malcolm · 2plantphysiology rung auf den. wuchsstoffgehalt, das langenwachs-tum und die...

2PLANT PHYSIOLOGY

rung auf den. Wuchsstoffgehalt, das Langenwachs-tum und die geotropische Krimmungsfahigkeit vonHelianthus hypokotylen. Planta 60: 109-30.

3. BRAY, G. A. 1960. A simple efficient liquid scintil-lator for counting aqueous solutions in a liquidscintillation counter. Anal. Biochem. 1: 279-85.

4. DIJKMNAN, M. J. 1934. Wuchsstoff und geotro-p)ische Kriimmung bei Lupinus. Rec. tray, bot.n~erland. 31: 391-450.

5. IOLK, H. E. 1930. Geotrol)ie en Groeistof. Diss.Utrecht. Eng. transl. 1936. Rec. trav. bot. neer-land. 33: 509-85.

6. FULLER, R. C. 1956. Modified end-window count-ing tube for paper chromatograms. Science 124:1253.

7. ('ILLESPIE, B. AND XV. R. BRIGGS. 1961. Mediationof geotropic response by lateral transport of auxin.Plant Physiol. 36: 364-68.

8. COLDSMITii, AM. H. M. AND Al. B. X\ATKINS. 1964.

Movemeitnt of auxin in coleoptiles of Zca tiv3s L.during geotropic stimulation. Plant Physiol. 39:151-62.

9. JACOBS, W. P. 1955. Studies on abscission: thephysiological basis of the abscission-speeding effectof intact leaves. Am. J. Botany 42: 594-604.

10. LYON, C. J. 1963. Auxin factor in branch epinastv.Plant Physiol. 38: 145-52.

11. LYON, C. J. 1963. Auxin transport in leaf eplnasty.Plant Physiol. 38: 567-74.

12. RAY, P. M. 1958. Destruction of auxin. Ai\.Rev. Plant Physiol. 9: 81-118.

13. THIMANN, K. V. 1934. Studies on the g-rowvtlhhormone of plants. VI. The distribution ef thegrowth substance in plant tissues. J. Gen. Phvsiol.18: 23-34.

14. VAN DER LAAN, P. A. 1934. Der Einfluss v-onAthylen auf die Wuchsstoffbildung Lei Ave.'a ulfnVicia. Rec. tra,. bot. nKrland. 31: 691-742.

Red light and the geotropic response of the Avena coleoptile 1'2Malcolm B. Wilkins3

The Biological Laboratories, Harvard University, Cambridge 38, Massachusetts

IntroductionFor many, years red light wvas regarded as a safe

light under which experimental mani pulations instudies of plant tropisms could be conducted. Thispractice arose from the finding that light of wave-length greater than 520 m/i did not induce phototropiccurvature. During the last few years, however, it hasbecome clear that red light has a marked effect onboth phototropic and geotropic responses of plantorgans. The changes induced in the phototropic re-sponses of coleoptiles have recently been reviewedby Briggs (9), and much of the literature describingeffects of light on geotropism has been collated by\Wilkins and Goldsmith (18).

Blaauw (3, 4) has investigated the effects of redanl far-redl light on the geotropic response of the.vZcna. cDleoptile. His findings on Avena differedfrom those of Wilkins and Goldsmith (18) on thecoleoptile of Zea ways in 3 major respects: Firstly,the geotropic response of the Avcna coleoptile wasincreased after exposure to red light whereas that of

I Received May 15, 1964.2 This work was supported by National Science

Foundation grant No. G 21799 to Professor Kenneth V.Thimann.

3Present address: School of Biological Sciences, Uni-versity of East Anglia, Norwich, NOR 77H, GreatBritain.

the Zca caleoptile xwas decreased. Secondly the in-crease(l responsiveness to a geotropic stimulus in.47Zc11a was greatest when stimulation began 30 min-utes after exl)osure to light and disappeared coi-pletely if this interval wvas 60 minutes. In Zea thedecreased sensitivity (lidi not develop until 6 to 8 hoursafter the light treatment but then lasted for at least22 hours. Thirdly, far-red irradiation also elicitedthe short-lived increase in the sensitivity of the Arena-coleoptile but did not reverse the effect of red light..In Zea, on the other hand, far-red irradiation alonehad no effect on the geotropic responsiveness of thecoleoptile but completely reversed the effect of rellight. Since coleoptiles of Azrena are rich in phyto--chrome (A'. R. Briggs, personal communication, and16), it is surprising that at least some reversal of theeffect of red radiation xvas not observed.

The differences between the reported responses ofcoleoptiles of Azrcna and Zea to red light seemed suf-ficiently great to warrant further investigation, since.they might reflect some fundamental differences illthe growth and organization of the 2 tissues. Theinfluence of red light on the geotropic response of the-Avena coleoptile has therefore been reinvestigatedwith respect both to the short-lived effect describedby Blaauw (3, 4) and to possible long-term effects.similar to those found by \Wilkins and Goldsmith ( 18),in Zea.

24,

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WILKINS-RED LIGHT AND GEOTROPIC RESPONSE OF AVENA COLEOPTILE

Materials and MethodsMaterial. Unhusked seeds of Avena sativa L. var.

Victory were soaked in water for 4 to 5 hours an I

then planted in vermiculite ("Terralite," ZonoliteCompany, Atlanta, Georgia) which had previouslybeen washed for 15 to 20 minutes in cold water andallowed to drain. The seeds were covered withvermiculite to a depth of approximately 5 cm to pro-vide support for the uninhibited mesocotyl, and thenthe trays were placed in a dark room maintained at25 to 26°. The seedlings were grown in absolute!arkness for 5 days.

Safelight. Because of the extreme sensitivity of

z2 io0

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z

z

i-

lz

4L

400 too goo M0O ao moWAVELENGTH Q0hl

400 600 0oo KM0 120 mwO

WAVELENGTH QnO)FIG. 1. Transmission curves for the green glass of

the incandescent lamp (continuous line) and for 4 layerseach of du Pont green and amber cellophane combined(broken line).

FIG. 2. Transmission curves for the red (---) andfar-red (-) interference filters, and for 4 layers of duPont red cellophane (-.-.-) and 4 layers of du Pont bluecellophane (...... ).

dark-grown Avena coleoptiles to red light it wasessential to have a safelight as free as possible fromradiation of these wavelengths. The most suitablelight source in this respect was a General Electricdeep-green incandescent lamp (Reference Number:60A 21/NG, General Electric Company, Nela Park,Cleveland 12, Ohio). Radiation from this lamp wasfiltered through 4 layers each of du Pont green andamber cellophane. The transmission curves for theglass of the lamp and for the combined green andamber filters are shown in figure 1. This safelightinduced no phototropic curvature in dark-grownAvena coleoptiles and had no significant influenceon their subsequent geotropic response (table I).The radiant flux from this source at the level of theplants during manipulation was 4.7 X 10-9 einsteinscm- sec ', an exposure never exceeded 10 minutes.

