomenon. On the other han(d the

existence of a second indifferent and a third positiveregion of the phototropic dose-response curve (4)has not been substantiated in these experiments.

The Action Spectrunt. In the highest intensityregion, where curvature decreases with increasingintensity, the spectral sensitivity of the responsecould not be investigated thoroughly since sufficientlyhigh intensity light was not available throughoutthe spectrum. However, measurements of the re-sponses of coleoptiles to a quantum flux for thesecond stimulus of 3.5 X 1012 quanta/cm2 sec'1,lasting 400 sec, indicated that light of wavelengthbetween 440 and 480 nm -was most eiffective inbringing about the decrease ,in response, and thatat this intenisity 495 nmiu and 355 nmn light did notappear to cause any depression in response.

The action spectrum for second positive photo-tropisni was determined in the low intenisity stimulusregion,̂ .where response increases with increasingintensity. Since the duration of the stimulus isthe major factor determining the response, the dura-tionis of the stimuli in all these experiments wereheld conistaint by using an initial high intensitystimulu.s of 100 sec, followed bv a second stimulusof 400 sec. 'lTo determine the effect of the intensityof the second stimulus on the seconid positive photo-tropic responlse at all the wavelengths investigated,2 (quantuitll fluix.es, 0.53 X 1(0' and 1.2 X 1012 quantacm sec-2 ,-were enmployed at each wvavelength. Thewaavelengths were spaced every 20 nm from 355 to435 nm and every 10 nm from 445 to 495 nm. Theexperiments at each waveleingth aind quantum fluxwere repeated 4 or 5 times, and the data are presentedin table I. As shown, the chaniige in response for achange iii (iuantumni flux of 1 log unit is found tobe reasonably conistant with wavelengtlh in this in-tensity regioni. The values for the slope calculatedat 455 and 475 iim are much smaller thaan the others,

C'5.I0

Is

2'l it 2's 705 -+

LOG iNTENSITY (ERGS/CM-/bECd OF

I,LCCtNU POSITIVE STIMULUS AT 475 nm

FIG. 4. Variation of responses to stimul'i of fixed(500 sec) duration at 5 intensiti,es of 475 nm light. Initial100 sec stimulus of 48 ergs cm-2 sec'1 received by allthe plants. The response is plotted against the logarithmof the inteinsity of the second stimulus (400 sec duration)Standard errors are indicated in figure 2.

.~~~~~~~~

1789

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Table I. Variation of Response With Intcnsitt and Wavelength ol the Second StimulusRespoinse is measured in degrees curvature + standard error. The slopes are determined in the intensity region

where response is increasinig with intensity. An initial 100 second stinailus of 57 ergs cm 2 sec at 470 nm wasgiveii to all the plants.

Wavelength

nm355375395415435445455465475485495

Avg slope

Response to400 sec of

0.53 X 1012 quanta/cm2 sec'l

7.912.611.512.914.917.717.817.416.014.16.8

+

+-I-

-4-+

+

0.80.91.30.61.21.01.10.60.80.90.8

Response to400 sec of1.2 X 101-' quanta

cm-2 sec-1

deg of curvature

19.3 ± 1.517.3 ± 1.418.2 + 1.720.2 + 0.922.0 + 0.9207 + 1.722.4 + 0.617.4 + 2.1

1 Values not included in calculation of average slope (see text).

and in these cases it seems reasonable to assumethat the higher quantum flux initiated the processes

causing the decreased curvatures observed at highintensities.

For the action spectrunm, the data on the responises

of the coleoptiles to the 2 quantum fluxes were usedto extrapolate or interpolate to the quantum fluxrequired for a standard response. The standardresponse chosen was 15°, and the quanta containedin a second stimulus, lasting 400 sec, required toproduce this standard response were determinedgraphically from the data on the responses to the 2quantum fluxes investigated. The reciprocal of thisvalue is plotted against wavelength in the actionspectrunm, figure 5. At the 2 wavelengths, 455 and475 nm, where the slopes of the lines relating re-

sponse to logarithm of stimulus intensity deviated

I

!1:

:T

I,

(Y

400 450 5sc

WAVELENGTH (nm)

FIG. 5. Action spectrum for the second positive pho-totropic responise in Avena coleoptiles. The reciprocalof the energy, in Einsteins/cm2, of the second stimulus.

when delivered in 400 sec, for a 150 curvature ,is p'ottedagainst the wave elngth of -the second stimulus.

z

LL

>L5L

ioO

75

50

25

360 400 440 480

WAVELENGTH (nm)

FIG. 6. A comparison of the action spectrum for firstpositive phototropism in oat coleoptiles, solid line from(16), with the action spectrum for second positive pho-totropismi (dashed line).