Source of Red and Far-Red Radiation. A projec-tor fitted with a 500 wv incandescent lamp was em-

ployed as a source of red and far-red radiation. Theprojector was placed in a box having an aperturethrough which the slightly diverging beam of lightemerged. The beam was filtered through 2 cm ofwater and then 4 layers of du Pont red cellophane.Ad camera shutter and iris diaphragm provided con-trol of exposure time and intensity, and a rheostatconnected in series with the lamp provided an addi-tional and independent means of intensity control.The beam finally passed through a Bausch and Lombnarrow-band, second order, interference filter. Oneof these had a peak transmission at 655 m~u and theother at 740 nVl. These filters were fixed in a slid-ing mount so that rapid change of wavelength couldhe achieved. The beam impinging on the inter-ference filter was slightly diverging and since filtersof this type transmit wavelengths shorter than thestate-I values when light rays do not strike themnormally, 4 layers of du Pont blue cellophane wereplaced in front of the 740 mnt filter to eliminate com-pletely any transmission below 700 m,4. The trans-mission curves for the interference filters are shownin figure 2 together with those for 4 layers each ofthe du Pont red and blue cellophanes. The transmis-sion of the interference filters has been determinedfrom 380 m~I to 1500 m/i on a Perkin-Elmer record-ing spectrophotometer. Since the water screen wouldeliminate 90 % of the transmission at the small firstorder peak of the 655 m/A filter at 1300 mu, and en-tirely prevent transmission at the first order peak

Table I. Effect of Exposing Coleoptiles to GreenSafelight at 200

Curvature after 2 hoursManipulation geotropic stimulationconditions (degrees) II*

10 minutes exposure Sample 1 18.1 1.6** 15to green safelight Sample 2 17.5 2.6** 11

Total darkness Sample 1 14.5 2.0** 13Sample 2 13.7 1.7** 13

* n = number of coleoptiles in sample.$* Standard error of mean.

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25




PLANT PHYSIOLOGX

Table 1I. Comparison of (eotropic Rcspmiscs of EJxcised Colcoptiles 011d 7Thost' of Intact Sccdlinygs

Treatment1)egree curvature after 2 Irs

geotrop)ic stimulation

Sample 1

-None (darkness)

Red light*

None (darkness

Red linht*

48.8 + 4.0*(I = 10) ***/74 0 + 2.9(nI = 11 )increase 51.6 ('.)30.8 2.6(nI = 21 )50. + .3.9(n1= 20)Increase 62.4 '((

Sample 2

48.8 -- 3.9*(I = 12 )82.8 + 2.5(n = 12)Increase 69 %t27.5 + 2.8*(In = 15)47.0 + 3.1(nI 18)Increase 70.8 %f

=- Stand(lard cirror of mean.- 6.() X 1( cinteins cI- given oxer 10 minutes

: Num11= bumher of coleoptiles in sample.

-I 0 1 2 3 4TIME FROM END OF LIGHT TREAT-MENT TO ONSETOFGEOTROP I CSTIMULATION (HOURS.

-I 0 1 2 3 4TIME FROM END OF LIGHT TREAT-MENT TO ONSET OFGEOTROPICSTIMULATION (HOURS).

IFi(,. 3. Effect of redl light on the geotropic response

of A.vzena coleopstiles. Samples taken from the red-lig-hlttreated tray of see(line)-s atre shown by the solid circles

inmediatelv before geotrinine Stimulation.

(1420 nilut of the 740 mips filter, the absolute valuesfor the raldiant fluxes at 655 mMi and 740 nitt quoted illthis paper are not comlilicated1 by the transmissionpeaks in the infira-red region of the spectrum.

Hlszirmcmcn t of RN/ifont Flutxecs. The absoluteener-gy measurements Nxvere made with an EppleyThermopile. the ElMF generate 1I eing recorded on aKintel El'ectronic (Gialxvanometer ( i\Molel 204A ). Thethermopile w as cal ilbrated agIainst a sItan lard lamp andthe calillrati.)n value (agreel x\-ithl those ol)taine(l inPrevious years.

E.xpcricniiital IProccc(hrc. In all experillments; ex-celt one, the trays of seedllings vere exposed fromabove to syviilietriceal red or far-red, light, or both, andthen retul-ned to (larkness. At dlefinite intervalsbefore and after the ligIht treatments samples ofcleoptile.i were taken from the treatell tray of seed-lings and fi om a el )trol trav wx which was not exposedto red or far-re l light. The coleoptile.s x ere excisedlust above the nod(le uiln er the green sa. eligit andl weretil)lpoitedl inl the vertical position 1b pushing theirbases into 2 1CD)i fco flacto-Ag-\ar contained ill rec-

tancgullar metal tr'oughs 15 x 2 5 x 3 0 cm. Exceptill experiments geotropic' stimuilation began lilillmm-(liatelv after the Nedetiles xxere excisell anI was

chiex e(l by turning the troughs through 90°. Thethrough \x\ere placeJ in a humidity chamber i (llark-ness during the time that the coleoptiles xxere ill thehorizontal plane. The curvature of the coleoptiles atthe en(l of the period of ge )tropic stimulation xas re-

Ce)l'(ledl ol 1Ko(labroinide. F4. hi-h-contrast paper.

nmi from the tray kept in dlarkness by open circles. I x-

postire to red light is inldicatetl by the hatched area. Thevertical lines extending from each p1ilnt equal si, x t

Fi.. 4. Effect of red light on the geotropic response of.iLen coleoptiles. Red-light treatment given after ex-ision and sh x)xvn by the hatched area. Samples takenfrom the red-light treatetl tray of seedlings are shoxxwn bysoliti circles andi from the tray kept in darkness by opencircles. Vertical lilies from each point equal sEM x tUo.'(urxvattire of samples taken 1.5 hours after the light treat-toent wxas (leterminie(l after 2 hours 20 minutes geotro )icstilniulation.

Material

Cioleoptilesof whole.SI-dlill r;

Fxcisedelekoj)tle..

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WILKINS-RED LIGHT AND GEOTROP.C RESPONSE OF AVENA COLEOPTILE

Although their absolute curvatures are consider-ably decreased by excision, detached coleoptiles re-

sponded to red light in a manner very similar to thatof intact coleoptiles (table II). All the geotropiccurvatures reported in this paper were determinedwith excised coleoptiles since these were much sim-pler to handle than whole seedlings having long meso-

cotyls. Details of the experimental procedure are

given with each experiment.Statistics. In most experiments there were 10 to

20 coleoptiles in each trough and in many cases

duplicate troughs were similarly treated. Each ex-

periment was carried out at least twice, but more

often 3 or 4 times. The vertical lines extending fromthe points in the figures are equal to the SE of themean multiplied by to 0 for the appropriate numberof degrees of freedom. The vertical lines thus rep-

resent one half the true 95 % confidence interval andshould be extended for an equal distance on the op-

posite side of the point to obtain the full confidenceinterval. This procedure has been adopted for clarity.

Results

The effect of red light on the geotropic response ofthe Avtena coleoptile is shown in figure 3. Samplesof coleoptiles were taken from 2 trays ( 0-* and0-0) of 5-day-old. dark-grown seedlings at inter-vals before and after 1 of the trays ( 0 0) was ex-

posed to light of wavelength 655 mu for 10 minutes

Table III. Relationship between

(dose = 6.6 X 10-8 einsteins cm-2). Each samplewas placed horizontally for 2 hours beginning im-mediately after the coleoptiles were excised.

Coleoptiles taken from the 2 trays before 1 was

exposed to red light showed closely similar geotropicresponses. The curvature of those geotropically stim-ulated immediately after irradiation, and after an

interval of 0.5 hour, was nearly twice that of theunirradiated coleoptiles, but with longer intervals theincreased responsiveness declined and disappeared.The enhanced geotropic sensitivity persists only forabout 60 minutes after the red-light stimulus.

The data shown in figure 3 were obtained by ex-

posing whole seedlings to red light and excising thecoleoptiles immediately before the geotropic response

of a sample was determined. In another experimentdark-grown coleoptiles were first excised and mountedin the troughs of agar and then half the numberof troughs were exposed to red light for 3 minutes(dose = 7.3 X 10-9 einsteins cm-2). An irradiatedand an unirradiated trough were placed on theirsides for 2 hours at predetermined times after theexposure to light. The geotropic response of thecoleoptiles was enhanced by an amount as much as

when whole seedlings were irradiated, and the effectwas again noticeable only during the 60 minutes fol-lowing exposure to light (fig 4).

Age and Length of Coleoptile. The effect of redlight on the geotropic response of dark-grown coleop-tiles aged from 4 to 7 days is shown in table III.

Age and Length of Coleoptile and the Effect of Red Light (RL)on the Geotropic Response

Degrees geotropic Degrees geotropicAge Length curvature in 2 hrs curvature in 2 hrs

(days) (mm) before 1 tray beginning immediatelywas exposed to RL after 1 tray wasexposed to RL

Tray I4 11.2 + 0.4* (treated) 27.6 + 1.7* 46.0 + 2.3*

(n = 29)** (n = 13) (n=16)Tray II

(control) 24.0 + 1.9 26.2 + 2.1(n=10) (n=11)

Tray I5 19.9 + 0.9 (treated) 28.2 ± 2.6 52.0 + 1.9

(n=21) (n= 12) (n= 15)Tray II

(control) 27.2 + 2.1 27.4 + 2.2(n = 13) (n = 14)

Tray I6 39.4 ± 1.7 (treated) 24.9 + 3.5 57.8 + 3.8

(n=31) (n=10) (n=15)Tray II

(control) 31.6 + 2.8 26.7 ± 2.6(n = 12) (n = 14)

Tray I7 57.3 + 1.2 (treated) 12.3 ± 1.8 43.0 ± 3.2

(n=28) (n 11) (n = 13)Tray II

(control) 13.4 ± 3.1 15.5 + 2.2(n = 10) (n = 10)

* = Standard error of mean.** n = Number of coleoptiles in sample.

27




PLANT PHYSIOLOGY

The seedlings were exposed to light before the coleop-tiles were excised. On the fourth, fifth, and sixthdays the geotropic responses of the unirradiatedcoleoptiles were similar, as were the increased re-

sponses of those exposed to red light for 10 minutes(dose = 6.6 X 10-8 einsteins cm-2). The response

of the unirradiated coleoptiles on the seventh day was

smaller than on previous (lays but was again increasedafter exposure to light of wavelength 655 mru. Five-lay-oll seedlings were used in the experiments thatfollow.

Effect of Prolonged Exposure to Red Light. Theloss of the ability of the Azvena coleoptile to give an

increased geotropic response approximately 1.5 hoursafter a few minutes' exposure to light might be attrib-utecl to the restoration of darkness. This possibilitywas tested by exposing seedlings to continuous redlight. After taking preliminary samples to ensure

that the geotropic responses of the seedlings in the2 trays w.ere essentially identical, 1 tray was exposedto continuous irradiation at wavelength 655 m/L andat an intensity of 5.5 X 10-12 einsteins cmn2 sec'. Atintervals thereafter samples of coleoptiles were takenfrom this tray ( 0-0 ) and from one that had beenkn2 t in darkness (0-0). Figure 5 shows clearlythat the capacity to give an increased geotropic re-

sponse persists no longer in continuously irradia'ce I

seedlings than in those irradiated for only a fev min-utes (fig 3, 4). It appears, therefore, that the firstfew minutes of the exposure to light sets off in tVesee 'lings a process which goes to completion eit'- e;in rc l lg.ht or in darkness.

Effect of Mlidtiple Exposures to Light. IntactSet lings which had been exposed to red light for 10mnuntes (,'ose = 6.6 X 10-8 einsteins cm- 2) were sub-

ject-d to a second. identical exposure after the imme-Ciate effects of the first one had disappeared. Thefirst exposure (A ), given at a time 0 hour (fig 6), in-creise l the geotropic responsiveness of the coleoptiles.Beginning apprximately 4.5 hours after the firstexposure to light, the seedlings were again irradiated(B) anl it is clear from figure 6 that this had no

effect upon the ge-otropic responsiveness of thecoleoptiles.

Another tray of seedlings which had also beenfirst exposed to red light (A) at time 0 hour (fig 6)was left in darkness until the next day. At 23 and23.5 hours after this exposure to red light the geo-

tropic responses of these coleoptiles and of otherswhich had been kept in darkness were compared.The curvature of the irradiated coleoptiles was now

approximately one half that of the unirradiated ones.