greatly from the other values. the qjuantumll fluxrequired for the standard response was determinedby assuming that the slope of the line relating re-

spoiuse to quantum flux through the point at 0.53 Xquianta cmn ' secI wvas the average slope of

1:5.40/log unit.In the action spectrum shown, the peaks fall at

about 375 and 450 nm, and no activity is observedabove 510 nm. The resem-blance of this action spec-trum to the 1 for first positive phototropism inAvena (17) is striking, and the 2 are graphedtogether in figure 6. In both action spectra thepeaks in the near ultraviolet are based on observa-tions at only a few wavelengths, so the details of thleshape in this spectral region are not well defined.Both action spectra have a valley at 400 nm, butabove this point certain differences are discernible.

Slope:Response/log(Intensity)

19.116.615.115.112.38.31

14.34.01

15.4

T

I I ~~~~~~~~~~~~~~~~~~~~~~~~~~~~.

1790 PLANT PHYSIOLOGY

5

.,3

.2

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EVERETT' AND TiIMIAN N-SECOND POSITIVE P'HO'I'OTROPISM

Because of the lengthy exl)erimuents involved, thep)oints for the second positive action spectrum arefuirther apart than those for the first positive re-sponse. For this reason a shoulder at 425 nmI inthe second positive action stpectrum may vwell havebeen missed. The nmajor peak in the action s-pectrumifor first positive phototropismn is at 445 nm and forthe seconid !positive phototropism it is at 445 to 455muii. The ratio of ithe height of the near ultravioletpeak to this one is in both cases very clo-e to 1 :2.The absenice of a second peak around 475 nm inthe second positive action spectrum is the majordiscrepancy between the 2 action spectra. Unfor-tunately there is a large change in the lanm,p outputat about this wvavelength. Additionial exlperiments onlthe second positive response in this spectral regionwere carried out using a second stimulus of 0.53 X1012 quanta cm-2 sec71 lasting 400 sec at the wav e-lengths 465, 470, and 475 nm. The experiment wasrepeated 6 times, and the results are shown in tableII. These experiments indicate that the seconldpositive phototropic action spectrum may well havea second peak around 475 nm. Since the presenceof such a peak has not yet been satisfactorily demon-strated in detailed experiments with varying stimuluisintensities, however, it has not been included in theaction spectrum for the respoinse. Nevertlheless, thesimilarities between the 2 action spectra are strong,and we conclude that the sanme, or verx similar,pigments appear to be involved in plhotoreceptionfor these 2 phototropic responses.

Table II. Additional --Iction Spectruml DattaResponses to an initial see stimiiulus of 57 ei-gs

nim-2 sec-I at 470 nin plins 400 sec of 0.53 X 10' quantacmI" sec-I at the test wavelenigth.

WVavelen-gth Curvature + standard errornm

465470475

deg16.80 1.3 a13.60 0.4 bh17.80 + 1.4 a'

a, b Valtues R ithl different letters are significantlydifferent at the 95 % coiifidence level.

Discussion

The method of isolating second positive l)hoto-tropislmi used in these experiments has permittedl a

characterization of the effects of stimulus duratioli.intensity, anld wavelength oIn the response. Hatupt('10) and Meyer zu Bentrup (12), working oni thephoto-induction of {polarity of germination in Fitcllszygotes and Equiiseturt spores, used a method similarto the one developed in these experiments. Thedose-response characteristics of the low and the highenergy types of polarity induction are similar tothose for first anid second positive phototropismii incoleoptiles, and the wavelengths of light effectivefor the responses are also in the blue and near

ultraviolet. However, studies on the spectral sensi-

tiv ity of the p)olarity inductioni processes indicatedthat the low and high energy responses miglht havedifferent action spectra (12), while it appears thatthe 2 kinds of positive phototropism have similaraction spectra.

The experimenits reported lhere agree with ear-lierevidenice (2, 5, 20) that seconid positive plhototropicresponses are determllined by the du,ration of thestimulus rather tlhain by the total energy of thestimulus. Zimmerman and Briggs (20) reporte(dthat a 10-fold change in intensitv had no influence.detectable wvith their nmethod, oni the linear relation-ship between response and stimulus duration. Howv-ever, the present ex,periments cover (a larger intens,ityrange with a more direct method aind bring out thefact that the stimulus intensity does indeed havesome modifying effect on the seconid positive r1e-spoInse. The meanis whereby stimiiulus intensity and(lduratioii affect the respoinse are not known. Perhapsthe decreased responsiveness at highl intensity iscaused by bleaching or photo-oxidation of the photo-receptor. Zimmerman and Briggs (20) suggestedthat the second positive response is determined bythe length of time durin,g which a photoequilibriumis maintained between an inactive and an activeform of the photoreceptor. A modificatioln of thisscheme in which the equilibrium concentration ofthe active form of the photoreceptor is intenisity -

dependent would provide 1 possible mechanismii toexplain the secondary effect of intensity.