The results shown in figure 6 indicate that there are

at least 3 phases of the response of Avena coleoptilesto red light: The first, in which the geotropic sensi-tivity is increased, occurs in the first 60 minutes or

so after illumination begins. The second, in whichthe irradiated and unirradiated coleoptiles have closelysimilar geotropic responses, extends from approxi-mately 1.5 hours to at least 6 hours after the lighttreatment. The third develops between the sixth

40-LULU a: 30-LU

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-I 0 1 2 3 4 5 6232425262728TIME FFOM END OF FIRSTLIGHT TREATMENTTO ONSET OF GEOTROPIC STIMJLATION Q1R

FIGu. 5. Effect of continuous red light on the persist-cnice of the increased geotropic responsiveness. Red lighttreatment is indicated by the hatched area. Curvature ofcoleoptiles taken from the irradiated tray is shown bysolid circles and from the unirradiated tray by open circles.

FIG. 6. The 3 phases of the response of Avena coleop-tiles to red light and the effect of a second red-light treat-ment given approximately 4.5 hours (B) or 24 hours (C)after the first one (A). Black bars indicate the lighttreatments. Curvature of the coleoptiles taken from theirradiated tray are shown by solid circles and from theunirradiated tray by open circles.

and twenty-third hour after irradiation and in thisthe response of the irradiated coleoptiles is only aboutone half that of the etiolated controls.

The tray of seedlings that exhibited the depressedgeotropic response resulting from the first exposureto red light (A) was subjected to a second identicalexposure (C) beginning about 24 hours after the firstone (fig 6). Again the coleoptiles showed no sig-nificant increase in their geotropic response which re-mained small in comparison with that of the controls.The final operation in this experiment was to exposethe remaining unirradiated coleoptiles to red light(D) to check that the absence of response in the pre-viously irradiated seedlings was not attributable tosome environmental condition. Figure 6 shows thatexposure to red light increased the geotropic responseof these coleoptiles in the usual way.

28




\VILIKI NS R1'D LI(;IIT AND) GEIOTROPIC RE-SPONSE, OF AVE..NA COLEOPTILE

Af~f ct opf F -Rod LIrli ti rwelvie silialltravs of sedhllnig-s were g-rl-CxxVn for (iavs il (iarkne.;.sanl then each one was eex;)o., el1, for (liitterent lengthlt time. t redi or farrled ra -'atdon. I mine alate

alter irraliation e lie,)ptiie ere exci'.e l nlld

geotropically .Stimulate 1. Ihe radiant fluxe.s w ere

4.4 X 10I1eisteinl co11 _;ec "t (); 1t and 3.2 X10

' einsteinls c ell- 1 at 740 nink. Ini add(itioll.' travs of seedxlix(-s\\ere l

inp('Iidarkness as controll.The results of this experiment are shown iln ftguire 7.Exposure for 1 seconldl at (55 nitL brought adbotut an

al-olst maximum increase ill thle g(e&tropic responseof the coleoptile9s; only a li gilt further increase \\-aachieved ith longer expostlire>. Onl the other handll.even 100 secollnd exposure t ) ra Iiiltia ait \Vxxelengllthl740 ni1u scarcely significantly enhance l the g-eotropicresponse of the cole.)Iptiles.

Iln a further experinlent see 1illn-i xxt'rlt eCXi(Ito red flight for 3 mlinutes ( 8.0 X 1() eillnste'ill.i cilm-to falr-re1 light for 3 llinute. 5.8 x 10( " ein.-teill.ncm1- ) or to 3 mllillute. red followe l at once hv 3ill Iltites far-rlel I rla iaitiO nl. SaIllplei Of coleopltill>xxere exciseci imllllledilatel afterward anIlli iliacel ilori-Z.)Iltallv together xxitl sapllellie from a tray which hladlbeell kept in darklles. The re~sults. WIhoIx1 in tigore

8. ill(licate that xlile far-rerl ra(iiation alone ii ith-

ont effect onl tile re.ipomls.i olf etiolatedl colipti~ e

to (rrvlxitv. it call rexverse tlhe effects of red radiiltiO1l.hlisi i.s true botil for tile short-term illcreae, anldi tilelon.g-terIll decreasee ill tlhe g-e )trl)ic re~sIolsiv elle>.i o

thle coleoptiles.D)osc-Rcsponsc RclutiI))lshl (1)d R.cci/l (ocit v.

lile resiults shoxx ill fignre ill ii cate tilat a lnearlilllsXilltlnnl increase ill tile geotlro)Iic I- oiMISC s

rong-lit ailhout by\ exposilng tile col eopties to a (i.>e(If red lgilt of. It tile 1110.t, 4.4 X 1 12 eill. einl c -

Tlle io-se-respiom>e reilationshipi) of tlis lphlellolllenolIlwas fnrtiler Stivie Il alli tlhe resnits are shlownillnnire 9. Tile cnrxe indicated h)v tlhe solid( circles .ilhowtlhe ge. )trolic reis)Illse (If c1iel)ti(.> after eX]postYCto (L raliiant flnx of 3.3 X 10 1` ein,-ein>s cII secl at

15Ill/I. A si nilihcallt illcrea.e ll geotropoic cnrva-ture occnrredi after 1 >econl's irradiiation 3.3

X 10 1

eillsteills Cil- l li tile l1llMXilll1lll effect after 10seconds> exp>lsure 3.3X 10 12 eil>teils cill ) . TIileo)pen circle> ilhow the resplmonie> of coleopties pre-(In1i y ex s e'.l to r1arad(lialt flnx i 3. X 10 eill-

s tei ns cll- s ec ; at lll(L Agaill a1zii*>li icalclt ill-creasewxsahl-ongilt albont hy 3.3X 10-': eilsteilln. cill-l111 tile lllaxillnmu hv 3.3X 10eillXtein> CbIhntill tiis case tue exposures xxwere 10 anl 100 secollixt

lpecti el v.Tl'e valii(itv(of tile unllsell-Ro1sce reciplrocityliax

10 as further testedi i)v selec tin- 2 (loise> andAntllg tleSe over al range(ofexl)II)nr-e tiles 0ll(1 inlten-

>iti e>. Fi gnre- 10 shoXx tilhat til ei nc geo

ti-opic resp"Illixene>sintlcedl hvlilit (loses of 3.3 X1() 1 einstei ns Cl pilolts li helle de 1 ) i 3.3 X

12 cinsteiln Csll p kiltXl Aheile ) areinlelpen1-enlt of exilosure tile and railailt ilux, at least over

tile railge tested ill thexe experilmlelnts.

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100

D R RF F R RF FTREATMENT.

I1.. Effect (If x arin>s expou.Ijres to red ((n); ;lu1)and far-red (740 ml, light 1111 the geotropic response (Ittile .Avcn1I coie( )lltie. Radiant flux at 655 mIll xxwas 4.4 X10 12 eil jte -ljj C,; -c11 ,a1l at ) 3. X 1( 12ellt eill,; C111 '2 ec-1.

I (. 8. EffectiVene.>0 (If fal-red F radiation ill re-

xersill tlhe effect, of red ligIlt (R) 11ti1 wxith respect totlle shiort-termll illcrea>ed geo(tropic responsivnlleiss S)and Iono-ternl (licreasel repmolnsiveless (L ). D ilclicateslarkln>>s alldl RIF reti follow el imllmlled-iateix hv far-redlracijatioll. Eacl plair (f blars lloxxs duplicate samllie>s.

Kif/ifcs(of t1/i (;lotroli(c Rcsponst,. Tlhe dleele)i-111ll tofgeI)tr)uiic cuirvxattIre ill i rrad.IiaLte(l a(nd(1 etioilte(dCole.ptiles as follo edx t flnctionlof time. Co1ho)p-tiles groxxn1 for daxs ill darkne.>s x-ere first excise(l11n1'1llotnIlted ill trotlg-lo (If agar. H ldfof tlese xxere

exp)sIecl to iigilt (If xxwaxeiellgtll (1 55 nil/ for 3 Illilutest.)t.ltl ir-ra(liaillCe 7.( X 10--> eil~teills C111- 2 andl(1 jIll-

lel hotel afterxwards. ail tile trilugilis were ttirile1

tiroumgil 90'. At interval tilereafter tile curvatures ot1 sallp)le of irradiated, alld 1 of tIlliirrct(liitel coeoII)-tiles xxere (determllile(d. Tlhe results. showx i fig-tre11, Ire cio>Cei- Sil111iLr to tlose puhlii>sle bh Blaauti1 ). The ilitiaLl rate (If curvxature of tlhe irralate]

coleoptile;xxas g-reater tlanll tlhatof tlhe dark control>.hut after 2 to 3 hlour>s ill tile h1orizonltal lpositioln tilerates decreased il hoth tlhe treated andl untreate I tis-tie. TllelIrge difterellce bletwxeen tile cutrvaltures of

I- / ----A-|-I----- - - ,o",mp

If I . It-,I.1i/

29




PLANT PHYSIOLOGY

v

w

0

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cKD0

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0 4 8 12 162024283236 165 330LIGHTDOSE CX 10& EINSTEINS CMo)

I I I I I

0 B

~o - r -t. T B

lo CONTROL

1%f a. . .A IJ-

0-01 0-1 I 10 100EXPOSURE TO LIGHT (SECONDS).

FIG. 9. Dosage-response curves for the effect of redlight in increasing the geotropic sensitivity of AZentacoleoptiles. Solid circles: radiant flux 3.3 X 10-13 ein-steins cm-2 sec-1 and exposures from 1 to 100 seconds.Open circles: radiant flux 3.3 X 1O14 einsteins cm2- sec1and exposures from 10 to 100 seconds.

FIG. 10. Effect of doses of red light of 3.3 X 10-12einsteins cm-2 (A) and 3.3 X 10-13 einsteins cm-2 (B)given over various times at different radiant fluxes. Cur-vature of unirradiated coleoptiles shown by horizontalline marked control.

the irradiated and etiolated material after 2 and 3hours' geotropic stimulation was maintained for atleast 12 hours.

It must be borne in mind when considering theshape of the curves in figure 11 that the coleoptileswvere detached from the rest of the seedling and may

thus have been depleted in nutrients and other essen-

tial factors for growth by the third or fourth hourafter excision. This may account for the rate of cur-

vature of both the irradiated and control coleoptilesdeclining almost to zero between the third and fifthhour after excision with the result that the large dif-ferences in curvature are maintained.

7lreatments Causing the Effect of Red Light toPersist for More Than 1.5 Hours. Further stu(l-

of the mechanism of the increased geotropic respon-sivreness developed by coleoptiles which have been ex-

posedl to red light would be considerably hamperedby the fact that the effect persists for such a shorttime after the light treatment. At 25° the effect is

maximal during the 30 minutes following the onsetof irradiation and thereafter rapidly declines and clis-appears. It has been found, however, that the dlis-appearance of the increased geotropic sensitivity canbe prevented by exposing the seedlings to low tempera-tures.

Four trays of 5-day-old clark-g-rowxn seedlings Werefirst sampled to determine wx hether the responses ot

the seedlings in the various trays xxere similar. Twoof the trays were then exposed to light of wavelength655 nms for 3 minutes (total irradiance 7.9 X 10 -`

einsteins cm-2 Immedliatelv afterwards samples

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so

60

40

20

0,

0 1 2 3 4 5 6 7 8 9 101112TIME IN HOURS

0 2 3 4 5 6 7 28 30

TIME FROMENDOF LIGHIT TREATMENT (A) TOONSET OFGEOTROPIC STIMULATION (HOURS).

FIG. 11. Development of geotropic curvature withtime in irradiated (655 mu, solid circles) and in un rradi-ated coleoptiles (open circles).

FIG. 12. Effect of low temperature on the persistenceof the red-light induced increased geotropic sensitivity ofAzvena coleoptiles. Time of red-light treatments shownby black bars labelled A and B. Curvature of irradiatedcoleoptiles shown by solid circles and of unirradiatedcoleoptiles by open circles. Curvature of chilled coleop-tiles shown by solid lines and of coleoptiles kept at ?5°by broken lines.

30

. I I I . . . I I--IT,- I T .

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. . . . . . . . . . . . .P





were taken from each of the 4 trays and then 1 irradi-ated and 1 unirradiated tray were transferred in dark-ness to a darkroom maintained at a temperature of2 to 3°. The other 2 trays were kept in darkness at250. At intervals thereafter further samples of cole-optiles were excised from each of the 4 trays and theirgeotropic response determined in the darkroom main-tained at 25°. The samples of coleoptiles from thechilled trays of seedlings were excised in the coldroom, transferred in darkness to the room kept at250, and then placed in the horizontal position.