In these experiments the minimum stimulus dura-tion required for initiation of second positive photo-tropic bendinlg is 100 sec. Curry (5) estimated thataL duration of about 240 sec was require(l. whileZimiimiermiiani anid Briggs' (20) data indicated thatthe second positive type of response was initiatedvith the start of the stimulus. These differencesare probably related to the different miiethods use(dto observe the response. With the 2 dose miiethodused in these experiments the intensity and durationof the initial dose might influence the point of onsetof the second positive responses, ibut it wvas notfounld possible to test this factor thoroughlY.

Th'lie similarity betweenl the action spectra forfirst anld second positive phototropismi accordsstrikingly w.ith the demiionstratioti of Pickard andThimann (13) that lateral transport of auxin occurs(luring both types of response in coleoptiles. Thereare 2 possible interpretations of these facts. Oneis that the 2 photoreceptors are indeed the sanie, theapparent discrepancy at 475 mIn being due to experi-mental imperfections. The oat coleoptile would thenlpresent the remarkable phenomenon of having 2different types of response (each witlh its character-istic kinetics) both mediated by the samle photo-receptor and brought about by the samie effector,namely auxin asymmetry. However, the great dif-ferences in dose-response characteristics hetwxeen the2 types of positive phototropisnm, as vell as theopposite effects of red light pretreatmiienit (in de-creasing the sensitivity of the first l)ositive and

17/91

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PLANT1PH YSIOlO,(\

increasinig that of the seconid positiv-e) ( 19, 20) havenot vet permitted the formulation of a model inwhich first and second positive phototropism arerelated to the same photoreceptive mechanism. Thealternative interpretation of the data is that thephotoreceptors in fact differ slightly; perhaps thesaqme pignient is conjugated with 2 different pro-teiwi. Zimmlieermiiani and Briggs (20) indeed sug-g,ested separate photoreceptor systems for first andsecOnd positive responses. Each systenm would hlaveits 0owtn dose-response characterisitics, b)ut the endresult, lateral transport of auxiin, would he the samiie.One wvouldl be tempted to draw7 an analogy w ithmammaliain vision where 2 different types of photo-receptors, the rods and the cones, operate in low orhigh light intensities respectively. but both brinigabout the sensation of light, and botlh contain veryclosely related pigmients.

The phototropic photoreceptor has not been iden-tified from the action spectrum for first positivephototropism. The same must at present be thesituation for the second positive photoreceptor. Theproblem has been discussed in detail in references(3), (9), and i(17). Suffice it to say here thatnieither type of pigment, flavin, or carotenoid, sug-gested as the photoreceptor. has an absorption spec-trum precisely matching the action spectra. Theidea, diEcussed by Briggs (3) and Thimanin (16).that flavins and carotenoids iparticipate together inlight absoription for the phototropic response is.indeed, reasonable, but proof for this or any othlertheory is lacking.

The identity of the photoreceptor for blue-light-reactions such as phototropism is an im.portant min-solved question in plant physiology. Reactions witlaction spectra similar to those for phototropisnm inoats are found tlhroughout the plant kinvgdom: afew examiples of such responses are phototropismiiin Phycomitvces (6, 8) wvhose actioni spectrum isvirtually identical wvith that for first positive photo-tropism in Avenia, photo-stimulation of respirationin Chlorella (11) with peaks at about 375 and 460nm, light-stimulated carotenoid syinthesis in Fusariuina(14) and chloroplast phototaxis in Lemna (21) withipeaks at 382, 452, and 485 nm. In none of thesehas the photoreceptor been unequivocally identified.The experiments reported here add yet another re-sponse to the list of these blue-light-reactions andprovide ani a(lditional reason for- contitnued effortsto identify this photoreceptor.

AcknowledgmentsThe authors acknowledge the continued interest of Dr.

W. R. Bri'ggs and his assistance in valuable and criticaldiscuss.ions.

Literature Cited

1. ARIZ, W. H. 1915. UnIltersuchungen uber den Pho-totropi.smnus. Rec. Trav. Botan. N'erl. 12: 44-216.

2. BRI;GS, W. R. 1960. Light dosage and phototropicresponises of corn and oat coleoptiles. Plant Phn sio'.35: 951-61.

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