The results are shown in figure 12. The familiarpattern of response was observed in the coleoptileswhich had been irradiated and kept at 250 (dashedline, solid points). In this experiment the increasedgeotropic responsiveness disappeared between 1.5 hourand 2.5 hours after the light stimulus-the longesttime the effect was observed to persist at room tem-perature. In contrast, coleoptiles which had beenirradiated and then chilled (solid line, solid points)retained the capacity for an enhanced geotropic re-sponse for at least 28 hours after the exposure to light.The response of the etiolated coleoptiles was unaf-fected by their exposure to low temperature (solidline, open points) which, in addition, had no influenceon the reaction of the coleoptiles to irradiation. Thisis also shown in figure 12. The remainder of theunirradiated batches of seedlings from which the con-trol samples had been taken throughout the experi-ment were finally exposed to light for 3 minutes (B)(655 mp, 7.9 x 10-9 einsteins cm-2). The increasesin the geotropic response of the chilled (solid line.open points) and unchilled (dashed line, open points)coleoptiles were identical.

Discussion

The effect of red light on the capacity of the Avc-nacoleoptile to respond to geotropic stimulation can bedivided into 3 phases. The first is characterized bythe short-lived increase in responsiveness, the secondby there being no detectable difference between thecurvatures developed by irradiated and unirradiatedcoleoptiles, and the third by the responsiveness of theirradiated coleoptiles being much less than that of theunirradiated controls.

The occurrence and approximate duration of thefirst phase of the response are confirmed, althoughin detail the present results differ somewhat fromlthose of Blaauw (3). The present study revealedthe increased response in coleoptiles that were geo-tropically stimulated immediately after irradiationwhereas Blaauw (3) detected the increased curvatureonly when geotropic stimulation began 30 minutesafter the onset of the light treatment. Blaauw (3)found an interval of about 60 minutes to be requiredbetween the onset of illumination and geotropic stimu-lation for the complete disappearance of the increasedresponsiveness whereas in the present study an inter-val of about 90 minutes was required. The maximumincrease in geotropic curvature reported by Blaauw

(3, 4) was somewhat smaller than that reported in thepresent study. This difference may be due toBlaauw's seedlings not having been grown in totaldarkness; the dehusked seeds were exposed to orangelight for 19 to 20 hours after soaking. In the presentpaper it is reported that 5-day-old seedlings whichhave once been exposed to a relatively small dose ofred light are incapable of responding to a secondexposure. It is not known at present to what extentthe ability of seedlings to respond to a second ex-posure to red light depends upon their age at the firstexposure.

In the present study far-red radiation was foundnot to enhance the geotropic curvature of Avena cole-optiles, even when the dose was 100 times that of redlight which just gave the maximum effect. It did,however, reverse the effects of red light, both withrespect to the short-term and to the long-term changesin geotropic sensitivity. Blaauw (3) found far-redlight to have the same effect as red if the total irradi-ance at 740 mp was 3 times that at 660 mp, and hecould detect no antagonism between the effects of redand far-red light. Accounting for these differencespresents great difficulties. It is possible that a littlered light was transmitted by the 740 myA filter em-ployed by Blaauw (3). This would certainly occurif the beam impinging on the filter were not perfectlycollimated or if there was an appreciable amount ofscattered light in the lamp-housing of the source.

The series of changes induced by red light in thegeotropic responsiveness of Zea coleoptiles is closelysimilar to that induced in Avenza except that phase I(increased responsiveness) does not appear to occur(18). There is no detectable difference between theresponsiveness of the treated and untreated seedlings(luring the 6 hours following exposure to red light.The decreased responsiveness develops between thesixth and eighth hour, increases in magnitude untilthe sixteenth, and persists until at least the twenty-second hour. The induction of the decreased respon-siveness in Zea coleoptiles is also reversed by far-redirradiation (18).

Dark-grown Avena coleoptiles are exceedinglysensitive to light of wavelength 655 m1A. Their geo-tropic responsiveness is increased in phase I by 3.3 X10-13 einsteins cm-2 and the maximum increase isachieved with 10 times this dose. Blaauw's (3) ex-periments also revealed the effect to be brought aboutbv small doses, 4.0 ergs cm-2 (2.2 X 10-12 einsteinscm-2) giving a detectable increase in geotropic curva-ture and 7000 ergs cMn2 (3.9 X 10-9 einsteins cm-2)the maximum increase. The lower sensitivity of theseedlings used by Blaauw (3) may also be attributableto their exposure to red light during the first 19 to 20hours of germination.

The changes induced in the geotropic responsive-ness of the Avena coleoptile by exposure to red lightare among the most sensitive photo-responses ofplants. The dose required to give a minimum detect-able increase (3.3 X 10-13 einsteins cm-2) is closelysimilar to the dose of blue light (at 435.8 mpu) re-

31




PLANT PHYSIOLOGY

quiredl to induce the minimum detectable phototropicresponse in Avena coleoptiles (19, 17). In Zca cole-optiles, 15 seconds' (560 ergs Cm l-2 sec-1) exposureto redl light induces the prolonged, decreased geotropicsensitivity (18), while the diageotropic rhizomes ofA1cgopodium podograria turn doxw-nwards after ex-

posure to weak red light for 30 seconds (2).The effects of red light on phototropism heave

been the subject of several recent investigations(1, 5, 6, 7, 8, 12, 19, 20 ). It is of interest to examine

whether these effects are in any way related to thle

red-light induced changes in geotropism. Thle reportof Curry (12) that exposure of Avena seedlings tored light for 1 hour decreased the phototropic sensi-

tivity of the coleoptiles was confirmed and stu liedl in

more dletail by Briggs (8). The dose-responilse curve

for phototropism is shifted by a factor of 10 aftercoleoptiles are exposed to light of wavelength 660 mll,for about 1.2 hours. Blaauw-Jansen (5), on theother hand, found the phototropic response of Avenacoleoptiles to be increased after exposure to red lightfor only 15 minutes. It is a matter of some difficultyto relate the responses describedd by Blaauw-Jansen(5) and by Briggs (8) [see Briggs (9) for discussionof these difficulties], and still more so to determine\w-hether either of these have anv relationship to theeffects of re(l light on geotropism reported by Blaaux\(3, 4), \Wilkins and Goldsmith (18) and the presentauthor. Several findings suggest that differentt mech-

alisms are involved in the responses describe l by

Briggs (8, 9) and in the present study. The maxi-

m1lum1l increase in geotropic curvature is brought about

bv 0.1 second's exposure to red light whereas about1 hour's exposure is required to achliexe the maxiimulmillchange in phototrol)ic sensitive ity (8). Although a

series of brief exposures given at 30-minute intervalsbefore phototropic in(luction seem also to be effective(8), it is clear that the induction of the shift in thephototropic dose response curve develops much moreslowly after exposure to red light begins an:1 reaches

a maximum at a time when the enhancedl geotropicsensitivity will have almost completely disappeared.The effect of red light on the geotropic resl)onse ofAvenla seems to occur in 3 distinct phases whereas

its effects on the phototropic response appear to beless complex. While the shift in the phototropic (loseresponse curve is apparently maintaine(l by prolong-ing the exposure to red light, at least for 2 hours, an 1

disappears only when (Iarkness is restore l, thle series

of changes in geotropic sensitivity is induced at theonset of irradiation anld proceeds both in light andl in

darkness. Even the duration of the initial increase

in geotropic responsiveness (phase 1) is unaffecte 1

bv keeping the seedlings in red light. It is just pos-sible that the increased phototropic sensitivity osAvena. reported by Blaauw-Jansen (5) may be relate l

to the changes in geotropic response since this coulclbe achieved with 15 minutes' exposure to red light.However, this relationship must remain very specula-tixye pending further evidence.

The distribution of curvature in etiolated and irra-

diatedi coleol)tiles is quite different. In etiolated cole-optiles curvature is fairly evenly distributed over theapical 20 mimii of the organ, although there is littlecurvature of the -apical 5 111111. WYhen geotropic.stimulation begins imme(liately after exposure to redflight (phase I) a sharp curvature developss in theapical 5 mm of the coleoptile, hut if it does not beginuntil a nunllmer of hours afterwards (phase Ill thereis little or nio curvature of the alpical 10 mm of thecoleoptile, and the geotropic response is greatly dle-pressedl. Blaauw (4) has also describedd the changein distribution of geotropic curvature along thlecoleoptile in phase I of the response and has foun Iit to be related t, thle dose] of red light which thecoleoptile has receivedl.

Turning now ti thle mechaniismn of the series ofchanges induced by red light in the geotropic respon-sivelenss of the Aqcnia caleoptile, 3 facts should be keptin min(l. Firstly, the effect is elicitedl in the first frac-tion of a secon(l of the exposure to flight andl passe.;through all its phases regardless of whether the see l-lings aire afterxw yards kept in redl light or (larkness.Secondly, once a 5-day-oAl coleoptile has been exposedto re l light it is incal)al)le of responding for a secondtime. Thirdly, the enhalnced capacity for geotropiccurvature can be retaine 1 if the steedlings are chilled1immedi atelk- after rradiati(un.

These facts suggest that redl light triggers off a

series of teinl)eratlir-e-(lel)eliclenit reactions in thecoleoptile. It is nolt knI )\ n at present whether theclhaged geotropic resp ,lnse is iliue to a chiianlge in thesenisitivity of thle geoperception mechanism of thecoleoptile, in the cal)acity of the tissue to transportclXin laterally upon geotropic stimulatioin, or il thegrowth rate of the tissue. If the changeJ geotl-opicrespoilse reflected changes in the growth rate of thecoleoptile, these might be accomplished by red lih,-I thrl-gillng about a marked increase in the rate of re-

lease of a growth stimulatillg substance from a bonn 1

fOrmll, a release that would otherwise occur at a lowrate. This could be auxin or al substance which ill-creases the sensitivity of the cells of the growing ZonIeof the coleoptile to auxini. The temperature-lepenlIl-ent dlisaappearance of the enhanced geotropic respoln-sivelness may be attributable either to the metalholicbreak(lown of the substance alter its release or to its

basipetal tran-isport away fromll the growinlOg le111 O.the coleoptile.

Curry, Thimaun11lll andA Rav ( 13 have showing that1 hour's exposure of an Aqcena seedling to redl lightincreases the growth rate of the alpical 10 mill of thecoleoptile in the 100 minutes after irradiatioll.l-riggs (8 ) has further slhownl that such a treatmenLtdecreases the plrodluctionl of auxin in the lz'cmcoleoptile tip. Unfortunately the minimum dosagerequirements for these effects were not (letermine(lso it is not know n whether they are induced by thesmall (loses of red light 'which have such a markedeffect oil the geotropic responsiveness of the coleop-tiles. Nevertheless, these findings demonstrate thatthe sensitivity of the tis.sutie to (aux-inl mlaust be enhanced,1

32





shortly after irradiation begins. Liverman and Bon-ner (15) have, in fact, found that pretreatment withred light increases the extension of Avena coleoptilesections in various concentration of IAA, but theymeasured the growth after a period of 16 hours.

The different phases of the effect of red light on

the geotropic response of the Avena coleoptile may,

therefore, be due to the interaction of several red-light induced changes, each of which reaches a maxi-mum at a different time after the onset of irradiation.The first phase (increased responsiveness) could bedue to an increased rate of release of a substance whichenhances the sensitivity of the tissue to auxin andwhich may persist for several hours in the growingzone of the coleoptile. Blaauw-Jansen (5) has ex-

tracted such a substance from coleoptiles of Avenawhich had been exposed to red light. It was found to

increase the growth-promoting effects of low auxinconcentrations which alone had little effect. The dis-appearance of phase I may be associated with thelowering of auxin production in the tip. Duringphase II of the response, the enhanced sensitivity toauxin and lower auxin supply may result in a rate ofgrowth similar to that of unirradiated coleoptiles.With the destruction of the sensitivity enhancing sub-stance, or its transport away from the growing zone

of the coleoptile, the lower auxin production may thenbe manifest in a lower growth rate of the tissue anJ1the development of a lower geotropic curvature (phaseIII). Briggs (8) has found, at least in Zea, thatauxin production by the tip of the coleoptile increasesto its original level about 3 hours after darkness isrestored. This finding would imply that the persist-ence of the decreased geotropic sensitivity (phaseIII) in Avena and in Zea (18) is the result of a per-

manent lowering of the sensitivity of the tissue toauxin due, perhaps, to the whole of the available sub-stance which increases the sensitivity of the tissue toauxin having been released from the bound form bythe exposure to red light.

The effectiveness of red light (655 muL) and itselimination by far-red radiation (740 min,) indicatesthat the pigment involved in this response is phyto-chrome. The data of Hendricks (14) and Butleret al. (11) suggest that the lack of response of thecoleoptile to a second exposure to red light may bedue to the absence of the pigment. In Zea coleoptilesthe amount of red absorbing form of the pigment (Pr)immediately after exposure to red light is very low(14), most of the pigment being in the far-red ab-sorbing form (Pfr). As time proceeds, however,some Pfr is reconverted to Pr but much of the Pfr isdestroyed so that the total amount of phytochrome(now all in the Pr form) after 4 hours in darknessis only about 30 % of that originally present. Thecomparable data for Avena coleoptiles are not yetavailable, but if destruction were even more prefer-ential to reconversion in Avena than in Zea, then thelevel of Pr present several hours after the exposure

to red light might be too low for perception of thelight stimulus. In addition to this, there may be little

or none of the bound form of the postulated auxinsensitivity enhancing substance reformed in 4-dayand 5-day-old coleoptiles so that even if the stimuluswere weakly perceived, there could be no further re-lease of this substance into the growing zone of thecoleoptile.

In conclusion it must be pointed out that, in comi-mon with several recent studies (3, 4, 5, 7, 8, 9, 12, 18),the present results show that the use of a red safelightfor phototropic and geotropic studies is fraught withdanger and should be avoided. So complex are thechanges induced by red light that it is impossible tocompare experiments carried out on seedlings grownunder different conditions of illumination (i.e. redlight given to inhibit the mesocotyl, or intermittentlyto see whether the seedlings were of suitable size forthe experiment) and under different manipulative con-ditions. It is to be hoped, therefore, that some uni-formity in growing conditions and the use of greensafelights can be agreed upon in the near future.

SummaryRed light (655 myL) induces a series of changes in

the geotropic responsiveness of dark-grown coleop-tiles of Avena sativa. The responsiveness is in-creased immediately after a few minutes' irradiationbegins but returns to a value similar to that of un-irradiated coleoptiles 60 to 90 minutes later. Theincreased responsiveness cannot be prolonged by ex-tending the exposure to light. Between the seventhand twenty-third hours after irradiation the respon-siveness of the coleoptiles declines to about one hal:that of the unirradiated coleoptiles.

Dark-grown coleoptiles are exceedingly sensitiveto red light; 3.3 x 10-13 einsteins cm-2 (0.6 ergscm-2) give a significant increase, and 3.3 X 10-12 ein-steins cm-2 (6.0 ergs cm 2) the maximum increasein the geotropic responsiveness. The Bunsen-Roscoereciprocity law is valid for both these dosages. Theeffects of red light (655 mf.) are reversed by far-redlight (740 mrL).

After having been exposed to red light for a fewminutes, 5-day-old coleoptiles are incapable of re-sponding to a second exposure regardless of whetherthis is given 4.5 hours or 24 hours after the first lighttreatment.

The increased geotropic responsiveness can be re-tained for at least 28 hours if the seedlings are chilledimmediately after exposure to red light.

The results suggest that the onset of the red-lighttreatment triggers off in the coleoptile a series oftemperature-dependent reactions which proceed tocompletion at 250 regardless of whether the coleoptilesare kept in red light or darkness, but which can bearrested by low temperatures. It appears that thisseries of reactions can be induced only once in thecoleoptiles.

AcknowledgmentsThe author wishes to express his sincere thanks to

Professor Kenieth V. Thimann for kindly inviting him

33




4PLANT PHYSIOLOGY

to accept a Research Fellowship at Harvard University,and for his keen interest in this project. His thanks arealso due to Drs. B. Bonner, M. Davies, M. Furuya,A. Hollowynsky, and Mr. T. P. O'Brien for many stim-ulating discussions.

Literature Cited1. ASOMANING, E. J. A. AND A. W. GAL-STON. 1961.

Comparative study of phototropic response and pig-ment content in oat and barley coleoptiles. PlantPhysiol. 36: 453-64.

2. BENNET-CLARK, T. A. AND N. G. BALL. 1951. Thediageotropic behaviour of rhizomes. J. Exp. Bot-any 2: 169-203.

3. BLAAUW, 0. H. 1961. The influence of blue, redand far-red light on geotropism and growth of theAvena seedling. Acta botan. Neerl. 10: 397-450.

4. BLAAUW, 0. H. 1963. Effects of red light on geo-tropism of AZcna and their possible relations toI)hototropic phenomena. Acta botan. Neerl. 12:424-32.

5. BJLAAUW\-JANSEN, G. 1959. The influence of redand far-red light on growth and phototropism ofthe .4ze'na seedling. Acta botany. Neerl. 8: 1-39.

6. BRIGGS, W. R. 1960. Light dosage and phototropicresponses of corn and oat coleoptiles. PlantPhysiol. 35: 951402.

7. BRIGGS, WV. R. 1960. Red light, auxin productionand phototropic behaviour in corn coleoptiles.(Abstr.) Plant Physiol. 35: xxxi-xxxii.

8. BRIGGS, W. R. 1963. Red light, auxin relationshipsand the phototropic responses of corn and oatcoleoptiles. Am. J. Botany 50: 196-207.

9. BRIGGS, W. R. 1963. The phototropic responses ofhigher plants. Ann. Rev. Plant Phvsiol. 14:311-52.

10. BUNSEN, R. AND H. RoSCOE. 1862. PhotochecmiscihUntersuchungen. Ann. Phys. Chem. 117: 529-62.

11. BUTLER, W. L., H. C. LANE, AND H. W. SIEGELMAN.1963. Nonphotochemical transformations of phyto-chrome in vivo. Plant Physiol. 38: 514-19.

12. CURRY, G. M. 1957. Studies on spectral sensitivityof phototropism. Doctoral thesis, Harvard Univer-sity, Cambridge, 'Mass.

13. CURRY, G. Ml., K. V. TjI-IMANN, AND) P. M. RAxY.1956. The base curvature response of -ilciiaseedlings to ultra-violet. Physiol. Plantarum 9:429-40.

14. HENDRICKS, S. B. 1960. Rates of change of pahyto-chrome as an essential factor determining photo-periodism in plants. Cold Spring Harbor Sympo-siumn 25: 245-48.

15. LIVERMAN, J. L. AND J. BONNER. 1953. The inter-action of aux n and light in the growth respon)sesof plants. P.N.A.S. 39: 905-16.

16. SIEGELMAN, H. XV. AND E. MI. FIRER. 1964. Puri-fication of phytochrome from oat seedlings. Bio-chemistry 3: 418-23.

17. THIIMANN, K. V. AND G. M. CtLRRY. 1961. Photo-periodism. Inl: Light and Life. XV. D. McElroyand B. Glass, ed. Johns Hopkinis Press, Baltinlorc,Maryland, 64070.

18. WILKINS, M. Bi. AND 'MARY HEIEN-.- M. Goi.i)s-\ir i,.1964. The effects of red, far-red and blue lighton the geotrop)ic response of coleoptiles of /c((1/1(1 I'S. J. Expl. Botany 1 5: 600-15.

19. ZINEINIERNIAN, B. K. AND WV. R. BIs.S. 1963).Phototrol)ic (losage-response curves for oat coleop-tiles. Plant Physiol. 38: 248-53.

20. ZIMMERMAN, B. K. AND XNV. R. BRIGGS. 1963. Akinetic model and phototropic responses of o~atcoleop)tiles. Plant Physiol. 38: 253-61.

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